Tolerance and recovery of the annual pasture legumes Melilotus siculus, Trifolium michelianum and Medicago polymorpha to soil salinity, soil waterlogging and the combination of these stresses

Aims: This study evaluated the responses to and recovery from soil waterlogging, soil salinity and the combination of these stresses of the annual pasture legumes Melilotus siculus cv. Neptune, Trifolium michelianum cv. Frontier and Medicago polymorpha cv. Scimitar. The major aim was to test the tolerance of these plants to salinity and waterlogging in soil inoculated with rhizobia, adding to previous controlled-environment studies, which have used nutrient solution cultures, as well as to examine post-stress recovery. Methods: Plants were grown in pots of soil inoculated with their appropriate species-specific rhizobia. Drained non-saline controls, and waterlogged non-saline, drained saline (100 mM NaCl) and waterlogged saline treatments were imposed for 3 weeks, followed by a 4 week recovery period. Results: In the waterlogged saline soil, shoot dry mass of M. siculus was 63% of the control, compared to 10% for T. michelianum and 5% for M. polymorpha, and after the recovery phase was 75% of the control for M. siculus, 58% for T. michelianum and 4% for M. polymorpha. Foliar Na⁺ concentrations of T. michelianum and M. polymorpha were 2.8-fold higher than for M. siculus. Conclusions: The greater tolerance of M. siculus can be attributed to regulation of leaf Na⁺ concentrations in saline conditions and the formation of aerenchymatous phellem to enhance O₂ supply to roots in waterlogged soils. This controlled glasshouse study using pots of soil supports earlier nutrient solution experiments and field observations that M. siculus possesses greater tolerance of combined waterlogging and salinity, than T. michelianum and M. polymorpha.