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 . 129 probably more stable in light than in dark (Molnár et al. 1998). Besides, the artificially generated intrathylakoid pH gradient effects an accumulation of zeaxanthin and a parallel increase of the thermal stability of thylakoid membranes (Weis 1982, Havaux and Gruszecki 1993). All this raises the possibility that the regulating/protecting mechanisms linked to low lumen pH have a role in the protection against the harmful effects of excess light as well as of heat stress. Poikilohydric plants rapidly lose water in high light and temperature conditions, and thus a third stress factor needs to be survived here. However, during water deficit the relative water content (Bajji et al. 2000) and the activity of some photosynthetic processes decrease (Cheves 1991; Cornic 1994; Cornic and Massacci 1996; Cornic 2000; Lawror 1995) there are observations to the effect that in higher plants the slow dehydration of removed leaves resulted in an increase of the thermal stability of PS II, detected on the basis of the temperature dependence of the initial level (F 0) of chlorophyll a fluorescence (Havaux 1992). Since parallel with the increase of irradiation and leaf temperature the water content rapidly decreases in poikilohydric plants, to achieve an adequate dry matter production and growth rate, an efficient photosynthetic functioning is necessary even under such unfavourable conditions: the effect of the three stress factors needs to be tolerated at the same time, which is impossible without high thermal stability of photosynthetic functions. On the other hand, the thermal tolerance of PSII measured in darkness and at full turgor is probably not adequate for tolerating the temperatures of the original habitat, since during the daytime the leaf temperature may rise as high as 40-45 °C. This short study reports the effects on the thermal stability of the photosynthetic apparatus of increasing light intensity and decreasing water content in the dessication tolerant Homalothecium lutescens moss. Materials and Methods Plant Materials All experiments were performed on green segments of Homalothecium lutescens (Hedw.) Robins, moss. Mosses were collected from a semidry rocky biotop in NE Hungary in early summer, and were stored in the territory of a weather station under similar exposition and conditions to those in the original biotop or in a greenhouse. Before the measurements the samples were rehydrated and transferred to a growth chamber for two days at 25 °C where mosses were stored under fluorescent illumination with a 16h photoperiod of white light at a photon flux density of 100 jimol m" 2 s~ l and at 100% relative air humidity. Short-term (30 min) osmotic treatments were
