Richard C. Sicher’s research while affiliated with Agricultural Research Service and other places

Changes in the stomatal aperture in response to CO2 levels allow plants to manage water usage, optimize CO2 uptake and adjust to environmental stimuli. The current study reports that sub-ambient CO2 up-regulated the low temperature induction of the C-repeat Binding Factor (CBF)-dependent cold signaling pathway in Arabidopsis (Arabidopsis thaliana) and the opposite occurred in response to supra-ambient CO2. Accordingly, cold induction of various downstream cold-responsive genes was modified by CO2 treatments and expression changes were either partially or fully CBF-dependent. Changes in electrolyte leakage during freezing tests were correlated with CO2′s effects on CBF expression. Cold treatments were also performed on Arabidopsis mutants with altered stomatal responses to CO2, i.e., high leaf temperature 1-2 (ht1-2, CO2 hypersensitive) and β-carbonic anhydrase 1 and 4 (ca1ca4, CO2 insensitive). The cold-induced expression of CBF and downstream CBF target genes plus freezing tolerance of ht1-2 was consistently less than that for Col-0, suggesting that HT1 is a positive modulator of cold signaling. The ca1ca4 mutant had diminished CBF expression during cold treatment but the downstream expression of cold-responsive genes was either similar to or greater than that of Col-0. This finding suggested that βCA1/4 modulates the expression of certain cold-responsive genes in a CBF-independent manner. Stomatal conductance measurements demonstrated that low temperatures overrode low CO2-induced stomatal opening and this process was delayed in the cold tolerant mutant, ca1ca4, compared to the cold sensitive mutant, ht1-2. The similar stomatal responses were evident from freezing tolerant line, Ox-CBF, overexpression of CBF3, compared to wild-type ecotype Ws-2. Together, these results indicate that CO2 signaling in stomata and CBF-mediated cold signaling work coordinately in Arabidopsis to manage abiotic stress.

Access to adequate irrigation resources is critical for sustained agricultural production, and rice, a staple cereal grain for half of the world population, is one of the biggest users of irrigation. To reduce water use, several water saving irrigation systems have been developed for rice production, but a reliable system to evaluate cultivars for water stress tolerance is still lacking. Here, seven rice cultivars that have diverse yield potential under water stress were evaluated in a field study using four continuous irrigation regimes varying from saturation to wilting point. To understand the relationship between water stress and yield potential, the physiological and leaf metabolic responses were investigated at the critical transition between vegetative and reproductive growth stages. Twenty-nine metabolite markers including carbohydrates, amino acids and organic acids were found to significantly differ among the seven cultivars in response to increasing water stress levels with amino acids increasing but organic acids and carbohydrates showing mixed responses. Overall, our data suggest that, in response to increasing water stress, rice cultivars that do not show a significant yield loss accumulate carbohydrates (fructose, glucose, and myo-inositol), and this is associated with a moderate reduction in stomatal conductance (gs), particularly under milder stress conditions. In contrast, cultivars that had significant yield loss due to water stress had the greatest reduction in gs, relatively lower accumulation of carbohydrates, and relatively high increases in relative chlorophyll content (SPAD) and leaf temperature (Tm). These data demonstrate the existence of genetic variation in yield under different water stress levels which results from a suite of physiological and biochemical responses to water stress. Our study, therefore, suggests that in rice there are different physiological and metabolic strategies that result in tolerance to water stress that should be considered in developing new cultivars for deficit irrigation production systems that use less water.

Impacts of potassium (K) deficiency and elevated carbon dioxide (eCO2) on seed constituents have rarely been explored in most crops including soybean. A controlled environment experiment was conducted with soybean grown under a sufficient (5.0 mM) and two deficient (0.50 and 0.02 mM) levels of K fertilization at ambient (aCO2) and eCO2 (400 and 800 µmol•mol −1 , respectively). Both treatments significantly affected several constituents, with the K deficiency having stronger impacts than eCO2. Out of 49 seed constituents, K deficiency and eCO2 influenced 41 and 16 constituents, respectively. The K deficiency primarily decreased on average 16 constituents including minerals (e.g., K, P, Mg, Mn, Zn, Fe, B), oil, and essential fatty acids (e.g., linoleic and linolenic acids) but enhanced 25 constituents such as protein, amino acids, simple sugars, and stress-responsive metabolites (e.g., sugar alcohols manni-tol and myo-inositol and proline). An accumulation of N while decreased C concentration resulted in the lower C:N ratio in the seeds of K-deficient plants. However, protein:oil, C:K, N:P, and N:K ratios were consistently greater under K deficiency. The eCO2 also decreased minerals such as P, S, Zn, B, and essential fatty acids but enhanced the concentration of six constituents including alanine, oleic acid, fructose, and sugar alcohols across K fertilization. In addition, the impact of eCO2 on several amino acids appeared to be dependent on the severity of K deficiency. For instance, eCO2 decreased essential amino acids (e.g., valine, phenylalanine, isoleucine) in the seeds of severely K-deficient plants but not in the other treatments leading to a K × CO2 interaction. Results showed that CO2 enrichment is likely to exacerbate the decline in the concentration of seed minerals such as P, K, S, Zn and B, essential fatty acids, and amino acids under K limited conditions.

Drought invokes a variety of metabolic alterations in plant leaves to cope with stress situations. To understand the effects of CO2 and drought stress for leaf metabolic changes in potato [Solanum tuberosum (L)], two contrasting potato cultivars Harley Blackwell (HB, an early maturing, newer cultivar) and Snowden (SD, an established, full-season cultivar) were tested under water-limited conditions and CO2 enrichment. The results revealed that most of the drought-triggered metabolites were lower in HB compared to SD. However, HB showed quicker adjustments in the metabolic processes such as conversion of starch into soluble sugars and biosynthesis of phenylalanine and other compatible solutes at the early stages of the drought progression. Moreover, the existence of genotypic differences for leaf water potential (LWP) in response to CO2 enrichment was evident. Our study provides insights into the possible metabolic strategies of drought tolerance in potato cultivars under ambient and elevated CO2.

Plants would experience more complex environments, such as sudden heat shock (SHS) stress combined with elevated CO2 in the future, and might adapt to this stressful condition by optimizing photosynthetic carbon metabolism (PCM). It is interesting to understand whether this acclimation process would be altered in different genotypes of maize under elevated CO2, and which metabolites represent key indicators reflecting the photosynthetic rates (PN) following SHS. Although B76 had greater reduction in PN during SHS treatment, our results indicated that PN in genotype B76, displayed faster recovery after SHS treatment under elevated CO2 than in genotype B106. Furthermore, we employed a stepwise feature extraction approach by partial linear regression model. Our findings demonstrated that 9 key metabolites over the total (35 metabolites) can largely explain the variance of PN during recovery from SHS across two maize genotypes and two CO2 grown conditions. Of these key metabolites, malate, valine, isoleucine, glucose and starch are positively correlated with recovery pattern of PN. Malate metabolites responses to SHS were further discussed by incorporating with the activities and gene expression of three C4 photosynthesis-related key enzymes. We highlighted the importance of malate metabolism during photosynthesis recovery from short-term SHS, and data integration analysis to better comprehend the regulatory framework of PCM in response to abiotic stress.

Analysis of uppermost fully expanded leaves is useful to detect a deficiency of mineral nutrients such as phosphorus (P) and potassium (K) in soybean. Although, the leaf P or K status aids in fertilizer management, information on nutrient seasonal relationships with growth and yield traits at maturity are limited. To investigate this, soybean was grown under varying P or K nutrition under ambient and elevated CO2 concentrations. Results show significant relationships of the relative total biomass and yield‐related traits with the foliar P and K concentrations measured several times in the season across CO2 levels. However, the relationships established earlier in the season showed that the growth period between 25 and 37 d after planting (DAP), representing the beginning of flowering and pod, respectively, is the best for leaf sampling to determine the foliar P or K status. The leaf P and K status as well as the critical leaf P (CLPC) and K (CLKC) concentrations for traits such as seed yield peaked around 30 DAP (R2 stage) and tended to decline thereafter with the plant age. The CLPC and CLKC of seed yield indicate that the leaf P and K concentration of at least 2.74 mg g⁻¹ and 19.06 mg g⁻¹, respectively, in the uppermost fully expanded leaves are needed between 25 and 37 DAP for near‐optimum soybean yield. Moreover, the greatest impact of P and K deficiency occurred for the traits that contribute the most to the soybean yield (e.g., relative total biomass, seed yield, pod and seed numbers), while traits such as seed number per pod, seed size, and shelling percentages were the least affected and showed smaller leaf critical concentration. The CLPC or CLKC for biomass and seed yield was greater under elevated CO2 24–25 DAP but varied thereafter. These results are useful to researchers and farmers to understand the dynamics of the relationship of pre‐harvest leaf P and K status with soybean productivity at maturity, and in the determination of suitable growth stage to collect leaf samples.

The effects of light on the growth, leaf morphology, physiological responses, and quality of Chinese cabbage (Brassica campestris L. ssp. pekinensis) were examined at five light intensity levels (200-1,000 μmol·m⁻²·s⁻¹) to determine the optimal light intensity for the production of high-quality cabbage heads. In each light treatment, peak irradiance values at midday were 200 (I), 400 (II), 600 (III), 800 (IV), and 1,000 (V) μmol·m⁻²·s⁻¹. Fresh and dry weights of plant shoots significantly increased with light intensity; leaf number and area were positively correlated with light intensity, although the total and mean leaf area for plants in group IV exceeded that of group V. The leafy head formation was observed in high light intensity (800 and 1,000 μmol·m⁻²·s⁻¹), but not in the low light intensity conditions. Total photosynthetic capacity in Chinese cabbages grown under 800 μ mol·m⁻²·s⁻¹ increased due to the greater leaf area of the plants, even though the photosynthetic rate was lower than that measured under 1,000 μmol·m⁻²·s⁻¹. In addition, the values of Fv/Fm indicated that Chinese cabbage was mildly stressed under 1,000 μmol·m⁻²·s⁻¹. We recommend a light intensity of 800 μmol·m⁻²·s⁻¹ for the production of high-quality Chinese cabbage. We hope that this study can help emphasize the importance of light intensity when maximizing the economic benefits of Chinese cabbage.

Detached leaves at top canopy structures always experience higher solar irradiance and leaf temperature under natural conditions. The ability of tolerance to high temperature represents thermotolerance potential of whole-plants, but was less of concern. In this study, we used a heat-tolerant (B76) and a heat-susceptible (B106) maize inbred line to assess the possible mitigation of sudden heat shock (SHS) effects on photosynthesis (PN) and C4 assimilation pathway by elevated [CO2]. Two maize lines were grown in field-based open top chambers (OTCs) at ambient and elevated (+180 ppm) [CO2]. Top-expanded leaves for 30 days after emergence were suddenly exposed to a 45°C SHS for 2 hours in midday during measurements. Analysis on thermostability of cellular membrane showed there was 20% greater electrolyte leakage in response to the SHS in B106 compared to B76, in agreement with prior studies. Elevated [CO2] protected PN from SHS in B76 but not B106. The responses of PN to SHS among the two lines and grown CO2 treatments were closely correlated with measured decreases of NADP-ME enzyme activity and also to its reduced transcript abundance. The SHS treatments induced starch depletion, the accumulation of hexoses and also disrupted the TCA cycle as well as the C4 assimilation pathway in the both lines. Elevated [CO2] reversed SHS effects on citrate and related TCA cycle metabolites in B106 but the effects of elevated [CO2] were small in B76. These findings suggested that heat stress tolerance is a complex trait, and it is difficult to identify biochemical, physiological or molecular markers that accurately and consistently predict heat stress tolerance.

Phytophthora megakarya and Phytophthora palmivora cause black pod rot of cacao. P. megakarya occurs in Africa while P. palmivora is distributed world-wide. P. palmivora has a higher temperature maximum (34 °C) than P. megakarya (30 °C). Factors contributing to temperature maxima in Phytophthora species have not been studied in detail. In culture, P. megakarya lost viability after 3 days at 32 °C while P. palmivora viability began decreasing after 3 days at 36 °C. When exposed to heat, genes encoding heat shock proteins and chaperones were most commonly induced in both species. Heat responsive genes in P. palmivora also tended to be induced at low temperatures (9 °C and 11 °C). In general, P. palmivora maintained higher metabolite pools than P. megakarya, although some temperature induced differences were recorded. When recovering from heat stress, P. megakarya produced a coralloid form of mycelia not previously observed in any Phytophthora species. P. megakarya's temperature range mimics the cacao production conditions in Africa and elevated seasonal field temperatures there are unlikely to persist long enough to eliminate P. megakarya although Phytophthora induced disease epidemiology may be influenced.