Effects of moisture stress on stomatal resistance and photosynthetic rate in cotton (Gossypium hirsutum) 1. Controlled levels of stress

The effects of moisture stress on stomatal resistance and on photosynthetic rate were studied in cotton (Gossypium hirsutum L., cv. Acala SJ-2) under field conditions with drip irrigation in 1984 and in 1985. Moisture stress was evaluated by measuring the minimum midday leaf water-potential, which was kept at two levels (-1.6 and -2.4 MPa) during each of two phenological periods. Stomatal resistance increased and photosynthetic rate decreased as a result of moisture stress. The correlation between stomatal resistance and radiation was lower when the plants were subjected to moisture stress. An exponential asymptotic relationship was found between photosynthetic rate and stomatal resistance. Stomata were found to limit the photosynthetic process in well-watered plants or in midly stressed plants, while mesophyll resistance was the main factor reducing it under more severe moisture stress. In the moisture-stressed plants, mesophyll resistance increased more than did stomatal resistance, and this was associated with a higher internal CO₂ concentration in the leaf (C_i) and a lower ratio of photosynthetic rate to C_i. Under moderate moisture stress the maximum photosynthetic rate was attained at an earlier leaf age, while a more severe stress resulted in a lower photosynthetic rate at all leaf ages.