The perennial forage alfalfa (Medicago sativa L.) may be affected by salinity at all stages of development. Selection for increased seed germination or seedling growth in saline environments has not resulted in improved forage yield under salt stress. The purpose of this study was to determine genetic and phenotypic relationships between plant performance in the presence of NaCl at three developmental stages in alfalfa. Understanding these relationships may improve the efficiency of breeding programs aimed at increasing crop survival and yields in saline environments. Fourteen half-sib families were randomly chosen from both an experimental alfalfa population produced from two cycles of mass selection for improved forage regrowth yield at 80 mM NaCl (A80), as well as from an unselected control population (AC1). In two separate experiments, individual plant performance was measured in these families at seed germination (radicle length at 7 days), and during seedling growth (forage yield at 40 days post-planting) and post-harvest regrowth (forage yield at 67 and 95 days post-planting) in the presence of 0 or 80 mM NaCl. Genetic, phenotypic, and family rank correlation coefficients, and broad-sense and narrow-sense heritability estimates were calculated within each growth stage, NaCl level, and population. Radicle length was not highly correlated with seedling or regrowth forage yield within a population or across NaCl levels. Phenotypic correlations between seedling and regrowth yields were also low. Heritability estimates were higher at 0 NaCl in AC1 between all growth stages, but were greater in A80 at 80 mM NaCl. Genetic correlations between seedling and regrowth yields were all positive. This suggests that selection for forage yield in saline environments at harvests-1,-2, or -3 should not decrease performance at other stages. Genetic correlations between seedling and regrowth yields were higher in A80 than in AC1 at 80 mM NaCl. The results indicate that selection for increased alfalfa forage yield in saline environments at germination may not be optimum. Family selection at germination or during seedling growth may be more effective than individual plant selection at any growth stage in saline environments. The results suggest than selection methods which include each critical growth stage may be required to develop alfalfa cultivars with increased forage yield in saline environments.
A major limitation to crop production throughout much of the world is soil salinity, which is increased by irrigation and fertilization. One way to better utilize saline soils is to grow salt tolerant crops. The objective of this research was the development of salt tolerance in germinating alfalfa (Medicago sativa L.) seeds. Five cycles of mass selection for NaCl tolerance during seed germination were conducted in the cv. Mesa‐Sirsa. Standard germination techniques were employed using blotter paper saturated with NaCl solutions of five different concentrations at a temperature of 26° C. The osmotic potential of the NaCl solution needed to produce 1% seed germination decreased from −1.40 to −2.45 MPa during the five cycles of selection. Germination at −1.30 MPa osmotic potential increased from 3% for the source population to 86% for the fifth cycle of selection. The broad sense heritability estimate for NaCl tolerance during seed germination, averaged over all cycles of selection and five levels of osmotic potential, was 50%. Significant progress was made in the development of salt tolerance in germinating alfalfa.
Identification of the physiological processes involved in salt tolerance is required to develop appropriate selection criteria for breeding salt‐tolerant crops. For this purpose selected physiological characteristics were evaluated in five alfalfa (Medicago sativa L.) populations selected for NaCl tolerance during seed germination and the source population, 'Mesa‐Sirsa'. Seed of the populations (Syn‐1) was germinated in distilled water and solutions of NaCl, NaNO3, KC1, KNO3, and mannitol ranging from −1.0 to −1.6 MPa osmotic potential. The mean difference in percent germination between Mesa‐ Sirsa and ‘AZST1982’, the most salt‐tolerant population, was greater in the NaCl solutions than the other salt solutions (P < 0.01). Seed from AZST 1982 also had higher percent germination than Mesa‐Sirsa in mannitol solutions (90.6 and 2.8%, respectively, at −1.6 MPa osmotic potential). Mesa‐Sirsa had higher (P < 0.01) average seed respiration rate than AZST 1982 between 3 and 24 h of germination in NaCl solutions of −0.6 to −3.0 MPa osmotic potential. There was no significant difference between Mesa‐Sirsa and AZST 1982 for Na+ or Cl− accumulation after 48 h of germination in NaCl solutions of −0.6 to −1.8 MPa osmotic potential, or absorption of tritiated water after 6 and 12 h of germination in NaCl solutions of −1.3 and −2.0 MPa osmotic potential. Selection for NaCl tolerance in germinating alfalfa seed results in two separate types of tolerance: tolerance of an inhibitory effect specific to NaCl and tolerance of lowered water potential. Selection for NaCl tolerance of alfalfa during germination does not appear to influence ion accumulation or the rate of imbibitional water uptake.
Identification of laboratory conditions for isolating alfalfa (Medicago sativa L.) plants which tolerate high temperature and high saline soil during germination and stand establishment would be useful for alfalfa improvement in and and semi‐arid environments. An experiment was designed to determine the germination percentages of ‘U.C. Salton’ and ‘Ladak 65’ alfalfa cultivars at all combinations between sodium chloride osmotic potentials (OP) of 0 to −15 bars and temperatures of 21, 27, 33, and 39 C. U.C. Salton was developed for the low desert Southwest. Ladak 65 was developed for Montana. U.C. Salton showed slight enhancement of germination when the OP was lowered from 0 to −1 bar at 21,27, and 33 C. Ladak 65 showed this enhancement only at 27 C. At lower OP's, the germination percentages declined sharply for both cultivars. There was an interaction between the effects of temperature and OP on seed germination. Cultivar‐temperature and cultivar‐OP interactions were highly significant. A statistical model was established from these data. Quadratic response surfaces were found which had high degrees of fit to the data for each cultivar. The optimal temperature‐OP combinations for germination from the response surfaces were 25.4 C at −1.1 bar for U.C. Salton and 21.4 C at 0 bars for Ladak 65. Germination trends involving salt and temperature could be useful for establishing selection pressures in alfalfa improvement programs.
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