Az Eszterházy Károly Tanárképző Főiskola Tudományos Közleményei. 2004. Sectio Biologiae. (Acta Academiae Paedagogicae Agriensis : Nova series ; Tom. 25)

Marschall, M. and Proctor M. C. F.: Aspects of stress tolerance in bryophytes

118 Marschall, M. and Proctor M. C. F. Relative water content Figure 3: Response of net photosynthesis to cell water deficit in two contrasting bryophytes (Tortula ruralis, Conocephalum conicum) and in spinach (Spinacia oleracea), a mesophytic vascular plant, from gasexchange measurements. Original figure from Proctor 2000. 3.2 Different light conditions and light responses Bryophytes have been regarded as showing unified shade plant-like characteristics on evidence including the fine structural features of the chloroplasts, and the typically low chl a/b ratios (of course in the range of shade-adapted vascular plants), although they are living between very different light conditions. Median chl a/b ratio for 45 mosses was 2.33, and for 18 liverworts 1.99 (Figure 4). There is wide variation in their light responses. The shade-loving species (exemplified by Plagiomnium undulatum

) saturate at a PPFD of 100-300 jLlmol rn

2 s"

1 (corresponding to 5-10% of Ml sunlight), the response curves of this species are generally similar to those of the shade-loving vascular plants (left graph of Figure 5). The sun-exposed species (exemplified by Racomitrium lanuginosum

) saturate at a PPFD of 1000 (imol m"

2 s"

1.(right graph of Figure 5). The sun-exposed bryophytes show two remarkable features in their PPFD response curves. First, REFR does not saturate, but continues to rise almost linearly with increasing irradiance. Second, these species show extraordinary high levels of NPQ, which often also continues to rise almost linearly to irradiances equivalent to full sunlight. At the same time 1-qP generally stabilises at around 0.3 to 0.4. Responses of this kind are found in a taxonomically and ecologically diverse range of bryophytes