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

114 Marschall, M. and Proctor M. C. F. active. It is important to determine the constitutive and inducible mechanisms of desiccation tolerance in bryophytes. Keywords: desiccation tolerance, stress responses, bryophytes, PPFDresponse curves, chlorophyll fluorescence Abbreviations used Chi: chlorophyll; DTT: dithiothreitol; water potential; NPQ: nonphotochemical quenching; PPFD: photosynthetic photon flux density; pv: pressure-volume; qP: photochemical quenching; REFR: relative electron flow; UV-B: ultraviolet-B; RWC: relative water content; WC: water content. 1. Introduction Bryophytes share most of their physiology with other green land plants, but there are also important differences; the similarities and differences do not necessarily fall in line with simple expectations (Proctor 2000). Because most bryophytes have simple 'stems' and 'leaves', therefore tradition has regarded them 'lower plants' or underdeveloped miniatures of vascular plants, which organisms that have evolutionarily not yet made the grade. The divergence of bryophytes and the various vascular plant groups happened 400 million years ago or earlier. 400 million years were enough for the development of evolutionary independent lines (phylum), as we call them hornworts, liverworts and mosses. Also contrast with expectations bryophytes physiologically are not primitive. With their succesful strategy they are making up a prominent part of the vegetation in oceanic temperate forests, tropical cloud forests, bogs and fens, polar and alpine fellfields and tundras. Their poikilohydric habit means they are taking up water and nutrients over the whole surface of the shoots via direct absorption from dry and wet deposition (Table 1). On the one hand they are limited by their lack of roots, but they can colonize hard and impermeable surfaces, like tree trunks, rock outcrops, roof surfaces, from which vascular plants are excluded. So they are successful in many nutrient-limited habitat and many of them are vulnerable to tolerate atmospheric pollutants. 2. Water relations in bryophytes To understand the various physiological processes and stress responses of bryophytes comparing with higher plants reactions it is essential to know the actual water status of the bryophyte tissue.