First Evidence of Leaf Architecture Change Driven By Elevated CO2 Plus Chronic Warming.

Plants can change their leaf architecture as a survival strategy in response to unfavorable abiotic environmental stimuli. Leaf hyponasty (upward bending of petiole and or blade) is one such adjustment which could be induced by climate change. Our recent findings show for the first time that the combination of elevated CO₂ and chronic warming can cause severe leaf hyponasty in tomato and soybean which subsequently reduce plant growth and productivity. To characterize this phenomenon, well-watered and -fertilized tomato and soybean plants were grown at two levels of CO₂ (400 vs. 700 ppm) and two temperature regimes (30 vs. 37^oC) for 18-21 days. According to our results, leaf hyponasty (petiole in tomato and blade in soybean) increased dramatically with the combination of warming and high CO₂, but changed little with either factor alone. Increases in leaf angle changes were not correlated with leaf temperature (T_leaf), water stress [based on stomatal conductance (G_s) and xylem water potential (Ψ_w)], or heat-related damage to photosynthesis [based on PSII efficiency of light-adapted leaves (F_v'/F_m') and steady-state photosynthesis measured as net CO₂ exchange (P_n)]. However, steeper leaf angles were correlated with decreases in leaf area and biomass, which we hypothesize, were caused by decreased light interception, and thus in situ P_n, as leaves became more-vertical. However, when high -CO₂ and -temperature treatments were stopped, plants recovered from hyponastic growth which indicates a direct effect of elevated CO₂ plus chronic warming on hyponasty. If increases in leaf hyponasty are common among species under high CO₂ and warming, then this has serious consequences for food production in coming decades.