Az Eszterházy Károly Tanárképző Főiskola Tudományos Közleményei. 2004. Sectio Biologiae. (Acta Academiae Paedagogicae Agriensis : Nova series ; Tom. 25)
Dulai, S., Csizi, K., Sass-Gyarmati, A., Orbán, S. and Molnár, I.: Combined effects of Thylakoid Energisation Level and Water Deficit on Thermal Stability of Photosystem II in a Dessication Tolerant Moss
136 Dulai, S. et al. the temperature dependence of the initial fluorescence level of H. lutescens. The increase of thermal stability under water deficit could have been detected only under continuous light (at steady-state photosynthesis level). The non-radiative dissipation of excess light depends on both the intrathylakoid p\H gradient and the activity of the xanthophyll cycle (Demming-Adams 1990). According to some studies, at a low lumen pH the activity of the xanthophyll cycle, besides its role in photoprotection, may have a role to play in the heat tolerance of PS II (Havaux and Tardy 1996; Molnár et al. 1998), since the zeaxanthin accumulation, besides intensifying high energy quenching, may increase the thermal sability of PS II with the rigidization of thylakoids (Havaux és Gruszeczki 1993). The NPQ values of treated plants at higher temperatures significantly surpass the control, with maximums close to Tc (Figure 4). Besides, the DTT treatment resulted in a decrease of NPQ values, and inhibited the increase of thermal stability both under light and osmotic treatment (not shown by data). On the other hand, in dark-adapted state water deficit did not result an increase of the thermal stability of PS II (Table 2). All this makes it likely that the thermal tolerance increase caused by water deficit only takes place in energized thylakoids. Therefore it might be possible that the protective processes (in the early stages) against the effects of excess light, high temperatures, and water deficit, at least in this kryptogam species, share certain characteristics, which may be related to the low lumen pU as well. Acknowledgements This work was supported by the research grant OTKA T43120. D. S. is grateful to the Hungarian postdoctoral (Békésy György) fellowship. References BAJJII, M., LUTTS, S., KINET, J. M. (2000): Water deficit effects on solute contribution to osmotic adjustment as a function on leaf aging in three durum wheat (Triticum durum Desf.) cultivars performing differently in arid conditions Plant Sci. 160, 669-681. BILGER, H. W., SCHREIBER, U., AND LANGE, O. L. (1984): Determination of leaf heat resistance: comparative investigation of chlorophyll fluorescence changes and tissue necrosis methods. Oecologia 63, 256-262. BJÖRKMAN , O. (1987): High-irradiance stress in higher plants and interactions with other stress factors. In 'Progress in photosynthesis' (Ed. J. Biggens) pp. 1118. (Martinus Nijhoff: Dordrecht). CHEVES, M. M. (1991): Effects of water deficits on carbon assimilation. Journal of Experimental Botany 42, 1-46.
