Evidence for regulation of transpiration in nonstomatal plants: insights from bryophyte gametophytes

Bryophyta (mosses) are a basal group of plants that lack stomata in their haploid form. Consequently, these plants are defined as poikilohydric, meaning poor control over water loss, and their cytosol is assumed to reach equilibrium with ambient humidity. This classification does not fully align with the diverse strategies observed in mosses. We studied gas exchange in 16 species under controlled dehydration conditions. Our results revealed that the cell wall water potential was below the cytosolic water potential in most species, as reflected by relative humidity at the cell wall surface decreasing to as low as 60% even under optimum hydric conditions. This indicates that cell membranes and/or cell walls involve barriers with higher resistance to water movement than previously assumed (2.7-23 MPa m² s mmol^-1). Additionally, species with better water control also exhibit traits of an avoidance strategy, including elastic tissues, high capacitance, and less negative osmotic pressure. These 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 nonstomatal control recently identified in angiosperms, which induces unsaturated conditions in the substomatal cavities. Bryophyta presents a valuable nonstomatal model for further investigating this mechanism.