Carbon Dioxide Enrichment Partially Alleviates the Impact of Drought Stress on Cotton Growth and Yield

This study investigated the effects of drought during reproductive and boll development stages under elevated CO2 conditions. A pot experiment was conducted in the Soil–Plant–Atmospheric-Research (SPAR) facility using the upland cotton cultivar DP 1646 B2XF grown under control (well-watered; 0.12 m3 H2O m−3 soil) and drought (0.09 m3 H2O m−3 soil) conditions at ambient (425 ppm; aCO2) and enriched (725 ppm; eCO2) CO2 concentrations. Under drought stress, photosynthesis and stomatal conductance decreased by 35% and 63%, respectively, under aCO2, whereas reductions were less pronounced under eCO2 (20% and 36%). Under drought and aCO2, intrinsic and instantaneous water-use efficiency increased by 74% and 45%, respectively, compared to control. Biomass partitioning shifted under drought, with increased allocation to shoots (55%) and roots (9%) and reduced allocation to reproductive organs (36%), compared to control conditions. Flowers produced under drought had 21% fewer ovules under both CO2 environments, while pollen production remained unaffected. Seed cotton and lint weights were reduced by 19% and 15% under drought, respectively. However, plants grown under eCO2 attained seed cotton and lint weights that were 20% higher than those grown under aCO2, under both control and drought conditions. Drought significantly affected fiber quality, increasing micronaire by 26% and reducing fiber length by 4%, regardless of CO2 level. eCO2-driven alleviation primarily acts through carbon assimilation and yield compensation, with limited capacity to regulate developmental programming and quality formation.