NITROGEN DEPENDENT CHANGES IN ANTIOXIDANT SYSTEM AND IN FATTY-ACID COMPOSITION OF CHLOROPLAST MEMBRANES FROM COFFEA-ARABICA L. PLANTS SUBMITTED TO HIGH IRRADIANCE

In the present work we investigated the contribution of N availability to the changes in some antioxidant systems and in fatty acid composit ion of chloroplast membranes. in order to evaluate their role in the h igh light acclimation in coffee plants. Young coffee (Coffea ici arabi ca L. cv. Catuai) plants, grown under low irradiance (up to 150 mu mol m(-2) s(-1)) with high (2N), medium (1N) and low (0N) N availability, were exposed to high irradiance (up to 1500 mu mol m(-2) s(-1)) for a maximal period of 15 days. Changes in several parameters were monitor ed during that period in the two top pairs of mature leaves. The forma tion rate of superoxide (O-2(.-)) estimated by ESR (K-f) did not diffe r among N-treatments before the onset of high light, but increased 70 and 30% in 0N and 1N plants, respectively, and decreased 33%, in 2N pl ants by the end of the stress. By this time, the chloroplastic activit ies of Cu,Zn-superoxide dismutase (Cu,Zn-SOD), ascorbate peroxidase (A Per) and glutathione reductase (GRed) increased, respectively, 18, 225 and 138% in 2N plants. The ON plants presented a 120%:, increase in C u,Zn-SOD activity (which agrees with the higher k(f) of O-2(.-)) and a 68% decrease in APer activity, suggesting an increase in H2O2 levels, while GRed activity decreased 20%. The total carotenoid content was h igher in 2N plants and increased (about 20%) only in these plants by t he end of stress. After 12 days of high irradiance, the total fatty ac id (TFA) content increased about 30% in 2N and 1N plants. but did not significantly change in 0N plants. In 2N and 1N plants a preferential synthesis of palmitic acid (16:0) and a decrease in the linolenic acid (18:3) percentage caused a decrease in the unsaturation level, which may have made the chloroplast membranes less susceptible to peroxidati on. The N dependent changes observed in 2N (and tu a lesser extent in 1N) plants could represent adaptive features under the new high irradi ance conditions, which increased the protection of the chloroplast str uctures against photooxidative stress. (C) 1998 Elsevier Science Irela nd Ltd. All rights reserved.