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Nitrate Reduction in Roots as Affected by the Presence of Potassium and by Flux of Nitrate through the Roots
Abstract
Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed to treatment solutions containing Ca(NO(3))(2), NaNO(3), or KNO(3); KNO(3) plus 50 or 100 millimolar sorbitol; and KNO(3) at root temperatures of 30, 22, or 16 C. In all experiments, the accelerated phase of NO(3) (-) transport had previously been induced by prior exposure to NO(3) (-) for 10 hours. The experimental system allowed direct measurements of net NO(3) (-) uptake and translocation, and calculation of NO(3) (-) reduction in the root. The presence of K(+) resulted in small increases in NO(3) (-) uptake, but appreciably stimulated NO(3) (-) translocation out of the root. Enhanced translocation was associated with a marked decrease in the proportion of absorbed NO(3) (-) that was reduced in the root. When translocation was slowed by osmoticum or by low root temperatures, a greater proportion of absorbed NO(3) (-) was reduced in the presence of K(+). Results support the proposition that NO(3) (-) reduction in the root is reciprocally related to the rate of NO(3) (-) transport through the root symplasm.
... Some studies evaluated the relationship between K and N metabolism. In contrast to the antagonistic relationship between K + and NH 4 + nutrition, the acquisition rates of K + and NO 3 − are often found to be positively correlated (Rufty et al., 1981;Coskun et al., 2016), and sufficient K supply can promote N metabolism and enhance the synthesis of amino acids and proteins (Ruan et al., 1998;Ruiz and Romero, 2002). Hu et al. (2016b) found that K deficiency could reduce Nitrate reductase (NR), Glutamine synthetase (GS), and Glutamate synthase (GOGAT) activities and inhibit nitrate absorption in cotton, whereas Armengaud et al. (2009) found that K deficiency could up-regulate the activities of GS and Glu dehydrogenase (GDH) in Arabidopsis. ...
... This suggests that appropriate K supply not only increases NO 3 − absorption in roots, but also promotes the transport from roots to shoots. Rufty et al. (1981) and Wang et al. (2003) also reported that K deficiency can seriously hinder the assimilation and translocation of nitrate, and higher N assimilation occurs in the roots. So it is necessary to focus on the effect of K on N assimilation in roots and leaves. ...
... Thus the activity of NR and GS will affect the absorption and assimilation of N. The inhibition of K deficiency on NR activity has been verified in cotton, cucumber, and Arabidopsis (Ruiz and Romero, 2002;Balkos et al., 2010;Hu et al., 2016b). Generally, with high external K + supply, the co-translocation of K + and NO 3 − to the shoot increases (Ben Zioni et al., 1971;Blevins et al., 1978), and both storage of NO 3 − and NR activity increases in leaves, while less N assimilation is found in roots (Blevins et al., 1978;Rufty et al., 1981). Consistent with previous results, we also found that for a certain range, with the increase of K supply, GS activity of roots and NR activity of leaves of M9T337 seedlings gradually increased, which promoted the assimilation of NO 3 − . ...
Nitrogen (N) is one of the most required mineral elements for plant growth, and potassium (K) plays a vital role in nitrogen metabolism, both elements being widely applied as fertilizers in agricultural production. However, the exact relationship between K and nitrogen use efficiency (NUE) remains unclear. Apple dwarf rootstock seedlings (M9T337) were used to study the impacts of different K levels on plant growth, nitrogen metabolism, and carbon (C) assimilation in water culture experiments for 2 years. The results showed that both deficiency and excess K inhibited the growth and root development of M9T337 seedlings. When the K supply concentration was 0 mM and 12 mM, the biomass of each organ, root-shoot ratio, root activity and NO3– ion flow rate decreased significantly, net photosynthetic rate (Pn) and photochemical efficiency (Fv/Fm) being lower. Meanwhile, seedlings treated with 6 mM K⁺ had higher N and C metabolizing enzyme activities and higher nitrate transporter gene expression levels (NRT1.1; NRT2.1). ¹³C and ¹⁵N labeling results showed that deficiency and excess K could not only reduce ¹⁵N absorption and ¹³C assimilation accumulation of M9T337 seedlings, but also reduced the ¹⁵N distribution ratio in leaves and ¹³C distribution ratio in roots. These results suggest that appropriate K supply (6 mM) was optimal as it enhanced photoassimilate transport from leaves to roots and increased NUE by influencing photosynthesis, C and N metabolizing enzyme activities, nitrate assimilation gene activities, and nitrate transport.
... Firstly, the model pertains to plants that reduce NO 3 À mainly in the shoot, which is a common but by no means universal occurrence (Pate 1973;Andrews 1986;Touraine et al. 1990). Interestingly, a low supply of K + can increase the degree of NO 3 À reduction in roots relative to shoots, possibly because of the lack of a xylem-mobile cation to accompany NO 3 À moving to the shoot (Rufty et al. 1981;Förster & Jeschke 1993; see Section on Metabolism). Secondly, even when shoot reduction is pronounced, and the upward movement of K + as a counterion with NO 3 À is observed, a significant downward recirculation of K-malate in the phloem is not always seen ( Kirkby and Knight 1977). ...
... One such connection involves the partitioning of NR activity between the root and shoot (Blevins et al. 1978;Barneix & Breteler 1985;Förster & Jeschke 1993;Casadesús et al. 1995;Armengaud et al. 2009), which also depends on plant species, external nitrate supply, temperature and light intensity (Pate 1973;Smirnoff & Stewart 1985;Andrews 1986). Generally, with high external K + supply, the co-translocation of K + and NO 3 À to the shoot increases (Ben Zioni et al. 1971;Blevins et al. 1978; see Section on Transport; Fig. 3), and both storage of NO 3 À and NR activity increases in leaves, while less N assimilation is found in roots (Blevins et al. 1978;Rufty et al. 1981). In contrast, with K + deprivation, less translocation of NO 3 À is observed, and hence, higher N assimilation occurs in roots ( Förster and Jeschke 1993;Rufty et al. 1981;Wang et al. 2003). ...
... Generally, with high external K + supply, the co-translocation of K + and NO 3 À to the shoot increases (Ben Zioni et al. 1971;Blevins et al. 1978; see Section on Transport; Fig. 3), and both storage of NO 3 À and NR activity increases in leaves, while less N assimilation is found in roots (Blevins et al. 1978;Rufty et al. 1981). In contrast, with K + deprivation, less translocation of NO 3 À is observed, and hence, higher N assimilation occurs in roots ( Förster and Jeschke 1993;Rufty et al. 1981;Wang et al. 2003). Interestingly, this has not been borne out in the model species Arabidopsis thaliana; Armengaud et al. (2009) observed significant decreases in root NR activity, relative to K + sufficiency, with prolonged (2 weeks) K + deprivation. ...
... In fact, potassium is further involved in the partitioning of NR activity between the root and the shoot (Blevins et al., 1978b;Armengaud et al., 2009), which also depends on plant species, external nitrate supply, temperature and light intensity (Pate, 1973;Smirnoff & Stewart, 1985;Andrews, 1986). As mentioned earlier, K promotes long-distance transport of NO3− from roots to the leaves and thus improves NO3− contents and nitrate reductase activity in leaves, facilitating N use efficiency and metabolism in tobacco (Zioni et al., 1971), barley (Blevins et al., 1978b) and soybean (Touraine et al., 1988), while less N assimilation occurs in barley (Blevins et al., 1978b) or corn (Rufty et al., 1981) roots. At the opposite, K deficiency leads to lower translocation of NO3− and then a higher N assimilation in corn roots (Rufty et al., 1981). ...
... As mentioned earlier, K promotes long-distance transport of NO3− from roots to the leaves and thus improves NO3− contents and nitrate reductase activity in leaves, facilitating N use efficiency and metabolism in tobacco (Zioni et al., 1971), barley (Blevins et al., 1978b) and soybean (Touraine et al., 1988), while less N assimilation occurs in barley (Blevins et al., 1978b) or corn (Rufty et al., 1981) roots. At the opposite, K deficiency leads to lower translocation of NO3− and then a higher N assimilation in corn roots (Rufty et al., 1981). Moreover, K was found to be involved in ammonium assimilation by up-regulating glutamine synthetase (GS), ferredoxin-glutamine-2-oxoglutarate aminotransferase (Fd-GOGAT) and glutamate dehydrogenase (GDH) (Armengaud et al., 2009;Hu et al., 2016b), suggesting a K effect as a compensatory response to maintain C flux through the TCA cycle and into amino acids and proteins. ...
Oil palm (Elaeis guineensis Jacq.) is one of the most productive oil crop in the world. Unfortunately, positive effect of K fertilization on fruit development with increasing in bunch weight and number, remain rather difficult to predict because of the lack of knowledge of underlying metabolic mechanisms. The objective of this thesis was precisely to assess the effect of K availability on oil palm metabolic pathways and determine if metabolic changes could be related to oil production. In addition to expected effects of potassium on vegetative traits and bunches, our results show that K availability affected carbon and nitrogen primary metabolism in both leaflets and developing fruits.This thesis presents, for the first time, a detailed metabolic exploration of oil palm in the field and shows that some metabolic traits (metabolites or enzymes) are linked to K availability, thereby opening avenues for the use of leaf biochemical markers to monitor oil palm mineral nutrition.
... K + , le cation majeur des cellules végétales est généralement considéré comme l'ion accompagnateur de NO 3 qui permet d'assurer l'électroneutralité du transport de l'ion. De fait les flux de K + et de NO 3 sont fortement corrélés, en particulier lors de l'étape de translocation vers les parties aériennes (Minotti et al., 1968, Frost et al., 1978, Rufty et al., 1981. ...
... L'effet stimulateur de K + sur le transport de NO 3 dans la racine est connu depuis longtemps (MacRobbie, 1971). La présence de K + affecte de manière différentielle les deux étapes du transport NO 3 de la racine (la stimulation est beaucoup plus marquée sur la translocation que sur l'absorption, (Rufty et al., 1981). Si la signification biologique de la stimulation de l'expression d'AtNRT1.1 par la carence en K + reste à élucider, la stimulation des trois autres gènes est cohérente avec les données physiologiques. ...
... Studies have demonstrated that the K supply increases the uptake and transport of K and NO 3 − to the plant shoots (Blevins et al. 1978;Fageria 2001;Rufty et al. 1981;Souto et al. 2018) and increases the Zn uptake and use efficiency by plants (Shukla and Mukhi 1980;Souto et al. 2018). In potato crops, the K supply also promotes changes in the concentrations of N, P, Ca, and Mg in certain plant tissues (Khan et al. 2012;Laughlin 1996;Neshev and Manolov 2015). ...
... 2a,b and 3a,c). Previous studies have found that K is related to NO 3 − transport to plant shoots via the xylem, increasing the NO 3 − storage and nitrate reductase activity in the leaves (Blevins et al. 1978;Rufty et al. 1981). In addition, as NO 3 − is taken up by plant roots through an active process (Marschner 2012), the NO 3 − uptake can be affected by the influence of K on photoassimilate translocation, which is required to support this active uptake process (Ashley and Goodson 1972;Fageria 2001;Marschner 2012). ...
Potassium (K) supply affects the growth and tuber yield of potato (Solanum tuberosum L.) as well as the uptake and removal of certain nutrients. However, information on this is scarce or inconsistent. Thus, this study was undertaken to evaluate the dry matter (DM) accumulation and the uptake and removal of nutrients by potato ‘Agata’ as affected by K fertilizer management (rates and application timings) in tropical clay soils with varied K availability. In soils with low K availability, K fertilization increased (P ≤ 0.05) the biomass of potato plants without differences among management types; however, in soils with medium and high K availability, K fertilizer did not alter the plant DM accumulation. K fertilization increased (P ≤ 0.05) N, K, Ca, Mg, S, B, Mn, and Zn uptake and removal in the soil with low K availability, while in soils with medium and high K availability, K fertilization had less influence on the uptake and removal of nutrients, except that K uptake and removal increased (P ≤ 0.05) under K fertilization, evidencing luxury uptake. In the soil with medium K availability, K application reduced (P ≤ 0.05) Mg uptake by plants, but this effect was not observed in soils with high K availability. Mn uptake and removal increased (P ≤ 0.05) under K fertilization in soils with low and high K availability. The increases in the uptake and removal of almost all nutrients in response to K fertilization were related to the increase in plant DM accumulation, but changes in the concentrations of K and some other nutrients also contributed to their increased uptake and removal.
... Meanwhile, dark respiration rate is increased and results in deterioration of plant growth and quality (Usherwood, 1985). K may also have indirect impacts on quality due to its interaction with other nutrients particularly with N and production practices (Rufty Jr et al., 1981;Coskun et al., 2017). It is important for the storage and preservation of fruit after harvest. ...
Essential macronutrients, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), significantly contribute to plant growth and developmental processes. The occurrence of nutritional stress, particularly the inadequate application of essential macronutrients, leads to the manifestation of numerous deficiency symptoms because of suboptimal nutrients supply. The utilization of visual deficiency symptoms has been widely employed in the field of plant nutrition for the purpose of assessing the nutritional status and optimizing the efficacy of fertilizer application. Hence, it is imperative to comprehend the morphological , physiological, and molecular changes exhibited by fruit trees in response to nutrient stress. This review discussed to establish a comprehensive connection between the visual phenotypic deficiency symptoms and the underlying molecular and physiological responses correlated with macronutrient dynamics in fruit trees. we proposed a range of nutrient management strategies, including integrated nutrient management and the principles of the 4 R's (right source, right rate, right time, and right place) that can be supportive for better plant growth and optimized production under nutrients stress environment. We also explored the significant contribution of advanced molecular tools such as omics tools (genomics, transcriptomics, proteomics, metabolomics, and ionomics) and automatized technologies (imaging technology, remote sensing, and in-field sensors). These advancements are integral to integrated nutrient management, as these can be harnessed for improving fruit tree nutrition and promoting sustainability in orchard systems. These recent advancements underlying plant responses to nutrients deficiencies offer an invaluable opportunity for a deeper understanding of macronutrients functionality in fruit trees.
... In the case of unbalanced fertilization, a lower K application can lead to significant depletion of available soil K reserves, resulting in a decline in soil fertility and an ongoing downward trend of negative K balance [5,12]. Soil K deficiency hinders normal crop plant development, reduces the absorption and utilization of water and nutrients, and affects the synthesis and transportation of photosynthates by affecting the diffusion of CO 2 in chloroplasts, phloem-loading, and the long-distance transport of photoassimilates, ultimately decreasing crop yield [13,14]. Potassium is closely related to the root absorption of NO − 3 and K + and the coordinated transportation of the phloem sap [15,16]. Further, K significantly affects the synthesis and degradation of amino acids and proteins during N metabolism [2], which, in K-deficient plants, is severely inhibited. ...
Field experiments were conducted to investigate the effects of K application on the nitrogen and potassium utilization efficiency and yield of foxtail millet (Setaria italica L.). The experiment was performed with a completely randomized design with two millet cultivars (Jingu 21 and Zhangza 10) and five K2O rates (0, 60, 120, 180, and 240 kg/hm2) in 2020 and 2021. We found that K promoted K and N absorption; significantly increased dry matter (DM), N, and K accumulation in millet organs; caused dry matter accumulation to peak earlier; and increased the DM accumulation rate. In addition, K accumulation preceded that of DM or N. Relative to the crop cycle, most K (61.07%) accumulated at booting, whereas N accumulated mostly (33.86%) during grain filling. N absorption efficiency increased by 31.87%, and the apparent and agronomic utilization rate of K fertilizer remained high, increasing millet yield, which peaked at a K rate of 180 kg/hm2 for both cultivars, by 29.91% and 31.51% in Jingu 21 and Zhangza 10, respectively, relative to untreated controls. Stepwise regression and path analysis showed that the leaf and spike K accumulation, stem N accumulation, and stem DM were the main factors affecting yield, with DM having the greatest direct effect, followed by leaf K accumulation. The K concentration (0.77–3.04%) in Zhangza 10 was higher than that in Jingu 21 (0.69–2.91%) in untreated plants. Under the same K application rate, N and K accumulation and the harvest index were higher for Zhangza 10 than those for Jingu 21, and the nutrient utilization ability was greater for Zhangza 10. The results demonstrated that rational K fertilizer application can increase K concentration and accumulation in leaves, promote N metabolism and accumulation, increase N and K utilization efficiency, and improve DM accumulation and millet yield.
... K and N metabolism relationship has been evaluated in some studies. In contrast to the antagonistic relationship between K + and NH 4 + nutrition, a positive correlation was found to exist between the acquisition rates of K + and NO 3 − [24,25] and the synthesis of amino acids and proteins can be enhanced by a sufficient supply of K, which promoted N metabolism [26,27]. Potassium (K) deficiency was found to reduce Nitrate reductase (NR), Glutamine synthetase (GS), and Glutamate synthetase (GOGAT) activities and this inhibited nitrate absorption in certain plants [28]. ...
Citrus fruit is one of the most important fruits all over the world. Citrus fruits are essential sources of food and energy and play a critical role in supplementing healthy diets. They contain vitamins A, E, and B (thiamine, riboflavin, and niacin), minerals, and antioxidants such as flavonoids, vitamin C, phenolic compounds, and carotenoids as nutrients present in them. Mineral nutrients are essential nutrients found in many different types of plant and animal-based foods. Macro-minerals are required in large amounts while trace minerals are needed in minute quantities such as iron, zinc, and copper. Potassium is a vital nutrient in citrus fruit which regulates fruit size, fruit appearance, fruit color, and vitamin content. Fresh size and mass, percentage of rind and juice, soluble solids content (SS), titratable acidity (TA), SS/TA ratio, and industrial yield, expressed in kg of sugar per 100 kg of processed fruit or SS per box (40.8 kg) are used to evaluate fruit quality in citrus fruits. The amount of potassium below 0.4% affects tree development; otherwise, over an extensive range of variation does not generally affect plant growth. Potassium is one of the abundant elements in citrus fruits that affect both yield and quality. This chapter reviews the role of mineral nutrients in citrus production and the roles play in the human body.
... Also, the N metabolism in cotton leaves was promoted by K fertilizer application. Potassium fertilizer application could improve NO 3 − uptake by roots (Rufty et al. 1981), leading to high N concentration and nitrate content in cotton leaves (Hu et al. 2016b;Zahoor et al. 2017b). Hu et al. (2017) found that K fertilizer application decreased the C/N ratio and the ratio of soluble sugar to free amino acid in cotton leaves, since after adding K fertilizer, the transport of C assimilation products to the reproductive organs was improved more significantly than that of N assimilation products. ...
Background
Many studies have indicated that straw-returning could meet part or even all of the potassium (K) demand for crop growth in the field, but few have compared the effects of crop straw as K source and inorganic K fertilizer on carbon–nitrogen (C–N) balance of cotton and the reproductive growth. To address this, field experiments were conducted using the cotton cultivar, Siza 3, under there treatments (CK as control group one, no crop straw and inorganic K fertilizer were applied; K150 as control group two, 150 kg·ha ⁻¹ of K 2 O was applied; and W9000, 9 000 kg·ha ⁻¹ wheat straw, which could provide K 2 O about 150 kg·ha ⁻¹ , was incorporated into soil).
Results
Although the final reproductive organ biomass did not differ between W9000 and K150, W9000 had a higher ratio of reproductive organ biomass to total biomass (RRT), suggesting that straw-returning was more conducive to the allocation of biomass to reproductive organs. The theoretical maximum biomass of reproductive organ was higher, but the average and maximum accumulation rates of reproductive organ biomass were 2.8%∼8.3% and 2.5%∼8.2% lower under W9000 than K150. Also, the duration of rapid-accumulation period for reproductive organ biomass (T) was 2.0∼2.8 d longer under W9000 than K150, which was a reason for the higher RRT under W9000. Straw-returning altered the dynamics of leaf K with the growth period, so that W9000 had a more drastic effect on leaf C metabolism than K150. Consequently, lower soluble sugar/free amino acid and C/N ratios were measured under W9000 than K150 at boll-setting (BSS) and boll-opening (BOS) stages. Higher leaf net photosynthetic rate, sucrose phosphate synthase and sucrose synthase activities, and lower acid invertase activity were observed under W9000 than K150 at BSS and BOS and these were more conducive to sucrose accumulation. However, less sucrose was measured under W9000 than K150 at these stages. This should be because straw-returning promoted the assimilate transport capacity when compared with inorganic K fertilizer application, which also explained the higher RRT under W9000 than K150. The lower acid invertase activity under W9000 inhibited the conversion of sucrose to other sugars, hence lower contents of soluble sugar and starch were measured under W9000 than K150.
Conclusion
Under low K condition, crop straw as K source can increase the assimilate transport from source to sink, leading to lower C/N ratio in leaf and higher allocation of biomass to reproductive organs than inorganic K fertilizer.
... The acquisition rates of cationic K + and anionic NO 3 are often found to be positively correlated, probably due to improved charge balance or activation of the enzymes involved in NO 3 assimilation (Hagin et al., 1990;Roosta and Schjoerring, 2008;Balkos et al., 2010;Yang et al., 2014;Xia et al., 2015). NO 3 is transported from root to shoot with K + as a counter ion in the xylem; thus, limited K + supply can result in high accumulations of NO 3 in roots (Rufty et al., 1981;Förster and Jeschke, 1993). Knockout of the nitrate transporter AtNPF7.3/NRT1.5 in Arabidopsis and OsNPF2.4 in rice not only decreased NO 3 loading to xylem sap, but also limited K + content in the xylem (Lin et al., 2008;Xia et al., 2015;Li et al., 2017), indicating the interaction of NO 3 and K + in plant cells. ...
The enzymatic controlled metabolic processes in cells occur at their optimized pH ranges, therefore cellular pH homeostasis is fundamental for life. In plants, nitrogen (N) source for uptake and assimilation, mainly in the forms of nitrate (NO3-) and ammonium (NH4+) quantitatively dominates the anion and cation equilibrium and the pH balance in cells. Here we review ionic and pH homeostasis in plant cells and regulation by N source from the rhizosphere to extra- and intra-cellular pH regulation for short and long-distance N distribution and during N assimilation. In the process of N transport across membranes for uptake and compartmentation, both proton-pumps and proton-coupled N transporters are essential, and their proton binding sites may sense change of apoplastic or intra-cellular pH. In addition, during N assimilation, carbon skeletons are required to synthesize amino acids, thus the combination of NO3- or NH4+ transport and assimilation results in different net charge and numbers of protons in plant cells. Efficient maintenance of N-controlled cellular pH homeostasis may improve N uptake and use efficiency, as well as enhance the resistance to abiotic stresses.
... Synthesis-impairment of starch and cellulose (high molecular weight compounds) and increase in soluble sugars, organic acids, amino acids, and nitrate (low molecular weight compounds) are the reasons of susceptibility of K-deficient plants to pathogen attack (Marschner, 1995). Potassium also controls the water and solute movement to and in the xylem (Rufty, Jackson, & Raper, 1981) and is responsible for phloem loading and transport (Marschner, 1995). Plant K thus affects allocation of photosynthates from source to sink. ...
... Root-to-shoot translocation and shoot homeostasis of K determine nutrient balance, growth, and stress tolerance of vascular plants (Drechsler et al. 2015). Triplett et al. (1980) demonstrated the close relationship between K ? and NO À 3 uptake and transport, which was later confirmed by Rufty et al. (1981). Potassium flow and partitioning can be transformed in response to the form of N (Szczerba et al. 2008;Zhang et al. 2010). ...
The nitrogen isotopic compositions of plant tissue could reflect its uptake of and preference for ammonium or nitrate. However, various factors may influence the field-collected δ¹⁵N values under field condition, which causes the interpretation problematic. The spatial variation of nitrogen (N) concentrations and the isotopic compositions were investigated in the soils and tissues of Chinese prickly ash from the southwest China to the east China. The objectives were to investigate the variation in soil and tissue δ¹⁵N values and N forms taken up by the plant. The leaf and root δ¹⁵N values varied significantly in response to the pattern of soil δ¹⁵N values. The difference in δ¹⁵N values between the leaves and roots was 2.57‰ and may be caused by an increase in the transport of unassimilated \( {\text{NO}}_{3}^{ - } \) and \( {\text{NH}}_{4}^{ + } \) to the leaves. Leaf nitrogen was significantly and positively correlated with leaf potassium and negatively related to leaf calcium. Because potassium is the favoured counter-cation for nitrate transport in the xylem, the enrichment of ¹⁵N in leaf relative to root induced by preferenced uptake of nitrate should be accompanied by significant and positive relationship of leaf nitrogen with leaf potassium concentrations. These results suggest that Chinese prickly ash prefers \( {\text{NO}}_{3}^{ - } \) over \( {\text{NH}}_{4}^{ + } \).
... Using Glycine max, which reduces ca 90% of NO3 in the leaves, however, evidence has been provided for the validity of the model (Touraine et al. 1988). The role for K* in NO3 translocation has been suggested to be through either K* stimulation of NO3 loading into the xylem or through transport of K* as a counterion for NO3 into the xylem (Rufty et al. 1981, 1982). The extent of engagement of the K*-shuttle may determine the site of NO3 reduction within the plant and may, therefore, differ in activity between the slow-growing and fast-growing plants. ...
... Since the concentration of NO3-in the root was declining during this period, the capacity for NO3storage was not saturated. The assimilatory pathway for NO3-in roots is composed of multiple components, which compete for available NO3- (9,27). The absence of accumulation beyond 6 h thus would suggest that a new equilibrium was attained between transport and reduction functions. ...
An experiment was conducted to investigate the reduction of endogenous NO3⁻, which had been taken up by plants in darkness, during the course of the subsequent light period. Vegetative, nonnodulated soybean plants (Glycine max [L]. Merrill, `Ransom') were exposed to 1.0 millimolar ¹⁵NO3⁻ for 12 hours in darkness and then returned to a solution containing 1.0 millimolar ¹⁴NO3⁻ for the 12 hours `chase' period in the light. Another set of plants was exposed to ¹⁵NO3⁻ during the light period to allow a direct comparison of contributions of substrate from the endogenous and exogenous sources. At the end of the ¹⁵NO3⁻ exposure in the dark, 70% of the absorbed ¹⁵NO3⁻ remained unreduced, and 83% of this unreduced NO3⁻ was retained in roots. The pool of endogenous ¹⁵NO3⁻ in roots was depleted at a steady rate during the initial 9 hours of light and was utilized almost exclusively in the formation of insoluble reduced-N in leaves. Unlabeled endogenous NO3⁻, which had accumulated in the root prior to the previous dark period, also was depleted in the light. When exogenous ¹⁵NO3⁻ was supplied during the light period, the rate of assimilation progressively increased, reflecting an increased rate of uptake and decreased accumulation of NO3⁻ in the root tissue. The dark-absorbed endogenous NO3⁻ in the root was the primary source of substrate for whole-plant NO3⁻ reduction in the first 6 hours of the light period, and exogenous NO3⁻ was the primary source of substrate thereafter. It is concluded that retention of NO3⁻ in roots in darkness and its release in the following light period is an important whole-plant regulatory mechanism which serves to coordinate delivery of substrate with the maximal potential for NO3⁻ assimilation in photosynthetic tissues.
One of the main factors regulating biomass production, enzymatic activity and the amount of nitrogen compounds in roots and leaves of snap beans (Phaseolus vulgaris L.) is the nutritional status of K, hence the objective of this research was to understand the effect of different potassium concentration on these physiological plant responses. The experiment was carried out with the snap bean cv. Strike in growth chambers during the spring of 2000 in Granada, Spain. The controlled environment conditions were: relative humidity 60-80%, temperature 28/22 °C (day/night), photoperiod 16/8 h (day/night) and luminosity of 350 µmol m-2 s-1. Potassium was applied as nutrient solution of KOH in increasing concentrations: K1= 1.0 mM, K2= 2.0 mM, K3= 4.0 mM, K4= 8.0 mM, K5= 12.0 mM, and K6=16.0 mM of K+. Results indicated that both deficiencies and excess of K affected the nitrogen metabolism in the same way that the effect caused by N stress. Under K+ deficiencies (K1 and K2) there was a decrease in the content of enzymes related to N assimilation such as nitrate reducíase, nitrite reductase, glutamine sintetase, glutamine sintase, and phosphoenolpyruvate carboxylase, causing in the plant low concentrations of amino acids, proteins, nitrogen organic compounds, all of which diminished biomass production in 15% when compared to K3. Treatments with higher concentrations of K+ such as K4, K5 and K6 had the greater concentrations of NO3- and NH4+, increased assimilation of N and higher concentration of nitrogen compounds. However, these increments were not related to biomass, acummulation, on the contrary, dry weight of roots and shoots were diminished in 35% due to the toxic effects observed in treatments K5 and K6. Therefore, K+ doses above the optimal were detrimental for root and shoot growth, being the shoot more diminished.
A field experiment was conducted over three growing seasons (2012–14) to study the effect of the foliar application of different potassium (K) fertilizers [potassium phosphate monobasic (KH2PO4), potassium nitrate (KNO3), and humic acid potassium (HAK)] on the fruit growth rate, yield, and quality of ‘Kousui’ japanese pear (Pyrus pyrifola) trees. Except the first year of study, foliar application of K fertilizers generally led to an increase in the concentration of fruit total soluble sugar, titratable acidity (TA) and sweetness, along with an elevated K accumulation in leaf and fruit at maturity. In 2013 and 2014, compared with the control, KNO3 treatment led to an average 16% higher yield, and HAK led to an average 15% higher soluble solid content (SSC). Furthermore, HAK resulted in 26% higher yield in 2014. KNO3 treatment showed 19% higher leaf K concentration, 38% leaf K accumulation, and 43% fruit K accumulation in maturity than the control in 2014. Different effects were found on the concentration of specific types of sugar and organic acid, of which fructose and malate were consistently increased by the K application. With regard to the amino acids, KNO3 and HAK treatments led to a significant increase in the concentration of aspartic acid, which was 12% and 22% higher than the control, respectively. In conclusion, foliar application of KNO3 is an efficient way to increase ‘Kousui’ japanese pear fruit yield, whereas spraying HAK is an effective way to improve the fruit quality. © 2016, American Society for Horticultural Science. All rights reserved.
The discovery of the convertibility between the energy of electrochemical proton gradients built up by charge separation at membranes and phosphate-ester-bond energy in ATP (Mitchell hypothesis) has led to a new orthodoxy in transport physiology. In principle all energy-dependent ion fluxes can be explained by secondary coupling to a proton electro-chemical gradient, \(
{\rm{\Delta}}{{\rm{\bar\mu }}_{{{\rm{H}}^{\rm{ + }}}}}\), with $$\Delta {\bar \mu _{{{\rm{H}}^{\rm{ + }}}}}{\rm{ = RT ln }}\frac{{{\rm{1}}{{\rm{0}}^{{\rm{- p}}{{\rm{H}}_{\rm{i}}}}}}}{{{\rm{1}}{{\rm{0}}^{{\rm{-p}}{{\rm{H}}_{\rm{o}}}}}}}{\rm{ + F }}\Delta {\rm{E}}$$ (1) or a proton-motive force, pmf, with $$pmf{\rm{ = 2}}{\rm{.3 }}\frac{{{\rm{RT}}}}{{\rm{F}}}{\rm{ }}\Delta {\rm{pH + }}\Delta {\rm{E}}$$ (2) where R is the universal gas constant, T the absolute temperature, △E the transmembrane electrical potential and F the Faraday constant; i and o mark inside and outside compartments respectively, △pH is the pH difference between the compartments.
The nitrogen (N) metabolism of the leaf subtending the cotton boll (LSCB) was studied with two cotton (Gossypium hirsutum L.) cultivars (Simian 3, low-K tolerant; Siza 3, low-K sensitive) under three levels of potassium (K) fertilization (K0: 0 g K2O plant−1, K1: 4.5 K2O plant−1 and K2: 9.0 g K2O plant−1). The results showed that total dry matter increased by 13.1-27.4% and 11.2-18.5% under K supply for Simian 3 and Siza 3. Boll biomass and boll weight also increased significantly in K1 and K2 treatments. Leaf K content, leaf N content and nitrate (NO3ˉ) content increased with increasing K rates, and leaf N content or NO3ˉ content had a significant positive correlation with leaf K content. Free amino acid content increased in the K0 treatment for both cultivars, due to increased protein degradation caused by higher protease and peptidase activities, resulting in lower protein content in the K0 treatment. The critical leaf K content for free amino acid and soluble protein content were 14 mg g-1 and 15 mg g-1 in Simian 3, and 17 mg g-1 and 18 mg g-1 in Siza 3, respectively. Nitrate reductase (NR), glutamic-oxaloace transaminase (GOT) and glutamic-pyruvic transaminase (GPT) activities increased in the K1 and K2 treatments for both cultivars, while glutamine synthetase (GS) and glutamate synthase (GOGAT) activities increased under K supply treatments only for Siza 3, and were not affected in Simian 3, indicating that this was the primary difference in nitrogen-metabolizing enzymes activities for the two cultivars with different sensitivity to low-K.
Soil steaming has become an important practice in commercial greenhouse production as an alternative to chemicals to control soil-borne pathogens. In addition to their effect on pests, due to soil temperature increase, steaming methods may also have an impact on soil chemical properties, on plant nutrient regime and, consequently, on crop yield. The aim of this work was to investigate the effect of two steam disinfection methods (deep or shallow steam injection) with or without the addition of an exothermic reacting compound (1000 kg CaO ha-1) on two consecutive greenhouse radish crops. The steam disinfection methods had a significant impact on soil thermal dynamics, on soil chemical properties and on radish crop production (with similar but more marked effects in the first than in the second crop cycle). The best results, in terms of both yield and quality, were obtained with shallow steam injection, while deep steam injection without CaO addition, resulted in a similar or lower yield than that obtained in the control soil. The main changes observed in soil chemical parameters were a temporary increase of pH following the incorporation of CaO and an increase of exchangeable K in the case of shallow steam injection.
The reduction of absorbed nitrate in the root and N transport to the shoot were studied in young barley (Hordeum vulgare L.) plants growing at low external nitrate levels. Plants were grown at three relative addition rates (RA) of nitrate: 0.04, 0.09, and 0.14 d-1, which represent different degrees of growth limiting nitrate supply. Root nitrate reduction and N transport in situ were estimated using 15N labelled nitrate exposures ranging from 5 to 60 min. With increasing RA, nitrate uptake in absolute terms increased, but the proportion of absorbed 15N-nitrate that was reduced in the root decreased markedly. After 10-20 min of exposure to the label, 75, 49, and 27 % of the 15N taken up was recovered as reduced 15N in the root at RAs 0.04, 0.09, and 0.14 d-1, respectively. The response pattern was supported by root nitrate reductase activities and xylem sap nitrate measurements. The decreasing proportion of reduced 15N in the root with higher nitrate supply rates was matched by a relative increase in 15N-nitrate storage and, to some extent, an increase in N transport to the shoot. Although small amounts of 15N were rapidly transported to the shoot, the accumulation of label in the shoot at the end of the 60 min period remained a relatively small proportion of total 15N uptake, which indicates delayed movement of 15N out of the root. The results clearly indicate that differing degrees of adjustment occur in important nitrate assimilation processes throughout the N deficiency range.
A study was made of some of the factors responsible for the accumulation of excessive amounts of nitrate in certain Kikuyu pastures. Accumulation, mainly in stem tissue, is the nett result of several contributing processes. Climatic conditions causing a decline in the requirements of the plant for assimilated nitrogenous substances, stimulate accumulation. High fertilizer-N applications and the re-cycling of nitrogenous substances in the pasture seem to be of considerable significance. These high soil nitrogen levels in conjunction with high potassium levels in the soil, aiding nitrate absorption, translocation and accumulation, seem to be major factors responsible for the accumulation of nitrate, especially in old Kikuyu pastures.
Respected and known worldwide in the field for his research in plant nutrition, Dr. Horst Marschner authored two editions of Mineral Nutrition of Higher Plants. His research greatly advanced the understanding of rhizosphere processes and trace element uptake by plants and he published extensively in a variety of plant nutrition areas. While doing agricultural research in West Africa in 1996, Dr. Marschner contracted malaria and passed away, and until now this legacy title went unrevised. Despite the passage of time, it remains the definitive reference on plant mineral nutrition. Great progress has been made in the understanding of various aspects of plant nutrition and in recent years the view on the mode of action of mineral nutrients in plant metabolism and yield formation has shifted. Nutrients are not only viewed as constituents of plant compounds (constructing material), enzymes and electron transport chains but also as signals regulating plant metabolism via complex signal transduction networks. In these networks, phytohormones also play an important role. Principles of the mode of action of phytohormones and examples of the interaction of hormones and mineral nutrients on source and sink strength and yield formation are discussed in this edition. Phytohormones have a role as chemical messengers (internal signals) to coordinate development and responses to environmental stimuli at the whole plant level. These and many other molecular developments are covered in the long-awaited new edition. Esteemed plant nutrition expert and Horst Marschner's daughter, Dr. Petra Marschner, together with a team of key co-authors who worked with Horst Marschner on his research, now present a thoroughly updated and revised third edition of Marschner's Mineral Nutrition of Higher Plants, maintaining its value for plant nutritionists worldwide. A long-awaited revision of the standard reference on plant mineral nutrition Features full coverage and new discussions of the latest molecular advances Contains additional focus on agro-ecosystems as well as nutrition and quality.
Higher plants acquire nitrogen from the soil mainly in the form of nitrate
andlor ammonium. The two N sources are taken up by the roots, where part
of the nitorgen can be utilized directly or stored (mainly as nitrate). If nitrate
or ammonium uptake exceed storage and utilization by roots, part of the
inorganic nitrogen will be transported to the shoot, where it can be reduced
and metabolized or stored as before. The first two sections of this chapter
review physiological aspects of nitrate and ammonium uptake by the roots
and their transport to the shoot. The third section focuses on the aspect of
a rapid regulation of nitrate reduction in roots and shoots by environmental
factors such as light, COr, or oxygen availability.
Potassium (K) is an essential macronutrient for plants involved in many physiological processes. It is important for crop yield as well as for the quality of edible parts of crops, as it is also required in human nutrition. Although K is not assimilated into organic matter, K deficiency has a strong impact on plant metabolism. Plant responses to low K involve changes in the concentrations of many metabolites as well as alteration in the transcriptional levels of many genes and in the activity of many enzymes. Today, these changes can be studied with high throughput technologies that allow a quantitative description of metabolic responses to K deprivation at multiple levels. To ensure K nutrition, plant roots are endowed with high‐ and low‐affinity uptake systems, some of which have been identified and characterised over the last decades of research.
Key Concepts
K is an essential macronutrient required for plant growth and development.
K deficiency affects many essential physiological and metabolic processes.
The K status determines the profile and distribution of primary metabolites in plant tissues.
K nutrition is closely related to sugar allocation, nitrogen assimilation and amino acid levels.
High‐throughput technologies allow researchers to determine simultaneously a large number of metabolites, transcript levels and enzyme activities in response to K supply.
K deficiency leads to build‐up of sugars, down‐regulation of nitrate uptake and synthesis of nitrogen‐rich amino acids.
Roots are endowed with high‐ and low‐affinity K uptake systems that ensure K nutrition in a wide range of external concentrations.
hak5 transporters mediate high‐affinity K uptake and akt1 channels mediate low‐affinity K uptake.
Under certain conditions akt1 channels can also mediate K uptake from very low concentrations.
Additional unidentified systems may also promote root K uptake.
The capacity of poplar (P. tremula x P. alba, clone INRA 717 1/B4) to respond to changes in the nutrient supply with modifications in the xylem exudate collected after decapitation was investigated with special respect to N-compounds. The composition (inorganic ions and amino-N) was analysed with respect to (a) the time after decapitation at different times of day, (b) a change in the nitrogen concentration from NO-/3 to N-free medium, (c) a change in the nitrogen source from NO-/3 to NH4+, (d) an increase in NO-/3 supply from 1 mM to 8 mM, and (e) the withdrawal of K+ supply. (a) The ion concentration in the xylem exudate was not affected up to 15 min after decapitation of the plants. Later a continuous increase in the concentration was observed. This increase was large if decapitation was performed in the middle of the light period and small at night-time. In both types of experiments (b, c) the NO3 concentration dropped immediately after the transfer, indicating the close connection between NO-/3 uptake and xylem loading. (b) After transfer to N-free medium poplar did not balance the charge in the xylem by increasing the concentration of other inorganic anions or decreased xylem loading of cations within 3 d of treatment. The N-status of the xylem exudate was reduced within 15 min. (c) After transfer of the NO3/--grown plants to NH2/+, as the sole N-source, the charge compensation in the xylem exudate was maintained by reducing the loading of cations, and 3 d later by an enhanced xylem loading of mainly SO2-/4. an immediate increase in glutamine concentration (2-fold in 15 min). (d) Increasing the NO-/3 supply to 8 mM had no effect on the ionic composition or the N-status of the xylem exudate. (e) The withdrawal of K+ from the medium for 11 d resulted in a limitation of the S- and N-supply of the plant, causing a decrease in the N-status of the xylem sap. The data are discussed with respect to charge compensation after changes in the nutrient supply and to the maintenance of the nitrogen status in the xylem sap.
Key message
Nitrate and potassium uptake are strongly correlated in deciduous trees from bud break to leaf fall. A conceptual model for potassium cycling is proposed to explain this result.
Abstract
We have studied the correlation between nitrate and potassium uptake in spring and autumn in two deciduous tree species: walnut and sycamore maple. Two-year-old trees were transplanted in early spring and cultivated on hydroponic recirculating nutrient solution systems in a greenhouse. Uptake of nitrate and potassium was surveyed daily during three consecutive weeks in mid-spring (i.e. during vegetative growth) and mid-autumn (i.e. during bud dormancy). Nitrate and potassium uptake was expressed per unit tree leaf area to account for tree size effect. Our results show that nitrate was much more absorbed than potassium in both species and its uptake remained nearly the same in spring and autumn. Contrary to this, potassium uptake was strongly reduced in autumn as a consequence of a strong reduction in vegetative growth. Although potassium uptake was strongly affected by seasonal variation in vegetative growth demand in both species, a positive and strong correlation between nitrate and potassium uptake was maintained in both species whatever the season. Essentially, any reduction in nitrate uptake as a consequence of decrease in nitrate availability in nutrient solution induced concomitantly a reduction in potassium uptake even if its concentration in the nutrient solution was sufficient to ensure potassium uptake. The results are discussed in the light of the accompanying role of potassium for nitrate uptake in plants. A conceptual scheme for internal/external potassium cycling in plant is proposed to explain the seasonal variations in nitrate and potassium uptake correlation in both deciduous tree species.
In field-grown maize (Zea mays L.) at different growth stages xylem exudate was collected and analyzed for nitrogen compounds and sugars. The plants were grown at two levels of nitrogen (N) fertilization (low N, high N). In an additional experiment in some plants the cobs were removed after anthesis.
Lettuce (Lactuca sativa L.) plants were grown under hydroponic system to characterize the diurnal change of nitrate concentration and nitrate uptake rate and to examine the effect of short term cold treatment to rhizosphere on nitrate concentration and uptake rate in lettuce plant. The nitrate concentrations in midrib were two times higher than those in leaf. Nitrate concentration in the shoot reached to minimum (8.7 mg-N/GDW) at 14:00 and, thereafter, increased continuously until 23:00. During 11:0017:00, nitrate uptake by lettuce plant was maximum (4.8 mg-N/GDW-Root/hr). Short term cold treatment reduced nitrate concentration in the shoot by 1418%, and nitrate uptake rate by 5055%, respectively. These results showed that short term cold treatment before harvest could be applied for the purpose of reduction of nitrate concentration in the leaf under hydroponic culture.
The effect of nitrate uptake, or its absence, on the utilization of nitrate previously accumulated by dark-grown, decpitated maize (Zea mays L., cv. DeKalb XL-45) seedlings was examined. Five-d-old plants that had been pretreated with 50 mM (14)NO 3 (-) for 20 h were exposed for 8 h to nutrient solutions containing either no nitrate or 50 mM (15)NO 3 (-) , 98.7 atom % (15)N. The ambient solution, xylem exudate, and plant tissue were analyzed to determine the quantities of previously-accumulated (endogenous) (14)NO 3 (-) that were translocated to the xylem, lost to the solution, or reduced within the tissue during the 8-h period. Energy was continuously available to the roots from the attached endosperm. In the absence of incoming nitrate, appreciable reduction and translocation of the endogenous (14)NO 3 (-) occurred, but efflux of (14)NO 3 (-) to the external solution was minimal. In contrast, during (15)NO 3 (-) uptake, there was considerable efflux of (14)NO 3 (-) as well as translocation of (14)NO 3 (-) to the xylem, but little (14)NO 3 (-) was reduced. Thus there appeared to be an inverse relationship between (14)NO 3 (-) efflux and reduction. The data are tentatively interpreted on the basis of a model which envisages (a) two storage locations within roots, one of which primarily supplies nitrate for translocation and the other of which primarily supplies nitrate for outward passage through plasmalemma, and (b) the majority of nitrate reduction as occurring during or immediately following influx across the plasmalemma, with endogenous (14)NO 3 (-) initially moving outward being recycled inward and thereby being reduced.
The effect of K+ as a countercation to NO3− on the reduction of NO3− in the root has been investigated using castor bean (Ricinus communis L.) and barley (Hordeum vulgare L.) grown with reciprocal splitroots. One part of the root system was supplied with a nutrient solution lacking potassium but containing NaNO3 instead of KNO3 (−K+N roots), whereas the other side obtained potassium but no nitrate (+K−N roots). The contribution of the roots to nitrate reduction was assessed firstly by analyzing the proportion of reduced to total nitrogen in the xylem sap and secondly by measuring nitrate reductase activity (NRA) in vivo and in vitro in the roots. −K+N roots of Ricinus contributed more to nitrate reduction in the whole plant than control roots, as shown by a higher ratio of reduced to total nitrogen in xylem sap and by a 2.3- to 2.8-fold higher in vivo NRA. This minus K stimulation of NRA was found also in roots of Hordeum in the NRA measured in vivo (1.9-fold) as well as in vitro (1.6-fold). The minus K stimulation in −K+N roots was reversible within 4 or 6 h and it decreased with a half-time of 43 or 75 min after supply of external K+ or Rb+, respectively. The development of the minus K stimulation, however, was slow. It occurred with a time lag of 4 days and was completed after 10 days exposure to minus K conditions, suggesting that regrowth of new root tissue was involved. NRA in +K−N roots was, as expected, low. These findings suggest that availability of easily permeating countercations to nitrate, such as potassium, plays a role in the regulation of the partitioning of nitrate reduction between shoot and root in the whole plant.
Ricinus communis seedlings were used to study the transfer of exogenously supplied nitrate and ammonium into the phloem and the xylem. Uptake into the phloem was tested after removal of the endosperm by incubating the cotyledons in nitrate and/or ammonium. Analysis of phloem sap proved that there is only restricted loading of nitrate into the phloem and ammonium was not taken up at all, even when offered at 10 mM to the cotyledons.
In contrast, when the roots were incubated in nitrate, rapid transfer into the xylem occurred. Both significantly higher exudation rates and nitrate concentrations in the root pressure exudate were detected when roots were incubated in KNO3 compared with NH4NO3. Ammonium when offered to the roots at 1 mM was not transported into the xylem, but when the roots were incubated in 10 MM ammonium, it was delivered into the xylem. Again, exudation rate and concentration of ammonium in the root pressure exudate were affected by the accompanying ion. When NH4NO3 was offered, the exudation rate was stimulated two-fold and ammonium concentration was as much as three times higher than when roots were incubated in NH4CI.
These complex mutually interdependent effects on water and nitrogen ion fluxes are discussed with respect to permeability and metabolism of the ions and also with respect to balanced charge flow into the xylem.
The effect of salinity on nitrate influx, efflux, nitrate net uptake rate and net nitrogen translocation to the shoot was assessed in a 15N steady state labelling experiment in the halophyte Plantago maritima L. raised for 14 days on solution supplied with 50, 100 and 200 mol m-3 sodium chloride or without sodium chloride. Additionally, salinity induced changes in root morphology were determined. Specific root length increased upon exposure to elevated sodium chloride concentrations due to variations in biomass allocation and length growth of the tap root. Changes in root morphology, however, had a minor effect on nitrate fluxes when expressed on a root fresh weight basis. The decreased rate of nitrate net uptake in plants grown on elevated levels of sodium chloride was almost entirely due to a decrease in nitrate influx. Expressed as a proportion of influx, nitrate efflux remained unchanged and was even lower at the highest salinity level. At all sodium chloride concentrations applied the initial rate of nitrogen net translocation to the shoot decreased relative to the rate of nitrate net uptake. It is concluded that under steady state conditions the negative effect of sodium chloride on the rate of nitrate net uptake at non growth-limiting salinity levels was due to the interaction between sodium chloride and nitrate transporters in the root plasma membrane and/or processes mediating the translocation of nitrogen compounds, possibly nitrate, to the shoot.
Maize (Zea mays L.) was grown in quartz sand culture either with a normal root system (controls) or with seminal roots only ('single-rooted'). Development of adventitious roots was prevented by using plants with an etiolated mesocotyl and the stem base was positioned 5-8 cm above the sand. Even though the roots of the single-rooted plants were sufficiently supplied with water and nutrients, the leaves experienced water deficits and showed decreased transpiration as transpirational water flow was restricted by the constant number of xylem vessels present in the mesocotyl. As a consequence of this restriction, transpirational water flow velocities in the metaxylem vessels reached mean values of 270 m h-1 and phloem transport velocities of 5.2 m h-1. Despite limited xylem transport mineral nutrient concentrations in leaf tissues were not decreased in single-rooted plants, but shoot and particularly stem development was somewhat inhibited. Due to the lack of adventitious roots the shoot:root ratio was strongly increased in the single-rooted plants, but the seminal roots showed compensatory growth compared to those in control plants. Consistent with decreased leaf conductance, ABA concentrations in leaves of single-rooted plants were elevated up to 10-fold, but xylem sap ABA concentrations in these plants were lower than in controls, in good agreement with the well-watered conditions experienced by the seminal roots. Surprisingly, however, ABA concentrations in tissues of the seminal roots of the single rooted plants were clearly increased compared to the controls, presumably due to increased ABA import via phloem from the water-stressed leaves. The results are discussed in relation to the role of ABA as a shoot to root signal.
Experiments were conducted to investigate the effect of concentration of NH4+ in nutrient solution on root assimilation of NO3− and to determine whether the NH4+NO3− interaction was modified in the presence of K+. Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed for 8 h to 0.15 mM Ca(NO3)2 and varying concentrations of (NH4)2SO4 in the absence or presence of 0.15 mM K2SO4. The accelerated phase of NO3− uptake appeared most sensitive to restriction by additions of 0.15 mM (NH4)2SO4. In the absence of K+, the restriction increased only slightly even when solution (NH4)2SO4, was increased from 0.15 mM to 12.5 mM which was accompanied by an increase of NH4+ in the tissue from about 7.0 to 35 μmol g−1 fr. wt. of root. Increasing concentrations of solution NH4+ progressively inhibited net K+ uptake. At the highest solution NH4+ concentrations, there was an initial net efflux of K+ and no net influx occurred during the treatment period. The severity of the NH4)SO4 restriction of NO3− uptake was moderated considerably in the presence of K+ as long as a net influx of K+ occurred. However, net influx of K+ was not associated with alteration of NH4+ uptake, assimilation, or accumulation in the root tissue. The lack of correlation between the severity of restriction of
NO3− uptake and endogenous NHJ suggested the restriction resulted from an effect exerted by exogenous NH4+ which tended to saturate at lower solution NHJ concentrations or by inhibitory factors generated during assimilation of NH4+. Several mechanisms were postulated to account for the moderating influence of K+. In all experiments, root NO3− reduction was restricted by the presence of ambient NH4+. The quantitative decreases in reduction tended to be less than decreases in NO3− uptake and therefore, could result from inhibition solely of uptake with subsequent limitation in availability of substrate
for the reduction process, but the possibility of a direct effect on reduction could not be excluded.
This study was conducted to evaluate the effect of nitrogen (N) and potassium (K) availability on root exudate composition of two sugarcane cultivars known to differ with regard to their resistance to drought and salinity stress. The plants were hydroponically grown in a greenhouse and subjected to three levels of N (0.1, 1.0, and 10 mM N) and three levels of K (0.02, 0.2, and 2 mM K). Nitrogen and K stress altered the xylem sap composition. Nitrogen stress significantly reduced nitrate (NO3 ), ammonium (NH4 ), calcium (Ca), magnesium (Mg), and amino acid content and increased the pH, phosphorus (P), and K content. Whereas, K stress significantly decreased pH, K, NH4 , and amino acid content but increased Ca, Mg, and P content. Nitrogen and K stress had opposing effects on xylem sap pH and osmolality. Results indicated that sugarcane plants recycle compounds between the phloem and xylem. The results also suggested that the NO3 and K concentration of xylem sap could be effectively used to estimate the N and K status of the soil solution.
Pretreatment of radish (Raphanus sativus L.) seedlings with exogenous hormones in the absence of external nitrate, resulted in a system having enough hormone levels to mediate the responses or activate the metabolic pathways which are necessary for nitrate (NO3) uptake and reduction. Effects of pretreatment of radish seedlings with KN, GA, and ABA on the induction of NO3 transport and corresponding NR activity, upon exposure to NO3 were investigated. A low NO3, uptake rate was observed with hormones when compared to DW‐control, while its induction pattern exhibited a sort of biphasic kinetics. It was observed that each hormone treatment affected NO3 uptake and reduction specifically at different ambient concentrations of NO3. On the basis of this, the operation of different constitutive and inducible (CHATS, LATS, HATS, and IHATS) transport systems was resolved for different hormonal treatments. Further, the hypothesis that nitrate uptake has direct role on nitrate reduction was supported by all treatments, except KN where highest NR activity despite lowest NO3 uptake, was recorded. The results with KN points towards enhanced NR mRNA and de novo synthesis of enzyme.
The nitrate and nitrite levels were determined in two carrot varieties, cultivated in three Experimental Stations of Varieties Assessment in different regions of Poland in 1987–1989. The results obtained were analysed against soil conditions (kind and composition, richness), agrotechnical background (length of vegetation period, harvest time) and climatic conditions (air temperature and precipitation). The nitrate level in carrot was estimated at av. 349 mg KNO3/kg. Significant differences were found in nitrate levels depending on place and year of cultivation. The most significant effects of the studied factors on the nitrate level had soil richness in potassium and magnesium as well as length of vegetation period. The studied carrot displayed a high level of nitrites (av. 3.1 mg NaNO2/kg), which might have resulted from a microbiological reduction of nitrates occurring between harvest and the time of performing analyses.Nitrat- und Nitritgehalte in MohrrübenIn den Jahren 1987–1989 wurden die Nitrat- und Nitritgehalte von zwei Mohrrübensorten bestimmt, die in drei Versuchsstationen in unterschiedlichen Regionen Polens angebaut worden waren. Die erhaltenen Ergebnisse wurden zu den Bodenbedingungen (Art und Zusammensetzung, Bodenschwere), den agrotechnischen Bedingungen (Länge der Vegetationsperiode, Erntezeitpunkt) und den klimatischen Bedingungen (Lufttemperatur, Niederschläge) in Beziehung gesetzt. Die Nitratgehalte der Möhren lagen im Durchschnitt bei 349 mg KNO3/kg. Signifikante Unterschiede der Nitratgehalte gab es in Abhängigkeit von Anbauort und Anbaujahr. Den stärksten signifikanten Einfluß auf den Nitratgehalt hatten sowohl der Kalium- und Magnesiumgehalt des Bodens als auch die Länge der Vegetations-periode. Die untersuchten Möhren wiesen einen hohen Nitritgehalt auf (im Durchschnitt 3.1 mg NaNO2/kg), der auf eine mikrobielle Reduktion der Nitrate zwischen Ernte und Analysenzeitpunkt zurückzuführen sein könnte.
This chapter discusses the potassium estimation, uptake, and its role in the physiology and metabolism of flowering plants. Potassium is one of the vital elements involved in inorganic plant nutrition. A number of methods have been developed for the qualitative and quantitative estimation of potassium. It is now feasible to determine accurately K+ content even at the cellular level. Two methods are available for the quantitative estimation of K+ in situ: electron probe X-ray microanalysis and potassium-sensitive microelectrodes. Electron probe X-ray microanalysis permits the simultaneous quantitative estimation of several elements in a microstructure. In principle, an electron beam is focused on a section of tissue that excites atoms to emit X-rays whose energy is characteristic of the elements. The X-rays are collected in a tune spectrometer, converted to electrical pulses, and counted. Electron probe X-ray microanalysis can also be used for qualitative localization of ions, a technique called X-ray mapping. The chapter reveals that potassium uptake is mainly metabolic at low external [K+] (<1 mM), while it becomes increasingly nonmetabolic at higher (> 1 mM) external [K+].
The capacity of poplar (P. tremula x P, alba, clone INRA 717 1/B4) to respond to changes in the nutrient supply with modifications in the xylem exudate collected after decapitation was investigated with special respect to N-compounds. The composition (inorganic ions and amino-N) was analysed with respect to (a) the time after decapitation at different times of day, (b) a change in the nitrogen concentration from NO3- to N-free medium, (c) a change in the nitrogen source from NO3- to NH4+, (d) an increase in NO3- supply from 1 mM to 8 mM, and (e) the withdrawal of K+ supply. (a) The ion concentration in the xylem exudate was not affected up to 15 min after decapitation of the plants, Later a continuous increase in the concentration was observed. This increase was large if decapitation was performed in the middle of the light period and small at night-time. In both types of experiments (b, c) the NO3- concentration dropped immediately after the transfer, indicating the dose connection between NO3- uptake and xylem loading, (b) After transfer to N-free medium poplar did not balance the charge in the xylem by increasing the concentration of other inorganic anions or decreased xylem loading of cations within 3 d of treatment, The N-status of the xylem exudate was reduced within 15 min. (c) After transfer of the NO3--grown plants to NH4+, as the sole N-source, the charge compensation in the xylem exudate was maintained by reducing the loading of cations, and 3 d later by an enhanced xylem loading of mainly SO42-. The N-status in the xylem exudate was maintained by an immediate increase in glutamine concentration (2-fold in 15 min), (d) Increasing the NO3- supply to 8 mM had no effect on the ionic composition or the M-status of the xylem exudate, (e) The withdrawal of K+ from the medium for 11 d resulted in a limitation of the S-and N-supply of the plant, causing a decrease in the N-status of the xylem sap. The data are discussed with respect to charge compensation after changes in the nutrient supply and to the maintenance of the nitrogen status in the xylem sap.
To examine effects of lead uptake on mineral nutrient contents, Pisumsativum was exposed to 0.5–9.4 mmol lead acetate kg−1 dwt soil for 12 and 21 days in potted soil cultures in a greenhouse. Over 21 days, more lead accumulated in the shoot at the 6.5 mmol kg−1 Pb treatment than at any lower or higher soil concentrations. The tolerance index (TI) for roots displayed a linear negative relationship with soil lead. Of 11 nutrient elements assessed, only iron and copper concentrations did not change significantly during 12- or 21-days Pb exposures. Lead at a 4 mmol kg−1 soil concentration altered the constant pattern of export of major elements from the cotyledons: at day 9, they were poorer in manganese but richer in magnesium, sulphur, sodium, and zinc than were the controls. The in vivo phytase activity of cotyledons was one of the parameters changing with age; at day 12 it was 70 and 75% inhibited by 2 and 4 mmol Pb kg−1, respectively. Imbalance in mineral nutrients was thus concluded to be a major impact of lead. Some of the alterations, the parallel patterns of change identified, and the mobilisation of major elements to the top of the shoot, are suggested to represent response mechanisms, which may help explain the fairly high tolerance to lead in terms of biomass and chlorophyll contents, of this species. Sensitivity to lead changed with age and soil lead concentration, with significant effects recorded below the maximum lead content suggested for agricultural soils (500 mg kg−1).
Two experiments were conducted to evaluate the effects of phenotypic recurrent selection for high and low post-anthesis leaf-lamina in vivo NRA on nitrate uptake, nitrate partitioning and in vitro NRA of seedling roots and leaves. In Experiment 1, intact plants of cycle 0, 4, and 6 of the high and low NRA strains were grown on NH4-N for 11 d, then exposed to 1.0 mol m⁻³ KNO3, and cultures sampled at 6 h and 28 h (induction and post-induction periods). Nitrate uptake, tissue nitrate concentration and in vitro NRA were determined. The pattern of response to selection in seedling leaf NRA was similar to that observed for in vivo NRA of field grown plants. Leaf NRA increased between 6 h and 28 h. Root NRA was not affected by selection or sampling time. Treatments differed in total fresh weight but not in reduction or uptake of nitrate per unit weight, indicating a lack of correspondence between NRA and reduction and supporting the idea that concomitant reduction by NR is not obligatorily linked to nitrate influx in the intact plant.
In Experiment 2, dark-grown plants of cycle 0, and 6 of the high and low NRA strains were cultured without N, detopped on day 6, transferred the following day to 0-75 mol m⁻³ KNO3 and sampled at 6 h and 28 h. In contrast to Experiment 1, selection populations differed in nitrate reduction and root NRA, which by 28 h reached higher average levels than root NRA of intact plants. Translocation and reduction were inversely related among strains within each sampling time. The high level of translocation in detopped plants of the low NRA strain was difficult to reconcile with its low leaf NRA level of Experiment 1. It is suggested that nitrate transport in detopped roots is altered relative to the intact system in a way which permits greater NRA induction and nitrate reduction. The results indicate that nitrate partitioning by detopped root systems should be interpreted with caution.
Upon first exposure to NO3-, the NO3- uptake and reduction capacities of dark-grown corn (Zea mays L.) roots are initially low, but increase markedly within 6 h. The development of the accelerated NO3- uptake rate appears to be substrate ‘induced’ as is NO3- reductase (NR), the first enzyme in the NO3- assimilatory pathway. However, the ‘induction’ of NO3- uptake is independent of NR induction.
The effect of NO3- deprivation was studied to determine the role of endogenous NO3-14 on subsequent NO3-15 uptake and reduction. Corn roots were ‘induced’ for 24 h in 0–5 mol m⁻³NO3-14 nutrient solution and then exposed for 0 to 32 h to NO3--free nutrient solution. Uptake and reduction of NO3- were determined periodically by exposing sets of roots to a 1 h pulse of 0.5 mol m⁻³NO3-15. Neither NO3-15 uptake (4.57 μmol root⁻¹ h⁻¹) nor the percentage of absorbed NO3-15 reduced (27%) was changed significantly (P⩽ 0.05) by exogenous NO3- deprivation. However, the estimated ‘induced’ component of NO3-15 uptake decreased significantly (50% after 32 h). Concurrently, the ‘non-induced’ basal component of NO3- uptake increased.
Previously accumulated NO3-14 decreased from 23 to 4.5 μmol root⁻¹ after 32 h of exogenous NO3- deprivation. Nearly equivalent quantities of endogenous NO3-14 were used for translocation and reduction during NO3- deprivation. During each 1 h NO3-15 pulse, the amounts of NO3-14 translocation and net efflux of NO3-14 to the uptake solution were similar. Net efflux of NO3-14 was strongly correlated (r = 0.991) to the amount of endogenous NO3-14. The remaining endogenous NO3-14 and its rate of utilization were apparently sufficient to minimize a rapid decline or complete loss in both the ‘induced’ NO3- uptake state and the rate of in vivoNO3- assimilation.
Changes in nitrate reductase activity in the youngest expanded leaves of the potato plant have been monitored in the field under a range of N regimes. Highest activity (in vivo assay, substrate non-limiting) occurred with the highest rate of N application, as did the highest leaf protein content. In high-N leaves the onset of the decline in nitrate reductase activity and protein content, which commenced around 80 days after planting, was not prevented by the application of high levels of nitrogen. This could not be explained by a fall in the concentration of nitrate in the petiole xylem sap supplying the leaves in question. The ratio of nitrate-N to amino acid-N in petiole xylem sap remained relatively constant throughout the growing period in plants receiving 8 g N m−2 at 14 day intervals, but declined under lower N inputs. Only in the highest N treatment did the ratio remain greater than unity. The reasons for these observations are discussed.
Nitrate reductase activity was also determined in leaves of field-grown plants with all of the N (20 g m−2) applied pre-planting. A comparison of in vivo assays carried out with and without nitrate in the assay medium showed that nitrate reductase was likely to limit nitrate assimilation only during the earliest stages of post-emergence growth.
The distribution of NO3− reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO3−, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 mM NH4+ and 0.5, 1.5 or 10 mM NO3−. Xylem transport of NO3− was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO3− reduction at all 3 levels of NO3− nutrition. However, the contribution of the shoots to the whole plant NO3− reduction increased with increasing external NO3− availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO3−, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO3− translocation from roots to shoots. It is proposed that the lack of NO3− export to the shoots at low NO3− uptake rate resulted from a competition between NO3− reduction in the root epidermis/cortex and NO3− diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO3− nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO3− assimilation, principally occurring in the roots, was mainly incorporated in the shoots.
Abstract Potassium (86Rb) influx from 200 mmol m −3 KCl into dark grown, decapitated maize seedlings 6 d old) was stimulated by nitrate pretreatment. The stimulus was clearly evident by 6h exposure to nitrate and required 12–24 h for maximal expression. Decay of the nitrate-stimulated potassium influx was more than 50% complete within 3 h after transfer to nitrogen-free solutions. The stimulation of potassium influx was entirely accounted for by an increase in the influx component that was resistant to inhibition by presence of 200 mmol m−3 ambient ammonium. In contrast, the component of potassium influx that was sensitive to inhibition by ambient ammonium was unaffected by nitrate pretreatment. Exposure to the glutamine synthetase inhibitor L-methionine-dl-sulphoximine (MSX) during nitrate pretreatment stimulated the resistant component but the sensitive component was nearly eliminated. Pretreatment with ammonium increased the resistant component of potassium influx within 3 h, i.e. before it was increased by nitrate pretreatment, but the sensitive component was concomitantly restricted. The latter recovered partially during extended pretreatment with ammonium. The data indicate that the resistant component responded positively to increases in tissue ammonium concentrations whereas the sensitive component was unaffected by tissue ammonium except at concentrations in excess of 10μmol g−1. Ammonium influx was also stimulated by nitrate pretreatment and to a greater extent than potassium influx. Presence of MSX with nitrate during pretreatment resulted in a further stimulation in ammonium influx. The parallel increases in root ammonium concentrations with the two pretreatments imply that part of the increase in ammonium influx was a consequnce of increased counter-transport with endogenous ammonium.
Shoot activity has been reported to affect rates of ion uptake by plant roots in other ways than merely through supply of assimilates. To elucidate the mechanisms by which a signal from the upper part of the plant controls the rate of K ⁺ and NO 3 ⁻ uptake by roots, both uptake of K ⁺ and NO 3 ⁻ and secretion into the xylem of young sunflower plants ( Helianthus annuus L.) were measured after changes in light intensity.
No close correlation was observed between the uptake of NO 3 ⁻ and that of K ⁺ ; an increase in light intensity produced a much greater stimulation of NO 3 ⁻ uptake than of K ⁺ uptake. On the other hand, secretion of NO 3 ⁻ into the xylem was tightly coupled to that of K ⁺ , and this coupling was strongly disturbed by excision of the root. The results suggest the involvement of the K 2 ‐malate shuttle on the regulation by the shoot of K ⁺ and NO 3 ⁻ secretion in the xylem, which is linked to NO 3 ⁻ uptake, while K ⁺ uptake is independent of this regulation mechanism.
Although nutrient stress is known to alter partitioning between shoots and roots, the physiological basis for the phenomenon is unresolved. Experiments were conducted to examine assimilation of 15NO3 by N-stressed plants and to determine whether apparent changes in assimilation in the root contributed to alterations in whole-plant partitioning of reduced-N. Tobacco plants (Nicotiana tabacum L. cv. NC 2326) were exposed to a low concentration of NO3− in solution (80 μM) for 9 days to effect a N-stress response. Exposure of plants to 1000 μM15NO3− for 12 h on selected days revealed that roots of N-stressed plants developed an increased capacity to absorb NO3−, and accumulation of reduced-15N in the root increased to an even greater extent. When plants were exposed to 80 or 1000 μM15NO3− in steady-state, 15NO3− uptake over a 12 h period was noticeably restricted at the lower concentration, but a larger proportion of the absorbed 15N still accumulated as reduced-15N in the root. The alteration in reduced-15N partitioning was maintained in N-stressed plants during the subsequent 3-day “chase” period when formation of insoluble reduced-15N in the root was quantitatively related to the disappearance of 15NO3− and soluble reduced-15N. The results indicate that increased assimilation of absorbed NO3−, in the root may contribute significantly to the altered reduced-N partitioning which occurs in N-stressed plants.
The effect of Ca2+, K+ and Na+ on uptake, transport and reduction of NO3− was studied in 10-day-old plants of Triticum aestivum L. cv. Sicco grown in water culture in relation to the ion balance in plant, medium and xylem exudate.
When Ca2+ (0.25 mM) was the only nutrient cation, NO3− uptake and accumulation of NO3− in the leaves were low as compared to treatments with K+ or Na+ (0.5 mM). With Na+, the uptake of NO3− was similar to that with K+, but virtually no Na+ and 75 % less NO3− accumulated in the leaves.
Medium pH dropped initially with 0.5 mM KNO3 but rose immediately when Ca2+ was the only cation. The carboxylate content rose in roots and leaves of K+ plants and, to a lesser extent, in Na+ plants. Roots of Ca(NO3)2 plants showed negative ash alkalinity indicating the presence of acidity.
The pH of the xylem exudate increased with the cation/NO3− ratio in the exudate and was about one unit higher with Ca(NO3)2 than with NaNO3 or KNO3.
We conclude that roots of plants grown in the absence of monovalent cations have serious pH problems because of a high rate of alkalinity release to the medium in exchange for NO3− uptake, and the secretion of alkalinity into the xylem.
Week-old wheat seedlings absorbed at least 40% NO(3) (-) from NaNO(3) when preloaded with K(+) than when preloaded with Na(+) or Ca(2+). Cultures of Triticum vulgare L. cv. Arthur were grown for 5 days on 0.2 mm CaSO(4), pretreated for 48 hours with either 1 mm CaSO(4), K(2)SO(4), or Na(2)SO(4), and then transferred to 1 mm NaNO(3). All solutions contained 0.2 mm CaSO(4). Shoots of K(+)-preloaded plants accumulated three times more NO(3) (-) than shoots of the other two treatments. Initially, the K(+)-preloaded plants contained 10-fold more malate than either Na(+)- or Ca(2+)-preloaded seedlings. During the 48-hour treatment with NaNO(3), malate in both roots and shoots of the K(+)-preloaded seedlings decreased. Seedlings preloaded with K(+) reduced 25% more NO(3) (-) than those preloaded with either Na(+) or Ca(2+). These experiments indicate that K(+) enhanced NO(3) (-) uptake and reduction even though the absorption of K(+) and NO(3) (-) were separated in time. Xylem exudate of K(+)-pretreated plants contained roughly equivalent concentrations of K(+) and NO(3) (-), but exudate from Na(+) and Ca(2+)-pretreated plants contained two to four times more NO(3) (-) than K(+). Therefore K(+) is not an obligatory counterion for NO(3) (-) transport in xylem.
Wheat seedlings (Triticum vulgare) treated with 1 mm KNO(3) or NaNO(3), in the presence of 0.2 mm CaSO(4), were compared during a 48-hour period with respect to nitrate uptake, translocation, accumulation and reduction; cation uptake and accumulation; and malate accumulation. Seedlings treated with KNO(3) absorbed and accumulated more nitrate, had higher nitrate reductase levels in leaves but less in roots, accumulated 17 times more malate in leaves, and accumulated more of the accompanying cation than seedlings treated with NaNO(3). Within seedlings of each treatment, changes in nitrate reductase activity and malate accumulation were parallel in leaves and in roots. Despite the great difference in malate accumulation, leaves of the KNO(3)-treated seedlings had only slightly greater levels of phosphoenolpyruvate carboxylase than leaves of NaNO(3)-treated seedlings. NADP-malic enzyme levels increased only slightly in leaves and roots of both KNO(3)- and NaNO(3)-treated seedlings. The effects of K(+) and Na(+) on all of these parameters can best be explained by their effects on nitrate translocation, which in turn affects the other parameters. In a separate experiment, we confirmed that phosphoenolpyruvate carboxylase activity increased about 2-fold during 36 hours of KNO(3) treatment, and increased only slightly in the KCl control.
Sugars (sucrose + hexoses) produced photosynthetically by isolated mesophyll protoplasts of wheat and tobacco were effluxed across the plasma membrane (3 to 10 micromoles hexose equivalents per milligram chlorophyll per hour). The efflux was sensitive to uncouplers and oligomycin which indicated a requirement for energy. A proton gradient was probably not coupled directly to the transport because changing the proton gradient across the plasma membrane by varying the pH of the medium or by adding sodium acetate had no significant effect on the rate of sugar release.A release of K(+) was associated with sugar efflux from the protoplasts. The molar ratio of K(+) to sugar varied between 1.5 and 2.5, depending on the species. Exogenous CKl, RbCl, and LiCl (50 millimolar each), but not NaCl or CsCl, significantly inhibited sugar efflux. Conditions that reduced sugar efflux (exogenous KCl, LiCl, mersalyl, or oligomycin) also reduced K(+) release and caused a time-dependent reduction in photosynthetic sucrose formation and increased amino acid and starch formation. Results obtained support the postulate that a K(+) symport is involved in the transport of sugar across the energized plasmalemma of photosynthetically active mesophyll cells.
Early spring is a period of widely fluctuating gradients between aerial and soil temperatures. Aerial temperatures affect growth and N demand of seedlings while temperature and N availability in the root zone alter the ability of roots to supply the plant with N. The purpose of this study, in which tobacco ( Nicotiana tabacum L.) plants were grown in a temperature controlled, flowing nutrient solution system within a controlled‐environment room, was to determine the independent effects of aerial temperature, root‐zone temperature, and NO 3 ‐ concentration of nutrient solution and their interactions on growth and N accumulation. In each of nine experiments consisting of all combinations of day/night aerial temperatures of 22/18, 26/22, and 30/26 C and solution NO 3 ‐ concentrations of 3.5, 7.2, and 14.6 m M , plants were grown for 38 days at root‐zone temperatures of 16, 24, and 32 C. For analysis of variance the study was considered as a 3 ³ factorial. Plants were sampled at 14, 19, 24, 31, and 38 days, dried, weighed, and analyzed for total N and soluble carbohydrates.
Root‐zone temperatures had greater effects on growth than either aerial temperature or NO 3 ‐ concentration. Plant dry weight, leaf area, and stem height were less at 16 C than 24 and 32 C root temperatures. The reduction in growth at 16 C apparently was attributable to initial water stress and reduced loading of N from the root into the vascular system. Reductions in dry matter and N accumulation of plants associated with reductions in root temperature were enhanced as aerial temperature was increased and solution NO 3 ‐ concentration was decreased. This suggests an interrelated and synergistic effect of the variables.
This chapter focuses on ion absorption by cells and other tissues—such as storage roots, leaves, algae—and in organelles such as mitochondria and chloroplasts. Ion absorption by cells, including the flux of ions across both the plasma membrane and tonoplast is presented. The chapter provides an overview of nutrient absorption by excised roots. Excised roots usually function for several hours, at least with regard to ion absorption, as if they had never been removed from the shoot. The value of using excised roots that are low in salt content for studying nutrient absorption is determined. These roots accumulate salts in a short time and the increase in ion content can be measured easily either chemically or by using the radioisotope of a specific element. The chapter discusses the kinetics and selectivity of ion absorption and proposes a model for ion absorption by roots. This model consists of two main features: a cation-activated ATPase in the plasma membrane and an anion carrier that brings about the exchange of anions across the plasma membrane.
A rapid automated method for determination of nitrate in plant and soil extracts is described. The method utilizes soybean nodule bacteroids for reduction of nitrate to nitrite. A portion of the nitrite is removed from the sample and bacteroids by continuous flow dialysis and is measured colorimetrically. The method is sensitive to as little as 0.01 μg. of nitrate nitrogen per ml. in the sample solution and is adaptable to extremely wide ranges of nitrate concentrations. No purification of either plant or soil extracts is necessary. The bacteroids possess an extremely active nitrate reductase and utilize succinate as an exogenous electron donor. No sterile techniques or aseptic culturing procedures are necessary. One 10-inch pot of well-nodulated soybeans will produce sufficient bacteriods for 300 to 500 nitrate analyses.
For single, unbranched roots of corn (Zea mays L., Pioneer Hybrid 309-B), the rate of exudation per square centimetre of root surface (Jw ) decreased logarithmically with increases in the external osmotic pressure (IIo).
In both the seminal axis and lateral roots of Cucurbita pepo L. the formation of large central xylem elements and the commencement of secondary cambial activity occur 10–20 cm from the root tip. Concomitant with or slightly preceding these developments there are changes in the structure of the walls of endodermal cells where the lignified casparian band spreads along the radial wall and substances staining with Sudan IV are deposited in both radial and tangential walls. At distances more than 30 cm from the tip of primary roots the radius of the stele increases considerably causing splits in the cortex. The endodermis is stretched and the suberin becomes organized in a lamellar form.
Against this background of anatomical change certain of the transport capabilities of the root are retained while others are lost. Using an apparatus for measuring the uptake of tracers by segments of intact roots it was found that neither the uptake nor translocation of potassium seem to be affected by the suberization of the endodermis or by secondary thickening, while the translocation of calcium is virtually eliminated when these processes begin. As the root ages its ability to absorb phosphate declines although the translocation of the phosphate absorbed is much less affected by structural development than that of calcium.
The observed rates of potassium uptake by complete root systems could be predicted quite accurately from the average of segment uptake data suggesting that the method used gives reliable results.
Vegetative soybean plants were exposed to root temperatures of 18, 24, and 30 °C in each of three aerial environments varying in light and CO2. Under high light (700 mUE m-2 s-1) at ambient CO2 (400 mUl l-1) nitrate absorption and root growth were less at 18 and 30°C than 24°C. Both nitrate absorption and root growth were more sensitive to increasing root temperature under low light (325 μE m-2 s-1) and less sensitive under enriched CO2 (1000 μl l-1). As indicated by dry matter accumulation, net flux of photosynthate to the roots was correlated with the changes in aerial environment. It was concluded from these relationships that the variation in root growth and nitrate uptake resulted from altered utilization of carbohydrates in the roots and that altered utilization of carbohydrates with increasing root temperature resulted from disproportionate partitioning of carbohydrate among root functions.
Although differences occurred in the amount of nitrogen translocated out of the root, nitrogen accumulation in the shoot was primarily a function of nitrate absorbed. Restrictions in dry matter accumulation in the shoot were similar to restrictions observed in the root. A decrease in emergence of new leaves was often the first response observed, with decreases in leaf area and dry wt occurring later. The integrated plant response to moderate root temperature stress is interpreted as evidence for an interdependent plant system, predominantly regulated by carbohydrate flux to the root and nitrogen flux to the shoot.
The assimilationof ammonium ion in plant cell cytoplasm produces at least one H ⁺ per NH ⁺ 4 ; N 2 fixation generates 0.1‐0.2 H ⁺ per N assimilated; NO ‐ 3 assimilation produces almost one OH ‐ per NO ‐ 3 . H ⁺ or OH ‐ produced in excess of that required to maintain cytoplasmic pH for H ⁺ or OH ‐ , the major process involved is H ⁺ efflux (frequently by active transport) from the cell. IN higher land plants, much of assimilated N occurs as hoot protein; the shoot cells have no direct acess to the H ⁺ and OH ‐ sink of the soil solution.
When ammonium ion is the N source it is assimilated into organic‐N in the roots. The shoot is supplied with a mixture of amino‐acids, amides and organic acids which an be incorporated (with neutral photosynthate) into cell material without damaging pH changes. Similar considerations apply to symbiotic N 2 assimilation in root nodules. IN both cases the excess H ⁺ generated in the root cell cytoplasm is exerted is excreted to the soil solution; there is no mechanism whereby photolithotrophic plant can, in the long term, counter intracellular acidity without resort to active H ⁺ efflux to an extracellular sink.
When nitrate is reduced in roots, the organic compounds involved in N transportged to the shoot are similar to those used when ammonium or N 2 is the N source with similar implications for the regulation of shoot pH. The excess OH ‐ generated in the roots is partly excreted to the soil solution, and partly neutralized by the ‘biochemical pH stat’ which produces strong organic acids from essentially neutral precursors.
When nitrate is assimilated solely in shoots, the excess OH ‐ is initially neutralized by the operation of the biochemical pH state. Storage of the inorganic cation‐organate in shoot cell vacuoles could lead to turgor and volume regulation problems in these cells. These are avoided when an insoluble salt (calcium oxalate) is the product of the pH stat, or when the cation organate is translocated to the roots where organate breakdown regenerates OH ‐ , whcih is lost to the soil solution.
This mixture of biochemical, and long and short distance transport processes, enables cells remote from a large sink for H ⁺ or OH ‐ to produce protein without unfavourable pH changes. These processes related to pH regulation during N assimilation have important consequences for the carbon and energy economy of the plant.
In vivo activities of nitrate reductase (NR) were determined in soybean (Glycine max) and cotton (Gossypium hirsutum) roots and leaves, with either exogenous nitrate or endogenous nitrate as substrate. Differences between endogenous-nitrate and exogenous-nitrate activities, if any, revealed limitations to nitrate assimilation by substrate availability. In soybean, substrate was completely nonlimiting in roots but was severely limiting in leaves. In cotton, substrate had a slight effect which did not vary between roots and leaves. Soybean accumulated a greater fraction of the plant's total nitrate in the roots, and translocated a much smaller fraction to the leaves, than did cotton. The results imply that the ability of soybean roots to transport nitrate into the xylem was restricted, compared to cotton roots. Such a postulated restriction can help to explain differences between the two species in their distribution of nitrate reductase activity between roots and leaves.
Nitrate uptake patterns by nitrogen‐depleted wheat seedlings ( Triticum vulgare var. Atlas 66) exhibited an initial lag phase which was lessened, but not completely overcome, by presence of solution Ca. Maintenance of a more rapid subsequent rate of nitrate uptake, which developed after the initial lag phase, depended upon presence of solution K. The rapid phase was enhanced when both Ca and K were present and was curtailed by shoot excision and by Ca deficiency. Sizeably larger amounts of nitrate were recovered in the shoots after 24 hours in KNO 3 as compared to Ca(NO 3 ) 2 , and with the former salt, smaller proportions of the absorbed nitrate were reduced. The data suggest a beneficial influence of Ca both on initial nitrate uptake by root tissue and on transport to shoots while a continual K supply was apparently beneficial in nitrate transport.
Progressive suberization and thickening of the endodermis along secondary seminal axes of Zea mays had little effect on the radial movement of phosphate into the vascular tissue. In all parts of the root, inhibitors of respiration, and low temperature, reduced uptake and xylem translocation of phosphate by more than 90%. With calcium, however, there appeared to be two components of the translocated fraction; one was sensitive to metabolic inhibition and was most prominent in young tissue near to the root tip, while the other was insensitive to metabolic inhibition. In older zones near the base of the root, where there was little calcium movement into the stele, only the second insensitive component of calcium transport was found.
A marked maximum in calcium translocation, but not in the translocation of phosphate, was found 12 cm from the root tip. This was not associated with a marked increase in total absorption but was due to a high proportion of the absorbed calcium which moved into the stele. This region is one where lateral roots are initiated in the pericycle and where the structure of the endodermis may change transiently.
At the extreme base of the roots the development of a suberized hypodermis appeared to restrict very severely the translocation of both phosphate and calcium.
Iron and manganese were also absorbed and translocated by all regions of the root examined; the proportion translocated was much greater for manganese than for iron.
A rapid colourimetric method of determination of nitrate was modified. Proposed modifications eliminated the use of barium sulphate and introduced diazotization of sulphanilamide by the nitrite ion obtained by the reduction of nitrate and subsequent coupling with N-1-naphthyethelenediamine dihydrochloride. Introduction of filtration in place of centrifugation of coloured solution simplified the procedure. Determinations were highly reproducible with coefficient of variation of 2.2 and 2.9% for soil and plant extracts respectively.
Five-or six-day old seedlings of corn (Zea mays L.) were exposed to 0.25 mm Ca(NO(3))(2), 1.0 mm sodium 2-[N-morpholino]-ethanesulfonate, 5 mug Mo per liter and 50 mug of chloramphenicol per ml at pH 6. Nitrate uptake was determined from depletion of the ambient solution. The pattern of nitrate uptake was characterized, after the first 20 minutes, by a low rate which increased steadily to a maximal rate by 3 to 4 hours. Transfer of nitrate to the xylem did not totally account for the increase. Development of the maximal accelerated rate did not occur at 3 C with excised roots nor with seedlings whose endosperm had been removed. Use of CaCl(2) rather than Ca(NO(3))(2) resulted in a linear rate of chloride uptake during the first 4 hours, and chloride uptake was not as restricted by endosperm removal as was nitrate uptake.Nitrite pretreatments or the addition of cycloheximide (2 mug ml(-1)), puromycin (400 mug ml(-1)) and 6-methylpurine (0.5 mm) restricted maximal development of the accelerated nitrate uptake rate. Actinomycin D (20 mug ml(-1)) inhibited the rate only after about three hours exposure. The RNA and protein synthesis inhibitors also restricted nitrate reductase induction in the apical segments of the root tissue. The data suggest that development of the maximal accelerated rate of nitrate uptake depended upon continuous protein synthesis, and the hypothesis that synthesis of a specific nitrate transport protein must occur is advanced. But the alternative hypothesis, i.e., that induction of nitrate reductase (and/or a consequence of the act of nitrate reduction) provided the required stimulus, remains tenable.
THE INDUCTION AND REINDUCTION OF NITRATE REDUCTASE IN ROOT TIP OR MATURE ROOT SECTIONS SHOW ESSENTIALLY A SIMILAR PATTERN: a lag, a period of rapid increase in enzyme activity and finally a period of relatively minor change. Both inductions are sensitive to 6-methylpurine and cycloheximide. Kinetic studies with 6-methylpurine suggest that the half-life of the messenger RNA for nitrate reductase in both sections is about 20 minutes. The rate of decay of nitrate reductase activity induced by transfer to a nitrate-free medium is slower in root tips (t(1/2) = 3 hours) than in mature root sections (t(1/2) = 2 hours). The enzyme from mature root sections is also less stable to mild heat treatments (27 C; 40 C) than the enzyme from root tip sections. The results indicate that factors regulating enzyme turnover show important changes as root cells mature and may be significant in determining steady state levels of the enzyme.
When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with ³⁰Cl and then transferred to unlabeled chloride, the activity in the xylem exudate reached a maximum about 4 minutes after pulse labeling was discontinued and then declined sharply. The rate at which labeled chloride was transported across the root into the xylem and basipetally therein was on the order of 75 to 250 centimeters per hour. Consequently, symplasmic movement of chloride in corn roots is fast and may not be rate-limiting in transfer from the root surface to the xylem. Experiments on pulse labeling with ²²Na gave similar results. A large fraction of the absorbed ²²Na was not translocated into the exudate but was tightly sequestered in a cell compartment, probably the vacuole.
Electron probe analysis was used to reveal the pattern of potassium distribution in cross sections taken 10 to 11 millimeters from the tip. The cytoplasm and vacuoles of the xylem parenchyma cells accumulated potassium to a much greater extent than cortical and other stelar cells. Ultrastructural studies showed that the cytoplasm of the xylem parenchyma cells contains numerous membrane systems. It was concluded that the xylem parenchyma cells secrete ions from the symplasm into the conducting vessels, and it was suggested that this secretion is driven across the plasmalemma by a carrier-mediated transport.
The compartmental analysis method was used to estimate the K(+) and Cl(-) fluxes for cells of excised roots of Zea mays L. cv. Golden Bantam. When the measured fluxes are compared to those calculated with the Ussing-Teorell flux-ratio equation, an active inward transport of Cl(-) across the plasmalemma is indicated; the plasmalemma K(+) fluxes are not far different from those predicted for passive diffusion, although an active inward transport cannot be precluded. Whether fluxes across the tonoplast are active or passive depends upon the vacuolar potential which is unknown. Assuming no electropotential gradient, the tracer flux ratios are fairly close to those predicted for passive movement. However, if the vacuole is positive by about 10 millivolts relative to the cytoplasm, the data suggest active inward transport for K(+) and outward transport for Cl(-).Fluxes to the xylem exudate were found to be more accurately estimated from the specific radioactivity of the cytoplasm (symplasm) than from the external solution specific radioactivity. The electrochemical gradients for K(+) and Cl(-) between the xylem vessels and the surrounding stelar parenchyma indicate active K(+) and passive Cl(-) movement into the vessels. The data are interpreted as being in accord with radial transport through the symplast into living vessels.
Five-day-old seedlings of corn (Zealpha mays L.) grown without nitrate were decapitated and exposed to 0.5 mm KNO(3) or 0.5 mm KCl in aerated solutions at 30 C. Uptake of nitrate, chloride, and potassium was determined by replacing solutions hourly and measuring their depletion. Translocation of these ions and of organic nitrogen was determined by hourly analysis of the vascular exudate. Nitrate reduction was estimated by the difference between nitrate uptake and nitrate recovered in the tissue and exudate. Nitrate uptake exhibited its usual pattern of apparent induction resulting in the development of an accelerated uptake phase. Chloride uptake remained fairly constant throughout the experimental period. Translocation of nitrate increased progressively for at least 7 hours whereas chloride translocation reached a maximum about the 3d hour and then declined to a lower rate than nitrate translocation. Nitrate uptake and translocation were restricted by anaerobiosis, by 20 and 40 C relative to 30 C, and by 0.05 mm 6-methylpurine, an RNA-synthesis inhibitor. Accumulation, reduction and translocation of nitrate had different sensitivities to all these factors. The effect of 0.05 mm 6-methylpurine was more detrimental to nitrate translocation and nitrate reduction than to nitrate uptake.Ambient nitrate, relative to chloride, enhanced the exudation volume and the translocation of organic nitrogen within 4 hours from initiation of the experiments. Translocation of nitrate and organic nitrogen decreased shortly after removal of external nitrate. The higher rates of organic nitrogen translocation which occurred during nitrate uptake indicates either (a) rapid translocation of amino acids synthesized from the entering nitrate, or (b) an accelerated rate of protein turnover and a resulting enhancement in translocation of endogenous amino acids.
The induction of nitrate reductase activity in root tips of cotton (Gossypium hirsutum L.) was regulated by several amino acids and by ammonium. Glycine, glutamine, and asparagine strongly inhibited induction of activity by nitrate and also decreased growth of sterile-cultured roots on a nitrate medium. Methionine, serine, and alanine weakly inhibited induction, and 11 other amino acids had little or no effect. Ammonium also decreased induction in root tips, but was most effective only at pH 7 or higher. The optimum conditions for ammonium regulation of induction were identical to those for growth of sterile-cultured roots on ammonium as the sole nitrogen source. Aspartate and glutamate strongly stimulated induction, but several lines of evidence indicated that the mechanism of this response was different from that elicited by the other amino acids. The effects of amino acids on induction appeared to be independent of nitrate uptake.In green shoot tissues, all attempts to demonstrate regulation of induction by amino acids failed. The great difference in observed responses of root and shoot to amino acids suggests that their nitrate reductase activities are regulated differently. Differential regulation of this enzyme is consistent with the responses of root and shoot nitrate reductase activity to nitrate.
The plasma membrane fractions from separated cortex and stele of primary roots of corn (Zea mays L. WF9 x M14) contained cation ATPase activity at similar levels but with somewhat different properties. ATPase activity from cortex was optimum at pH 6.5, showed a simple Michaelis-Menten saturation with increasing ATP.Mg, and showed complex kinetic data for K(+) stimulation similar in character to the kinetic data for K(+)-ATPase and K(+) influx in primary roots. The results for cortex indicate that homogenates of primary roots are dominated by membranes from cortical cells.ATPase activity from stele was optimum at pH 6.5 and showed another maximum at pH 9. At pH 6.5, activity from stele had properties similar to that from cortex except that the kinetics of K(+) stimulation closely approached that expected for a Michaelis-Menten enzyme. At pH 9, the enzyme activity from stele was inhibited by 5 mug/ml oligomycin, suggesting that a significant portion of the activity was of mitochondrial origin. Sucrose density gradient analysis indicated some contamination of mitochondrial membranes in the plasma membrane fraction from stele. The results for stele are consistent with the view that stelar parenchyma cells are not deficient in ion pumps.
ATPase activity in xylem parenchyma cells of barley (Hordeum vulgare L.) roots was demonstrated cytochemically with a lead precipitation reaction. The methodical parameters of this cytochemical test were optimized for distinction between ATPase-specific and nonspecific precipitates. Optimum conditions were prefixation in 1% glutaraldehyde for 1 hour and incubation for 2 hours in a medium containing 2 mm each of ATP, Ca(2+), and Pb(2+) at pH 7 and 25 C. Problems of cytochemical localizations are discussed.ATPase activity occurred mainly at the plasmalemma, the endoplasmic reticulum nuclear envelope, and outer mitochondrial membranes of xylem parenchyma cells. The tonoplast of these cells showed only little ATPase activity. High K(+) concentrations stimulated ATPase activity, particularly at the plasmalemma. Diethylstilbestrol prevented the formation of ATPase-specific precipitates. The cytochemical demonstration of a K(+)-stimulated ATPase at the plasmalemma of xylem parenchyma cells is discussed in relation to the possible role of this membrane in ion transport to the vessels.
When amino acids or ammonia are added to plant systems, the effects on the development of nitrate-dependent nitrate reductase activity are variable. In addition, amino acids added singly or as casein hydrolysate may not support a normal growth. A physiologically correct mixture of amino acids, one similar in composition to amino acids released by the endosperm, has been shown to support normal growth and protein synthesis in corn (Zea mays) embryos. In this investigation, we have used the mixture of corn amino acids to determine whether amino acids have an effect on the appearance or disappearance of nitrate reductase activity. The results show that these amino acids partially inhibit the induction of nitrate reductase in corn roots. The effect is more pronounced in mature root than in root tip sections. When glutamine and asparagine are included along with the "corn amino acid mixture," the inhibition is more severe. Amino acids or amino acid analogues added singly to the induction medium have a similar effect: i.e. when the induction of nitrate reductase is inhibited in the root tips (lysine, canavanine, azaserine, azetidine-2-carboxylic acid, dl-4-azaleucine, asparagine, and glutamine), that inhibition is more severe in mature root sections. Arginine enhanced the recovery of nitrate reductase in root tips but inhibited it in mature root sections. The effect of the amino acids is apparently on some phase of the induction processes (i.e. the uptake or distribution of nitrate or a direct effect on the synthesis of the enzyme) and not on the turnover of the enzyme.
Nitrate uptake by dark-grown , K+, N03, C1 ), corn seedlings
- Jackson Wa
- D Flesher
JACKSON WA, D FLESHER, RH HAGEMAN 1973 Nitrate uptake by dark-grown , K+, N03, C1 ), corn seedlings. Plant Physiol 51: 120-127
WB FROST 1978 Role of potassium and malate in nitrate uptake and translocation by wheat seedlings
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BLEVINs DG, NM BARNETr, WB FROST 1978 Role of potassium and malate in nitrate uptake and translocation by wheat seedlings. Plant Physiol 62: 784-788
Influence of K+ on the uptake, translocation, and reduction ofnitrate by barley seedlings
- Blevins Dg Hiarr
- Lowe
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BLEVINS DG, AJ HiArr, RH LowE, JE LEGGETr 1978 Influence of K+ on the uptake, translocation, and reduction ofnitrate by barley seedlings. Agron J 70: 393-396
The influence ofnitrogen nutrition on ion uptake and translocation by leguminous plants
- Dw Israel
- Wa Jackson
IsRAEL DW, WA JACKSON 1978 The influence ofnitrogen nutrition on ion uptake
and translocation by leguminous plants. In CS Andrew, EJ Kamprath, eds,
Mineral Nutrition of Legumes in Tropical and Subtropical Soils. Commonwealth Scientific and Industrial Research Organizations, Canberra, Australia.
pp 113-129
Nitrate acquisition and assimilation by higher plants: processes in the root system Soil-Plant-Nitrogen Relationships
- Jackson Wa
JACKSON WA 1978 Nitrate acquisition and assimilation by higher plants:
processes in the root system. In DR Nielsen, JG MacDonald, eds, Nitrogen in
the environment, Vol 2 Soil-Plant-Nitrogen Relationships. Academic Press,
New York. pp 45-88
Nitrate uptake by dark-grown corn seedlings
- Jackson Wa
- D Flesher
- Rh Hageman
JACKSON WA, D FLESHER, RH HAGEMAN 1973 Nitrate uptake by dark-grown
corn seedlings. Plant Physiol 51: 120-127