Drought-introduced variability of mesophyll conductance in Gossypium and its relationship with leaf anatomy

Mesophyll conductance (g_m ) is one of the major determinants of photosynthetic rate, for which it has an impact on crop yield. However, the regulatory mechanisms behind the decline in g_m of cotton (Gossypium. spp) by drought are unclear. An upland cotton (Gossypium hirsutum) genotype and a pima cotton (Gossypium barbadense) genotype were used to determine the gas exchange parameters, leaf anatomical structure as well as aquaporin and carbonic anhydrase gene expression under well-watered and drought treatment conditions. In this study, the decrease of net photosynthetic rate (A_N ) under drought conditions was related to a decline in g_m and in stomatal conductance (g_s ). g_m and g_s coordinate with each other to ensure optimum state of CO₂ diffusion and achieve the balance of water and CO₂ demand in the process of photosynthesis. Meanwhile, mesophyll limitations to photosynthesis are equally important to the stomatal limitations. Considering g_m , its decline in cotton leaves under drought was mostly regulated by the chloroplast surface area exposed to leaf intercellular air spaces per leaf area (S_c /S) and might also be regulated by the expression of leaf CARBONIC ANHYDRASE (CA1). Meanwhile, cotton leaves can minimize the decrease in g_m under drought by maintaining cell wall thickness (T_cw ). Our results indicated that modification of chloroplasts might be a target trait in future attempts to improve cotton drought tolerance.