How wet can a leaf get? It depends which leaf you’re talking about and where it came from. It’s often seen that leaf wettability changes within a species based on the altitude of a plant. Aryal and colleagues investigate water-repellency of leaves of Arabidopsis halleri subsp. gemmifera (Brassicaceae) growing in contrasting montane habitats along an altitudinal gradient at Mt Ibuki, Japan.
The authors also look at the importance of a leaf’s position on a plant. In rosette-forming plants, rosette leaves are near the ground and their adaxial (stem facing or upper) surfaces are exposed. However, cauline leaves (leaves on the flower stem) are lifted from the ground throughout the reproductive season, and their abaxial (facing away from the stem, or lower) surfaces are more exposed.
Aryal et al. carried out field investigations and a growth chamber experiment to determine whether field-observed variation in leaf wettability was caused by genetic differences. They also performed gene expression analysis of a wax-related gene, i.e. AhgCER1, a homologue of A. thaliana ECERIFERUM1 (CER1) that may be involved in differentiation of leaf wettability.
The team found cauline-leaf specific genetic differentiation in leaf wettability between contrasting montane habitats. Cauline leaves of semi-alpine plants, especially on abaxial surfaces, were non-wettable. Cauline leaves of low-altitudinal understorey plants were wettable, and rosette leaves were also wettable in both habitats. AhgCER1 expression corresponded to observed leaf wettability patterns.
Why would some leaves be more wettable than others? Aryal and colleagues think it might be because the cauline leaves have another job as well as photosynthesis. They also wrap the flowering buds in early spring. Low wettability means they repel water and so they help protect the bud inside from frost damage. The authors state: “To our knowledge, this is the first report of genetic differentiation in leaf wettability between natural plant populations within a single species. The lower wettability specific to cauline leaves of the semi-alpine plants is supportive of the postulate that the dry surface of cauline leaves is necessary for the protection of floral buds from frost under exposure to prevailing cold wind. The theory needs to be tested in future field and experimental studies.”
They also propose a genome-wide study of the species. They add that they: “…identified a candidate gene that could explain the differential leaf wettability between cauline and rosette leaves and between cauline leaves from the two habitats. A genome-wide study on segregated populations derived from crosses between plants from the two habitats will likely reveal the identity of the responsible SNP(s) underlying the discovery in this study.”
Aryal, B., Shinohara, W., Honjo, M. N., & Kudoh, H. (2018). Genetic differentiation in cauline-leaf-specific wettability of a rosette-forming perennial Arabidopsis from two contrasting montane habitats. Annals of Botany, 121(7), 1351–1360. https://doi.org/10.1093/aob/mcy033