TEMPERATURE-DEPENDENCY OF CHANGES IN THE RELAXATION OF ELECTROCHROMICSHIFTS, OF CHLOROPHYLL FLUORESCENCE, AND IN THE LEVELS OF MESSENGER-RNA TRANSCRIPTS IN DETACHED LEAVES FROM PISUM-SATIVUM EXPOSED TO SUPPLEMENTARY UV-B RADIATION

Detached Pisum sativum leaves were used to study temperature-dependent and temperature-independent effects of supplementary UV-B radiation o n chloroplast-associated components and properties. Temperature-indepe ndent effects were considered to be due to direct physical damage by t he UV-B radiation itself, whereas temperature-dependent damage was tho ught to be a result of UV-B-triggered chemical reactions. Supplementar y UV-B radiation caused a lowering in chlorophyll fluorescence (F-v/F- m) at both 2 and 22 degrees C (by about 20 and 2%, respectively); it i s likely to be caused by direct interaction of UV-B quanta with Photos ystem II (PS II), since no additional decrease was seen in F-v/F-m whe n leaves irradiated at 2 degrees C were subsequently incubated in dark ness for 4 h at 22 degrees C. However, chilling made PS II more prone to UV-B damage. The development of the UV-B effect on the relaxation o f the electrochromic shift (t(1/2)), caused by the trans-thylakoid mem brane potential, showed a greater temperature dependency which indicat es that UV-B induces chemical reactions that in turn cause the damage. Exposure for 4 h to supplementary UV-B radiation led to a 75% decreas e in t(1/2) at 22 degrees C, whereas the effect at 2 degrees C was muc h smaller (<50%), even smaller than the effect of the chilling itself. Leaves which were first exposed to UV-B at 2 degrees C, and then left in the dark for 4 h, displayed an additional 37% drop in t(1/2) if th e leaves were held at 22 degrees C during the dark period but only a 1 2% drop if they were held at 2 degrees C in the darkness. This indicat es that the UV-B exposure triggers changes in the thylakoid membrane a t 2 degrees C which are not fully manifested at this temperature. The lowering of lhc mRNA levels were 34% compared to the control in leaves UV-B-treated at 22 degrees C for 4 h, but only 11% when exposed at 2 degrees C. Interestingly, the mRNA levels of lhc transcripts were incr eased at 2 degrees C compared with the corresponding sample held at 22 degrees C under both control and UV-B-supplemented light regimes, e.g . a 2.5-fold increase was obtained in control leaves at 2 degrees C.