4.2 Foliar water uptake ability in Crassula is
independent of geographical distribution
Our observations suggest that FWU ability in Crassula is
independent of geographical distribution and the associated
environmental conditions, thus challenging our initial assumptions. The
coastal areas of the (semi-)arid western and southwestern sides of
southern Africa, which fall under the influence of the fog belt,
comprise most of the species diversity of Crassula (Jürgens
1995), particularly in subgenus Crassula (Bruyns et al. 2019; Lu
et al. 2022). In the Succulent Karoo, nighttime and early morning fog
and dew are more reliable and even more abundant water sources than the
overall low rainfall (Williamson 1997; Cowling et al. 1999; Desmet and
Cowling 1999). There, shallow-rooted dwarf Crassula species
usually grow in extremely xeric microhabitats on rock outcrops, which
makes them highly dependent on regular water supply (Esler and Rundel
1999). Accordingly, most of these species grow on south- and west-facing
slopes that face the ocean, where interception of wind-driven advective
fog is more efficient (Tölken 1974, 1977; Jürgens 1995). Previous
studies of FWU in Crassula have focused on these species
occurring within the fog belt, as the combination of extremely low soil
moisture and periodically high air humidity makes them an ideal case
study (von Willert et al. 1992; Martin and von Willert 2000). Even in
the slightly less arid Little Karoo, the southernmost region of the
Succulent Karoo and habitat to C. tecta , dew can contribute to
significant water deposition despite less fog influence (Weiss and Yapp
1906; Desmet and Cowling 1999).
Overall, these are compelling arguments in support of hydathode-mediated
FWU as an ecophysiological strategy that allows dwarf Crassulaspecies to exploit even tiny amounts of atmospheric water during long
droughts. This may be even more crucial for species occurring within the
hyper-arid Gariep centre (sensu van Wyk and Smith 2001), the
northernmost region of the Succulent Karoo biome and habitat to C.
ausensis , C. deceptor , C. plegmatoides and C.
sericea (Fig. 1 ). Besides Crassula , other plants that
occur sympatrically in western southern Africa are also believed to
benefit from frequent fog and dew through fog drip, self-irrigation and
maybe even FWU (Snow 1985; Andrews et al. 2011; Vogel and Müller-Doblies
2011; Roth-Nebelsick et al. 2012), including leaf succulents in the
Aizoaceae (Niesler 1997; Matimati et al. 2013) and the
desiccation-tolerant resurrection plant Myrothamnus
flabellifolius (Myrothamnaceae; Drennan et al. 2009). Interestingly,Myrothamnus is also one of the rare cases in which laminar
hydathodes occur, suggesting the possibility of FWU.
Far from the influence of the fog belt, Crassula species
occurring on the southeastern and eastern sides of southern Africa
experience overall higher relative humidity and year-round to summer
rainfall (Fig. 1B ; van Wyk and Smith 2001; Mucina and
Rutherford 2006). Most of the species in this geographical range belong
to subgenus Disporocarpa , which tends to extend beyond the GCFR
(Bruyns et al. 2019). In C. multicava and C. ovata , a
white mineral crust forms on the hydathodes (Figs. 3–5 ), which
has also been noted in other species of subgenus Disporocarpa ,
such as C. lactea (Whittaker 2015). This crust has generally been
regarded as a sign of frequent guttation (Tölken 1974; Chen and Chen
2005; Michavila et al. 2021; Mehltreter et al. 2022). Thus, under
conditions of high soil moisture and low transpiration, theseCrassula species most likely experience guttation. Guttation
through the hydathodes can promote water flux and xylem transport of
nutrients when transpiration is limited, while preventing excessive
water accumulation and detrimental mesophyll flooding (Feild et al.
2005; Cerutti et al. 2019; Bellenot et al. 2022). Nevertheless, our
results show that C. multicava and C. ovata are also as
capable of FWU as the Crassula species from within the fog belt,
and they are able to quickly redistribute the absorbed water within the
leaf (Fig. 9 ). Even though droughts are less severe on the
eastern side of southern Africa and fog has minimal, if any, influence,
FWU may allow them to utilize any available water from dew formation or
brief rainfall events that can wet the leaves, even if they do not lead
to significant soil wetting.