| Nitrogen and phosphorus are the very important elements that are vital to plants. In plants, 'many chemicals that contain nitrogen and/or phosphorus, such as proteins, phospholipids and nucleotides, play very important roles in physiology and biochemical functions. The deficiency of nitrogen and phosphorus in plants must induce the impedance of growth and development, and reduce plant products. Plants get nitrogen and phosphorus from soils by the activities of roots under natural conditions. Therefore, the environmental supplies of nitrogen and phosphorus to plants affect plant products to a great extent. However, plants adapt to environmental stresses by many mechanisms, ensuring their normal growth and development. Studies on the adaptive mechanisms to nutrition environment are of significance to plant physiology and practice.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. Betula alnoides and Pinus kesiya are two kinds of important woody species for industry in southwestern China. At present, the soils in the distribution regions of B. alnoides and P. kesiya are red soil, brown soil and yellow mountain soil. Nitrogen and phosphorus is deficient in these soils. The two species have been adaptive to the deficiency of nitrogen and phosphorus because of their long habit. Sequoia sempervirens, coming from USA, is not native to southwestern China. Its adaptation to deficiency of nitrogen and phosphorus in soils of southwestern China is still unclear. 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, experiments were carried out in greenhouse and in field. In greenhouse, one-year-old seedlings were potted in sand. The nutrient solution was prepared as Hoagland receipt. The concentration gradients were designed as follow: 0%, 25%, 50%, 75%, 100%, 150%, 200%, 300%, 100% as control. Five different ratios of NO3-/NH4+ were 100:0, 75:25, 50:50, 25:75, and 0:100. In field experiments, experimental materials were one-year-old seedlings planted in harvest areas: 1.5X1.5 m for Betula alnoides and Pinus kesiya, and 2 X 2 m for S. sempervirens. Then all indexes were determined from sample. The results suggested:(1) When seedlings were supplied with deficient nitrogen and phosphorus, growth of seedlings was slow; When supply of nitrogen and phosphorus to seedlings was over the metabolic and physiological capacity, oversupply of nitrogen and phosphorus did harm to seedlings. According to the results of the experiments under different levels of nitrogen and phosphorus, when the seedlings of B .alnoides, P. kesiya, and S sempervirens were supplied with normal levels of nitrogen and phosphorus, they grew well; when they were oversupplied with nitrogen and phosphorus or supplied with less nitrogen and phosphorus, they grewslowly.(2) When the seedlings of the three species grew under the conditions of deficient nitrogen, partitioning of biomass to roots was more in order to increase root growth. Biomass partitioning to roots was less when the seedlings of B.alnoides and S sempervirens were oversupplied with nitrogen, but biomass partitioning of seedlings of P. kesiya hardly changed. Biomass partitioning to roots increased when the seedlings of the three species were supplied with less phosphorus. Oversupply of phosphorus caused relative deficiency of nitrogen, leading to increase of biomass partitioning to roots. In field experiments, there were positive relationships between biomass of roots, stems and leaves with their own contents of nitrogen and phosphorus, but there was an exception of biomass of B. alnoides roots.(3) When the seedlings of the three species were supplied with less nutrients, these seedlings changed below-ground and above-ground structures by regulating partitioning of photosynthates between them, and utilize...
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