Simulating short-term light responses of photosynthesis and water use efficiency in sweet sorghum under varying temperature and CO2 conditions

Abstract

Climate change, characterized by rising atmospheric CO₂ levels and temperatures, poses significant challenges to global crop production. Sweet sorghum, a prominent C₄ cereal extensively grown in arid areas, emerges as a promising candidate for sustainable bioenergy production. This study investigated the responses of photosynthesis and leaf-scale water use efficiency (WUE) to varying light intensity (I) in sweet sorghum under different temperature and CO₂ conditions. Comparative analyses were conducted between the A _n-I, g _s-I, T _r-I, WUE_i-I, and WUE_inst-I models proposed by Ye et al. and the widely utilized the non-rectangular hyperbolic (NRH) model for fitting light response curves. The Ye's models effectively replicated the light response curves of sweet sorghum, accurately capturing the diminishing intrinsic WUE (WUE_i) and instantaneous WUE (WUE_inst) trends with increasing I. The fitted maximum values of A _n, g _s, T _r, WUE_i, and WUE_inst and their saturation light intensities closely matched observations, unlike the NRH model. Despite the NRH model demonstrating high R ² values for A _n-I, g _s-I, and T _r-I modelling, it returned the maximum values significantly deviating from observed values and failed to generate saturation light intensities. It also inadequately represented WUE responses to I, overestimating WUE. Across different leaf temperatures, A _n, g _s, and T _r of sweet sorghum displayed comparable light response patterns. Elevated temperatures increased maximum A _n, g _s, and T _r but consistently declined maximum WUE_i and WUE_inst. However, WUE_inst declined more sharply due to the disproportionate transpiration increase over carbon assimilation. Critically, sweet sorghum A _n saturated at current atmospheric CO₂ levels, with no significant gains under 550 μmol mol^-1. Instead, stomatal closure enhanced WUE under elevated CO₂ by coordinated g _s and T _r reductions rather than improved carbon assimilation. Nonetheless, this response diminished under simultaneously high temperature, suggesting intricate interplay between CO₂ and temperature in modulating plant responses. These findings provide valuable insights into photosynthetic dynamics of sweet sorghum, aiding predictions of yield and optimization of cultivation practices. Moreover, our methodology serves as a valuable reference for evaluating leaf photosynthesis and WUE dynamics in diverse plant species.

Keywords: CO2 concentration; instantaneous water use efficiency; intrinsic water use efficiency; leaf gas exchange; model; photosynthetic light response; sweet sorghum; temperature.