Integrated effects of abiotic stresses on inoculant performance, legume growth and symbiotic dependence estimated by 15N dilution

Temperature, water, salinity, sodicity, acidity and nutrient disorders are major abiotic stresses that can affect legume growth or the establishment and function of the legume-Rhizobium symbiosis. We have examined the literature where the application of the N-15 isotope dilution methodology permits the effect of individual abiotic stresses to be independently and quantitatively separated into plant growth-mediated and BNF (biological N-2 fixation)-mediated components. The response of the symbiosis to a particular stress depends on a host of factors, including legume genotype, cultivar, Rhizobium inoculant, climatic conditions, and the duration, timing and severity of the stress. Published data are analysed in terms of the above variables and their interactions. As a general rule, severe stress inhibits both legume dry matter (DM) and the proportional dependence of the legume on BNF as a source of N. The symbiosis is resilient to low to moderate stress, but there may still be a penalty on legume DM. Gaps in knowledge are identified, and general guidelines on the identification and amelioration of abiotic stresses are provided.