| Nitrogen (N) and phosphorus (P) are the very important elements that are vital to plants. Nitrogen and phosphorus availability influence tree growth directly, furthermore, it affects the tree yield and ecosystem productivity. Nitrate N and ammonium N is two major type form of Nitrogen. Because they are different in form, the electric charge, their effect on plant growth is different. Tree growth and stand productivity are greatly related to nutrient availability in soils. This has been very important for silviculture to cultivate trees under the deficiency of nitrogen and phosphorus. It is very important to cultivate the three species in order to improve their productivity and to resolve the current crisis of short supply of wood in China.In this present dissertation, one-year-old seedlings and sowing Machurian Ash (Fraxinus mandshurica) seedlings were potted in sand and cultivated with nutrient solution in greenhouse. The seedling were grown in different nitrogen concentration(1,4, 8 and 16 mmol.L-1), different phosphorus concentration(0.125, 0.5, 1 and 2 mmol.L-1) and different NO3-:NH4+ ratios (0:100, 25:75, 50:50, 75:25, 100:0). During the seedlings cultivating, sampled monthly and determined some physiological indexes, then analyzed and studied the growth, biomass, nutrient concentration, chlorophyll content, soluble protein content, Rubisco activity, photosynthetic rate and so on. And the results suggested:(1) The nitrogen and phosphorus concentration in seedling leaf, stem and root is highly relative to the nutrient concentration supplied. Increasing nitrogen concentration supplied lead to the increase of nitrogen concentration in leaf, stem and root. When nitrogen is insufficient supplied, seedling nitrogen absorbing was influenced and the nitrogen concentration in tissue was decline. The effect of phosphorus on the nutrient concentration is not distinct. In different nitrogen form treatment, nitrogen and phosphorus concentration were the highest when the ratio of nitrate N to ammonium N is 75:25. In nitrogen treatment experiment, nitrogen concentration in leaf is different in different month. In June and July, nitrogen concentration in leaf is higher than that in September.(2) The effect of nitrogen concentration on the biosynthesis rate of 5-aminolevulinic acid (ALA) and activity of porphobilinogen (PEG) synthase were not significant. When nitrogen concentration increased, seedling leaf chlorophyll content, carotenoid and soluble protein content increased significantly. The same situation is found in soluble sugar. Leaf nitrogen concentration and soluble protein content is highest when supplied with 8 mmol-L" nitrogen. Otherwise, theircontent is insensitive to phosphorus concentration treatment. As for nitrogen form treatment, seedling soluble protein changed distinct, while not on chlorophyll content. Although Machurian Ash seedling prefer to nitrate N, the seedlings show the advantage in leaf nitrogen concentration and soluble protein content when treated with 75% nitrate N and 25% ammonium N solution.(3) Ribulose-1, 5- bisphosphate carboxylase (Rubisco) is the key enzyme of photosynthesis, its activity is highly correlated with leaf nitrogen content. In August, Rubisco initial activity and total activity increased with nitrogen concentration increasing. It is highest when supplied with 16 mmol.L-1 nitrogen. Rubisco initial activity and total activity has the same change pattern. When treated with deficiency phosphorus, the activity is low. When treated with enough phosphorus, the activity is high. Rubisco initial activity and total activity of sowing seedling is hither when grown in. mixed nitrogen form than that when grown in sole nitrogen form. Whereas, Rubisco initial activity and total activity of one-year-old seedling increased when the proportion of nitrate N increased.(4) Nitrogen concentration supplied has remarkable effect on the seedling photosynthesis. There is a strong positive correlation between the light- saturated rate of photosynthesis of a leaf and its nitrogen concentrat... |
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