Diffusional and Biochemical Limitations to Photosynthesis Under Water Deficit for Field-Grown Cotton.

Water deficit stress limits net photosynthetic rate (AN), but the relative sensitivities of underlying processes such as thylakoid reactions, ATP production, carbon fixation reactions, and carbon loss processes to water deficit stress in field-grown upland cotton require further exploration. Therefore, the objective of the present study was to assess (1) the diffusional and biochemical mechanisms associated with water deficit-induced declines in AN and (2) associations between water deficit-induced variation in oxidative stress and energy dissipation for field-grown cotton. Water deficit stress was imposed for three weeks during the peak bloom stage of cotton development, causing significant reductions in leaf water potential and AN. Among diffusional limitations, mesophyll conductance was the major contributor to the AN decline. Several biochemical processes were adversely impacted by water deficit. Among these, electron transport rate and RuBP regeneration were most sensitive to AN-limiting water deficit. Carbon loss processes (photorespiration and dark respiration) were less sensitive than carbon assimilation, contributing to the water deficit-induced declines in AN. Increased energy dissipation via non-photochemical quenching or maintenance of electron flux to photorespiration prevented oxidative stress. Declines in AN were not associated with water deficit-induced variation in ATP production. It was concluded that diffusional limitations followed by biochemical limitations (ETR and RuBP regeneration) contributed to declines in AN, carbon loss processes partially contributed to the decline in AN, and increased energy dissipation prevented oxidative stress under water deficit in field-grown cotton.

Differential sensitivities of photosynthetic component processes govern oxidative stress levels and net assimilation rates in virus-infected cotton.

Cotton (Gossypium hirsutum L.) leafroll dwarf virus disease (CLRDD) is a yield-limiting threat to cotton production and can substantially limit net photosynthetic rates (AN). Previous research showed that AN was more sensitive to CLRDD-induced reductions in stomatal conductance than electron transport rate (ETR) through photosystem II (PSII). This observation coupled with leaf reddening symptomology led to the hypothesis that differential sensitivities of photosynthetic component processes to CLRDD would contribute to declines in AN and increases in oxidative stress, stimulating anthocyanin production. Thus, an experiment was conducted to define the relative sensitivity of photosynthetic component processes to CLRDD and to quantify oxidative stress and anthocyanin production in field-grown cotton. Among diffusional limitations to AN, reductions in mesophyll conductance and CO2 concentration in the chloroplast were the greatest constraints to AN under CLRDD. Multiple metabolic processes were also adversely impacted by CLRDD. ETR, RuBP regeneration, and carboxylation were important metabolic (non-diffusional) limitations to AN in symptomatic plants. Photorespiration and dark respiration were less sensitive than photosynthetic processes, contributing to declines in AN in symptomatic plants. Among thylakoid processes, reduction of PSI end electron acceptors was the most sensitive to CLRDD. Oxidative stress indicators (H2O2 production and membrane peroxidation) and anthocyanin contents were substantially higher in symptomatic plants, concomitant with reductions in carotenoid content and no change in energy dissipation by PSII. We conclude that differential sensitivities of photosynthetic processes to CLRDD and limited potential for energy dissipation at PSII increases oxidative stress, stimulating anthocyanin production as an antioxidative mechanism.

Mutation in the Endo-ß-1,4-glucanase (KORRIGAN) Is Responsible for Thick Leaf Phenotype in Sorghum.

Sorghum [Sorghum bicolor (L.) Moench] is an important crop for food, feed, and fuel production. Particularly, sorghum is targeted for cellulosic ethanol production. Extraction of cellulose from cell walls is a key process in cellulosic ethanol production, and understanding the components involved in cellulose synthesis is important for both fundamental and applied research. Despite the significance in the biofuel industry, the genes involved in sorghum cell wall biosynthesis, modification, and degradation have not been characterized. In this study, we have identified and characterized three allelic thick leaf mutants (thl1, thl2, and thl3). Bulked Segregant Analysis sequencing (BSAseq) showed that the causal mutation for the thl phenotype is in endo-1,4-ß-glucanase gene (SbKOR1). Consistent with the causal gene function, the thl mutants showed decreased crystalline cellulose content in the stem tissues. The SbKOR1 function was characterized using Arabidopsis endo-1,4-ß-glucanase gene mutant (rsw2-1). Complementation of Arabidopsis with SbKOR1 (native Arabidopsis promoter and overexpression by 35S promoter) restored the radial swelling phenotype of rsw2-1 mutant, proving that SbKOR1 functions as endo-1,4-ß-glucanase. Overall, the present study has identified and characterized sorghum endo-1,4-ß-glucanase gene function, laying the foundation for future research on cell wall biosynthesis and engineering of sorghum for biofuel production.