5 CONCLUSIONS
Although FWU has recently been recognized as a rather common phenomenon among plants (Berry et al. 2019), it remains largely unexplored and thus hampers a comprehensive understanding of plant hydraulic functioning. In arid-adapted lineages such as Crassula , FWU is expected to be even more crucial for their survival in severe drought. Our observations clarify previous findings (Martin and von Willert 2000), confirming that hydathode-mediated FWU occurs in Crassula and that it is probably widespread across the genus.
From their primordial function of guttation, these hydathodes have been co-opted for water absorption, which has likely had a strong influence in the evolution and diversification of the genus in (semi-)arid environments with periodically high air humidity. However, we suggest that the ability for FWU in Crassula is independent of geographical distribution and its associated environmental conditions, as well as phylogenetic relationship. While FWU provides an ecophysiological advantage to Crassula species occurring on the (semi-)arid western side of southern Africa, where frequent dew and fog can ameliorate the harsh droughts, FWU is also expected to be beneficial to species occurring on the more humid eastern side, where plants may still be able to exploit dew or brief rainfall events. Furthermore, we did not find a strong link between FWU ability and leaf surface wettability. Instead, FWU in Crassula may be facilitated by hierarchically sculptured leaf surfaces and differential wettability. In highly hydrophilic leaves, such as those of C. tecta , hemiwicking spreads the water film over the whole surface, thus leading to efficient FWU. However, even seemingly hydrophobic species such as C. deceptor may be able to experience frequent leaf surface wetting and subsequent FWU thanks to hydrophilic leaf surface microdomains.
Several questions remain unanswered regarding FWU in Crassula , such as the molecular mechanisms behind it and the possible involvement of the epithem. Moreover, other phylogenetically diverse groups of succulent plants may also be benefiting from this phenomenon for their survival, yet empirical proof is needed. Further exploration of FWU in these plants can advance our understanding of adaptation and ecophysiology of succulents and other arid-adapted plants. Surfaces ofCrassula and other succulent plants benefiting from atmospheric water could also provide inspiration to improve systems for atmospheric water harvesting, which is becoming an increasingly valuable water resource in many parts of the world (Zhang and Guo 2020; Wang et al. 2021).