Effect of nitrogen source on growth response to salinity stress in maize and wheat

The effect of ammonium and nitrate nutrition on maize and wheat grown hydroponically and salinity stressed was assessed from measurements of growth rate and gas exchange. In both maize and wheat the ammonium-grown plains were much more sensitive to salinity toxicity than nitrate-grown plants particularly when exposed to 60-80 mM salinity. Shoot growth was retarded to a far greater extent than root growth in salinity-stressed plants of both wheat and maize with either nitrogen source. There was no significant decrease of photosynthetic rate in salinity-stressed plants of either species fed nitrate or ammonium, except in severely wilted plants of both species fed nitrate or ammonium at the highest (80 mM) salinity concentration. The same was true for stomatal conductance, transpiration rate and transpiration ratio (water use efficiency). In nitrate-fed wheat, raising the calcium concentration from 2 to 12 mM in the presence of 60 mM salinity produced an 11% increase in growth. This effect is ascribed to improved nitrate uptake due to calcium protection of the nitrate transporter and was not evident in ammonium-grown wheat. Possible reasons for the differential effects of ammonium and nitrate nutrition are discussed.