An Integrated Hydraulic-Hormonal Model of Conifer Stomata Predicts Water Stress Dynamics.

The ability of plants to dynamically regulate water loss through stomata under conditions of changing evaporative demand or water availability is paramount for avoiding excessive desiccation, hydraulic failure, and plant death under conditions of water stress. Despite apparently similar functional demands on stomatal evolution, not all lineages employ the same mechanism of controlling water loss, but rather show a directional shift from passive hydraulic control in ferns and lycophytes to active metabolic control mediated by the phytohormone abscisic acid (ABA) in angiosperms (Brodribb and McAdam, 2011; McAdam and Brodribb, 2014). Phylogenetically midway between the ferns and angiosperms are the gymnosperms, which appear to employ passive hydraulic control for short-term perturbations in leaf water status, in common with earlier lineages, but are also capable of switching to ABAmediated control following extended water stress (Brodribb and McAdam, 2013; McAdam and Brodribb, 2014; Martins et al., 2016). The proposed evolutionary trajectory from simple to more complex mechanisms of stomatal control of leaf water status may provide a useful framework for the general modeling of stomatal control, starting from passive hydraulic models in ferns and lycophytes, with the end goal of modeling stomatal control in angiosperms where both hydraulics and metabolism are important (Buckley et al., 2003; Brodribb andMcAdam, 2011). Of the many hydraulic models proposed (e.g. Tuzet et al., 2003; for review, see Damour et al., 2010), including models combining both metabolic and hydraulic components (Buckley et al., 2003), most do not describe stomatal dynamics to short-term perturbations, nor do they specifically include the effect of ABA. Older hydraulic models that do describe stomatal dynamics were principally interested in the “wrong-way” response or stomatal oscillations (e.g. Cowan, 1972; Delwiche and Cooke, 1977). Models that do include ABA (Tardieu and Davies, 1993; Dewar, 2002) rely on the hypothesis that ABA is produced in the roots following water stress and is transported via xylem sap to the shoots, where it accumulates in leaves (Davies and Zhang, 1991). However, the root-derived ABA hypothesis has been challenged on multiple fronts, with recent data supporting leaf-synthesized ABA as the source of stomatal control (Holbrook et al., 2002; Christmann et al., 2007; Manzi et al., 2015; McAdam et al., 2016). Moreover, the sensitivity of stomatal conductance (gs) to ABA level in current stomatal models is highly empirical, with no obvious mechanistic basis (Tardieu and Davies, 1993; Gutschick and Simonneau, 2002). ABA acts to close stomata through the activation of outward-rectifying anion channels, including SLAC1 in guard cells (Kollist et al., 2014). Modeling stomatal movements on the basis of ion fluxes into and out of guard cells will provide the ultimate mechanistic basis for changing aperture, and although Hills et al. (2012) developed a model of stomatal movement based on known ion channel behavior from electrophysiological studies, the intention of their model at this stage is not to predict leaf-level stomatal behavior. In light of recent developments, modeling the effect of ABA on stomatal conductance needs re-evaluation. Dynamic stomatal responses to changes in plant water status are well described by a passive hydraulic model in ferns and lycophytes due to ABA insensitivity (Brodribb andMcAdam, 2011; Martins et al., 2016), and in gymnosperms over short intervals of water stress due to a limited influence of ABA, which is synthesized slowly in conifers (McAdam and Brodribb, 2014). However, over longer periods of water stress, the synthesis of ABA leads to an uncoupling of stomatal conductance from bulk water potential in gymnosperms (McAdam and Brodribb, 2014). Here, we developed an analytic passive hydraulic model based on leaf water relations, augmented by a simple, semimechanistic ABA effect at the guard cell. The model was tested on the gymnosperm Metasequoia glyptostroboides Hu and Cheng, first fitting to steady-state behavior, then perturbing the plant water status by: (1) excising branches in air to stop hydraulic supply and allowing stomata to close as the leaves dry out, before recutting underwater to reconnect the hydraulic supply; and (2) droughting plants to allow branches to experience longer periods of water stress and accumulate ABA, before rehydrating leaves by recutting underwater to reconnect the hydraulic supply.