Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/5/2016
Publication Date: 6/5/2016
Citation: Chastain, D.R., Snider, J.L., Choinski, J.S., Collins, G.D., Perry, C., Whitaker, J., Grey, T.L., Sorensen, R.B., Iersal, M.V., Byrd, S.A., Porter, W. 2016. Leaf ontogeny strongly influences photosynthetic tolerance to drought and high temperature in Gossypium hirsutum. Journal of Plant Physiology. dx.doi.org/10.1016/j.jplph.2016.05.003.

Interpretive Summary: The current study addressed the hypotheses 1) that young leaves of Gossypium hirsutum (= 24% of fully expanded leaf area) would exhibit greater photosystem II heat tolerance than more mature leaves on the same plant, and 2) that PN in young leaves would be more tolerant to both drought and high temperature exposure under field conditions than more mature leaves. Recent studies have demonstrated that young leaves exhibit limited transpirational cooling, have higher midday temperatures than fully expanded, and that primary photochemistry is more heat tolerant during the early stages of leaf expansion than at full. We demonstrate here for the first time that leaf ontogeny strongly impacts PN response to leaf temperature. In particular, Node 2 leaves showed no relationship with leaf temperature even over leaf temperature ranges sufficient to cause 66% (July 26) and nearly 100% (all sample dates combined) declines in PN for Node 4 leaves, and well-above the 35°C threshold (> 42°C when considered over all sample dates) for PN inhibition in G. hirsutum. Thus, we conclude that young G. hirsutum leaves are more tolerant than fully expanded leaves, both in primary photochemistry and carbon reactions, to the combined effects of drought and high temperature that are common under field conditions. Elucidating the mechanisms associated with these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.

Technical Abstract: 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 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 different leaf stages were sampled on three dates during an abnormally dry summer. Early in the growing season, ontogenic PSII heat tolerance differences were observed. Photosystem II was more thermotolerant in young leaves than mature leaves. Later in the growing season, no decline in young leaf net photosynthesis (PN) was observed as leaf temperature increased from 31 to 37 °C, as average midday leaf water potential ('MD) declined from -1.25 to -2.03 MPa. In contrast, mature leaf PN declined 66% under the same conditions. Stomatal conductance (gs) accounted for 84 to 98% of variability in leaf temperature, and gs was strongly associated with 'MD in mature leaves but not in young leaves. We conclude that young leaves are more photosynthetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms causing these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.