[Truncated abstract] Salinity and waterlogging interact to reduce the growth of most crop and pasture species. Species that are productive on saline-waterlogging land are needed for Australian farming systems. One option is Lotus tenuis, a perennial legume widely grown for pasture in the flood-prone and salt-affected Pampa region of Argentina. To identify mechanisms responsible for the adverse interaction between salinity and waterlogging, Lotus tenuis with a reputation for tolerance was compared with L. corniculatus, the most widely cultivated Lotus species. The physiology of salt and waterlogging tolerance in L. tenuis (4 cultivars) was evaluated, and compared with L. corniculatus (3 cultivars). Overall, L. tenuis cultivars accumulated less Na+ and Cl-, and more K+ in shoots than L. corniculatus cultivars, when exposed to 200 mM NaCl for 28 d in aerated or in anoxic (stagnant agar) solutions. In a NaCl dose response experiment (0 to 400 mM NaCl in aerated solution), Lotus tenuis (cv. Chaja) accumulated half as much Cl- in its shoots than L. corniculatus (cv. San Gabriel) at all external NaCl concentrations, and about 30% less shoot Na+ in treatments above 250 mM NaCl. Ion distributions in shoots were determined for plants at 200 mM NaCl; L. tenuis (cv. Chaja) accumulated about half as much Cl- in old leaves, young leaves and stems, compared with concentrations in L. corniculatus (cv. San Gabriel). There were not, however, significant differences between the two species for Na+ concentrations in the various shoot tissues under aerated NaCl treatment. '...' Therefore, during early stages of exposure to salinity, L. tenuis accumulated a higher proportion of total Na+ in the roots under combined stagnant-plus-NaCl treatment (55% versus 39% for L. corniculatus). Na+ transporters, particularly those relying on H+ gradients across membranes, which in turn require adequate ATP levels, could be impaired under O2 deficits that inhibit respiration. To study the effect of O2 deficiency on a Na+ transporter, an NHX1-like gene was cloned from L. tenuis and identity established via sequencing and yeast complementation studies. Real-time qPCR showed expression of NHX1 in L. tenuis roots increased under stagnant-plus-NaCl treatment, whereas it was reduced in L. corniculatus. Thus, maintaining O2 transport to roots, together with up-regulation of an NHX1-like gene for Na+ accumulation in vacuoles, contributes to tolerance of L. tenuis to combined salinity and waterlogging stresses. This study highlights the importance of minimising Cl- transport to shoots as a mechanism of salt tolerance and has identified a CCC-like gene in L. tenuis as a candidate for mediating root-to-shoot Cl- transport. Under combined stagnant-plus-NaCl treatment, control of Na+ transport is another mechanism contributing to tolerance in these Lotus species. Enhanced root aeration in L. tenuis maintains root Na+ transport processes, such as accumulation in vacuoles via NHX1-like genes, to diminish xylem loading to the shoot. Overall, this thesis has contributed new knowledge on the potential of Lotus tenuis as a saltland pasture and has significantly enhanced current understanding on the mechanisms of salinity and waterlogging tolerance in plants.