The behavior of some Romanian alfalfa genotypes to salt and water stress

Abiotic stress conditions cause extensive losses to agricultural production worldwide (Bray, 2002). Drought and salinity stress can significantly affect plant yield in and and semi-arid regions and not only. Climatic changes will conduct to severe drought conditions and to aridity of some important regions in Romania. One of the most important strategies which could reduce the influence of drought and salinity on alfalfa (Medicago sativa) production is to breed for increased cultivar tolerance. The present paper reports the reactions of some Romanian alfalfa genotypes to salt and water stress. The aim was to elucidate some physiological and metabolic aspects of those stresses, in order to establish screening criteria to facilitate the development of genotypes with enhanced tolerance to field stress conditions. Seeds of nine alfalfa genotypes were sown in Mitcherlich plots filled with a soil-sand mixture. The plant were grown in vegetation house under optimal condition up to just before flowering, when for water stress variant the watering was reduced for 10 days; salt stress was imposed on plants by adding 300 mM NaCl/l and under combined stress the plants were treated with 300 mM NaCl/l one week before reducing watering. The alfalfa yield of all studied genotypes was significantly reduced under water and salt stress while stresses combination caused a reduction on fresh biomass, too. Salt stress significantly decreased biomass by more than 37% while water stress by more than 73%. The effects of salt and water stresses on yields were additive but not equal. Alfalfa responded to drought by decreasing leaves transpiration. Between biomass accumulation and leaves transpiration under water and salt stress there was a linear relationship (r = 0.76*; r = 0.82*). Under optimal condition the proline content was very small (1.7-5.4 mg proline/g f.w.) but there were obviously higher proline contents under salt stress (156-441 mu M proline/g f.w.), water stress (45-68 mu M proline/g f.w.) and stress combination (120-330 mu M proline/g f.w.). The negative effect of salinity and combined stresses on alfalfa growth could be attributed to osmotic effects. Osmotic stress inhibits water uptake from the soil and requires the plant to use energy and carbohydrates in synthesizing organic solutes to adjust its internal osmotic potential. Yield loss results from closing stomata and from energy and carbohydrates use in osmoregulation. The leaves transpiration and biomass accumulation were correlated, suggesting the use of transpiration as a screening tool for drought and saline tolerance of alfalfa genotypes.