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.

Experiments were conducted at Tucson, Arizona during a 3-year period to determine the effect of water management on several characteristics in 'Mesa-Sirsa' alfalfa (Medicaqo sativa L.) grown under field conditions. Three water management treatments were imposed: (a) irrigating to field capacity when 50% of available soil moisture had been utilized to a depth of 1.2 m (high), (b) irrigating to field capacity when 100% of the available soil moisture had been utilized to the same depth (medium), and (o) irrigating similar to the high treatment on first and last harvests of the growing season, but withholding irrigation water during the balance of the growing season (low). Plants receiving medium and high water treatments were harvested at 25% bloom through the entire growing season while those receiving the low water treatment were harvested twice--at the first and last harvest for plants receiving medium and high water treatments. In plants grown under both the high and medium water treatments it was found that dry matter production, plant height at harvest, efficiency of water use, and total water utilization per growth period decreased during the summer months while the daily rate of water utilization increased. Treatments did not have an effect on any of these variables. When irrigation water was returned to plants grown under the low water treatments, dry matter production and plant height at the subsequent harvest were not significantly different from plants grown under the medium and high water treatments. Plant population per unit land area declined steadily during the three years of production. Neither stand decline nor root distribution was significantly influenced by water management over the 3-year period. Variation in root morphology among genotypes was observed. The majority of the variability was attributed to genetic expression. Water stress resulted in significant increased proline and decreased arginine content of root tissue. Sucrose content increased significantly in roots of plants subjected to water stress while fructose and glucose levels were not affected. Water insoluble N and water soluble protein levels were lower at midsummer when compared with the beginning and completion of the growing season. Total available carbohydrates (TAC) and water soluble protein levels decreased following forage harvest, while levels of water insoluble N, water soluble NI amino acid composition of proteins, and free sugar levels did not change during regrowth. TAC levels under the medium and high water treatments decreased during midsummer each of the 3 years of the study. The trend was not as evident during the initial production year as in subsequent years. Net carbohydrate utilization during the 2 weeks following harvest varied among years and harvests. Differences among harvests were attributed to climatic conditions as no treatment effect was observed. Variation among years may have resulted from root maturation and genotypic changes in the plant population. TAC levels of nonharvested summer-stressed plants in the low water treatment remained relatively constant unless rainfall occurred in sufficient magnitude to stimulate top growth, in which case a decrease in TAC levels was observed.

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Graduation date: 1987

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Physiological and agronomic responses of two pearl millet (Pennisetum americanum L. K. Schum) parents and their hybrid were evaluated under different water levels on a Brazito sandy loam soil at Tucson, Arizona in 1983 and 1984. A line source sprinkler irrigation gradient was used to create the treatments. Soil moisture and physiological parameters were measured under field conditions at weekly intervals. Growth was analyzed every other week, while yield and yield components were evaluated at the end of the experiment. Harvest Index, drought tolerance index, and water use efficiency (WUE) were calculated. Percent crude protein of seeds was analyzed, and photosynthesis was measured in 1984 only. Regression analysis was used to compare the performance of the cultivars for stability. Water stress significantly affected all parameters measured, and the stress effects were more pronounced in 1983. There was no significant difference between entries at each water level for most parameters. Plant height, dry matter, and total leaf area decreased as watering level decreased. The leaf area of the hybrid was significantly higher at high water level in 1983. Firing ratio was increased significantly by stress in both seasons and was significantly lower for the male at low water level in 1983. Yield and yield components were reduced significantly by stress in both seasons, which was reflected in the reduced harvest index. Relative yield of dry matter was higher for the male in both seasons. The hybrid had the highest drought tolerance index in 1983, while the female had the highest drought tolerance index in 1984. WUE of dry matter increased with stress, while WUE of grain yield decreased with stress in both seasons. The significant reduction in transpiration was 57, 56 and 66% in 1983 and 45, 53, and 52% in 1984 for female, male, and hybrid respectively. Leaf diffusive resistance and leaf temperature were increased by water stress in both seasons. Stability regression analysis provided no adequate differentiation among cultivars. The percent crude protein (weight basis) was increased significantly by stress by 38, 43, and 28% in 1983 and by 47, 33, 46% for female, male, and hybrid, respectively.