Role of Foliar Application of Nicotinic Acid and Tryptophan on Onion Plants Response to Salinity Stress

This experiment was conducted at the National Research Centre, Dokki, Cairo, Egypt during 2006 and 2007 winter season to evaluate the effects of spraying with tryptophan (TP) and nicotinic acids (NA) on onion plants grown under varying degree of salinity stress (irrigation water with 3000 and 6000 ppm salinity) as compared to tap water (300 ppm) irrigation. Irrigation by high salinity water decreased the number of leaves, mean bulb diameter, fresh and dry weight of bulb and whole plant. Foliar application of TP and NA significantly increased the top height and fresh weight, and bulb dry weight. The above beneficial effects were greater in the plants sprayed with NA as compared to those of the plants received TP spray. At the lower salinity level (3000 ppm), TP was more effective than NA in mitigating the negative effects of salinity stress. At the high salinity stress (6000 ppm), however, the converse was evident. The electrolyte leakage (EL) was not influenced by 3000 ppm salinity irrigation water as compared to that of the plants which received tap water (300 ppm) irrigations. However, it increased significantly in the plants irrigated by 6000 ppm salinity water. Lipid peroxidation (LP) and EL were significantly lower in the plants sprayed with TP or tapwater. In addition, spraying of NA or TP increased the plant growth and biomass weight. Therefore, the above foliar sprays appear to mitigate the negative effects of increased salinity.