Salt tolerance and avoidance mechanisms at germination of annual pasture legumes: importance for adaptation to saline environments

Mechanisms for tolerance and avoidance ofsalinity at germination were examined in five selfregeneratingannual pasture legumes of Mediterraneanorigin (Medicago polymorpha, Melilotus siculus,Trifolium subterraneum, T. michelianum and T.tomentosum). The maximum NaCl concentrations,for which no reduction in germination percentageoccurred, were 300 mM for M. siculus, 240 mM forM. polymorpha and 120 mM for T. michelianum, T.subterraneum and T. tomentosum. Following 21 daysexposure to 300 mM NaCl, imbibed seeds of T.subterraneum showed reduced germinability upontransfer to non-saline solution, whereas those of thefour other species recovered full germinability. Followingexposure to 600 mM NaCl, however, only T.michelianum and Melilotus siculus had some degreeof recovery, with 38% and 31% germinability,respectively. Na+ in imbibed seed tissues of allspecies increased markedly with increasing NaClconcentration, while K+ decreased in all but Melilotussiculus. Seed coat impermeability (‘hard seeds’) actedas protection against the toxic effects of salinity.Melilotus siculus, Medicago polymorpha and T.tomentosum showed a delay in hard-seed breakdown(‘seed softening’) over the summer–autumn period,compared to T. subterraneum and T. michelianum,which acts to defer germination until soil surfacesalinity levels are likely to be lower. These threespecies also had higher levels of residual hard seeds.The results from this study support field observationsthat Melilotus siculus and Medicago polymorpha arethe best adapted annual pasture legumes for highlysaline soils in southern Australia, T. tomentosum andT. michelianum have some adaptation to moderatelyand mildly saline soils, respectively, whereas T.subterraneum has no adaptive traits for even mildly saline environments. A model for annual legumegermination on saline land in southern Australia isalso presented.