Comparative Effects of N–Sources on Growth and Physiological Responses of Soyabean Exposed to NaCI–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.