LAUSR

Legumes are among the most utilised agronomic plant species due to their symbiotic association with N2‐fixing bacteria. Since N2 fixation entails high ATP cost, salt stress disrupts N2 fixation in the symbiont, but increases the production of osmolytes and antioxidant systems in the host plant. This results in competition for C allocation between osmoprotection in the host and continued supply to the symbiont for N acquisition, which may result in different plant responses to salinity. Two‐nodule types of plant species with contrasting carbon requirements for organic N2 fixation can be found within legume species; determinate and indeterminate. In this study, we tested responses of respiratory carbon metabolism, nitrogen assimilation and antioxidant machinery in leaves and roots of Phaseolus vulgaris (determinate nodules) and Pisum sativum (indeterminate nodules) 24 and 72 h after salt treatment (300 mM of NaCl). In P. sativum, we observed that nitrogenase activity was maintained at 24 h, but showed a strong decrease at 72 h together with cytochrome activity. On contrast, in P. vulgaris, respiration rates were maintained by an enhanced antioxidant activity under salinity although at the expense of nodule metabolism. Despite of the severity of the salt stress for N2 fixation, both species showed similar mechanisms to cope with salinity, like the maintenance of alternative respiration and increased antioxidant defence, that are worthy to be tested in the long term under field conditions.