A stomatal optimization model integrating leaf stomata-photosynthetic capacity regulation in response to soil water stress

Leaf stomatal regulation of water-carbon exchange processes plays a crucial role in the water-carbon cycle. Uncovering the response mechanism of leaf gas exchange to soil water stress is challenging due to the complex effects of both the stomatal regulation (i.e., stomatal conductance, $g_s$) and non-stomatal regulation (i.e., photosynthetic carboxylation capacity, $V_{c\max 25}$). Different from previous studies that achieved stomatal and non-stomatal regulation in stomatal optimization models by linearly simplifying or independently optimizing $V_{c\max 25}$, this study hypothesizes that $V_{c\max 25}$ and $g_s$ are co-regulated by balancing intercellular ${\mathrm{CO}}_{₂}$ concentration ($C_i$). By adjusting stomatal opening to minimize water-carbon cost, a stomatal optimization model (SRSC model) that integrates the synergistic regulation of $g_s$ and $V_{c\max 25}$ was developed. Experimental and numerical results show that the SRSC model accurately reproduces the stomatal response to environmental changes, especially for the low soil water potential conditions $({\Psi }_{\mathrm{soil}}<-2\mathrm{MPa})$ compared to the previous models, which increased the $R^2$ of $g_s$, photosynthetic rate ($A_n$), and $C_i$ reaching 2.56 %, 1.97 %, and 9.04 %, respectively. Additionally, the SRSC model reasonably predicted a coordinated decline in $g_s$ and $V_{c\max 25}$ and concurrently mitigated the classical models that simulate $g_s$ and leaf water potential responses deviating from the actual values under drought conditions. More importantly, the SRSC model revealed that experiencing drought and flooding stresses in rice improved intrinsic water use efficiency by increasing photosynthetic capacity. This study refines the application of the stomatal optimization model and enhances the mechanistic understanding of the stomatal optimization model to a certain extent.