The Effects of Salinity and Sodicity upon Nodulation and Nitrogen Fixation in Chickpea (Cicer arietinum)

Taking nitrogen with a pinch of salt

As highlighted in last month's Annals of Botany, the formation and function of nitrogen-fixing nodules in legumes relies on a subtle and complex interplay between host and symbiont, an interplay that is readily disrupted by certain abiotic stresses. Salinity, for example, inhibits nodule formation (even though the Rhizobium can still infect the plant), disrupts the development of nitrogen-fixing capability in differentiating nodules and reduces N₂-fixation in fully formed nodules. This is bad news for anyone wishing to grow legumes in arid or semi-arid regions where saline and/or alkaline soils may be widespread. However, one grain legume, Cicer arietinum (chick pea), is widely grown in semi-arid regions and it is important to try to improve its performance under stressful conditions. An international team (Rao et al., pp. 563-570) has studied nodule formation and the N₂-fixing capability of nodules in a range of C. arietinum cultivars in field- and laboratory-grown plants. The cultivars differed significantly in their nodulation and N₂-fixation activity under stress, although in all cultivars nodulation and N₂-fixation were reduced significantly as salinity increased. However, one particular cultivar stood out as performing much better than the others in saline conditions. Furthermore, when the cultivars were compared under control conditions, it was apparent that cultivar performance under stress was correlated with nodulation capacity in normal soils. But what of the other partner, the Rhizobium symbiont? Earlier authors had shown that the bacterium is, in general, more salt-tolerant than the host and the data in this paper show that the bacterium does not feature as an important factor in the effects of salinity on nodulation. The authors conclude that '. . . selection for improved nitrogen fixation should proceed by first screening . . . to identify genotypes with the best available nodulation and grain yield characteristics under salt stress'. Good advice which should be heeded.