Physiological factors related to water-use efficiency of alfalfa (Medicago sativa L.) genotypes

Abstract

A study was conducted at Tucson, Arizona to determine the variation in water-use efficiency among 150 genotypes of Medicago sativa L. cultivar ‘Mesa-Sirsa' grown under three soil moisture regimes and to determine if there were physiological and anatomical parameters related to efficiency of water use and growth components of alfalfa genotypes at three different growth stages. Information was obtained that would contribute to the development of techniques in the selection of alfalfa genotypes efficient in water use. Mesa-Sirsa genotypes varied in their water requirement. Some plants required three times more water to produce one gram of dry matter than other plants. Soil-moisture content significantly affected the water-use efficiency in the seedling stage but not in the mature stage of growth. The amount of available soil. moisture significantly influenced the production of dry forage, stems, and leaves at both seedling and mature stages of growth. The relationship between water-use efficiency and growth components studied was significantly influenced by the amount of available soil moisture at the 'seedling stage but not at the mature stage of growth. The water-use efficiency of Mesa-Sirsa seedlings grown under the medium soil moisture regime increased when dry matter production from forage, stems, and leaves increased. There was also a significant relationship between the total transpired water per plant and dry matter produced, number of stems and leaflets. Total water transpired was not significantly associated with the water-use efficiency of alfalfa seedlings. At the mature stage of growth, there was a significant correlation between the water-use efficiency and each of the following factors: dry weight of forage and stems, number of leaflets, and stem-leaf ratio. The amount of water transpired per plant was significantly and positively correlated to dry matter production from forage, stems, and leaves, and to number of stems and leaflets per plant. There was no association between the total water transpired from a plant and the water requirement. The relationship between the water requirement and transpiration rate significantly varied with the three growth stages. The most efficient alfalfa genotypes at floral bud or flowering stage transpired significantly less water per unit leaf area per unit time compared to the less efficient genotypes. The rates of net photosynthesis and respiration varied among alfalfa genotypes and with growth stages. The rates of both decreased as plants progressed toward maturity. At the floral bud stage plants with the highest water requirement also had the highest rates of photosynthesis and transpiration. At the vegetative stage of growth, respiration was directly associated with leaf-stem ratio and transpiration rate. The number of xylem vessels was significantly associated with the water-use efficiency of alfalfa genotypes at the vegetative stage. The most efficient alfalfa plants had significantly less xylem vessels in the leaf midvein when compared to the least efficient plants. The thickness of palisade layer was also significantly related to water requirement of alfalfa genotypes at the floral bud stage but not at other stages. Efficient alfalfa genotypes developed a thinner palisade layer.