Idioblasts as pathways for distributing water absorbed by leaf surfaces to the mesophyll in Capparis odoratissima

Capparis odoratissima is a tree species native to semi-arid environments of the northern coast of South America where low soil water availability coexists with frequent nighttime fog. Previous work with this species demonstrated that C. odoratissima is able to use water absorbed through its leaves at night to enhance leaf hydration, photosynthesis, and growth. Here, we combine detailed anatomical evaluations of the leaves of C. odoratissima, with water and dye uptake experiments in the laboratory. We used immunolocalization of pectin and arabinogalactan protein epitopes to characterize the chemistry of foliar water uptake pathways. The abaxial surfaces of C. odoratissima leaves are covered with overlapping, multicellular peltate hairs, while the adaxial surfaces are glabrous but with star-shaped "structures" at regular intervals. Despite these differences in anatomy, both surfaces are able to absorb condensed water, but this ability is most significant on the upper surface. Rates of evaporative water loss from the upper surface, however, are coincident with cuticle conductance. Numerous idioblasts connect the adaxial leaf surface and the adaxial peltate hairs, which contain hygroscopic substances such as arabinogalactan proteins and pectins. The highly specialized anatomy of the leaves of C odoratissima fulfills the dual function of avoiding excessive water loss due to evaporation, while maintaining the ability to absorb liquid water. Cell-wall related hygroscopic compounds present in the peltate hairs and idioblasts create a network of microchannels that maintain leaf hydration and promote the uptake of aerial water.