Revisiting an ecophysiological oddity: hydathode-mediated foliar water uptake in Crassula species from southern Africa

Abstract

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 Crassula (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 Crassula species occur and where periodically high air humidity leads to fog and/or dew formation. To investigate if FWU is operational in different Crassula 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 Crassula and that it is probably widespread across the genus. Hydathodes in Crassula have been repurposed 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 FWU ability is independent of geographical distribution and its associated environmental conditions, as FWU is possible in species occurring within the fog belt of western southern Africa but also in those from the rather humid eastern side. We did not find a strong apparent link between FWU ability and leaf surface wettability. Instead, the hierarchically sculptured leaf surfaces of several Crassula species may facilitate FWU due to hydrophilic leaf surface microdomains, even in seemingly hydrophobic species. Overall, these results confirm the ecophysiological relevance of FWU in Crassula and reassert the importance of atmospheric humidity for some arid-adapted plant groups.