CHILLING-RESISTANCE OF PHOTOSYNTHETIC PERFORMANCES IN DIURON-ADAPTED EUGLENA-GRACILIS

Photosynthetic oxygen evolution and various PSII fluorescence induction parameters were analysed to study the chilling-resistance of sensitive, Z, and diuron-adapted Euglena cells [ZR(25) adapted to 25-mu-M and ZR(250) adapted to 250-mu-M]. The temperatures assayed ranged from 5 to 45-degrees-C, the cells being grown at 25-degrees-C. The ZR(25), and even more so the ZR(250) cells, displayed shifts to lower temperatures of the breakpoints characterizing the changes of either oxygen evolved, or ratios of variable to constant fluorescence (F(v)/F(o)). The phenomena of chlorophyll redistribution, as a function of the temperature, were accompanied by modifications of the antenna sizes. The breakpoints in the changes of the different fluorescence parameters might correspond to critical temperatures for the lipid phase behaviour within the chloroplast membranes. As a matter of fact when compared with the control Z cells, the thy-lakoids of cells resisting to 25 and 250-mu-M had been shown (Troton et al., Phytochemistry, 25 (1986) 393-399) to contain, on a per chlorophyll basis, lower proportion of MGDG to the benefit of DGDG, and lower proportions of the sum 16:0 + 16:1-trans in phosphatidylglycerol. Such differences might be correlated to the observed chilling-resistance of the diuron-adapted cells.