COMPARATIVE EFFECTS OF N-SOURCES ON GROWTH AND PHYSIOLOGICAL-RESPONSES OF SOYBEAN EXPOSED TO NACL-STRESS

To study the effects of N-sources (NO3-, NH4+ and NO3-/NH4+ solutions) on NaCl-responses, soybeans (Glycine max (L.) Merr. cv. Maple arrow) were grown for 28 d on complete nutrient media containing three different N-sources (NO3-, NH4+, NO3-/NH4+ 2.6 mol m-3) and 0 or 100 mol m-3 NaCl. Root and stem biomasses of soybean were not altered, irrespective of NaCl and N-treatments, while leaf biomass of NO3- and NH4+-fed plants decreased significantly due to NaCl. Ion contents (Na+, Cl-, K+, Ca2+, N, and P) did not change in leaves regardless of N-source. Enhanced decreases in protein contents under saline stress were found when NO3- and NH4+ were the sole N sources. Root NRase activities of NO3- and NH4+/NO3--fed plants increased in saline medium, while only root GS activity of the former decreased significantly in NaCl medium. NiRaSe, GS, GOGAT, PEPCase, RuBPCase activities and chlorophyll contents of NO3-/NH4+-fed plants were less affected by NaCl-stress than other N-treatments. In leaves from NaCl-stressed plants. similar decreases in electron transport rate (PSII) and increases in glycolate oxidase activities were found, regardless of N-source. In the same way, total free amino acid and free proline contents of leaves did not change regardless of N-source; conversely, levels of organic acids, and particularly malate, were the highest in NO3-/NH4+-fed plants. Two different pathways of NaCl-toxicity, as regards N-sources and organic solute composition of tissues, are discussed.