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Effects of magnesium deficiency and variation in nitrate to ammonium ratio on needle histology and chlorophyll concentration were investigated in current-year and one-year-old needles of clonal Norway spruce trees ( Picea abies  (L.) Karst.). Six-year-old trees were grown for one year in sand culture with circulating nutrient solutions containing a sufficient (0.2 mM) or a limiting (0.04 mM) concentration of Mg. The nitrogen concentration was not varied (5 mM), but the NO 3  − /NH 4  + -ratio was adjusted to 0.76 in Mg-sufficient and to 1.86, 0.76 or 0.035 in Mg-limited plants. Visible symptoms of Mg deficiency occurred only in current-year needles, indicating adequate Mg nutrition before the experiment. Under conditions of Mg limitation, chlorophyll and Mg concentrations were lowest in needles of trees supplied with NH 4  +  as the major nitrogen source and highest in needles of trees supplied with NO 3  −  as the major nitrogen source. In current-year and one-year-old needles, starch accumulation induced by Mg deficiency was increased when NH 4  +  was the major nitrogen source. The accumulation of tannin spherules in current-year needles, which occurred in response to Mg deficiency, also increased with decreasing NO 3  − /NH 4  + -ratios. Deficient Mg supply caused premature aging in tissues of the vascular bundle, as indicated by modifications of the cambium and increased amounts of collapsed sieve cells. The number of collapsed sieve cells was slightly lower in needles grown in a NH 4  + -dominated nutrient regime than in needles grown in a NO 3  − -dominated nutrient regime. We conclude that was not directly toxic to Norway spruce trees at the applied concentrations. However, effects of Mg deficiency were considerably greater in an NH 4  + -dominated nutrient regime than in a NO 3  − -dominated nutrient regime.

Histology of magnesium-deficient Norway spruce needles influenced by nitrogen source