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.