Effects Of Nitrogen, Calcium, And Light On Glutamine Synthetase And Nitrogen Assimilation In Wheat

Nitrogen is a dominant factor for crops' production. How to improve the crops' utilization efficiency of nitrogen is a question on which scientists pay much concern. Light, as an important ecological factor, and calcium, as an essential element and a second messenger, have significant influence on nitrogen metabolism in plants, but the mechanism that light and calcium exert their regulation on plant nitrogen metabolism is now unclear. In this paper, solution culture and soil culture were conducted to study the effects of calcium on absorption and assimilation of nitrogen in wheat seedlings, the relationship between calmodulin-regulatory system and primary nitrogen assimilation in wheat, the effects of exogenous nitrogen, calcium, and tight on the activities of glutamine synthetase (GS) and its isoenzymes in leaves and roots of wheat, the effects of calcium on the ear characteristics and content of nitrogen of wheat grains in acidic Ca-deficient soil, and the relationship between the mobilization of nitrogen in flag leaves and GS isoenzymes during grain filling in wheat, in order to provide theoretical and technical instructions to reasonable application of calcium-fertilizer in Ca-deficient soil to improve the utilization efficiency of nitrogenous fertilizer, grain yield and quality in cereals. The main results are as follows:1. Calcium accelerated the absorption and assimilation of nitrate in wheat seedlings. Under the condition of 5mmol/L nitrate-N, the suitable calcium concentration was Immol/L, and 4mmol/L Ca2+ should be supplied to nutrient solution under the level of 25mmol/L nitrate-N. So the suitable ratio of nitrate-N to calcium was about 5 to 7 in nutrient solution for wheat. Under the suitable ratio of nitrate/calcium, calcium enhanced the absorption of nitrate in wheat, promoted the activities of nitrate reductase (NR) and glutamine synthetase (GS) in leaves and roots, accelerated the transformation of nitrate to protein, and increased the total nitrogen and dry matter accumulation in shoots and roots of wheat.2. Wheat is an nitrate-philic plant, but calcium has no promotion on the absorption and assimilation of ammonium in wheat.3. Calcium chelator EGTA or calmodulin antagonist chlorpromazine (CPZ) suppressed the absorption of nitrate in wheat, decreased the activities of NR and GS, inhibited the transformation of nitrate and/or amino-nitrogen to protein, and reduced the dry matter accumulation of shoots and roots in wheat.4. Exogenous nitrate, especially ammonium, significantly enhanced the activity of GS2 in leavesof wheat, induced the de novo synthesis of GS isoenzyme-GS_rb, in roots. The ratio of synthetase to transferase of GS_rb was larger than that of GS_ra, but GS_rb was more unstable in vitro than GS_ra. GS_ra and GS_rb, had almost the same mobility in PAGE, and the same subunit molecular weight in SDS-PAGE. GS_rb, an enzyme only induced by exogenous nitrogen, probably played an important role in primary assimilation of nitrogen in roots.5. The synthesis of GS₁, especially GS₂ was stimulated by white light, and light mainly regulated the synthesis of GS on the protein translational level during a relatively short period of light-induction. Calcium increased the activities of GS₂ and GS_rb in leaves and roots of wheat, respectively.6. The results that the synthesis of GS₁ and GS₂ was evidently inhibited by cycloheximide (CHI) indicated that GS₁ and GS₂ polypeptides were synthesized in cytoplasm. Lincomycin (LCM) didn't influence the activity of GS₁, but strongly suppressed the activity of GS₂ during white light-induction. This implied that the active holoenzymes of GS₂ were assembled in chloroplast.7. Red light (R) activated the activity of GS in etiolated wheat leaves, and far-red-light (FR) partly reversed the effect of red-light activation of GS. This suggested that phytochrome involved in the process of light-activation of GS in leaves. Triflupromazine (TFP), a calmodulin antagonist,decreased the effects of red-light on activation of GS. The...