Figure 1. The boundary conditions in terms of key variables (i.e., soil water content, soil oxygen level, leaf-surface oxygen level, leaf-surface CO2 level, and leaf-surface light intensity) under drought, non-stressed, soil waterlogged, and completely submerged conditions.
Integrating eco-physiological responses from drought to flooding
3.1 Generic responses to drought
The plant adaptive response to drought has been well captured by existing SPAC models (Elfving et al., 1972; Manzoni et al., 2014; Zhang et al., 2021). With the onset of soil water deficit, root water uptake due to transpiration demand will further decrease the soil water content, resulting in more negative soil water potential (Havranek & Benecke, 1978). As a result the root xylem water potential becomes more negative (Kaldenhoff et al., 2008), leading to stressed xylem water conductance due to xylem embolism (Martínez-Vilalta & Garcia-Forner, 2017). The limited whole-plant water transport and the leaf-level transpiration demand will further lower leaf water potential (Ehrler et al., 1978), thereby downregulating stomatal conductance through reduced turgor (Brown et al., 1976). Additionally, this more negative root xylem water potential has been suggested to be detected by plants and trigger the synthesis and release of abscisic acid (ABA) (Finkelstein, 2013), which leads to further guard cell turgor loss and downregulation of stomatal conductance (Hauser et al., 2017). When soil water is in extreme deficit, xylem water potentials become more negative and increase the risk of hydraulic failure by triggering embolism which can temporarily or permanently block water transport (Venturas et al., 2017). Thus, drought reduces stomatal conductance both directly through hydraulic processes and through biochemical hormonal signaling processes. Reduced stomatal conductance then causes a lower transpiration rate and a lower photosynthesis rate. This is based on Fick’s law that the amount of gas exchange (i.e. , water vapor diffusing out and CO2diffusing in) is determined by stomatal conductance and pressure deficit inside and outside the leaves. The variables and their relationships in the SPAC model, and the coupling of the SPAC model with photosynthetic processes are outlined in figure 2, SPAC-photosynthesis modelcompartment.