Trichome-like emergences in Croton of Brazilian highland rock outcrops: Evidences for atmospheric water uptake

Background and aims Although trichomes are typical features of the species-rich genus Croton, little is known about their development, anatomical structure and function. This study aims to characterize the anatomy of leaf trichomes of three Croton species restricted to Brazilian rock outcrops, and investigate their functional role in an environment with limited soil water availability. Methods Samples of leaves at different stages of maturity were submitted to standard anatomical light and scanning electron microscopy techniques to perform a structural and developmental study. Atmospheric water uptake was investigated through the absorption of aqueous solutions of berberine hemisulfate and Indian ink. Key results The so-called trichomes actually correspond to emergences because they originate from protodermal and ground meristem activity rather than from epidermal tissue. These emergences show a complex anatomy, are closely associated with the mesophyll sclereids and might be involved in leaf atmospheric water uptake. The outermost cells of the emergences are arranged radially and present non-lignified thick walls. We demonstrate that an aqueous solution can effectively penetrate through the leaf emergences, continuing through sclereids and reaching the vascular bundle or flowing across the mesophyll maximizing water distribution in the leaf tissue. Conclusions The complex structure and hypothesized function of Croton trichome-like emergences represent a novel finding in Euphorbiaceae. In the three species studied, these leaf emergences might play a role in atmospheric water absorption and they may be one of the clues for the occurrence of Croton in habitats with limited soil water supply where this genus is usually species-rich and abundant.This work was supported by Conselho Nacional de Desenvolvi-mento Científico e Tecnológico [477867/2013-8]; Fundac¸ ão deamparo à Pesquisa do Estado de Minas Gerais [CRA-APQ]; and Cen-tro de Ciências Ambientais Floresta Escola [FUNARBE-6991].Peer Reviewe