INFLUENCES OF DIFFERENT NITRATE TO AMMONIUM RATIOS ON CHLOROSIS, CATION CONCENTRATIONS AND THE BINDING FORMS OF MG AND CA IN NEEDLES OF MG-DEFICIENT NORWAY SPRUCE

Effects of Mg deficiency and variations of the NO3-/NH4+-ratio on chlo rophyll, mineral nutrient concentrations the binding forms of Mg and C a were investigated in current-year, one- and two-year-old needles of clonal Norway spruce (Picea abies [L.] Karst.). Six-year-old spruce pl ants were grown for one year in sand culture with circulating nutrient solutions containing sufficient (0.2 mM) or limiting (0.04 mM) concen trations of Mg. The NO3-/NH4+-ratio in the nutrient solutions administ ered to the experimental trees was adjusted to 0.76 in the Mg-sufficie nt treatment and to 1.86, 0.76 and 0.035 in Mg-limited treatments. Mg and chlorophyll concentrations, were strongly influenced by the applie d nitrogen source in current-year needles and - to a less extent - als o in one-year-old needles. NH4+-dominated nutrition resulted in decrea sed height growth and significantly lower Mg and chlorophyll concentra tions in current-year and one-year-old needles compared to NO3--domina ted nutrition. Decreases in total Mg were linearly correlated to reduc tions of water-soluble Mg and water-unsoluble Mg not bound to chloroph yll. Mg bound to chlorophyll, however, was only reduced, when total Mg decreased below a physiological threshold value of 2% of the total ni trogen concentration in the respective needles. Total Ca concentration s in the needles, which were reduced by Mg deficiency especially when nutrition was NH4+-dominated, were strongly correlated to the portion of Ca bound to oxalate. The amount of water-soluble Ca and pectate-bou nd Ca remained nearly constant, independent from changes of total Ca c oncentrations. Negative effects of increasing NHS supply on concentrat ions of Mg and other cations in the needles can be attributed to an in hibition of cation uptake induced by ion antagonism and/or reduced roo t growth.