Effect of Foliar Application of Urea, Proline and Ascorbic Acid on some Physiological Characteristics and Yield of Forage of Yellow Sweet Clover (Melilotus officinalis L.) under Salinity

IntroductionSalinity, as one of the world-wide limiting factors, can restrict the crop quality as well as world food products, particularly in arid and semi-arid regions. Yellow sweet clover (Melilotus officinalis L., Fabacea family), an herbaceous plant with both medicinal and domestic utilizes that the most salt-tolerant legumes, can improve the sustainable agriculture in saline soils. In order to minimize the harmful effects of salinity, various strategies including cost-effective and efficient use of the foliar spraying are adopted to increase plant tolerance by mainly alleviating Na and Cl injuries to the plants. Ascorbic acid (AA), a natural water-soluble antioxidant and proline, a beneficial solute amino acid, protects the plants as an anti-oxidative defense molecule during various stresses. Foliar urea application directly affects nitrogen metabolism in saline soils, and consequently amino acids synthesis.Materials and methodsTwo years (2017-2018) field experiment were performed at a research field of Urmia University. For each year, the experiment was arranged in a split-plot design based on a randomized complete block design with three replications. The main factor was soil salinity (0.9 dS/m – as none-saline and 6.7 dS/m – as saline soil), and also the sub factor was foliar application including proline (20 mM, 2.3 g/L), ascorbic acid (5 Mm, 0.9 g/L), urea (46% N, 10 g/L) and control (water spraying). plant biomass (oven-dried at 80 ºC for 48 h) and seed yield (with 15% moisture content) measured from harvested plants from 1 m2 of each experimental plot. The chemical composition of samples was obtained based on the standard procedures (The quantities of Photosynthetic pigments and osmolytes) and forage qualities. Comparison of means were performed by Duncan’s multiple range test (DMRT) at 5% and 1% probability levels by using SAS and MSTAT-C statistical software.Results and DiscussionThe analysis of variance showed that the interaction between salinity and foliar application on biomass, grain yield, chlorophyll a, carotenoids, proline, soluble sugars and all the characteristics related to forage quality were significant. Also, the effect of salinity on chlorophyll b and glycine betaine, and the effect of foliar application on chlorophyll b were significant. The salinity increased the amount of acid-soluble fiber, water-soluble fiber and total ash, but the amount of crude protein, digestible dry matter, water-soluble carbohydrates and crude fiber were higher in non-saline condition. In non-saline condition, the use of ascorbic acid increased the amount of crude protein by 6.8% compared to the control, but in saline conditions was observed no significant effect. Under saline condition, foliar application of urea had the greatest effect on acid-soluble fiber and water-soluble fiber, so that in these conditions, urea-sprayed plants had 0.5, 7.86 and 1.69% more acid-soluble fiber than control plants and plants treated with proline and ascorbic acid, respectively. Also in saline conditions, the amount of water-soluble fiber in urea spraying was 0.46%, 3.32% and 6.31% higher than control, proline and ascorbic acid treatments, respectively. Proline improved biological yield and grain yield by reducing the adverse effects of salinity, while foliar application of plants with ascorbic acid did not show an additive effect on yield. Osmotic regulators can act as mechanisms to maintain cellular water potential in plants under salinity stress.ConclusionIn general, biomass yield and grain yield were lower in saline conditions than in non-saline conditions. In both conditions, urea foliar application had the greatest effect on yellow sweet clover yield and increased its amount compared to control and other treatments. In general, yield reducing under salinity conditions may be due to inhibition of photosynthesis, which causes the plant to absorb fewer nutrients. Our data show that foliar application of urea in saline conditions counteracts the harmful effects of salinity on plant yield. Based on the results, the amount of NDF and ADF increased in saline conditions compared to non-saline conditions. The highest amount of acid-soluble fiber and water-soluble fiber was obtained in salinity conditions and by spraying yellow sweet clover with urea.Acknowledgements  The authors of the article thank Mr. Vakili, Crop Physiology Laboratory, Department of Plant Production and Genetics, Urmia University, for his helps in conducting the experiments of this research.