Physiological implications of FWU pathways
Even with open stomata, FWU is insufficient to provide any meaningful absorption of water to sustain prolonged transpiration. To put it in the quantitative context, the maximum FWU rate with open stomata measured in this study was ~12 g m -2h-1 which is ~80 times lower than typical transpiration rates reported for the studied species (~1,000 g m -2 h-1or 15 mmol m-2 s-1; Romero et al. 2004). Thus, 10 hours of fog water absorption at maximum FWU rates would support 7 minutes of transpiration. However, water stress results in a decreased gs and significantly reduced transpiration. In such conditions, FWU may be decisive for survival by temporarily recovering Ψ and leaf turgidity. For example, a tree crown with 10 m2 of foliage could absorb ~400 g of water over 10 hours via the cuticle only (~1,200 g if stomata are open). While this amount seems small in comparison to transpirational requirements, if placed in the perspective of total leaf water content (2,000 g assuming 300 µm leaf thickness and 66% cellular content), 400 g is not only more than enough to recover turgor (~200 g) and Ψ, but can also provide water to locally refill embolized conduits (Earles et al. 2016), allow for maintenance of phloem activity, and partially rehydrate basal organs (Cassana et al. 2016).
The contribution of stomata to FWU highlights the importance of favorable timing when both plant and atmospheric conditions are conducive to absorption (e.g. conditions that promote stomatal opening during a precipitation event, and at night). This agrees with Berry et al. (2014) who found that the timing of fog exposure had a greater impact on FWU than the duration of surface wetness, probably associated with the role of open stomata in FWU. Under such favorable conditions, a number of species keep stomata open (Merilo et al. 2018; Resco de Dios et al. 2019; Schulze et al. 1972), thus maximizing FWU. In this regard, increasing FWU capacity can be a potential function for nocturnal stomatal conductance which is in line with its positive effect on plant growth (Resco de Dios et al. 2019). However, the contribution of the cuticle to FWU should not be underestimated. In addition to constituting a ubiquitous FWU pathway, its cuticular permeability can be essential to enable stomatal opening in response to Ψ increase, potentially extending the photosynthetic period and enhancing CO2 uptake rates. This could lead to higher plant water use efficiency (Vesala et al. 2017). Differential uptake of water by guard cells and ordinary epidermal cells may restore turgidity in guard cells faster forcing stomata to open as they push against the still dehydrated epidermal cells (Buckley 2019). The hypothesized higher permeability of the exposed guard cell cuticle (Maier-Maercker 1983; Schlegel et al. 2005), or the presence of amorphous cellulose regions in the cell wall underneath (Shtein et al. 2017) might favor water fluxes into guard cells.