Morphological and physiological characteristics associated with tolerance to summer irrigation termination in alfalfa

Deliberately withholding irrigation to alfalfa (Medicago sativa L.) during summer, a management strategy referred to as summer irrigation termination (SIT), has been suggested to conserve water in long-season desert environments. Summer irrigation termination may reduce productivity of alfalfa stands, although such effects may be lessened if cultivars with improved tolerance to SIT could be developed. This study was conducted to identify characteristics associated with low crown mortality as the primary component of tolerance to SIT. Using such traits in selection may be superior to direct selection for post-SIT yield, a trait potentially characterized by low heritability. Single spaced nondormant alfalfa plants were grown in a field trial in Tucson, AZ, and subjected to two SIT treatments during two years (Treatment 1, 84 d in 1994 and Treatment 2, 42 d in 1994 and 75 d in 1995). Traits evaluated prior to SIT (forage yield, stem number, plant height, leaf area, and root diameter) were not significantly associated with crown mortality. Significant but weak correlations were detected between crown mortality and days to wilting (r = -0.25*) and plant height during SIT (r = -0.29*). Accumulation of osmotically active substances, such as proline and sucrose, was not related to crown mortality. However, crown mortality was significantly correlated with concentration of total nonstructural carbohydrates (TNC) in crown tissue on Days 28 (r = -0.57*) and 35 (r = -0.79*) of SIT. The TNC reserves may be essential for continued respiration during periods of drought. Depletion of TNC reserves could have caused plant death in intolerant plants. Concentrations of TNC in crown tissue may represent a criterion in indirect selection for decreased crown mortality during SIT in selected environments.