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

Mesophyll conductance (gm ) 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 gm 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 (AN ) under drought conditions was related to a decline in gm and in stomatal conductance (gs ). gm and gs 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 gm , its decline in cotton leaves under drought was mostly regulated by the chloroplast surface area exposed to leaf intercellular air spaces per leaf area (Sc /S) and might also be regulated by the expression of leaf CARBONIC ANHYDRASE (CA1). Meanwhile, cotton leaves can minimize the decrease in gm under drought by maintaining cell wall thickness (Tcw ). Our results indicated that modification of chloroplasts might be a target trait in future attempts to improve cotton drought tolerance.