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.

Variation in thermotolerance of photosystem II energy trapping, intersystem electron transport, and photosystem I electron acceptor reduction for diverse cotton genotypes.

Cotton breeding programs have focused on agronomically-desirable traits. Without targeted selection for tolerance to high temperature extremes, cotton will likely be more vulnerable to environment-induced yield loss. Recently-developed methods that couple chlorophyll fluorescence induction measurements with temperature response experiments could be used to identify genotypic variation in photosynthetic thermotolerance of specific photosynthetic processes for field-grown plants. It was hypothesized that diverse cotton genotypes would differ significantly in photosynthetic thermotolerance, specific thylakoid processes would exhibit differential sensitivities to high temperature, and that the most heat tolerant process would exhibit substantial genotypic variation in thermotolerance plasticity. A two-year field experiment was conducted at Tifton and Athens, Georgia, USA. Experiments included 10 genotypes in 2020 and 11 in 2021. Photosynthetic thermotolerance for field-collected leaf samples was assessed by determining the high temperature threshold resulting in a 15% decline in photosynthetic efficiency (T15) for energy trapping by photosystem II (ΦPo), intersystem electron transport (ΦEo), and photosystem I end electron acceptor reduction (ΦRo). Significant genotypic variation in photosynthetic thermotolerance was observed, but the response was dependent on location and photosynthetic parameter assessed. ΦEo was substantially more heat sensitive than ΦPo or ΦRo. Significant genotypic variation in thermotolerance plasticity of ΦEo was also observed. Identifying the weakest link in photosynthetic tolerance to high temperature will facilitate future selection efforts by focusing on the most heat-susceptible processes. Given the genotypic differences in environmental plasticity observed here, future research should evaluate genotypic variation in acclimation potential in controlled environments.

Differential sensitivities of photosynthetic processes and carbon loss mechanisms govern N-induced variation in net carbon assimilation rate for field-grown cotton.

Nitrogen (N) deficiency limits the net carbon assimilation rate (AN), but the relative N sensitivities of photosynthetic component processes and carbon loss mechanisms remain relatively unexplored for field-grown cotton. Therefore, the objective of the current study was to define the relative sensitivity of individual physiological processes driving N deficiency-induced declines in AN for field-grown cotton. Among the potential diffusional limitations evaluated, mesophyll conductance was the only parameter substantially reduced by N deficiency, but this did not affect CO2 availability in the chloroplast. A number of metabolic processes were negatively impacted by N deficiency, and these effects were more pronounced at lower leaf positions in the cotton canopy. Ribulose bisphosphate (RuBP) regeneration and carboxylation, AN, and gross photosynthesis were the most sensitive metabolic processes to N deficiency, whereas photosynthetic electron transport processes, electron flux to photorespiration, and dark respiration exhibited intermediate sensitivity to N deficiency. Among thylakoid-specific processes, the quantum yield of PSI end electron acceptor reduction was the most sensitive process to N deficiency. It was concluded that AN is primarily limited by Rubisco carboxylation and RuBP regeneration under N deficiency in field-grown cotton, and the differential N sensitivities of the photosynthetic process and carbon loss mechanisms contributed significantly to photosynthetic declines.

OJIP-fluorescence parameters as rapid indicators of cotton (Gossypium hirsutum L.) seedling vigor under contrasting growth temperature regimes.

Vigorous seedling growth in cotton is desirable because it minimizes the negative impact of multiple early season stresses, and seedling vigor can be impacted by early season growth temperature or cultivar. OJIP fluorescence provides rapid information on a broad range of photosynthetic component processes and may be a useful surrogate for seeding vigor, but this possibility has not been evaluated previously in cotton. To this end, a controlled environment study was conducted with six cultivars selected based on seed characteristics that are widely indicative of vigor and under two growth temperature regimes (sub-optimal = 20/15 °C day/night temperature; optimal = 30/20 °C) for the first two weeks after seed germination. Thereafter multiple whole-plant vigor assessments were conducted along with extensive OJIP-fluorescence characterization in cotyledons. Growth temperature was the primary factor influencing multiple plant responses. Specifically, all whole-plant indicators of seedling vigor were negatively impacted by sub-optimal temperature as were all photosynthetic performance indices and quantum efficiencies. By comparison, most photosynthetic structural indicators or reaction center-specific fluxes were either unaffected or positively impacted by low growth temperature, largely because PSII antenna size increased. The performance index, PIABS, and the quantum efficiency, φEo, were the most sensitive to low growth temperature and exhibited the strongest relationships with whole-plant seedling vigor. Thus, OJIP parameters incorporating intersystem electron transport beyond PSII but not additional downstream processes may represent the most useful surrogates for whole-plant seedling vigor in cotton.