EFFECT OF MAGNESIUM-DEFICIENCY ON ANTIOXIDATIVE SYSTEMS IN NEEDLES OFNORWAY SPRUCE [PICEA-ABIES (L) KARST] GROWN WITH DIFFERENT RATIOS OF NITRATE AND AMMONIUM AS NITROGEN-SOURCES

The effect of magnesium deficiency and variations of the ammonium to n itrate ratio on chlorophyll, soluble protein and antioxidative systems was investigated in current year's needles of clonal spruce trees [Pi cea abies (L.) Karst.]. The trees were grown for 1 yr in sand culture with circulating nutrient solutions containing sufficient (0.2 mM) or limiting (0.041 mM) concentrations of Mg. The nitrogen concentration o f the nutrient solutions was not varied (5 mM) but the NO3-/NH4+-ratio was adjusted to 0.76 in Mg-sufficient and to 1.86, 0.76 and 0.035 in Mg-limited plants. Under Mg-limitation, the chlorophyll, soluble prote in and Mg-content were lowest in needles of trees supplied with NH4+ a nd highest in trees supplied with NO3- as major N-source. Apoplastic p eroxidase activities were not affected by changes in nutrition. In tot al needle extracts, NADH oxidase, ascorbate peroxidase, monodehydroasc orbate reductase, glutathione reductase and ascorbate content were not , or only little, affected by changes in nutrition, except for trees g rown with the highest NH4+-concentrations. Glutathione content and gua iacol peroxidase were increased in Mg-deficient needles. Superoxide di smutase activity was decreased in Mg-limited needles of trees grown wi th NH4+ and increased in needles of trees grown with NO3- as a major N -source. Except for superoxide dismutase, the activities of antioxidan t enzymes and substrates were not correlated with the degree of needle chlorosis. Superoxide dismutase activity was low compared to enzymati c activities involved in H2O2 detoxification. Supposing that the needl es suffered from enhanced oxidative stress, these results suggest that scavenging of O-2(-) was a limiting factor in stress compensation rat her than the detoxification of H2O2.