New perspectives in the biophysics and physiology of bryophytes

The paper summarizes recent advances in the biophysics of membrane transport, and the biochemistry of CO₂ fixation, in bryophytes. The fundamentals of transport of the plasmalemma and the tonoplast of bryophytes closely resemble those of their nearest relatives, i.e. charophycean algae and vascular plants. A good start has been made in relating these transport phenomena to the ecology of the organisms, and to their development (e.g. gametophyte to sporophyte solute fluxes), but further investigation is needed. A particularly interesting case is that of the 'action potentials' with a metabolic component in Anthoceros and Conocephalum which may have a role in signal transduction. An analysis of the relevant literature shows that the great majority of the bryophytes tested (mosses, liverworts, and the pyrenoid-less hornwort Megaceros) have C₃ physiology (diffusive CO₂ transport to RUBISCO), although two aquatic mosses (Fissidens cf. mahatonensis; Fontinalis antipyretica) show indications of the presence of a CO₂-concentrating mechanism. There is no evidence of C₄ or CAM photosynthesis in bryophytes. Those hornworts with pyrenoids (e.g. Anthoceros, Phaeoceros) have, on the basis of gas exchange and ¹³C/¹²C discrimination evidence, a well developed CO₂ concentrating mechanism. Quantitative analysis of the conductances of CO₂ in various parts of the pathway of CO₂ transport (fluid diffusion boundary layer; transport within the cell to RUBISCO; carboxylation in situ using ¹³C/¹²C discrimination; measurement of boundary layer conductances, measurements of biochemical capacity of photosynthesis) shows that less than half of the limitation of photosynthesis in aquatic mosses, and less than a third of that in terrestrial bryophytes, relates to CO₂ transport. In aquatic mosses in rapidly flowing water, and in some terrestrial bryophytes, there is little restriction of CO₂ fixation by CO₂ diffusion. However, there are several unexplained findings and both those bryophytes with C₃ physiology and the minority with a C0₂-concentrating mechanism need further investigation.