TEMPERATURE EFFECTS ON CHLOROPHYLL FLUORESCENCE INDUCTION IN TOMATO

Chlorophyll fluorescence induction of tomato leaf discs was measured at a low actinic light intensity of 10 mu mol.m(-2).s(-1) and at decreasing temperatures from 30 degrees to 0 degrees C. F-o remained constant within the temperature range assessed. In contrast, the peak of fluorescence induction, F-p, showed a strong temperature dependence. F-p increased by lowering the temperature within two temperature regions, from 30 degrees to 22 degrees C and from 14 degrees to 0 degrees C, respectively. F-p was hardly affected by temperatures between 14 degrees and 22 degrees C. Consequently, two temperature breakpoints of F-p were observed at 14 degrees and 22 degrees C.F-p reflected a maximum reduction state of the primary electron acceptor of photosystem II, Q(A). The relation between F-p and maximal Q(A) reduction was used to explain the observed temperature effects on F-p. The reduction state of Q(A) depended on both photosystem I oxidizing and photosystem II reducing activity. Consequently, breaks in the temperature response of F-p were attributed to dissimilar effects of temperature on photosystem I and photosystem II functioning.The steep increase of F-p below 14 degrees C was attributed to a low temperature induced impaired electron transport. This was sustained by the absence of a low temperature break when electron transport was inhibited by DCMU. A limitation of Q(A) oxidizing activity below 14 degrees C could be deduced from the temperature dependence of F-p of control discs.The second break in the temperature response of F-p at 22 degrees C was not caused by temperature effects on electron transport. The break of F-p at 22 degrees C was also found for F-m of electron transport inhibited leaf discs. While F-m decreased above 22 degrees C, the ratio of F-m determined at 687 and 730 nm (F(m)730/F(m)687) increased. This indicates a redistribution of excitation energy between the photosystems.Kinetic analysis of fluorescence induction traces showed a temperature effect on PSII heterogeneity. From 22 degrees to 30 degrees C, the PSII alpha fraction decreased from 70 to 50 % while the PSII beta fraction increased from 35 to 50 %. These results also suggest a temperature induced redistribution of excitation energy above 22 degrees C in favour of PSI. Implications of temperature effects on thylakoid membrane associated processes and plant functioning are discussed.