What can we learn from mosses and liverwort gas-exchange in the light… and in the dark!

Mosses and liverworts (phylum Bryophyta) are poikilohydric organisms that can tolerate oscillations in their tissue water content, but they also need a certain threshold level of tissue water content to remain metabolically active. Mosses usually depict numerous finely divided leaflets formed by one unique layer of cells that hold an external layer of capillary water to delay desiccation. Mosses lack the regulatory mechanisms to control water loss (i.e. stomata) and hence, carbon assimilation is limited by diffusion of CO2 first through an outer liquid boundary layer and then through the cell wall and internal membranes. Thus, when there is a positive vapour pressure deficit between the ambient air and the leaflet surface, at first the surface water evaporates increasing assimilaton as a result of increased diffusivity of CO2 into the cells and then as the tissue gradually becomes more desiccated, assimilation decreases. Along these processes, the CO2 and H2O molecules –and other trace gases- exchanged between the mosses and the atmosphere change their isotopic composition depending on the hydration state of the moss and the environmental conditions.