1 INTRODUCTION
Uptake of atmospheric water by plant organs other than roots has been a long-standing debate, yet it is currently believed to be more widespread than previously thought. Foliar water uptake (FWU; Rundel 1982; Dawson and Goldsmith 2018; Berry et al. 2019) and stem water uptake (Oliveira et al. 2005; Earles et al. 2016) have been reported in many plant families and across most biomes. A decreasing water potential (Ψ) gradient from water on the leaf surface into the mesophyll is presumed to be a basic requirement for FWU (Goldsmith 2013; Oliveira et al. 2014), so it is a presumably common phenomenon in habitats with generally high atmospheric humidity (Binks et al. 2019; Boanares et al. 2019; Regalado and Ritter 2021; Chin et al. 2022). However, steep Ψ gradients leading to efficient FWU are more likely to occur in (semi-)arid and saline habitats that experience seasonal rainfall or periodically high air humidity (Rundel 1982; von Willert et al. 1992; Reef and Lovelock 2015), in which FWU is expected to be a highly favourable strategy. Accordingly, FWU has been reported in mangrove species (Fuenzalida et al. 2019; Hayes et al. 2020; Coopman et al. 2021), in several drought-tolerant shrubs and trees (Breshears et al. 2008; Yan et al. 2015; Hill et al. 2021), and in epiphytes growing in xeric microhabitats (Reyes-García et al. 2012; Gotsch et al. 2015; Pan et al. 2021).
Deposition of liquid water on leaf surfaces is governed by their wettability, which in turn is determined by features such as trichomes, hygroscopic salts and epicuticular waxes (Konrad et al. 2015; Barthlott et al. 2017), and water entering the leaves through FWU must overcome a series of hydraulic resistances until it reaches the cells or the vasculature (Buckley 2015; Boanares et al. 2020). Several entry points into the leaf have been proposed, such as the cuticle and stomata (Fernández et al. 2021; Guzmán-Delgado et al. 2021; Chin et al. 2023), but also specialized trichomes and scales (Fernández et al. 2014; Eller et al. 2016; Pina et al. 2016; Raux et al. 2020; Prats and Brodersen 2021), which may facilitate FWU by relaxing some of the hydraulic resistances. Even more specialized leaf surface structures, usually found in arid- and saline-adapted plants, can be coupled with structures within the mesophyll which may enhance FWU even further and aid in water distribution. Notable examples include the peltate hairs associated with thick-walled idioblasts in the xerophyte Capparis odoratissima(Capparaceae; Losada et al. 2021) and the cork warts associated with sclerified idioblasts in species of the mangrove Sonneratia(Lythraceae; Bryant et al. 2021). However, linking leaf morpho-anatomical traits to FWU remains challenging, given that different and apparently opposite trait syndromes that support FWU have been reported among different plant groups (dos Santos Garcia et al. 2022; Chin et al. 2023).
Many drought-avoiding succulent plants (for review see Ogburn and Edwards 2010 and Males 2017) occur in habitats with extremely low soil moisture yet with periodically high air humidity due to a strong oceanic influence resulting in fog and/or dew formation, such as the Atacama desert and the Baja California desert (part of the Sonoran Desert) in the Americas (Rundel et al. 1991; Webb and Turner 2015), and the Succulent Karoo in southern Africa (Desmet and Cowling 1999; Matimati et al. 2010). Indeed, fog-harvesting strategies and fog drip have been widely reported among succulents (Mooney et al. 1977; Martorell and Ezcurra 2002; Schulz et al. 2011; Matimati et al. 2013; Kundanati et al. 2022), and water uptake by aerial parts has often been suggested. In the Americas, stem water uptake through the areoles has been long suspected in many Cactaceae (Schill and Barthlott 1973; Barthlott and Capesius 1974; Porembski 1994), likely facilitated by fog-harvesting properties of spines and trichomes (Ju et al. 2012; Liu et al. 2015; Kim et al. 2017). In southern Africa, FWU has been suggested for succulent species of Anacampserotaceae and Aizoaceae, most of which possess specialized trichomes or scales (Marloth 1910; Barthlott and Capesius 1974; Seely et al. 1977; Niesler 1997). However, many of these cases lack solid experimental evidence and thus remain speculative.
A particular case that has attracted more attention is that of the genusCrassula (Crassulaceae) in southern Africa, in which FWU has long been suspected. Southern Africa comprises several biomes with contrasting environmental conditions, including differences in aridity, rainfall seasonality and fog influence (Fig. 1 ). Along the (semi-)arid western coast of southern Africa, ocean moisture and topographical features give rise to a coastal fog belt extending up to 100 km inland (Fig. 1C ) (Olivier 1995, 2002; Atlas of Namibia Team 2022), which strongly influences the coastal areas of the floristically megadiverse Succulent Karoo biome, part of the Greater Cape Floristic Region (GCFR; see Fig. 1A ) (Cowling et al. 1998; Mucina and Rutherford 2006; Snijman 2013). Within this belt, nighttime and early morning fog and dew are more predictable and even more abundant water sources than rainfall, particularly on rock outcrops, ridges and inselbergs (i.e. isolated mountains) that efficiently intercept moisture (Williamson 1997; Cowling et al. 1999; Desmet and Cowling 1999). In contrast, the eastern side of southern Africa exhibits a dry sub-humid to humid climate, with significantly more abundant rainfall (Fig. 1B ).
The leaf-succulent genus Crassula is a characteristic element of the southern African flora and occurs across all southern African biomes, but its centre of diversity is in the Succulent Karoo and the rest of the predominantly winter-rainfall GCFR, where it has undergone a recent radiation (Manning and Goldblatt 2012; Snijman, 2013; Lu et al. 2022). According to Tölken (1977), the genus can be divided in two subgenera: the paraphyletic subgenus Disporocarpa Fisch. & C.A.Mey., which encompasses the two most early-diverging clades, and the highly diverse monophyletic subgenus Crassula , which corresponds to the most speciose clade. The genus displays a high degree of morphological diversity (Dortort 2009a, 2009b), as well as striking variation of leaf surface sculpturing (Jürgens 1985; Whittaker 2015; Fradera-Soler et al. 2021). One of the most defining features ofCrassula is the presence of marginal and/or laminar hydathodes on the leaves of nearly all species (van Jaarsveld 2003; Thiede and Eggli 2007). Hydathodes are often overlooked foliar structures that are relatively common among vascular plants (for review see Cerutti et al. 2019), being responsible for the process of guttation (i.e. the exudation of apoplastic fluid; Bellenot et al. 2022). Marginal and apical hydathodes are the most prevalent (Cerutti et al. 2019; Jauneau et al. 2020; Rios et al. 2020), while laminar hydathodes, which are found over the entire leaf surface, are restricted to Crassulaceae and three other eudicot families, which have very few or no succulent representatives: Moraceae, Urticaceae and Myrothamnaceae (Lersten and Peterson 1974; Lersten and Curtis 1991; Chen and Chen 2005; Drennan et al. 2009). The noteworthy anatomy of hydathodes in Crassula and their contrasting foliar distribution among different species have led to several exquisitely illustrated publications through the years (de Bary 1884; Sporer 1915; Rost 1969; Smirnova 1973; Voronin et al. 1976). However, the fact that structures usually associated with excess water and guttation occur so abundantly in arid-adapted Crassulaspecies constitutes an ecophysiological enigma. Early studies already speculated that FWU may occur in Crassula through trichomes and/or hydathodes (Marloth 1910; Schönland 1910; Sporer 1915), an idea that was revisited and linked more robustly to hydathodes (Barthlott and Capesius 1974; Tölken 1974, 1977; Voronin et al. 1976). The highly variable foliar distribution of hydathodes among Crassulaspecies, often occurring on the surfaces most exposed to the atmosphere and thus most likely to experience water deposition, is a compelling indication that FWU may be a widespread adaptation in the genus (Voronin et al. 1976; von Willert et al. 1992). Informal observations by Tölken (1974, 1977) of uptake of a crystal violet solution suggested that nearly all Crassula species examined were able to absorb water through the hydathodes if dehydrated enough. This culminated in the most comprehensive work on hydathode ecophysiology in Crassula by Martin and von Willert (2000), who demonstrated that FWU is possible in many species by measuring changes in leaf thickness after surface wetting. However, their results did not show a clear distinction between the direct effects of wetting through FWU and the possible indirect effects through transpiration reduction, nor did they empirically link FWU to hydathodes.
Hydathode-mediated FWU in Crassula has become a widely assumed phenomenon that often appears in the literature, yet, to our knowledge, no visual proof has been available until now to unequivocally link hydathodes to FWU in this genus. The goal of this study was to corroborate that hydathode-mediated FWU does indeed occur inCrassula by using a fluorescent tracer and different imaging techniques. We hypothesized that FWU would occur mostly, if not exclusively, in Crassula species occurring in or near the fog belt of western southern Africa. Furthermore, we hypothesized that FWU ability in Crassula would be strongly influenced by leaf surface sculpturing and wettability. Finally, we aimed to interpret the results from an evolutionary perspective and assess their ecophysiological relevance.