高等植物对有机氮吸收与利用研究进展

Advances in Study of Plant Organic Nitrogen Nutrition

主要综述植物氨基酸营养生理生化和分子生物学研究方面的最新进展。长期以来 ,人们一直认为植物只能吸收无机态 N,有机 N必须矿化为无机 N后才能被植物吸收利用 ,而近年来越来越多实验证明植物能吸收有机 N,特别是氨基酸 ,其吸收能力因植物种类而异。生长在有机 N丰富的北极、高山和亚高山生态环境中的植物甚至嗜好氨基酸。因此 ,不应过分夸大有机 N矿化的重要性。迄今一些植物细胞质膜上的氨基酸转运子基因已被描述并加以克隆。

Traditional models of nutrient cycling assume that soil microorganisms must decompose organic matter, releasing inorganic N, before that N becomes available for plant uptake. But, there is a growing evidences that plant can take up organic N. In the moist tundra of arctic tundra, most wetland are poor inorganic, plant-available nutrients because mineralization is restricted due to low temperatures and anoxic soils. But tundra soils have higher concentrations of water-extractable free amino acids than of inorganic N. Two tundra sedges, Eriophorum vaginatum and Carex aquatilis, take up amino acids at least as rapidly as they take up NH + 4 over a range of concentrations, and compete well for glycine and aspartate N relative to NH + 4. Eriophorum vaginatum, a non-mycorrhizal sedge that dominantes moist upland tundra throughout the circumpolar Arctic, can absorb free amino acids, accounting for at least 60% of the nitrogen absorbed by this species in the field, and grow on them as its sole N source. So the dynamics of labile organic, rather than of inorganic N, appears to be the critical component of the tundra N cycle, at least in terms of controlling plant uptake and growth. Alpine and arctic ecosystems are similar in that N mineralization rates are heavily constrained by climate, and plant N demands cannot be met through the uptake of inorganic ions. In the ecosystems where sedge commonly occurs, amino acids were present in the soil pore water, but in highly variable amounts. Amino acid concentrations in soil pore water are 13 ~ 15 μmol/L in alpine dry meadow sits and 15 ~ 20 μmol/L in a subalpine fen habitat. The alpine and subalpine Cyperaceae species exhibited higher rates of glycine uptake relative to NH + 4 and NO - 3 uptake, compared to species from the more temperate habitats. A alpine sedge (Kobresia myosuroides) lacks the ability to take up NH + 4. This may reflect specialization toward the uptake of organic N in the alpine and subalpine species. Mycorrhizae can enhance the capacity of the plant to absorb amino acids. Deciduous shrubs, which are ectomycorrhizal, have the highest rates of amino acid absorption (particularly of glycine); evergreen species with ericoid mycorrhizae are intermediate; and graminoids, which are largely nonmycorrhizal, tend to have low rates of amino acid absorption. Mycorrhizal endophyte associated with ericaceous species can absorb and then releases free amino acids that are subsequently taken up by the host plant without ammonification suggests that plants take a more active role in nutrient acquisition and element cycling. At concentrations of free amino acids in arctic tundra, these plant's uptake rates of the three amino acids (glycine, aspartatic acid, and glutamic acid) together may account for between 10 and 82% of the total N uptake in the field, depending on species. The dominant plant species in the boreal forest where climate is similar to that in arctic, irrespective of their type of mycoorrhiza, all compete well for, and use, glycine as a N source. At least, 91, 64 and 42% of the N from the absorbed glycine was taken up in intact glycine by the dwarf shrub Vaccinium myrtillus, the grass Deschampsia flexuosa and the trees Pinus sylvestris and Picea abies, respectively. Rates of glycine uptake of these plants were similar to those of NH + 4. In agricultural systems, organic N potentially is important for plant N acquisition. Agricultural plants are also shown to absorb amino acids in laboratory studies and in the field. Four agricultural plants (Phleum pratense, Trifolium hybridum, T. pratense, and Ranunculus acris) can take up glycine in the form of intact amino acid. A minimum of 19%~23% of the glycine-derived N is take up as intact amino acid by these species. Rice also can take up glycine-N in sterile sand-culture and the contribution of glycine-N uptake to the plant's N budget is dependent largely on the relative levels of glycine-N in a mixture solution. The contribution of glycine-N increases with increasing the level of glycing-N an