4.1 Hydathode-mediated foliar water uptake is probably a
widespread phenomenon among Crassula species
After decades of speculation, our results reaffirm hydathode-mediated
FWU as a common phenomenon in both subgenera of Crassula (i.e.Disporocarpa and Crassula ), regardless of growth form,
habitat preference and phylogenetic relationship. Our results
unequivocally prove that FWU through hydathodes is possible in all but
one of the Crassula species examined. The fact that LYCH
fluorescence was present throughout the uptake path demonstrates that
this is an entirely, or at least predominantly, apoplastic process
(Fig. 8 ). The foliar distribution of hydathodes, occurring on
the most exposed leaf surfaces (Table 2 ; Voronin et al. 1976;
Martin and von Willert 2000), and the highly developed 3D venation that
interconnects them (Rost 1969; Melo-de-Pinna et al. 2016; Fradera-Soler
et al. 2021) are anatomical features that further imply that FWU and
subsequent redistribution of absorbed water provide an ecophysiological
advantage to Crassula species. Since succulents experience very
little water loss, even small amounts of water from fog and dew can
considerably improve their water balance (von Willert et al. 1992).
Furthermore, Martin and von Willert (2000) reported that
CO2 uptake rates in Crassula increase in response
to FWU and that absorbed water can be transferred from wetted older
leaves to younger ones, which highlights the far-reaching physiological
consequences of this phenomenon.
Other genera in the Crassulaceae, such as Kalanchoe ,Aichryson and Sedum , have only marginal hydathodes or a
single (sub)apical one (Caballero and Jiménez 1977; ‘t Hart and Bleij
2003; Thiede and Eggli 2007; Moreira et al. 2012), and even though there
are reports of laminar hydathodes in some Cotyledon species
(Weingart 1935), further anatomical investigations are needed to confirm
this. This makes Crassula one of the few, if not the only, plant
group in which laminar hydathodes and leaf succulence converge.
Succulent organs are characterized by high values of hydraulic
capacitance (C ) (i.e. the change in water content relative to the
change in Ψ), which reflects their ability to maintain relatively high Ψ
values even during periods of net water loss (Ogburn and Edwards 2010;
Fradera-Soler et al. 2022; Leverett et al. 2023). As hypothesized by
Berry et al. (2019), high values of C should result in lower
overall FWU rates that are sustained for longer: an increase in Ψ
resulting from FWU will be slower and will take longer to reach
equilibrium, so that the Ψ gradients that underlie FWU can persist for
longer. Indeed, a trade-off between C and FWU has been
postulated, with species with high C exhibiting lower FWU rates
and/or capacity (Gotsch et al. 2015; Boanares et al. 2018). Moreover,
higher C means that succulents capable of FWU such asCrassula will be able to absorb greater amounts of atmospheric
water, thus buffering declines in Ψ during drought. This suggests that
the ecophysiological implications of hydathode-mediated FWU have likely
played an important role in the diversification of Crassula ,
particularly in (semi-)arid habitats with periodically high air humidity
in western southern Africa, such as the Succulent Karoo.
However, not all Crassula species seem to be capable of
hydathode-mediated FWU. In this study FWU could not be induced inC. perforata (Fig. 8 ), which agrees with the
observations by Martin and von Willert (2000). Tölken (1974, 1977) did
not observe signs of FWU in C. rupestris , C. macowanianaor C. brevifolia either. In contrast with most Crassulaspecies, these four species share some commonalities: belonging to
subgenus Crassula , a mostly glabrous leaf surface with a
hydrophobic waxy bloom, a relatively large shrubby growth form, and the
ability to dominate in exposed zonal habitats under considerable aridity
(Jürgens 1995; Bruyns et al. 2019; Lu et al. 2022). It has been
hypothesized that, since these larger species have more extensive root
systems, high leaf hydrophobicity may increase water throughfall during
precipitation events and provide more water to the roots (Rosado and
Holder 2013; Fradera-Soler et al. 2021), which may render FWU less
essential for their survival. This may also explain the loss of
hydathodes in C. brevifolia (von Willert et al. 1992; Martin and
von Willert 2000).