Hidrofóbicas ou hidrofílicas: como as folhas de espécies caducifólias e sempre verdes da caatinga se comportam?

Abstract

In the Brazilian semiarid, two phenological behaviors of deciduous and perennial leaves stand out. This phenology is related to the seasonal availability of water in the region, with short rainy periods and prolonged drought. As such traits occur together, other mechanisms related to water usage may favor the evergreen condition as the leaf wettability, where hydrophilic surfaces can favor leaf water absorption, contributing to a greater water turgor. Therefore, the study of leaf surfaces may contribute to the understanding of strategies involved in the use of water resources by plants in seasonally dry environments. We expect leaves of evergreen and late deciduous species to be less hydrophobic than leaves of early deciduous species. Leaves of three species with different phenologies, Croton blanchetianus (early deciduous), Combretum leprosum (late decidous) and Cynophalla hastata (perennial) were studied. The contact angle of a sessile droplet of water on the leaf surface (abaxial and adaxial) of young and mature leaves, the cuticle thickness and the density of trichomes were measured. Our data demonstrated that the density and types of trichomes affect the contact angle and leaf hydrophobicity, as observed in C. blanchetianus (102,9º) which is hydrophobic, due to its high density of non-secretory trichomes with cell walls rich in lignin, a substance that has low affinity for water. On the other hand, C. leprosum (68.9°) and C. hastata (73.0º) are hydrophilic. Combretum leprosum has a high density of peltate trichomes with cell walls rich in pectic compounds, as in its epidermals cells, a substance with high affinity for water. Cynophalla hastata has a thick cuticle, which may be related to photoprotection. Its hydrophilicity may be due to the absence of waxes on the epicuticular surface and the composition of the cuticle. Young leaves were more hydrophobic than mature leaves due to the higher density of trichomes and surface roughness. The abaxial side of the leaves was more hydrophobic than the adaxial one, which may favor the carbon absorption by avoiding the occlusion of the stomata with water on the main gas exchange surface as all three species are hipoamphistomatic. However, cuticle thickness did not contribute to increase the hydrophobicity of leaves. Further studies are needed to unreveal the role played by the cuticule in leaf hydrophobicity.