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

Acta Acad. Paed. Agriensis, Sectio Biologiae XXV (2004) 127-12

6 Combined effects of Thylakoid Energisation Level and Water Deficit on Thermal Stability of Photosystem II in a Dessication Tolerant Moss Dulai, S.

1,

4, Csizi, K.

1, Sass-Gyarmati, A.

3, Orbán, S.

3, and Molnár, I.

2 'Dept. of Plant Physiol. Eszterházy College, Eger, Hungary;

2 Agricultural Research Institute of the Hungarian Academy of Sciences, Martonvásár; Research Group for Bryology of the Hungarian Academy of Sciences, Dept. of Botany, Eszterházy College, Eger;

4 Corresponding author; e-mail: ds@ektf.hu Abstract. The thermal stability of photosystem II was examined under different (light and water deficit) treatments in the moss Homalothecium lutescens (Hedw.) Robins. In the dark-adapted samples, according to the critical temperatures of the F

0 vs.T curves (T

c

, T

p, F

1/

2), the temperature tolerance measured in the default state (non-energised thylakoids at full turgor) was inadequate to withstand the thermal conditions of the original habitat. Similarly to some higher plants, the temperature dependence of the steady-state level of fluorescence resulted in a significant increase of thermal stability of PS II at a low excitation energy level (100 pmol m

2 s"

1 photon flux density). This decrease in heat sensitivity was more expressed at higher levels of excitation energy. Parallely, compared to the optimal quantum yield (FJF

m) measured in the dark-adapted state, the heat dependence of the effective quantum yield of PSII (AF/F

m') measured in samples with a steadystate photosynthesis level signalled a higher thermal stability. The decrease of water content under continuous light effected a similar heat-tolerance increase, further intensified by increasing excitation energy levels. The breakpoints (T

c, T

p, F

m) of the Fs vs.T curves were significantly shifted towards higher temperatures even under a 30-minute moderate (-1.3 MPa) osmotic treatment, which was inhibited by DTT. Both a moderate and a higher water deficit (-2.5 MPa) resulted in an increase of thermal stability,