Leaf anatomical alterations reduce cotton's mesophyll conductance under dynamic drought stress conditions.

Drought stress significantly affects cotton's net photosynthetic rate (A) by restraining stomatal (g s ) and mesophyll conductance (g m ) as well as perturbing its biochemical process, resulting in yield reductions. Despite the significant progress in dissecting effects of drought on photosynthesis, the variability observed in cotton's g m , and the mechanisms contributing to that variability under dynamic drought stress conditions are poorly understood. For that reason, a controlled-environment experiment with two cotton genotypes (Dexiamian 1, Yuzaomian 9110), three water levels (soil relative water content: control [75 ± 5]%, moderate drought [60 ± 5]%, severe drought [45 ± 5]%), and two drought durations (10 and 31 days) were conducted. The results indicated that the cotton boll biomass was significantly decreased under 10-day severe drought and 31-day moderate and severe drought. Decreases in g s were later accompanied by decreases in g m and further combined with reductions in electron transport rate, as drought stress progressed in duration and severity, ultimately resulting in significant reductions in A of subtending leaf. Stomatal and mesophyll conductance constraints were the primary factors limiting photosynthesis, while biochemical constraints decreased, as drought stress progressed. Considering g m , its decline was ascribed to increases in the diffusion resistance of CO 2 through cytoplasm (r cyt ), under short- or long-term drought, as well as to increases in leaf dry mass (LMA), and decreases in the chloroplast surface area exposed to intercellular air space (S c /S), under long-term drought. It was concluded that A could be enhanced, under dynamic drought stress conditions, by increasing g m through increasing S c /S and reducing LMA and r cyt .