Introduction
Plants can absorb water through their non-woody aerial surfaces (Dawson and Goldsmith 2018; Rundel 1982; Stone 1957). This ability, commonly referred to as foliar water uptake (FWU) is shared across phylogeny, and may have profound implications for the water and carbon balance at both the plant and ecosystem level (Binks et al. 2019; Boanares et al. 2019; Hayes et al. 2020). Foliar water uptake can increase plant water status and primary productivity (Berry et al. 2014; Gouvra and Grammatikopoulos 2003; Eller et al. 2013; Fernández et al. 2014; Kerhoulas et al. 2020; Pina et al. 2016; Simonin et al. 2009), and enhance survival by potentially allowing the restoration of xylem transport capacity (Fuenzalida et al. 2019; Laur and Hacke 2014) or facilitating xylem/phloem transport during crucial phenological stages (e.g. fruit development or leaf senescence; Guzmán-Delgado et al. 2017, 2018). Nevertheless, there are many critical knowledge gaps that limit our understanding of FWU (Berry et al. 2019). For instance, the pathways and mechanisms underlying the process are poorly understood but are pivotal to deciphering the physiological functions and evolutionary significance of this trait.
Water may be absorbed through parallel pathways, including the cuticle/wax layer of ordinary epidermal cells, trichomes or guard cells, and stomatal or hydathode pores (Fernández and Eichert 2009; Martin and von Willert 2000; Schreel et al. 2020). Cuticular water uptake was found to be a very slow process involving fluxes in both liquid and vapor phases (Schreiber and Schönherr 2009). In contrast, studies evaluating stomatal FWU have provided divergent results. Water dissolved solutes or suspended particles applied to the leaf surface were either not observed to penetrate stomata (Eller et al. 2016; Schreel et al. 2020), found on the walls of stomatal pores (Arsic et al. 2020; Li et al. 2018), or on mesophyll cells lining the substomatal cavity (~43 nm diameter suspended particles; Eichert et al. 2008). It is argued that stomatal anatomical and physico-chemical features prevent liquid water entry into the pore, unless it is forced by an external pressure or water surface tension and cuticular hydrophobicity are reduced by the action of surfactants or other substances such as deliquescent particles or bacteria present along the pores (Burkhardt et al. 2012; Eichert et al. 2008; Schönherr and Bukovac 1972). In particular, the hydrophobic cuticular ledges over guard cells present in many species with diverse phylogenetic origins and ecologies may serve this function (Cullen and Rudall 2016; Edwards et al. 1998; Hunt et al. 2017; Merced and Renzaglia 2013). Other epidermal structures like stomatal wax plugs, papillae or striations found in fossil and extant species of humid habitats may also contribute to keep stomatal pores free of liquid water thus allowing for gas exchange (Aparecido et al. 2017; Brodribb and Hill 1997; Jordan et al. 1998; Feild et al. 1998). Therefore, if and how stomata contribute to FWU remain open questions.
In general, ‘closed’ stomata (i.e. (near) null pore apertures) severely constrain the flow of both liquid and gaseous water. ‘Open’ stomata (i.e. greater pore apertures) enable significant vapor diffusion out of the leaf (transpiration; Lawson et al. 1998) and potentially also absorption when the vapor pressure inside the leaf is lower than the air outside (Vesala et al. 2017). Water droplet/film formation on the leaf surface may also allow the inflow of liquid water through open stomata (Eichert and Burkhardt 2001), which could dramatically increase FWU rates. However, since stomata seem to be designed to protect internal leaf tissues from liquid water entry, we hypothesize that their contribution to FWU is mostly realized by vapor diffusion, not a liquid path. To test our hypothesis, we applied a novel method that allows quantifying the temporal dynamics of leaf rehydration and hydraulic parameters associated with FWU using measurements of leaf mass and water potential changes in a fog chamber (Guzmán-Delgado et al. 2018). To determine the contribution of stomata to FWU we chemically modified stomatal aperture by the application of abscisic acid to keep stomata closed and of fusicoccin to force stomata open. We used leaves of two species that do not have trichomes or hydathodes to limit confounding water entry pathways. We found that ‘open’ stomata can accelerate the rehydration of moderately stressed leaves over rates observed for a cuticle ‘only’ path, although the increased rates fall short of suggesting the formation of a direct hydraulic path.