Revisiting an ecophysiological oddity: Hydathode‐mediated foliar water uptake in Crassula species from southern Africa.

Hydathodes are usually associated with water exudation in plants. However, foliar water uptake (FWU) through the hydathodes has long been suspected in the leaf‐succulent genus (Crassulaceae), a highly diverse group in southern Africa, and, to our knowledge, no empirical observations exist in the literature that unequivocally link FWU to hydathodes in this genus. FWU is expected to be particularly beneficial on the arid western side of southern Africa, where up to 50% of species occur and where periodically high air humidity leads to fog and/or dew formation. To investigate if hydathode‐mediated FWU is operational in different species, we used the apoplastic fluorescent tracer Lucifer Yellow in combination with different imaging techniques. Our images of dye‐treated leaves confirm that hydathode‐mediated FWU does indeed occur in and that it might be widespread across the genus. Hydathodes in serve as moisture‐harvesting structures, besides their more common purpose of guttation, an adaptation that has likely played an important role in the evolutionary history of the genus. Our observations suggest that ability for FWU is independent of geographical distribution and not restricted to arid environments under fog influence, as FWU is also operational in species from the rather humid eastern side of southern Africa. Our observations point towards no apparent link between FWU ability and overall leaf surface wettability in . Instead, the hierarchically sculptured leaf surfaces of several species may facilitate FWU due to hydrophilic leaf surface microdomains, even in seemingly hydrophobic species. Overall, these results confirm the ecophysiological relevance of hydathode‐mediated FWU in and reassert the importance of atmospheric humidity for some arid‐adapted plant groups. This study demonstrates that foliar water uptake (FWU) through hydathodes, previously suspected but unconfirmed, is indeed operational in most species. This phenomenon occurs across different habitats and highlights the ecophysiological relevance of atmospheric humidity for these plants.