HISTOLOGY OF MAGNESIUM-DEFICIENT NORWAY SPRUCE NEEDLES INFLUENCED BY NITROGEN-SOURCE

Effects of magnesium deficiency and variation in nitrate to ammonium r atio on needle histology and chlorophyll concentration were investigat ed in current-year and one-year-old needles of clonal Norway spruce tr ees (Picea abies (L.) Karst.). Six-year-old trees were grown for one y ear in sand culture with circulating nutrient solutions containing a s ufficient (0.2 mM) or a limiting (0.04 mM) concentration of Mg. The ni trogen concentration was not varied (5 mM), but the NO3-/NH4+-ratio wa s adjusted to 0.76 in Mg-sufficient and to 1.86, 0.76 or 0.035 in Mg-l imited plants. Visible symptoms of Mg deficiency occurred only in curr ent-year needles, indicating adequate Mg nutrition before the experime nt. Under conditions of Mg limitation, chlorophyll and Mg concentratio ns were lowest in needles of trees supplied with NH4+ as the major nit rogen source and highest in needles of trees supplied with NO3- as the major nitrogen source. In current-year and one-year-old needles, star ch accumulation induced by Mg deficiency was increased when NH4+ was t he major nitrogen source. The accumulation of tannin spherules in curr ent-year needles, which occurred in response to Mg deficiency, also in creased with decreasing NO3-/NH4+-ratios. Deficient Mg supply caused p remature aging in tissues of the vascular bundle, as indicated by modi fications of the cambium and increased amounts of collapsed sieve cell s. The number of collapsed sieve cells was slightly lower in needles g rown in a NH4+-dominated nutrient regime than in needles grown in a NO 3--dominated nutrient regime. We conclude that NH4+ was not directly t oxic to Norway spruce trees at the applied concentrations. However, ef fects of Mg deficiency were considerably greater in an NH4+-dominated nutrient regime than in a NO3--dominated nutrient regime.