Effect of NH₄⁺-N and NO₃^–-N on plant growth and root respiration of pea seedlings under salt stress

Objectives Soil salinization is one of abiotic factors affecting agricultural production in arid and semi-arid irrigated areas, and the areas of salinization have been gradually expanded in recent years. Reasonable controlling measures can alleviate damage of salinization to plants. Therefore, this study researched the physiological and ecological mechanism of nitrogen sources regulating the salt stress of legume plants.

Methods A sand culture experiment was conducted with three pea cultivars of ‘Yinwan No.1’, ‘S5001-1’ and ‘Ha’ as tested materials. Three salt stress levels of NaCl 0, 50 and 100 mmol/L were set up, NH₄⁺-N and NO₃^–-N were supplied separately in the set of salt treatment. The biomass, root growth parameters, root respiration and salt ion contents of pea seedlings were measured at 29 days after transplanting.

Results 1) Among the three NaCl stress treatments, the biomass of pea seedlings, the root dry matter, root length and root surface area for the three cultivars were all highest under 50 mmol/L NaCl treatment. The effects of nitrate-N were significantly better than those of ammonium-N. 2) Compared with no salt stress, the water contents of pea seedlings for all three varieties were significantly lower under 100 mmol/L NaCl treatment, and those with nitrate-N supply were significantly lower than with NH₄⁺-N. 3) The root respiration rates of three varieties decreased with the increase of salt concentration and plant growth period. The root respiration rates of the pea cultivars responded differently to the two N forms. Those of ‘Yinwan No.1’ in NH₄⁺-N treatments were higher than in NO₃^–-N treatments at the same salt concentration, ‘Ha’ was opposite, and ‘S5001-1’ was similar. Under 50 mmol/L NaCl stress, the cumulative root respiration of both ‘Ha’ and ‘S5001-1’ in the NO₃^–-N treatments were significantly higher than those in the NH₄⁺-N treatments, and those of ‘Yinwan No.1’ were on the contrary. Under 100 mmol/L NaCl stress, the cumulative root respiration in ‘Ha’ were significantly higher in the NO₃^–-N treatments than in NH₄⁺-N treatments, there was no significant differences in other two varieties. 4) The contents of Na⁺ and Cl^– of pea seedlings were increased with increase of salt concentrations. The effect of nitrogen sources on Na⁺ accumulation in pea seedlings depended on different pea cultivars.

Conclusions Under moderate salt stress, the alleviation ability of NO₃^–-N on the growth of pea seedlings is higher than that of NH₄⁺-N. Under severe salinity stress, the alleviation of nitrogen sources becomes less effective. Although the response differences of root respiration to NH₄⁺-N and NO₃^–-N among different pea cultivars, NO₃^–-N performs better than NH₄⁺-N in alleviating salt stress in general.