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.