LAUSR

The data available so far indicate that photosynthetic and relative growth rates of bryophytes are a 10% of those reported for tracheophytes. By revising the existing literature and reanalysing data published in over a hundred of manuscripts, this review scrutinizes the ecophysiological, biochemical and structural reasons behind. The limiting rubisco content and surface for gas exchange are the internal factors that can explain more clearly the low rates of bryophytes. The role of their thicker cell walls in the limitation of CO2 diffusion depends on the current uncertainties regarding their porosity and permeability to CO2. From this review, it is also evident that, despite having low photosynthetic rates, the positive carbon balance of bryophytes is tightly related with their capacity to deal with extreme conditions. These include their capacity to deal with big daily oscillation between day and night temperatures, and to delay the cessation of photosynthesis under water deficit (or to tolerate desiccation in extreme situations). Although further studies on bryophytes are needed before more solid conclusions can be drawn, it seems that their success relies on a remarkable tolerance to a highly variable environment, possibly at the expense of their maximum photosynthetic rates.