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.     

Seasonal development of the photosynthetic performance of Norway spruce (Picea abies [L.] Karst.) under magnesium deficiency

In order to investigate the influence of different magnesium nutrition on photosynthesis, one hundred 6-year-old spruce trees derived from one clone were planted in October 1990 into a special out-door experimental construction, where they were cultivated in sand culture with an optimal supply of nutrients, except magnesium, via circulating nutrient solutions. Magnesium was added to the nutrient solutions in three different concentrations, varying from optimal to severe deficient supplies. During the first vegetative period in 1991, photosynthetic performance and carboxylation efficiency were measured under saturating light, controlled CO₂ conditions, optimal temperature and humidity, using a minicuvette system.During summer, the trees under moderate magnesium deficiency developed tip yellowing symptoms on older needles, while the youngest needles remained green with unchanged chlorophyll contents. Trees under severe magnesium deficiency showed yellowing symptoms on all needle age classes combined with decreased chlorophyll contents in the youngest needles as well. In comparison with the controls, the photosynthetic performance of the 1-year-old needles was significantly lower in both deficiency treatments. The same was observed in the youngest needles of the trees under severe deficiency. Trees under moderate deficiency treatment decreased in photosynthetic performance during the summer without reduction of chlorophyll contents. The reduction of photosynthetic rates corresponded to a decrease in carboxylation efficiency, which is taken as a measure of the activity of the enzyme ribulose-1,5-bisphosphate carboxylase. This reduction, together with the observed increase of carbohydrate contents in needles of trees growing under magnesium deficiency, led to the assumption that the photosynthetic carbonfixation is reduced as a consequence of the accumulation of carbohydrates.     

Magnesium deficiency treatment causes reductions in photosynthesis of well-nourished Norway spruce

In order to investigate effects of magnesium deficiency on Norway spruce [Picea abies (L.) Karst.] photosynthesis, 100 well-nourished 5-year-old spruce trees were grown in sand culture, individually supplied with circulating nutrient solutions. Mineral nutrients were added to the nutrient solutions in optimal quantities and optimal relations to nitrogen. Magnesium was supplied at 0.203, 0.041 and 0.005 mM in order to simulate optimal nutrition, moderate deficiency and severe deficiency. Parameters of photosynthetic gas exchange, chlorophyll, magnesium and starch concentrations were determined in current-year and 1-year-old needles during one growing season. By mid May — 6 months after onset of the Mg deficiency treatments in late autumn — CO₂-assimilation rates of 1-year-old needles were significantly decreased independent of the severity of the deficiency treatment, whereas the chlorophyll concentrations did not differ from the controls. The occurrence of yellowing symptoms during July did not further influence the Mg deficiency effect on photosynthesis. In contrast to 1-year-old needles, significant reductions of photosynthesis and chlorophyll in current-year needles were only caused by severely deficient Mg supply. Mg deficiency affected carboxylation efficiency but not light use efficiency. From the accumulation of starch in the needles, up to 30-fold of the controls, the conclusion has been drawn that reactions of CO₂-fixation were affected by reduced carbohydrate export. The light-dependent pigment reduction, leading to the typical tipyellowing of needles, clearly reflects a secondary effect of Mg deficiency.