EFFECT OF MAGNESIUM-DEFICIENCY ON ANTIOXIDATIVE SYSTEMS IN NEEDLES OF NORWAY 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 nitrate ratio on chlorophyll, soluble protein and antioxidative systems was investigated in current year's needles of clonal spruce trees [Picea 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 of 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 protein and Mg-content were lowest in needles of trees supplied with NH4+ and highest in trees supplied with NO3- as major N-source. Apoplastic peroxidase activities were not affected by changes in nutrition. In total needle extracts, NADH oxidase, ascorbate peroxidase, monodehydroascorbate reductase, glutathione reductase and ascorbate content were not, or only little, affected by changes in nutrition, except for trees grown with the highest NH4+-concentrations. Glutathione content and guaiacol peroxidase were increased in Mg-deficient needles. Superoxide dismutase activity was decreased in Mg-limited needles of trees grown with NH4+ and increased in needles of trees grown with NO3- as a major N-source. Except for superoxide dismutase, the activities of antioxidant enzymes and substrates were not correlated with the degree of needle chlorosis. Superoxide dismutase activity was low compared to enzymatic activities involved in H2O2 detoxification. Supposing that the needles suffered from enhanced oxidative stress, these results suggest that scavenging of O-2(-) was a limiting factor in stress compensation rather than the detoxification of H2O2.