EFFECTS OF TEMPERATURE AND LIGHT INDUCTION OF CHL A FLUORESCENCE IN SITU : AN ECOPHySIOLOGICAL vIEw

The effects of PAR and temperature on induction of Chl a fluorescence were observed on leaves of 20 plant species in their natural environments. �ight affects the acceptor side of PS2. Temperature regulation of PS2 activity proceeds by affecting the RC and acceptor side of PS2. The impact of higher temperature can be attributed to greater fluidity of the thylakoid membranes. Photosynthetically active radiation and temperature under the given conditions are in highly significant positive correlation, so it is not clear whether this impact is due to individual or complementary mechanisms of PAR and/or temperature affecting photo-synthetic processes in the thylakoid membranes. Also, it is not clear whether species specificity has any significance in the plant photosynthetic response to changes of PAR and temperature. Changes of PAR and temperature during induction of Chl a fluorescence do not affect Pindex, as a parameter of total photosynthesis.


INTRODUCTION
As a process of energy conversion, photosynthesis depends on the intensity (PAR) and quality (R/ FR acclimation) of light.Photosynthetically active radiation or PAR is probably the most important variable environ-mental factor affecting plants (B j ö r k m a n , 1981).This factor varies over seasons, days, and regions.
Plants adapt to the prevailing light system: there are sun-requiring plants (heliophytes) and shaderequiring plants (sciophytes) (B j ö r k m a n , 1981).Generally, sun-requiring plants have greater intensities of photosynthesis in light with higher PAR than those observed in shade-requiring plants under these conditions (B j ö r k m a n , 1981).Besides light, temperature is another important ecophysiological factor that affects photosynthesis (B e r r y and B j ö r k m a n , 1980).Temperature affects photosynthesis through a series of enzymic reactions according to Michaelis-Menten kinetics (� d w a r d s kinetics (� d w a r d s (� d w a r d s and Wa l k e r , 1983), as in a case of photosynthetic reactions in the stroma (dark phase of photo-synthesis).Moreover, temperature affects photosyn-).Moreover, temperature affects photosyn-.Moreover, temperature affects photosynthetic processes in the chloroplast membrane (light phase of photosynthesis), which mainly depends on ), which mainly depends on the state of thylakoid lipids (� y o n s et al., 19�9�� (� y o n s et al., 19�9�� N i s h i d a and M u r a t a , 1996).Photosynthetic reactions within the membrane are related to photosynthetic processes in the stroma (� d w a r d s and Wa l k e r , 1983�� G e i g e r and S e r v a i t e s , 1994).Total photosynthesis can therefore be estimated by observation of the light phase.Measuring Chl a fluorescence is a common technique for monitoring membrane photosynthetic reactions (K r a u s e and We i s , 1991).This method is used to estimate the state of membrane reactions, as well as the level of total photosynthesis in various ecophysiological circumstances

Chl a fluorescence measurements, other measurements and statistical processing of obtained results
Induction of Chl a fluorescence induction was monitored with a Handy-P�A portable fluorometer (Hansatech, UK), which operates on the principle of non-modulated fluorescence.
This device has a software for calculation, numerical presentation, and memorization of Chl a fluorescence parameters.Parameters F 0 , Fm, Fv/Fm and Fv/F 0 were observed as defined by K r a u s e and We i s (1991), S t r a s s e r et al. (1995), and M a x w e l l and J o h n s o n (2000).The Pindex parameter (photosynthesis relative vitality index) index) ) and parameters Tfm (msec�� time taken to reach Fm, an indicator of Q A reduction rate of the PS 2 acceptor, i.e., the rate of PS 2 electron transport) and A (area bmS�� the area above the fluorescence induction curve between F 0 and Fm, measurement of size of the plastiquinone pool in PS 2 ), defined in accordance with S t r a s s e r et al. (1995), were also monitored.Photosynthetically active radiation (PAR�� µmol m -2 s -1 ) was measured with a �i-1000 light meter (�i-Cor, USA), while temperatures were measured with a BIG DIGIT Dual Thermo thermometer (measuring range from -50 o C tо +�0 o C).As light and temperature represent the energy (light and thermal) to which plants are exposed, the obtained results were, prior to statistical processing, transformed by the method of Arrhenius as modified by M a r k o v i ć et al. (1996).This transformation of results was done by logarithmization of parameters not presented as ratios (F 0 , Fm, Tfm, and A), while PAR and the absolute temperature (T�� o K) were given as reciprocal values (1/A, 1/T).Parameters defined as ratios (Fv/Fm, Fv/F 0 , and Pindex) were not transformed.Statistical processing of results (determinations of means, correlation, and regression analyses of relations of fluorescence parameters vs. 1/PAR or 1/ T) was performed in the �xcel program package (Microsoft, USA).
What is the meaning of this?Increase of PAR and temperature raises leaf energy content (Marković et al., 1996), which thermodynamically accelerates dif-s dif-different reactions in PS 2 (electron transport, the state of the acceptor side of PS 2 , etc.).Due to higher temperatures, thylakoid membrane lipids become more motile, while lower temperatures cause rigidification of these membranes (�yons et al., 19�9�� Nishida and Murata, 1996�� Marković et al., 1996�� Havaux, 1998), which affects the functioning of photosynthetic complexes in the membranes.Various modifications of the state of thylakoid lipids (changes in the role of the xanthophyll cycle: Havaux, 1998�� changes in the degree of unsaturation of fatty acids: Nishida and Murata, 1996) affect the degree of membrane reactions when temperatures are changed.Hence, temperature increase lowers the quantum yield in PS 2 , at least under the given conditions in the stated plants (Table 1, Figs. 1 and 2), which can be attributed to increased fluidity of the thylakoid membranes (Marković et al., 1996�� Havaux, 1998).Furthermore, increased temperatures change the Q A redox state (ln Tfm�� Fig. 3) and the capacity of the plastiquinone pool in PS 2 (ln A�� Fig. 4), which means that higher temperatures affect the RC and the state of the acceptor side of PS 2 .All this points to different mechanisms by which amplified light and increased temperatures affect photosynthetic processes in the thylakoids.It is our opinion that amplified light directly affects the acceptor side of PS 2 and electron transport in photosynthesis, while high temperatures possibly affect the state of lipids and thereby influence photosynthetic processes in the thylakoids, above all in the RC of PS 2 .It is not yet clear whether the mechanisms by which PAR and temperatures affect photosynthetic processes in the thylakoids are individual or complementary.In such a context, it is important to note that the Pindex parameter, as an indicator of total photosynthesis (Strasser et al., 1995), depends on processes in the RC of PS 2 (Fv/Fm, Fv/F 0 ) and does not depend on processes on the acceptor side of PS 2 (ln Tfm, ln A), at least in these plants under the given ecophysiological conditions (Table 1).It is not yet evident whether the plant species has any significance in the observed processes.Neither PAR nor temperature significantly affect Pindex (Table 1).
(B j ö r k m a n and D e m m i g , 198��� G e n t y et al., 1989�� B u r k e , 1990�� D e m m i g -A d a m s and A d a m s , 1992�� O b e r h u b e r and � d w a r d s , 1993).�cophysiological studies on effects of light and temperature on photosynthesis in situ are mostly done by so-called PAM fluorometry (M a x w e l l and J o h n s o n , 2000).The application of non-modulated fluorometry is much less present in ecophysiological studies.