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.