4.3 Foliar water uptake in Crassula is likely facilitated by hierarchical leaf surface sculpturing
Leaf surfaces exhibit notoriously diverse wettability across the plant kingdom, governed by features such as surface sculpturing and the chemical properties of epicuticular waxes (Prüm et al. 2012; Barthlott et al. 2017). In the Crassula species examined we observed a wide range of leaf wettability (Fig. 7 ) that did not seem to be clearly linked to the leaf indumentum. The highly diverse hierarchical sculpturing of the leaf surfaces (Barthlott et al. 2017; Riglet et al. 2021) among the studied Crassula species ranged from glabrous to different types of indumentum (Figs. 3–5 ; Table 2 ). Besides its long-established functions in reflecting excessive insolation and limiting transpirational water loss (Bickford 2016; Karabourniotis et al. 2021; Buckley et al. 2022), the indumentum has been implicated in initiating dew formation (von Willert et al. 1992; Burkhardt and Hunsche 2013; Konrad et al. 2015) and collecting fog droplets (Andrews et al. 2011; Ju et al. 2012). However, even among theCrassula species with indumentum, leaf wettability ranged widely, from superhydrophilic to highly hydrophobic (Fig. 7 ). We observed hemiwicking (i.e. capillarity-driven spread of liquid on a rough hydrophilic surface; Quéré 2008; Kim et al. 2016; Telecka et al. 2018) in the two Crassula species with the most hydrophilic leaf surfaces. This phenomenon occurred very rapidly on the asymmetrically sculptured leaf surface of C. tecta and led to efficient spread of the water film (Video S1 ) (Shin et al. 2016; Jiang et al. 2022), as previously reported (Tölken 1974, 1977; Fradera-Soler et al. 2021), while it was rather slow and barely noticeable in C. ausensis (Video S2 ). At the other end of the spectrum, we observed near superhydrophobicity in C. deceptor and C. plegmatoides , known as the ‘lotus effect’ (i.e. high θCand low θC hysteresis, see Bhushan and Nosonovsky 2010; Schneider et al. 2016; Okulova et al. 2018), which is likely caused by the highly hierarchical sculpturing of leaf surfaces, involving the epicuticular waxes (Fig. 5 ) (Barthlott et al. 2017; Riglet et al. 2021).
The interplay of leaf surface wettability and FWU is not strictly relational. Although higher wettability has often been linked to increased FWU capacity or rate (Pan et al. 2021; Tianshi and Chau 2022), and species with hydrophobic leaves are assumed to be less likely to benefit from water deposition from fog and dew, other studies have noted no significant relationship between FWU and leaf surface wettability (Matos and Rosado 2016). This also reflects the diversity of strategies underlying FWU among different plant groups (dos Santos Garcia et al. 2022; Chin et al. 2023). In Crassula , even species with seemingly hydrophobic leaves are capable of FWU. Our experimental θC measurements are based on a water droplet size typical of large raindrops (i.e. 5 μl volume, ~ 2 mm diameter) (Glickman 2000), so wetting behaviours that deviate from our observations are plausible under natural conditions with different droplet sizes, such as those from fog deposition (< 200 μm droplet diameter) and rainfall (> 500 μm droplet diameter). As seen in other plant groups (Pierce et al. 2001), Crassulaspecies with seemingly hydrophobic leaf surfaces may still be able to induce condensation or collect fog droplets within hydrophilic leaf surface microdomains. For instance, the wax-free epidermal tubercle tips and hydathode water pores in C. deceptor (Figs. 4G, 5G ) (Barthlott and Capesius 1974; Jürgens 1985) may be able to experience condensation and droplet coalescence (Narhe and Beysens 2006; Sharma et al. 2019; Xing et al. 2020). Abrasion of these waxes may also explain why older leaves of some Crassula species, such as C. plegmatoides , can be more wettable than young ones (pers. obs.) (Ensikat et al. 2011; Rosado and Holder 2013). Furthermore, higher hydrophobicity of the indumentum compared to the surface underneath, coupled with relatively low trichome density (Brewer et al. 1991; Brewer and Smith 1997; Bhushan and Jung 2008), can lead to changes in overall wettability if water droplets exceed a critical size that allows them to sink between the trichomes and spread along the surface, as in some hairy-leaved species of Echeveria (Crassulaceae) (Godeau et al. 2017). This may explain similar observations in some Crassulaspecies, such as C. sericea , in which leaf surface wettability can range from highly hydrophobic to nearly hydrophilic (Fig. 7 ). All things considered, the complex leaf surface sculpturing inCrassula and the existence of leaf surface microdomains resulting in differential wettability may facilitate water deposition and/or channelling towards hydathode water pores, thus facilitating FWU even in seemingly hydrophobic species.