Differences in dehydration tolerance affect survival of white clover (Trifolium repens) and lucerne (Medicago sativa) during a drying cycle

There is very little robust, experimentally based knowledge comparing drought tolerance of one legume species with another. Dehydration tolerance and plant survival of the perennial legumes white clover (Trifolium repens L., considered quite sensitive to drought) and lucerne (Medicago sativa L., considered drought tolerant) were compared in a drying cycle experiment conducted in pots in a glasshouse, with the deep rooting of lucerne constrained. White clover used more soil water, drying the pots to a final soil gravimetric water content (θ_g) of 4.7%, compared with 8.3% in lucerne pots. Rates of water use were also different: white clover used 0.47% of θ_g per day and lucerne 0.3%. The more conservative water use allowed lucerne to survive for longer into the drying cycle than white clover. Lucerne partitioned more of its total dry matter into root growth and had much higher root:shoot ratios than white clover. Leaf/stolon elongation is one of the first plant processes to cease as water deficit increases; however, elongation was greater in white clover than lucerne at the beginning of the drying cycle, and this trend continued until lower soil water contents were reached. Conversely, leaf senescence generally commenced at quite high levels of water stress and progressed more rapidly to complete senescence in white clover than in lucerne. Lucerne retained tissue relative water content at a higher level than white clover, with final minimum values of 25% and 13.6%, respectively. In lucerne, 50% mortality was observed at θ_g of 9%, compared with 6% in white clover, albeit with greater variability. In conclusion, lucerne maintained a higher relative water content than white clover even though it endured the drying cycle for longer and without access to water at depth, evidence of its superior dehydration avoidance and better adaptation to dry conditions. However, white clover was more able to extract water from surface soil layers. This study provides valuable insight into the adaptive traits of both species and identifies some traits that might be useful in the quest to improve white clover adaptation.