New strains of rhizobia that nodulate regenerating messina (Melilotus siculus) plants in saline soils

Messina [Melilotus siculus (Turra) Vitman ex B. D. Jacks (syn. M. messanensis (L.) Mill.)] is the most promising annual pasture legume for saline waterlogged soils in southern Australia. Messina forms a symbiosis with the commercial Sinorhizobium medicae strain, WSM 1115, used for many annual medic (Medicago) species. However, WSM 1115 does not persist over the summer months in saline soils and fails to adequately nodulate regenerating messina plants, restricting its commercial development as a new species for agriculture. To overcome this symbiotic constraint, two field experiments (swards and rows) and a glasshouse symbiotic effectiveness experiment were undertaken to identify strains of S. medicae able to persist in saline soils and adequately nodulate regenerating messina plants. In the sward experiment, no rhizobia were detected in WSM 1115 plots in the first autumn following seed set, whereas 3519 rhizobia per g of soil were measured for strain SRDI 554. Compared with WSM 1115, SRDI 554 increased regenerating messina nodulation from 32 to 100%, the number of nodules per plant from 1.3 to 32.4 and shoot dry weights from 23.6 to 80.2 mg/plant. The row field experiment found SRDI 554 had greater saprophytic competence than WSM 1115 and increased overall mean plant nodulation from 11 to 74%. The symbiotic effectiveness experiment found plant shoot weight of the majority of S. medicae strains under non-saline conditions was similar to WSM 1115. These experiments have identified new strains of rhizobia that overcome the symbiotic constraint in regenerating messina plants in saline soils. Further evaluation, particularly in acidic saline, waterlogged soils, is required to confirm adaptation of the most promising strains to soils within the full range of messina target environments.