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

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 1mmol/L). The photosynthetic rates of F. mandshurica seedlings decreased as the P concentration decreased. The maximum net photosynthetic rate at the lowest P concentration (0.125mmol/L) was 61% of that at the highest P concentration (1mmol/L). The light compensation points were higher at lower P concentrations (0.125, 0.25 and 0.5mmol/L) than at the highest P concentration (1mmol/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 0.5mmol/L) than at the highest P concentration (1mmol/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 0.25mmol/L), nitrate reductase activity was lower in shoots than in roots, while the opposite trend was observed under higher P concentrations (0.5 and 1mmol/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.