Thylakoid membranes isolated from either spinach or chickpea leaves were used as a model system for evaluating the capacity of cyclitols to act as cryoprotectants. The effect of freezing for 3 h at -18 degrees C on cyclic photophosphorylation and electron transport was measured. The cyclitols, ononitol, O-methyl-muco-inositol, pinitol, quebrachitol and quercitol at 50-150 mol m(-3) decreased membrane damage by freezing and thawing to a similar degree as the well known cryoprotectants sucrose and trehalose. On addition of the cryotoxic solute NaCl (100 mol m(-3)) to the test system these methylated cyclohexanhexols again provided a protection comparable to that of the two disaccharides. Quercitol (cyclohexanpentol) was not effective when added in lower concentrations (50-100 mol m(-3)) and in case of this cyclitol a ratio of membrane toxic to membrane compatible solute of 0.66 was apparently needed to prevent a loss of cyclic photophosphorylation. Little difference was observed in the results from spinach or chickpea thylakoids although these plants naturally accumulate different cyto-solutes (spinach: glycinebetaine; chickpea: pinitol).
Sodium chloride-tolerant calli were selected from leaf-derived embryogenic calli of Dactylis glomerata L. on agar solidified medium supplemented with 200 mM NaCl, a concentration lethal to non-selected calli. Growth characteristics, water relations and proline accumulation pattern were compared in selected and non-selected lines. The objective was to gain an understanding of the mechanism(s) of tolerance in the NaCl-tolerant line. Growth in the selected line, as expressed in terms of tolerance index (ratio of fresh wt. on NaCl medium:fresh wt. on NaCl free medium x 100), was greater than that of the non-selected line at all levels of NaCl between 50 and 300 mM. There was no significant difference in proline accumulation in the selected and non-selected lines. Maintenance of turgor by osmotic adjustment was observed in the non-selected line despite decreased growth. In contrast, the selected line lost either the need or the ability to adjust osmotically. There was little or no increase in symplastic osmolality in the selected line when exposed to NaCl. Presumably, selection was made for a salt-excluding tissue that has lost the ability to accumulate solutes and adjust turgor with NaCl stress.
Loss of nutrients following pulses of nutrient input in northern hardwood forests and the general effects of atmospheric deposition on forest communities are of concern. Uptake of nutrients by ground layer vegetation, including herbs and tree wildlings, may be important in both of these processes. We brought plants from the field (Catskill Mts, New York) and grew them under controlled environment conditions at two nutrient input levels to determine responses of Oxalis acetosella and Acer saccharum to increased nutrient input and tree wildling density. Oxalis nutrient concentration increased for many nutrients compared to field plants. Both species doubled their P concentration when P input doubled. Biomass of Oxalis was unaffected by both nutrient input level and Acer wildling density. Acer showed a similar response to increased nutrient input and its density did not affect the response of Oxalis to increased nutrient input. Results indicate that both species may be important to nutrient retention in northern hardwood forests.
The influence of pre-measurement storage length and season of harvest of stem segment samples on hydraulic conductance and percentage embolism was determined for two tree species because no published guidelines exist concerning storage. Stem sections from Fraxinus americana L. 'Autumn Applause' (white ash) and Acer rubrum L. x saccharinum L. 'Autumn Blaze' (hybrid red maple) were collected from well-established trees in fall 1995 (October), spring 1996 (April), and summer 1996 (July). Ends of stem sections collected in the fall were either covered with wax or left exposed. Entire sections from all dates were placed in closed plastic bags to prevent desiccation during transport and subsequent storage. Stem sections were either analyzed immediately (0 storage) or held at 2 degrees C for 2 or 4 days. Hydraulic conductance before embolisms were cleared with positive pressure (initial k(h)), hydraulic conductance after embolisms were cleared (maximum k(h)), and percentage embolism were similar for all pre-embolism measurement storage lengths within each of the three seasonal sampling periods for hybrid red maple and spring- and summer-collected white ash. Fall-collected white ash samples with 0 storage had higher initial k(h), and percentage embolism increased if samples were stored. Embolism was greatest for summer-collected samples and lowest for spring-collected samples for hybrid red maple, but values were similar for white ash. Stem covering did not influence measured parameters. Our data indicate that hybrid red maple stem segments can be stored without significant loss of hydraulic conductance for up to 4 days, but white ash should not be stored in the fall. Unless maximum levels of native embolism have been reached, as determined from laboratory analysis, stem segments of species on which storage data are not available should be processed as soon as possible.
The uptake of hydrogen sulfide (H2S) by shoots of curly kale (Brassica oleracea) showed saturation kinetics with respect to the atmospheric concentration. The kinetics are largely determined by the rate of metabolism of the absorbed H(2)S into cysteine, catalyzed by O-acetylserine (thiol)lyase, and can be described by the Michaelis-Menten equation. When B. oleracea was grown under sulfate (SO42-)-deprived conditions, plants developed sulfur (S) deficiency symptoms and H(2)S uptake kinetics were substantially altered. Shoots of SO42--deprived plants had a lower affinity to H(2)S uptake, whereas the maximal H(2)S uptake rate was higher. When SO(4)(2-)-deprived plants were simultaneously exposed to 0.2 &mgr;l l(-1) H(2)S all S deficiency symptoms disappeared and H(2)S uptake kinetics returned rapidly to values observed for S-sufficient shoots. The activity of the H(2)S-fixating enzyme O-acetylserine (thiol)lyase was hardly affected upon either prolonged H(2)S exposure or SO(4)(2-) deprivation. Evidently, the activity of O-acetylserine (thiol)lyase was not the rate-limiting step in the H(2)S uptake by shoots. The significance of the in situ availability and rate of synthesis of the substrate O-acetylserine for O-acetylserine (thiol)lyase as determining factor in the uptake kinetics of H(2)S needs further evaluation.
Stemflow from Japanese red cedar (Cryptomeria japonica) enters forest soil at a low pH. We evaluated the responses of the root system of Japanese red cedar saplings to acidic conditions, used to simulate this situation, in two different growth media, a brown forest soil (BS) and a Yahagi sand (YS). Soils were acidified by the addition of solutions at pH 2.0, 3.0 and 5.5 (control). Root morphology, root surface area index, root respiration activity and root biomass were measured. In the pH 3.0 treatment, no significant effects were found on the root systems compared with the controls in either soil, except for a slight difference in root-tip diameter in the Yahagi sand. In the pH 2.0 treatment, the surface area index and dry weight ratios of the whole root in the Yahagi sand were significantly lower than those in the other treatments. No significant effects on the whole root were observed in the brown forest soil. These results suggest that detrimental effects of acidic solutions on the root systems would be less significant in brown forest soil, which contains humus, than in the Yahagi sand, which lacks humus. They also suggest that the threshold pH value causing visible morphological changes on the roots of Japanese red cedar saplings falls in the pH range between 2 and 3. White roots in the pH 2.0 treatment had low respiration activity and showed visible morphological changes in both soils. These responses were presumably related to the effects of excess Al in the soil solution. White roots in the pH 2.0 treatment typically produced exodermis. The results suggest that stemflow with a pH of 3.0 has no effects on the root systems of Japanese red cedar, and that the morphology of white roots was adversely affected not by treatment at pH 2.0 but by excess water-soluble Al in the soil.
Uptake capacity of organic nitrogen was studied in solution experiments on eight grasses and two forbs growing in acid soils with relatively high nitrogen mineralisation in southern Sweden. Uptake of a mixture of amino acids (alanine, glutamine, glycine), that varied between 1.6 and 6.3 µmol g(-1) dw root h(-1), could not be explained by soil data from the species' field distributions (pH, total carbon and nitrogen, potential net mineralisation of ammonium and nitrate). The ratio between organic and inorganic nitrogen (methylamine) uptake was <0.05 for the forbs, higher for the grasses with a maximum of 1.42 for Deschampsia flexuosa. The ratio was negatively correlated with measures related to soil acidity (Ellenberg's R-value, soil nitrate and total carbon) but not, as hypothesised, with the total amount of mineralised nitrogen. The total demand on nitrogen by all components of the ecosystem would probably have described the extent to which competition among and between plants and microbes induced nitrogen limitation. In a methodological study two grasses were exposed to pH 3.8, 4.5 and 6.0 and to 50, 100 and 250 µmol l(-1) of three amino acids. Uptake was also compared between intact plants and excised roots. The treatment response varied considerably between the species which stresses the importance of studying intact plants at field-relevant pH and concentrations.
In this study, we test the hypothesis that exposure to environmentally significant concentrations of aluminum (Al, 80 µM) causes the microfilament array of Vaucheria longicaulis var. macounii vegetative filaments to become fragmented and disorganized. Changes in F-actin organization following treatment of vegetative filaments by Al are examined using vital staining with fluorescein phalloidin. In the cortical cytoplasm of the apical zone of pH 7.5 and pH 4.5 control cells, axially aligned bundles of F-actin lead to a region of diffuse, brightly stained material. Dimly stained focal masses are noted deeper in the cytoplasm of the apical zone whereas they are absent from the zone of vacuolation. The F-actin array is visualized in the cortical cytoplasm of the region of the cell, distal to the apical tip, which exhibits vigorous cytoplasmic streaming (zone of vacuolation) as long, axially aligned bundles with which chloroplasts and mitochondria associate. Thirty minutes following treatment with aluminum, and for the next 8-16 h, the F-actin array is progressively disorganized. The longitudinally aligned F-actin array becomes fragmented. Aggregates of F-actin, such as short rods, amorphous and stellate F-actin focal masses, curved F-actin bundles and F-actin rings replace the control array. Each of these structures may occur in association with chloroplasts or independently with no apparent association with organelles. Images are recorded which indicate that F-actin rings not associated with organelles may self-assemble by successive bundling of F-actin fragments. The fragmentation and bundling of F-actin in cells of V. longicaulis upon treatment with aluminum resembles those reported after diverse forms of cell disturbance and supports the hypothesis that aluminum-induced changes in the F-actin array may be a calcium-mediated response to stress.
Minirhizotrons provide a nondestructive, in situ method for directly viewing and studying fine roots. Although many insights into fine roots have been gained using minirhizotrons, a review of the literature indicates a wide variation in how minirhizotrons and minirhizotron data are used. Tube installation is critical, and steps must be taken to insure good soil/tube contact without compacting the soil. Ideally, soil adjacent to minirhizotrons will mimic bulk soil. Tube installation causes some degree of soil disturbance and has the potential to create artifacts in subsequent root data and analysis. We therefore recommend a waiting period between tube installation and image collection of 6-12 months to allow roots to recolonize the space around the tubes and to permit nutrients to return to pre-disturbance levels. To make repeated observations of individual roots for the purposes of quantifying their dynamic properties (e.g. root production, turnover or lifespan), tubes should be secured to prevent movement. The frequency of image collection depends upon the root parameters being measured or calculated and the time and resources available for collecting images and extracting data. However, long sampling intervals of 8 weeks or more can result in large underestimates of root dynamic properties because more fine roots will be born and die unobserved between sampling events. A sampling interval of 2 weeks or less reduces these underestimates to acceptable levels. While short sample intervals are desirable, they can lead to a potential trade-off between the number of minirhizotron tubes used and the number of frames analyzed per tube. Analyzing fewer frames per minirhizotron tube is one way to reduce costs with only minor effects on data variation. The quality of minirhizotron data should be assessed and reported; procedures for quantifying the quality of minirhizotron data are presented here. Root length is a more sensitive metric for dynamic root properties than the root number. To make minirhizotron data from separate experiments more easily comparable, idiosyncratic units should be avoided. Volumetric units compatible with aboveground plant measures make minirhizotron-based estimates of root standing crop, production and turnover more useful. Methods for calculating the volumetric root data are discussed and an example presented. Procedures for estimating fine root lifespan are discussed.
Differential responses to elevated atmospheric CO(2) concentration exhibited by different plant functional types may alter competition for above- and belowground resources in a higher CO(2) world. Because C allocation to roots is often favored over C allocation to shoots in plants grown with CO(2) enrichment, belowground function of forest ecosystems may change significantly. We established an outdoor facility to examine the effects of elevated CO(2) on root dynamics in artificially constructed communities of five early successional forest species: (1) a C(3) evergreen conifer (longleaf pine, Pinus palustris Mill.); (2) a C(4) monocotyledonous bunch grass (wiregrass, Aristida stricta Michx.); (3) a C(3) broadleaf tree (sand post oak, Quercus margaretta); (4) a C(3) perennial herbaceous legume (rattlebox, Crotalaria rotundifolia Walt. ex Gemel); and (5) an herbaceous C(3) dicotyledonous perennial (butterfly weed, Asclepias tuberosa L.). These species are common associates in early successional longleaf pine savannahs throughout the southeastern USA and represent species that differ in life-form, growth habit, physiology, and symbiotic relationships. A combination of minirhizotrons and soil coring was used to examine temporal and spatial rooting dynamics from October 1998 to October 1999. CO(2)-enriched plots exhibited 35% higher standing root crop length, 37% greater root length production per day, and 47% greater root length mortality per day. These variables, however, were enhanced by CO(2) enrichment only at the 10-30 cm depth. Relative root turnover (flux/standing crop) was unchanged by elevated CO(2). Sixteen months after planting, root biomass of pine was 62% higher in elevated compared to ambient CO(2) plots. Conversely, the combined biomass of rattlebox, wiregrass, and butterfly weed was 28% greater in ambient compared to high CO(2) plots. There was no difference in root biomass of oaks after 16 months of exposure to elevated CO(2). Using root and shoot biomass as a metric, longleaf pine realized the greatest and most consistent benefit from exposure to elevated CO(2). This finding suggests that the ability of longleaf pine to compete with sand post oak, a common deciduous tree competitor, and wiregrass, the dominant understory herbaceous species, in regenerating ecosystems may be significantly enhanced by rising atmospheric CO(2) concentrations.
Tradescantia pallida cv. purpurea, a popular garden plant in Brazil, was used for the Tradescantia micronucleus (Trad-MCN) assay. In situ monitoring of the genotoxicity of air pollutants was carried out by sentinel approach, using the plant grown in the field or using the plants in pots which were carried to the monitoring sites. Two highly polluted sites, in São Paulo city (Cerqueira Cesar and Congonhas) and two rural sites (the cities of Pirassununga, 200 km and Caucaia do Alto, 50 km from São Paulo, respectively) were chosen for this study, in order to determine the gradient difference of the air pollution levels. Sentinel plants in Congonhas site presented the highest frequency of micronuclei (4.4%), in comparison with 2.2 and 2.3% found in plants from Pirassununga and Cerqueira Cesar sites, respectively (Kruskal-Wallis; P<0.020). Significant increases (F test; P<0.0001) in the frequency of micronuclei were observed in plants exposed in the polluted urban sites (Cerqueira Cesar: 5.7%; Congonhas: 7.1% and Caucaia do Alto: 2.3%). The increase in the frequency of micronuclei observed indicates the potential risk of mutagenicity in presence of high concentrations of pollutants.
To test the hypothesis that growth-CO(2) concentrations affect stress susceptibility, leaves of poplar trees (Populus alba x tremula) grown under ambient or about twofold ambient CO(2) concentrations were subjected to chilling temperatures at high light intensities or were exposed to paraquat. Photosynthesis was less diminished and electrolyte leakage was lower in stressed leaves from poplar trees grown under elevated [CO(2)] as compared with those from ambient [CO(2)]. Severe stress caused pigment and protein degradation but to a lower extent in leaves from elevated as compared with those from ambient [CO(2)]. The protection was accompanied by rapid induction of superoxide dismutase activities (EC 22.214.171.124). Ascorbate and glutathione-related detoxification systems as well as catalase (EC 126.96.36.199) activities were less resistant than superoxide dismutases and declined in stress-exposed leaves from poplars grown under elevated [CO(2)] to a similar extent as in those from trees grown under ambient [CO(2)]. These results suggest that the CO(2)-mediated amelioration of stress was confined to SOD and limited since the destruction of H(2)O(2)-degrading systems was not prevented.
Research was conducted on Aloe vera, a traditional medicinal plant, to investigate the effects of light on growth, carbon allocation, and the concentrations of organic solutes, including soluble carbohydrates and aloin. The plants were vegetatively propagated and grown under three irradiances: full sunlight, partial (30% full sunlight), and deep shade (10% full sunlight) for 12-18 months. After 1 year of growth, five plants from each treatment were harvested to determine total above- and below ground dry mass. Four plants from the full sunlight and the partial shade treatments were harvested after 18 months to assess the soluble carbohydrate, organic acid and aloin concentrations of the clear parenchyma gel and the yellow leaf exudate, separately. Plants grown under full sunlight produced more numerous and larger axillary shoots, resulting in twice the total dry mass than those grown under partial shade. The dry mass of the plants grown under deep shade was 8.6% that of plants grown under full sunlight. Partial shade increased the number and length of leaves produced on the primary shoot, but leaf dry mass was still reduced to 66% of that in full sunlight. In contrast, partial and deep shade reduced root dry mass to 28 and 13%, respectively, of that under full sunlight, indicating that carbon allocation to roots was restricted under low light conditions. When plants were sampled 6 months later, there were only minor treatment effects on the concentration of soluble carbohydrates and aloin in the leaf exudate and gel. Soluble carbohydrate concentrations were greater in the gel than in the exudate, with glucose the most abundant soluble carbohydrate. Aloin was present only in the leaf exudate and higher irradiance did not induce a higher concentration. Limitation in light availability primarily affected total dry mass production and allocation, without substantial effects on either primary or secondary carbon metabolites.
Aluminum-induced cell death was investigated in root-tip cells of barley (Hordeum vulgare). The growth of roots in 0.1-50 mM Al treatments was inhibited after 8 h treatments, and could not be recovered after 24 h recovery culture without Al. Viable detection with fluorescein diacetate-propidium iodide (FDA-PI) staining shows that most of the root-tip cells have lost viability. These results suggest that the irreversible inhibition of root growth after 8 h Al treatments or 24 h recovery culture is mainly caused by cell death. DNA ladders occurred in root tips only after 8 h Al treatments (0.1-1.0 mM), but no apoptotic bodies in root tips were observed. Thus, the cell death caused by Al stress is likely to be Al-induced programmed cell death (PCD). The reactive oxygen species (ROS) in root-tip cells measured by ultraweak luminescence indicated that the oxidation status in root-tip cells basically ceased after exposure to 10-50 mM Al for 24 h, but was very violent in the root-tip cells treated with 0.1-1.0 mM for 24 h. Exposure to 0.1-1.0 mM Al for 3-12 h led to ROS burst. Therefore, our results suggest that 0.1-1.0 mM Al treatments for 8 h induce cell death (Al-induced PCD) possibly via a ROS-activated signal transduction pathway, whereas 10-50 mM Al treatments may cause necrosis in the root-tip cells. These results have an important role for further studies on the mechanism of Al toxicity in plants.
Relationships between growth and different gas exchange characteristics of two amphidiploid salt tolerant species, Brassica napus, and B. carinata with respect to their salt sensitive parents, B. oleracea, and B. nigra were investigated. Twenty three-day old plants of these four species along with those of another amphidiploid moderately salt tolerant B. juncea (developed by hybridization of diploids, B. campestris and B. nigra), and a diploid moderately salt tolerant, B. campestris, were subjected for 28 days to salinized sand culture containing 0, 100 or 200 mol NaCl m(-3) in Hoagland's nutrient solution. The species B. napus and B. carinata produced significantly greater shoot fresh and dry matters than their parents under saline conditions. A close association was found between growth, and assimilation rate for all species differing in degree of salt tolerance. Stomatal conductance (g(s)) was reduced due to salt stress in all species but this variable had no significant correlation with assimilation rate (A). However, the amphidiploid salt tolerant species, B. napus and B. carinata had significantly greater photosynthetic rate, water use efficiency (A/E), intrinsic water use efficiency (A/g(s)) than those of their diploid parents. In conclusion, high salt tolerance of the two amphidiploid species, B. napus and B. carinata was associated with a high assimilation rate, water use efficiency and intrinsic water use efficiency but there was little association of the tolerance of these species with stomatal conductance, leaf water potential or transpiration rate (E).
Levels of endogenous glycine betaine in the leaves were measured in response to cold acclimation, water stress and exogenous ABA application in Arabidopsis thaliana. The endogenous glycine betaine level in the leaves increased sharply during cold acclimation treatment as plants gained freezing tolerance. When glycine betaine (10 mM) was applied exogenously to the plants as a foliar spray, the freezing tolerance increased from -3.1 to -4.5 degrees C. In addition, when ABA (1 mM) was applied exogenously, the endogenous glycine betaine level and the freezing tolerance in the leaves increased. However, the increase in the leaf glycine betaine level induced by ABA was only about half of that by the cold acclimation treatment. Furthermore, when plants were subjected to water stress (leaf water potential of approximately -1.6 MPa), the endogenous leaf glycine betaine level increased by about 18-fold over that in the control plants. Water stress lead to significant increase in the freezing tolerance, which was slightly less than that induced by the cold acclimation treatment. The results suggest that glycine betaine is involved in the induction of freezing tolerance in response to cold acclimation, ABA, and water stress in Arabidopsis plants.
Symplastic growth of plant organs may be described by a continuous growth tensor field. In tensorial analysis of meristems, the trajectories of periclinal and anticlinal cell walls represent trajectories of the principal directions of growth (PDGs); this follows from the maintenance of mutual orthogonality between periclinal and anticlinal wall trajectories during growth. Periclinal and anticlinal cell divisions are also oriented in the principal planes of growth. The growth tensor for the root apex is specified in such a way that the principal directions of the tensor fit the pattern of periclinal and anticlinal walls in the apex, and that the grid formed by material particles aligned along PDG trajectories preserve this alignment during growth. Two growth tensors are formulated--one giving a maximum and the other giving a minimum of the volumetric relative elemental growth rate at the region of the initial cell(s). Temporal sequences of deformation of a grid formed by lines coinciding with the principal directions of growth are shown. The formation of cellular patterns in root apices is simulated. Two types of patterns are obtained: one with an apical cell and merophytes, and another with files of cells converging towards a quiescent centre.
Sunflower (Helianthus annuus) was grown in both open-field and outdoor potted conditions in Southern Italy, and irrigated with water having electrical conductivity ranging between 0.9 and 15.6 dS m(-1) obtained by different NaCl concentrations. The aim of the work was to study the leaf area and photosynthetic responses of sunflower to mild salt stress. The response curve (A/c(i)) of assimilation (A) to leaf internal CO(2) concentration (c(i)) was used to determine leaf gas-exchange parameters, in order to evaluate stomatal and non-stomatal limitations to photosynthesis in relation to salt stress. In the field, a reduction of 19% in leaf area expansion occurred, while no correlation was observed between Psi(l) and stomatal conductance to water vapour (g(sw)) ranging between 0.76 and 1.35 mol m(-2) s(-1). This result was also evident at a higher salinity level reached in the pot experiment where leaf osmotic potential (psi(s)) varied from -1.35 to -2.67 MPa as compared with the field experiment, where psi(s) ranged from -1.15 to -1.42 MPa. Considering the two experiments as a unique data set, the assimilation rate, the stomatal conductance to CO(2) (g(sc)) and the sensitivity of A to c(i) variation (g*) were not significantly influenced by salinity in the whole range of psi(s). As a consequence, the stomatal and non-stomatal limitations to photosynthesis were not affected by salt treatment, averaging around 20 and 80%, respectively. The variation in A (from 44 to 29 µmol m(-2) s(-1)) was paralleled by the variation in g(sc) (from 0.47 to 0.84 mol m(-2) s(-1)), with a remarkable constancy of both c(i) (200+/-12.5 µmol mol(-1)) and normalized water-use efficiency (5+/-0.7 µmol mmol(-1) kPa), showing the optimal behaviour of the plant processes. These findings indicate that, under mild salt stress, the same as observed under water deficit, sunflower controls assimilation mainly by modulating leaf area rather than by stomatal closure, and that non-stomatal limitation of photosynthesis was not affected at all by the level of salinity reached in this study.
Tropisms and other movements of a plant organ result from alterations in local rates of cell elongation and a consequent development of a growth differential between its opposite sides. Relative elemental rates of elongation (RELELs) are useful to characterize the pattern of growth along and round an organ. We assume that the value of the RELEL at a given point is dependent on distance from the tip and that the distribution of values along the organ surface can be characterized in terms of the spread and the position of the maximum value. A computer model is described which accommodates these parameters and simulates tropic curvatures due to differential growth. Additional regulatory functions help to return the simulated organ to its original orientation. Particular attention is given to the simulation of root gravitropism because here not only do each of the various growth and regulatory parameters have a known biological counterpart, but some can also be given an actual quantitative value. The growth characteristics relate to the biophysical properties of cells in the elongation zone of the root, while the regulatory functions relate to aspects of the graviperception and transmission systems. We believe that, given a suitably flexible model, computer simulation is a powerful means of characterizing, in a quantitative way, the contribution of each parameter to the elongation of plant organs in general and their tropisms in particular.
Using iron-deprived (-Fe) chlorotic as well as green iron-deficient (5 μM Fe) and iron-sufficient supplied (50 μM Fe) leaves of young hydroponically reared Brassica napus plants, we explored iron deficiency effects on triggering programmed cell death (PCD) phenomena. Iron deficiency increased superoxide anion but decreased hydroxyl radical (•OH) formation (TBARS levels). Impaired photosystem II efficiency led to hydrogen peroxide accumulation in chloroplasts; NADPH oxidase activity, however, remained on the same level in all treatments. Non-autolytic PCD was observed especially in the chlorotic leaf of iron-deprived plants, to a lesser extent in iron-deficient plants. It correlated with higher DNAse-, alkaline protease- and caspase-3-like activities, DNA fragmentation and chromatin condensation, hydrogen peroxide accumulation and higher superoxide dismutase activity. A significant decrease in catalase activity together with rising levels of dehydroascorbic acid indicated a strong disturbance of the redox homeostasis, which, however, was not caused by •OH formation in concordance with the fact that iron is required to catalyse the Fenton reaction leading to •OH generation. This study documents the chain of events that contributes to the development of non-autolytic PCD in advanced stages of iron deficiency in B. napus leaves.
During phototropic curvature, indolyl-3-acetic acid (IAA) remains evenly distributed in the hypocotyl of sunflower (Helianthus annuus L.) and in the oat (Avena sativa L.) coleoptile. At the irradiated side, growth inhibiting substances accumulate. In sunflower, basipetal movement of a growth factor is not involved, since the top of the seedling can be covered or removed without affecting the photo-tropic response; this response, moreover, is independent of the rate of elongation growth. The chemical nature of the growth-inhibiting substances is only partly known. In the hypocotyl they occur in the neutral fraction: in sunflower cis-xanthoxin is one of them, in radish (Raphanus sativus L.) cis- and trans-raphanusanins, and possibly raphanusamide, are involved. The inhibitor(s) in the oat coleoptile are acidic. During curvature, their amount remains rather constant but the distribution changes with an accumulation at the irradiated side. It is concluded that phototropic curvature is brought about by an accumulation, at the irradiated side, of growth-inhibiting substances that unilaterally reduce cell elongation even though the IAA distribution is uniform.
Loblolly pine (Pinus taeda L.) seedlings were grown in competition with native weeds using soil and seed bank collected from recently chopped and burned areas near Appomattox, Virginia. One-year-old seedlings were planted and weeds allowed to germinate from the native seed bank while being exposed to CO(2) (ambient and elevated - approximately 700 ppm) and water (water stressed and well watered) treatments for approximately one growing season in a greenhouse. Elevated CO(2) did not influence total weed biomass; however, C(3) weed community development was favored over C(4) weed community development in elevated CO(2) regardless of water availability. This suggests that weed community composition may shift toward C(3) plants in a future elevated CO(2) atmosphere. Pine growth was significantly greater in the well watered and elevated CO(2) treatments compared to the water stressed and ambient treatments, respectively, even though they were competing with native herbaceous weeds for resources. There was a significant water and CO(2) interaction for pine root:shoot ratio. Under elevated CO(2), root:shoot ratio was significantly greater in the water stressed treatment than the well watered treatment. In contrast, there was no significant difference in the root:shoot ratio under the ambient CO(2) treatment for either water treatment. These results suggest that loblolly pine seedlings will respond favorably in an elevated CO(2) atmosphere, even under dry conditions and competing with herbaceous weeds.
Oat (Avena sativa) plants were grown in the field near the urban area of Valencia, Eastern Spain. The data on air quality showed that ozone was the main phytotoxic pollutant present in ambient air reaching a 7-h mean of 46 nl l(-1) and a maximum hourly peak of 322 nl l(-1). The effect of ambient ozone on PSII activity was examined by measurements of chlorophyll (Chl) a fluorescence. In leaves with visible symptoms, the function of PSII was changed at high actinic irradiances. Nonphotochemical quenching (NPQ) was higher and quantum efficiency of PSII (Phi(PSII)), photochemical quenching (q(p)), quantum efficiency of excitation capture and PSII electron flow (F(v)'/F(m)') were lower. An enhanced susceptibility to photoinhibition was observed for symptom-exhibiting leaves compared to leaves that remain free of visible symptoms. Both the lowering of photosynthesis efficiency and the increased sensitivity to photoinhibition probably contribute to reduced crop yield in the field, to different extents, depending on growth conditions. To our knowledge, this is the first report that demonstrates that quantum efficiency of exciton trapping in PSII is associated with foliar injury in oat leaves in response to ambient concentration of ozone.
Two field experiments in 1993 and 1994 as well as a laboratory germination experiment were conducted on the Egyptian cotton cultivar Giza 75 (Gossypium barbadense) to determine the effect of six concentrations of kinetin (6-furfurylaminopurine) ranging from 0 to 10.0 mg l(-1) and three different methods of application: (A) seeds were soaked for 24 h before germination (laboratory experiment) or sowing (field experiment) in solutions of different kinetin concentration, (B) cotton plants were sprayed twice with different kinetin concentrations at 60 and 75 days after sowing (DAS) during the square initiation and the beginning of bolling stages, at volume solution of 480 l ha(-1). (C) Seeds were soaked in kinetin solutions before sowing as method A. In addition cotton plants were sprayed twice as method B with the same kinetin concentrations. Kinetin application improved seed viability and seedling vigour as shown by lengths of the hypocotyl, radicle and the entire seedling, as well as seedling fresh weight. Moreover, significant increases were recorded in the number of open bolls/plant, boll weight, lint and seed indices, seed cotton yield/plant, and seed cotton and lint yields/plot. The highest means were obtained at 5 mg kinetin l(-1) concentration and under method C of application. Treatments generally, had no significant effects on lint percentage, yield earliness and fiber properties. These results show that, the use of kinetin at 5 mg l(-1) for pre-soaking seeds before planting and spraying cotton plants at 60 and 75 DAS with the same concentration could improve cotton germination, seed cotton and lint yields.
Senescence is a highly regulated process which is under genetic control. In monocarpic plants, the onset of fruit development is the most important factor initiating the senescence process. During senescence, a large fraction of plant nutrients is reallocated away from vegetative tissues into generative tissues. Senescence may therefore be regarded as a highly effective salvage mechanism to save nutrients for the offspring. CO(2) enrichment, besides increasing growth and yield of C(3) plants, has often been shown to accelerate leaf senescence. C(3) plants grown under elevated CO(2) experience alterations in their nutrient relations. In particular their tissue nitrogen concentrations are always lower after exposure to elevated CO(2). We used a monocarpic C(3) crop - spring barley (Hordeum vulgare cv. Alexis) - grown in open-top field chambers to test the effects of CO(2) enrichment on growth and yield, on nitrogen acquisition and redistribution, and on the senescence process in flag leaves, at two applications of nitrogen fertilizer. CO(2) enrichment (650 vs. 366 µmol mol(-1)) caused an increase both in biomass and in grain yield by 38% (average of the two fertilizer applications) which was due to increased tillering. Total nitrogen uptake of the crops was not affected by CO(2) treatment but responded solely to the N supply. Nitrogen concentrations in grains and straw were significantly lower (-33 and -24%) in plants grown at elevated CO(2). Phenological development was not altered by CO(2) until anthesis. However, progress of flag leaf senescence as assessed by chlorophyll content, protein content and content of large and small subunit of RubisCO and of cytochrome b559 was enhanced under elevated CO(2) concentrations by approximately 4 days. We postulate that CO(2) enhanced flag leaf senescence in barley crops by increasing the nitrogen sink capacity of the grains.