PHYSIOLOGY OF SALT TOLERANCE IN ALFALFA (BREEDING, MEDICAGO SATIVA).

Abstract

The application of fertilizers and saline irrigation water have resulted in increased soil salinity and the removal of large land areas from crop production. One method to overcome the effects of soil salinity is to increase the salt tolerance of crops. The objective of this research was to investigate the physiological, genetic, and agronomic differences between alfalfa (Medicago sativa, L.) bred for increased salt tolerance and salt sensitive alfalfa. The materials used in these studies were the result of five cycles of selection for germination NaCl tolerance, AZST 1978 to 1982, and the source population, 'Mesa-Sirsa'. All salt-tolerant cycles and Mesa-Sirsa were evaluated for ability to germinate in NaCl, NaNO₃, KCl, KNO₃, mannitol and polyethyleneglycol (PEG) solutions ranging from -1.0 to -1.6 MPa of osmotic potential and a control of distilled water. Germination in the lower osmotic potentials of all germination medias was significantly higher with each succeeding cycle of selection for germination NaCl tolerance. Selection for tolerance to NaCl during germination also resulted in increased tolerance to the other salts as well as mannitol and PEG. Germination in mannitol was higher than in any of the salt solutions. This suggests that ion toxicity also inhibits germination. There was no significant difference between Mesa-Sirsa and AZST 1982, the most salt-tolerant cycle, in seed respiration in NaCl solutions or in uptake of tritiated NaCl solution during germination. Broadsense heritability of germination NaCl tolerance was estimated at 49%. All the Arizona Salt Tolerant cycles and Mesa-Sirsa were evaluated for several mature plant characteristics under non-saline field conditions. There were no significant differences among germplasm sources in forage yield, apparent photosynthesis, transpiration, or diffusive resistance. Seedlings of Mesa-Sirsa and AZST 1982 were grown in NaCl solutions ranging from 0 to 18000 ppm NaCl in the greenhouse. The plants were evaluated for several plant growth characteristics to determine whether selection for germination NaCl tolerance resulted in increased salt tolerance at more mature plant growth stages. There was no evidence that germination salt tolerance is related to salt tolerance at later growth stages in alfalfa. Salt tolerance during germination and later growth stages may be controlled by different physiological and genetic mechanisms.