Effects Of Nitrogen And Phosphorus On Photosynthesis And Nitrogen Assimilation Of Fraxinus Mandshurica Seedlings

Nitrogen and phosphorus are the necessary mineral nutrition to trees. They determine the growth of individual, the development of community and the productivity of ecosystem in the nature. Thus, the study and knowledge about the mechanism of adaptation and use of nutrition under different concentration of nitrogen and phosphorus is important for seedling growth, they are the basis of exertion of trees' potential capacity, improvement of forest productivity, and silviculture and management of forest.In this experiment, one-year-old seedlings of Machurian Ash (Fraxinus mandshurica) were potted in sand and treated with different nutrient solution in greenhouse. Treatment of nitrogen concentration consisted of 1, 8 and 16 mol L^-1; different phosphorus concentration included 0.125, 0.25, 0.5 and 1 mmol L^-1; and different ratio of NH₄⁺-N and NO₃^--N comprised 0:100.25:75, 50:50, 75:25 and 100:0. The seedlings were treated for two months. Then we began to sample and determine physiological indexes of photosynthesis, the content of different form nitrogen and the enzymes' activity in nitrogen assimilation of the leaves and roots. The results suggested:(1) Higher concentration of nitrogen supply can promote photosynthesis of leaves. With the increment of nitrogen concentration, the apparent quantum yield (AQY), the light-saturated photosynthetic rate and the chlorophyll content of leaves increased, while the content of soluble protein decreased. Maximal efficiency of PSII photochemistry(Fv/Fm) and efficiency of PSII photochemistry(φ PSII) was higher, but photochemical quenching(qP) and non-photochemical quenching(qNP) was lower, when nitrogen concentration was abundance.(2) Photosynthetic carbon assimilation was determined by phosphorus supply too. Low phosphorus decreased AQY, the light-saturated photosynthetic rate, the content of soluble protein and the chlorophyll content of leaves. When phosphorus concentration was 0.25 mmol L^-1, Fv/Fm and φ PSII were highest, while qP was lowest.(3) The mixing of NH₄⁺-N and NO₃^--N benefited CO₂ assimilation. When the percentage of NO₃^-N was higher, AQY, the light-saturated photosynthetic rate, and the chlorophyll content of leaves reached the highest. PSII reaction centers activity declined, when the percentage of NH₄⁺-N was higher.(4) The content of inorganic and soluble protein nitrogen in leaves was higher than those in roots under different nitrogen concentration. With the improvement of nitrogen concentration, the percentage of inorganic and soluble nitrogen in roots increased. Nitrate reductive enzyme(NR) activity of leaves decreased, but NR activity of roots increased under lower nitrogen concentration, and the reductive capacity of leaves was bigger than that in roots. The activities of glutamine synthetase (GS), NADH-glutamate synthetase (NADH-GOGAT), NADH-glutamate dehydrogenase (NADH-GDH) and aspartate aminotransferase (AAT) in leaves decreased, and the ammonia reductive capacity of roots increased, when nitrogen concentration was higher.(5) Phosphorus supply concentration can affected distribution and assimilation of nitrogen.The content of total nitrogen and inorganic nitrogen increased in the roots and leaves, while the concentration of phosphorus supply is bigger. Nitrate was mostly assimilated in roots of P-deficient seedlings, however leaves were primary organ of nitrate assimilation with replete P-supply. GS activity changed acutely in different P-supply concentration, but AAT activity in higher P-concentration increased.(6) When the percentage of nitrate supply is 75%, plants exhibited relatively lower inorganic nitrogen and soluble protein nitrogen content in leaves, but it was reverse in roots. NR activity of leaves is higher than that of roots, and it was enhanced by increasing the percentage of nitrate supply. GS activity of leaves decreased with more nitrate, but the activity of GS, NADH-GOGAT, NADH-GDH, AAT of roots increased.In general, nitrogen and phosphorus can affected photosynthetic CO₂ assimilation, and this was associated nitrogen assimilation. In leaves, the assimilation of nitrate increased with photosynthesis rate augmenting, while ammonia is opposite...