Physiological limitations to photosynthetic carbon assimilation in cotton under water stress

Water stress may reduce leaf net photosynthetic carbon assimilation (AN) through both stomatal effects, which reduce the leaf internal CO2 concentration (C-i), and nonstomatal effects, which result in reduced AN 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 AN 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.