Physiological consequences of moisture deficit stress in cotton

Moisture deficits can depress cotton (Gossypium hirsutum L.) lint yield in all cotton production regions. However, most cotton physiological drought stress research has been conducted in and production regions, growth chambers, or greenhouses. The objective of this research was to document the effects of moisture deficit stress on the physiology of cotton grown in the humid southeastern USA. Field studies were conducted under dryland and irrigated conditions from 1998 to 2001 with eight genotypes, including an okra-normal leaftype near-isoline pair and transgenic lines paired with their recurrent parents. Dry matter partitioning, light interception, canopy temperature, leaf water potential, gas exchange, chlorophyll (Chl) fluorescence, and leaf Chl content data were collected. Genotypes responded similarly to both soil moisture regimes. Drought stress reduced overall plant stature with a 35% leaf area index (LAI) reduction, prompting an 8% reduction in solar radiation interception. Dryland leaves had 6% greater CO2 exchange rates (CER) and 9% higher light-adapted photosystem II (PSII) quantum efficiency than irrigated leaves during the morning. However, as water potential of the dryland plants became more depressed during the afternoon, the CER and light adapted PSII quantum efficiency of the dryland plants became inhibited and was 6 and 10% lower, respectively, than irrigated leaves. A 19% greater Chl content for the dryland leaves contributed to their higher CER during the morning. This polarity of photosynthesis throughout the day for the dryland plants relative to irrigated plants may explain some of the irrigation yield response inconsistencies in the southeastern USA.