EFFECT OF SOIL-WATER AVAILABILITY ON WATER-USE STRATEGIES AND DRY-MATTER PRODUCTION BY 2 ALFALFA CULTIVARS DIFFERING IN WINTER DORMANCY

Several authors report that the main drought tolerance mechanism in al falfa (Medicago sativa L.) is the capability of the root system to elo ngate and thus feed on water from subterranean water tables. But when relatively shallow water tables are absent, water stored in the soil p rofile must be used. The water use strategy developed by alfalfa in si tuations of severe drought is not completely understood. The purpose o f the present work is to characterize the effect of soil water availab ility on water use strategies and dry matter production by two alfalfa cultivars with different degrees of dormancy. Dry matter production, soil cover, relative water content, stomatal conductance, leaf tempera ture and volumetric water content in the soil, are evaluated in rainfe d and no-rain treatments. In soils without available water tables, cro p water-use strategies depend on the crop's level of stress and are no t influenced by degree of dormancy of the cultivar. Water extraction f rom soil layers below 2.5 m occurs when the relative water content and stomatal conductance drop below 70% and 0.3 cm s(-1). At this point, when the crop begins to draw water from soil below 2.5 m, dry matter p roduction starts to be affected, and production in the no-rain plots i s 41% lower than in rainfed plots. Soil water stored in the upper 50 c m produces more dry matter in rainfed plots.