低磷胁迫对水曲柳幼苗光合速率与氮素同化的影响

Effects of phosphorus stress on photosynthesis and nitrogen assimilation of Fraxinus mandshurica seedlings

在温室内采用盆栽砂培的方法,对不同供磷水平(0.125、0.250、0.500和1.000 mmol/L)的水曲柳幼苗进行了光合速率与氮素同化的研究。结果表明,水曲柳幼苗最大净光合速率随供磷减少而降低,供磷水平最低(0.125mmol/L)时水曲柳幼苗叶片最大净光合速率为正常供磷(1 mmol/L)的61%,而低磷水平处理(0.125、0.250、0.500mmol/L)时的光补偿点均高于正常供磷处理。各供磷水平下,叶片中的全氮、硝态氮和铵态氮含量均高于根系。随供磷的减少,叶片与根系中硝态氮含量均降低,同时,低磷水平处理(0.125、0.250、0.500 mmol/L)时根系铵态氮含量均高于正常供磷(1 mmol/L),表明低磷胁迫下水曲柳偏向铵态氮的吸收。但是,叶片全氮含量随供磷水平的降低而下降。供磷水平较低时(0.125和0.25 mmol/L)叶片硝酸还原酶(NR)活性低于根系,而在供磷水平较高时(0.5和1 mmol/L)则相反。随供磷的减少,叶片和根系NR活性,以及叶片天冬氨酸转氨酶(AAT)活性均逐渐降低,而叶片谷氨酰胺合成酶(GS)活性变化较小。上述结果表明,在低磷胁迫下水曲柳幼苗氮素吸收数量和同化酶活性均降低,减少了氮素向叶片的分配,导致植株光合速率下降。

The aim of this study was to investigate the effects of phosphorus （P） deficiency on the photosynthetic rate and nitrogen assimilation of Fraxinus mandshurica seedlings. One-year-old F. mandshurica seedlings were grown in sand medium and supplied with P at four different concentrations （0. 125, 0.25, 0.5 and 1 mmol/L）. The photosynthetic rates of F. mandshurica seedlings decreased as the P concentration decreased. The maximum net photosynthetic rate at the lowest P concentration （0. 125 mmol/L） was 61% of that at the highest P concentration （1 mmol/L）. The light compensation points were higher at lower P concentrations （0. 125, 0.25 and 0.5 mmol/L） than at the highest P concentration （ 1 mmol/L）. At all four P concentrations, the concentrations of total nitrogen, nitrate, and ammonium were higher in shoots than in roots. As the P concentration decreased, the nitrate concentrations in leaves and roots also decreased, suggesting that nitrate uptake was limited under low-P conditions. The ammonium concentrations in roots were higher at low P concentrations （0. 125, 0.25 and 0.5 mmol/L） than at the highest P concentration （ 1 mmol/L）, suggesting that ammonium uptake by seedlings increased under low-P conditions. Nevertheless, the total nitrogen concentrations in leaves decreased as the P concentration decreased. Under lower P concentrations （0. 125 and 0.25 retool/L） ,nitrate reductase activity was lower in shoots than in roots, while the opposite trend was observed under higher P concentrations （0.5 and 1 mmol/L）. As the P concentration decreased, the activities of nitrate reductase in leaves and roots and aspartate aminotransferase in leaves decreased, while there were smaller changes in glutamine synthetase activity. These results suggeste that nitrogen uptake, the activities of nitrogen assimilation enzymes, and nitrogen allocation to leaves of F. mandshurica seedlings decrease under low-P conditions, resulting in a decreased photosynthetic rate.