Effect of Nitrate on Root Development and Nitrogen Uptake of Suaeda physophora Under NaCl Salinity

The effects of NaCl salinity and NO−3 on growth, root morphology, and nitrogen uptake of a halophyte Suaeda physophora were evaluated in a factorial experiment with four concentrations of NaCl (1, 150, 300, and 450 mmol L−1) and three NO−3 levels (0.05, 5, and 10 mmol L−1) in solution culture for 30 d. Addition of NO−3 at 10 mmol −1 significantly improved the shoot (P < 0.001) and root (P < 0.001) growth and the promotive effect of NO−3 was more pronounced on root dry weight despite the high NaCl concentration in the culture solution, leading to a significant increase in the root:shoot ratio (P < 0.01). Lateral root length, but not primary root length, considerably increased with increasing NaCl salinity and NO−3 levels (P < 0.001), implying that Na+ and NO−3 in the culture solution simultaneously stimulated lateral root growth. Concentrations of Na+ in plant tissues were also significantly increased by higher NaCl treatments (P < 0.001). At 10 mmol L−1 NO−3, the concentrations of NO−3 and total nitrogen and nitrate reductase activities in the roots were remarkably reduced by increasing salinity (P < 0.001), but were unaffected in the shoots. The results indicated that the fine lateral root development and effective nitrogen uptake of the shoots might contribute to high salt tolerance of S. physophora under adequate NO−3 supply.