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CROP PHYSIOLOGY & METABOLISM

Physiological Limitations to Photosynthetic Carbon Assimilation in Cotton under Water Stress

^a Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602-7272
^b Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada N1G 2W1

^* Corresponding author (hjearl{at}uoguelph.ca)

Water stress may reduce leaf net photosynthetic carbon assimilation (A_N) through both stomatal effects, which reduce the leaf internal CO₂ concentration (C_i), and nonstomatal effects, which result in reduced A_N at a given level of C_i. However, the leaf gas exchange techniques used to calculate C_i are susceptible to important artifacts when applied to water-stressed leaves, making such C_i estimates unreliable. As an alternative to C_i, the CO₂ concentration in the chloroplast (C_C) can be calculated from simultaneous measurements of A_N from gas exchange measurements, and the thylakoid electron flux from chlorophyll fluorometry. This permits diffusional effects (stomatal plus mesophyll limitations to CO₂ diffusion) to be differentiated from chloroplast-level effects. We used this method to investigate physiological restrictions to photosynthesis in leaves of water stressed cotton (Gossypium hirsutum L.) plants in a series of greenhouse experiments. A null-balance lysimeter was used to slowly induce four distinct levels of water stress. Combined leaf gas exchange/chlorophyll fluorescence measurements differentiated the treatments more effectively than gas exchange measurements alone. All treatments reduced C_C, but only the two most severe stress treatments significantly increased nondiffusional restrictions, detectable as a reduction in the slope of A_N on C_C. In a second experiment, recovery of leaf photosynthesis was determined 24 and 48 h after relief of a severe stress by rewatering. Recovery of the A_N/C_C relationship was substantial but incomplete after 24 h and did not recover further by 48 h after rewatering, indicating lasting chloroplast-level injury as a result of the stress. Similar experiments should be conducted under field conditions to determine if water stress results in irreversible chloroplast-level injury in field-grown cotton.

Abbreviations: , leaf fractional absorption of incident PPFD • A_N, leaf net CO₂ assimilation rate • A_G, leaf gross CO₂ assimilation rate • A_N-C, leaf net CO₂ assimilation rate corrected for sample chamber leakage • A_N-M, measured leaf net CO₂ assimilation rate (uncorrected for sample chamber leakage) • C_A, ambient CO₂ concentration • C_C, CO₂ concentration at the carboxylation site in the chloroplast • C_i, leaf internal CO₂ concentration • C_S, CO₂ concentration in the sample chamber • DAP, days after planting • _II, quantum efficiency of Photosystem II • _II, fraction of absorbed PPFD absorbed by the antennae of Photosystem II • F_S, steady state chlorophyll fluorescence signal • F^'_M, maximum (light saturated) chlorophyll fluorescence signal • G_L, sample chamber conductance to CO₂ flux (leakage) • G_M, mesophyll conductance to CO₂ in the liquid phase • G_S, stomatal conductance to water vapor • IRGA, infrared gas analyzer • J_e, electron flux through Photosystem II • K_S, CO₂/O₂ specificity ratio of RubisCO • L, leaf area in the sample chamber • LRWC, leaf relative water content • O_C, oxygen concentration at the carboxylation site in the chloroplast • PPFD, photosynthetic photon flux density • R_D, rate of leaf respiration in the dark • RSWC, relative soil water content • v_C, velocity of the carboxylation reaction of RubisCO • v_O, velocity of the oxygenation reaction of RubisCO • W_D, pot plus soil dry weight • W_P, plant fresh weight • W_S, pot plus soil water saturated weight • W_T, pot target weight

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Crop Science 2005 45: vii. [Full Text]  

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