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).