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

Thermal Stability Changes of PS II in a Dessication . 135 20 25 30 35 40 45 50 55 60 T(° C) Fig. 4 Effect of decrease of relative water content (RWC) on the temperature dependence of non-photochemical quenching (NPQ) for green segments of Homalothecium lutescens at 100 pmol m' s

1 AL intensity (100%RWC, triangles; 98% RWC, squares; 90% RWC, circles). The results are means of data from five independent measurements on different segments from different plants. As a result of short osmotic treatments, in samples with a steady-state photosynthesis at 100 pmol m"

2 s"

1 AL intensity the critical values of the F

s vs.T curves were shifted significantly higher, compared to the 100% relative water content control (Table 3), and contrary to some results concerning pea (Dulai et al. unpublished data), this increase of heat tolerance prevails under longer water deficit (Figure 2). This enhanced thermal tolerance could have been further increased by the increase of AL (Figure 1). Besides, at given AL intensity greater water deficit also shifts upwards the T

c and T

p values of the F

s vs. T curves, indicating the increased thermal stability of PS II (Figure 1, Table 3). This is also manifested by the temperature dependence of the effective quantum yield of PSII: with the increase of water deficit, the AF/Fm values start decreasing drastically at higher temperatures (Figure 3). Previous studies have also shown that slow dehydration of intact leaves causes an increase of the thermal stability of PS II, based on the F

0 vs. T curves (in darkness), intensified by the strong pre-illumination (Havaux 1992). Contrary to these results, the rapid osmotic treatment or a longer water deficit at growth light intensity did not cause a significant change in