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

130 Dulai, S. et al. performed using polyethylene glycol solutions with —1.3 and -2.5 MPa osmotic potential. Longer term treatments were carried out by dessicators at given air humidity at least for 24 hours but no more than 48. Chlorophyll a Fluorescence Measurements The responses of the in vivo chlorophyll a fluorescence to temperature change were measured in dark-adapted leaves with a pulse amplitude modulation fluorometer (PAM 101-103, Walz, Effeltrich, Germany) and recorded with a potentiometric chart recorder (NE-244, EMG, Budapest, Hungary) and a computer as described by Dulai et al. (1998). The initial level (Fo) of fluorescence was excited by a weak 650-nm light beam modulated at 1.6 kHz (0.01 pmol m"

2 s"

1). The fluorescence was detected by a PIN S

1723 photodiode. The maximal fluorescence level (F

m) of the darkadapted leaves was induced by a white saturating flash (7000 pmol m 2 s"

1) of 0.8-s duration, provided by a Schott KL-1500 light source. After a lag phase of 120 s, a fluorescence transient of 15-min duration was induced by continuous actinic light (AL) of 100-1000 pmol m"

2 s"

1. To analyse the quenching mechanisms, saturation pulses were triggered at steady state fluorescence level and at given temperature values. The variables and equations for quenching analysis were determined according to van Kooten and Snel (1990). The quantum efficiency of photochemistry was calculated as AF/F

m', as described by Genty et al. (1989), and the Stern-Volmer coefficient (NPQ) was also used. Heat-induced Chlorophyll Fluorescence For the determination of the breakpoints {T

c, Fm and T

p) of F

0 vs. T or F

s vs. T curves the method of heat induction of fluorescence was applied as described by Schreiber and Berry (1977). The leaves were dark-adapted for 30 min, and then placed on the thermoelectric module. During heating from 25 °C to 60 °C at a rate of 1 °C min"

1, the temperature was monitored by a thermocouple thermometer. Heating for F

s vs. T curves was started when the photosynthesis was steady. T

c, F

m and T

p were determined from the F

0 or F

s vs. T curves. Results and Discussion The sensitivity of the photosynthetic apparatus to heat stress is closely linked to the thermal stability of PSII, which is well characterised by the critical values of the temperature dependence of the initial fluorescence level (F

0) of dark-adapted leaves (Schreiber and Berry 1977; Smillie and Nott 1979; Bilger et al. 1984). Some studies have already reported that the