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

128 Dulai, S. et al. independent of the excitation energy level. This effect of water deficit remained observable in a longer period (24-48 h). Since in a dark-adapted state the critical values of the F

0 vs.T curves did not shift towards significantly higher temperatures with the increase of water deficit, it seems likely that the water deficit induced rapid thermal stability increase of PSII happens only in energized photosynthetic membranes. The temperature dependence of non-photochemical quenching and the changes under DTT treatment seem to suggest that the low lumen pH and the related processes might have a role in the protecting mechanisms concerning both heat stress and water deficit: it seems likely the protection against excess light, high temperature, and water deficit reveals common characteristics, in this kryptogam plant, at least. Keywords: thermal tolerance, photosystem II, water deficit, Homalothecium lutescens Abbreviations used AL: actinic light; DTT: dithiothreitol; AF/F

m

': effective quantum yield of PS II; F

0

y F

s

: initial and steady state levels of chlorophyll a fluorescence; FJF

m

: optimal quantum yield of PS II; NPQ: non-photochemical quenching; PS II: photosystem II; T: temperature; T

c

: critical temperature; T

v: peak temperature. Introduction In natural habitats the different ecological factors (temperature, light, water condition) vary in interdependence with each other. Simultaneous stress factors may elicit a response different from that given to a single factor, resulting in intensification, overlapping or antagonistic effects (Osmond et al. 1986). The heat sensitivity of plants is closely connected to the thermal stability of PSII. It is more or less clear that the heat sensitivity of the photosynthetic apparatus, and the thermal stability of PSII, can change rapidly (within tens of minutes) as a result of heat pre-treatment (Havaux and Tardy 1996). However, in spite of the wide-ranging research in the field (Björkman 1987; Gamon and Pearcy 1990; Havaux 1992), it is still not widely recognized how these short-term responses to heat are influenced by other stress factors like light and water deficit, or dessication. The study of these problems is further justified by the fact that under natural conditions high light intensity, heat stress, and water deficit occur in combination with each other. A good example for this is that the presence or absence of light can significantly modify the measure of heat-induced damage: the photosynthetic apparatus is