Lucerne root growth as a function of summer watering

The reaction of the root system of lucerne (Medicago sativa) plants subjected to water stress was studied in a 24-core lysimeter. Plants were subject to summer soil moisture stress or given one irrigation to simulate a summer rainfall. Roots were observed through minirhizotron tubes. Root development in the irrigated cores was significantly greater than that in the dry cores with plant roots growing into the deeper zones as the soil dried. Ongoing drought failed to promote root extension. Root development was not significantly influenced by soil type or cultivar dormancy characteristic. Some development of the root system was observed at the 2 metres which was independent of the irrigation, soil type or cultivar. We conclude that a growth period to accumulate carbohydrate reserves may be necessary for a lucerne plant to extend roots in response to drought. Threshold root extension rates relative to soil water availability and transpiration requirement may determine the root response to drought.