TEMPERATURE-DEPENDENCE OF CHLOROPHYLL FLUORESCENCE PARAMETERS OF PEA-SEEDLINGS

The thermosensitivity of the photosynthetic apparatus of 12 day-old seedlings of Pisum sativum cv. Ran was investigated within the temperature range 2 - 50-degrees-C at intervals of 2-degrees-C. The parameters of prompt chlorophyll fluorescence were used as a criteria of photosynthetic capacity, measured after 5 min temperature treatment (simultaneously with dark adaptation) at the respective temperature. It was established that in the temperature interval 2 - 42-degrees-C the ground fluorescence (F(o)) did not change significantly, but above 42-degrees-C it increased dramatically. This gave us reason to suppose that temperatures higher than 44-degrees-C induced irreversible injuring of pea thylakoid membranes. The variable fluorescence (F(v)) remained unchanged from 2-degrees-C to 20-degrees-C, after 22-degrees-C started to decrease first monotonously, then sharply and at 50-degrees-C it comprised only 6 % of its initial level. It was also shown that in the temperature range 2-30-degrees-C the ratio F(v)/F(m), considered as a measure of PS2 activity, did not change significantly, but began to decrease at higher temperatures. After 42-degrees-C its level strongly decreased and at 50-degrees-C it was only about 10 % of the initial value of F(v)/F(m). Similar changes were found for the F(v)/F(o) ratio. It is concluded that the physiological state and capacity of PS2 for electron transport do not changes significantly in a wide temperature range.