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 chlorophyll, mineral nutrient concentrations and the binding forms of Mg and Ca were investigated in current-year, one- and two-year-old needles of clonal Norway spruce (Picea abies [L.] Karst.). Six-year-old spruce plants were grown for one year in sand culture with circulating nutrient solutions containing sufficient (0.2 mt M) or limiting (0.04 mt M) concentrations of Mg. The NO₃^-/NH₄⁺-ratio in the nutrient solutions administered to the experimental trees was adjusted to 0.76 in the Mg-sufficient treatment and to 1.86, 0.76 and 0.035 in Mg-limited treatments. Mg and chlorophyll concentrations, were strongly influenced by the applied nitrogen source in current-year needles and - to a less extent - also in one-year-old needles. NH₄⁺-dominated nutrition resulted in decreased height growth and significantly lower Mg and chloropyhll concentrations in current-year and one-year-old needles compared to NO₃^--dominated nutrition. Decreases in total Mg were linearly correlated to reductions of water-soluble Mg and water-unsoluble Mg not bound to chlorophyll. Mg bound to chlorophyll, however, was only reduced, when total Mg decreased below a physiological threshold value of 2% of the total nitrogen concentration in the respective needles. Total Ca concentrations 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-bound Ca remained nearly constant, independent from changes of total Ca concentrations. Negative effects of increasing NH₄⁺ supply on concentrations of Mg and other cations in the needles can be attributed to an inhibition of cation uptake induced by ion antagonism and/or reduced root growth.