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 . 131 photosynthetic apparatus of pre-illuminated leaves reveals higher thermal stability than the untreated control (Havaux and Gruszeczki 1993). It has also been reported that in higher plants slow, continuous dehydration increases the thermal stability of PSII (Havaux 1992). In the original habitat of H. lutescens the temperature often rises to about 45 °C, which is always coupled with high irradiation. The data in Table 1 show that the heat tolerance of PSII determined on the basis of the F

0 us. T curves is not sufficient for tolerating such high temperatures. However, since the recording of the temperature dependence of F

0 is done in dark, it is inadequate for determining the thermal tolerance of samples in a lightadapted state at a steady-state photosynthesis level. Similarly to FQ, the temperature dependence of F

S is also biphasic, and the breakpoints of the curve - according to recent results - appropriately show the thermal stability of samples with a steady-state photosynthesis level (Molnár et al. 1998). In the energized state of tylakoids increasing the intensity of actinic light, compared with the T

c and T

p values of the F

0 vs.T curves (recorded in darkness), the same values of the F

S vs.T curves are shifted towards significantly higher temperatures, indicating the higher thermal tolerance of PSII (Table 1). This shift was inhibited by DTT, and in the untreated samples, parallel with the upwards shift of the critical values, the effective quantum yield values also indicated a decreased heat sensitivity (not shown by data). In connection with this, in higher plants a close connection has been found between the activity of photoprotective mechanisms and the thermal stability of PSII (Molnár et al. 1998). The critical values of the FO vs.T curves of untreated leaves and of leaves which received a 30 min. -1.3, -2.5 MPa osmotic treatment did not show a significant difference. Similarly, in dark-adapted state a longer water deficit does not effect an increase of thermal stability, either (Table 2 and Figures 1, 2). Table 1 The breakpoints of the F

0 vs. T (in darkness) and of the F

s vs. T curves at different actinic light intensities in green segments of Homalothecinm lutescens at 100% relative water content (RWC). Heating for the Fq VS. T curves was started after 30 min dark relaxation andfor the F

s vs. T curves was started when the photosynthesis was steady at given AL intensity. The results are means of data from five independent measurements on different segments from different plants. Treatment T 1 c T 1 P F1/2 Control (dark, F

0-T) 41.2±0.86 46.1±2.06 43.2±1.6 100 nmol m*

2 s"

1 45.2±0.52 52.

1±1.47 47.3±0.87 400 |imol m"

2 s"

1 46.3±0.39 52.3+1.16 48.5±0.94 1000 jimol rn

2 s"

1 46.6±0.49 54.3±0.94 49.3±1.07