Are Bryophytes Shade Plants? Photosynthetic Light Responses and Proportions of Chlorophyll a, Chlorophyll b and Total Carotenoids

Bryophytes see the light

Early in our botanical education we learn the differences between sun and shade plants, but later we come to understand that it is not so straightforward. This is well illustrated in the paper by a joint Hungary-UK team, Mariann Marschall and Michael Proctor (pp. 593�603). They point out that bryophytes, even those that grow in the open, are generally classified as shade plants, based on chlorophyll a : b ratios and the irradiance levels at which photosynthetic rates plateau. However, they have questioned whether this classification should or indeed can be applied to all bryophytes. They worked with a large range of bryophytes from a variety of habitats, measuring chlorophyll a : b ratios, chlorophyll : carotenoid ratios, chlorophyll fluorescence and, for some species, gas exchange. The main feature that stands out from their data is that it is not safe to generalize. Pigment ratios and photosynthetic parameters vary widely between species, although there are strong correlations between chlorophyll a : b ratios, chlorophyll : carotenoid ratios and photosynthetic photon flux density saturation curves (obtained via chlorophyll fluorescence). Further, some of the values for bryophyte species from open, well-illuminated habitats are at the high end of the range for shade plants. Nevertheless even these cannot be regarded as true sun plants. So, are these bryophytes of open habitats really shade plants that are poorly adapted to their environment? It is not that simple: in most bryophytes with �unventilated� photosynthetic tissue, CO₂ uptake is limited by leaf structure and this is the probable reason for the relatively low photosynthetic saturation values. In support of this, the authors show that the Polytrichaceae, with their ventilated photosynthetic tissue and hence greater surface areas for CO₂ uptake, are the nearest approach to sun plants among the bryophytes. Thus, as the authors state, �bryophytes include but are not inherently shade plants�.

Professor J. A. Bryant

University of Exeter, UK

j.a.bryant{at}exeter.ac.uk