Evidence for active regulation of transpiration in non-stomatal plants

O_LIBryophyta (mosses) are a basal group of plants that lack stomata in their haploid form, as well as developed vascular tissue and a hydrophobic cuticle. Consequently, these plants are classified as poikilohydric, meaning poor control over water loss and are often assumed to reach equilibrium with ambient humidity. This classification does not fully align with the diverse strategies observed in mosses. C_LIO_LIWe studied gas exchange in 14 species from Albuquerque and Boston, USA, under controlled dehydration conditions. C_LIO_LIOur results revealed significant variation in transpiration rates, cell wall equilibrium humidity, and desiccation times across species. These differences could not be explained by tissue water storage relative to the transpiring surface area, suggesting that water loss is not entirely passive. Additionally, species with better water control also presented traits of an avoidance strategy, including elastic tissues, high capacitance, and less negative osmotic potential, suggesting an adaptive constraint. C_LIO_LIThese findings point to a basal, non-stomatal mechanism of water loss control through cell membranes and/or cell walls. Potentially, this mechanism is homologous to the non-stomatal control recently identified in angiosperms, which induces unsaturated conditions in the substomatal cavities. Bryophyta presents a valuable non-stomatal model for further investigating this mechanism and its evolutionary significance. C_LI