Improvement of tolerance to summer irrigation termination in alfalfa

Abstract

Withholding irrigation to alfalfa (Medicago sativa L.) during summer, a management strategy referred to as summer irrigation termination (SIT), has been suggested as a way to conserve water in desert environments. SIT may decrease productivity of alfalfa stands, although such negative effects may be reduced if cultivars with improved tolerance to SIT could be developed. This research was undertaken to determine how improved tolerance to SIT could be achieved through plant breeding. Single spaced plants of an extremely nondormant alfalfa population were grown in a field trial in Tucson, AZ and exposed to SIT in 1994 and 1995. These plants were used to identify traits associated with tolerance to SIT and represented parental material in a selection experiment. Direct selection for minimal reduction of forage yield following SIT was conducted under two stress intensities (lengths of SIT) and compared to indirect selection for characteristics potentially associated with dehydration avoidance. None of these selection criteria improved post-SIT forage yield relative to a random sample of plants from the parental population. This lack of response from selection was attributed to stress intensities that were not sufficiently high to fully expose genetic variation for yield following SIT. Physiological studies showed that high concentrations of total nonstructural carbohydrates (TNC) in crown tissue are positively associated with tolerance to SIT. Using TNC concentrations as an indirect selection criterion may therefore represent a more promising approach in improving tolerance to SIT than direct selection for post-SIT yield. Crown tissue was shown to die if the tissue moisture content fell below about 42%. This threshold value was used to predict whole-plant mortality of alfalfa grown in solid-seeded plots comparable to commercial fields. Crown samples were taken at five locations within the field along a soil gradient that caused whole-plant mortality to vary from 0.5 ± 0.5 to 48.7 ± 4.1%. Predicted values closely followed this change in observed mortality rates (r² = 0.97*) but tended to overestimate actual mortality on average by 4.2%. Alfalfa growers may be able to minimize mortality using this simple method to predict mortality during SIT and to reschedule irrigation accordingly.