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.