Leaf Ontogeny Strongly Influences Photosynthetic Tolerance to Drought and High Temperature in Gossypium hirsutum

High temperature and drought are major abiotic limitations to crop productivity worldwide. While abiotic stress physiology research has focused primarily on fully expanded leaves, no studies have investigated differences in photosynthetic tolerance to concurrent drought and high temperature during leaf ontogeny. To address this, Gossypium hirsutum plants were exposed to five irrigation treatments, and two distinctly different leaf stages were sampled on three dates during an abnormally dry summer. When ontogenic differences were observed during the season, photosystem II was more thermotolerant in young leaves than mature leaves, and no decline in net photosynthesis (P_N) was observed as leaf temperature increased from 31 to 37 °C or as average midday leaf water potential (Ψ_MD) declined from -1.25 to  -2.03 MPa, whereas P_N in mature leaves declined 66% under the same conditions. Stomatal conductance (g_s) accounted for 84 to 98% of variability in leaf temperature, and g_s was strongly associated with Ψ_MD in mature leaves but not in young leaves. We conclude that young leaves are substantially more photosythetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms associated with these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.