Pea responses to saline stress is affected by the source of nitrogen nutrition (ammonium or nitrate)

The effect of the source of nitrogen nutrition (ammonium or nitrate), on the response of pea plants to a moderate saline stress (30 mM NaCl) was studied. Growth declined under saline stress but nitrate-fed plants were less sensitive to salinity than ammonium-fed plants. This different sensitivity was due mainly to a better maintenance of root growth in nitrate-fed plants. Organic nitrogen content decreased significantly in roots of ammonium-fed plants. Water relations changed slightly under saline stress leading to a decrease in stomatal conductance, which was correlated to a decline in carbon assimilation rates regardless of nitrogen source. Salinity affects the uptake of several nutrients in a different way, depending on the nitrogen source. Thus, chloride was accumulated mainly in nitrate-fed plants, displacing nitrate, whereas sodium was accumulated mainly in ammonium-fed plants, especially in roots, displacing other cations such as ammonium and potassium. It is concluded that the nitrogen source (ammonium or nitrate) is a major factor affecting pea responses to saline stress, plants being more sensitive when ammonium is the source used. The different sensitivity is discussed in terms of a competition for energy between nitrogen assimilation and sodium exclusion processes.