Root and shoot growth in salinity-stressed alfalfa in response to nitrogen source

The influence of nitrate-nitrogen (N) or ammonium-N on shoot and root growth in salinity-stressed alfalfa was investigated under field conditions. Two alfalfa cultivars, Batini, a relatively salinity-tolerant cultivar, and Trifecta, a susceptible cultivar, were used. Seeds were sown in transparent PVC cylinders filled with loamy sand and arranged in large plastic pots embedded in the soil. Salinity treatments were 0, 3.0, 6.4, and 12.2 dSm(-1) prepared with NaCl, while the N treatments were 0, 3.0, and 6.0 mM. Shoot and root growth, as well as shoot: root ratio, were inhibited by increased salinity, inhibition being more severe in the shoot than the root. Addition of nitrate-N or ammonium-N reduced the deleterious effect of salinity, but their effectiveness did not follow a consistent pattern. Ammonium-N was superior to nitrate-N in counteracting the adverse effects of salinity in the first cut, but in the regrowth where the rate of growth was higher than the first cut, the reverse was true. Root elongation and root multiplication rate (RMR) were also significantly reduced by increased salinity. but nitrate-N was a more effective antidote in cv. Batini, while ammonium-N was more effective in cv. Trifecta.