Differential behavior of Medicago truncatula to calcareous soil is explained by modulation of stomatal responses, antioxidant activity, and iron use efficiency

The effect of calcareous soil on growth, photosynthetic gas exchanges, symbiotic nitrogen fixation (SNF), iron nutrition, and antioxidative enzymes activities was investigated in two lines of Medicago truncatula: TN8.20 and Jemalong. The results showed that growth (especially in shoots) was significantly lower on calcareous soil in both lines, but Jemalong was more impacted than TN8.20. Plants cultivated on calcareous soil showed significantly lower values for photosynthesis activity (as net CO2 assimilation and stomatal conductance) and nodule performance (nodulation, nodule growth and SNF), but this trend was more pronounced in the sensitive line (Jemalong). The better performance of TN8.20 as compared to Jemalong could be ascribed due to its aptitude to preferentially allocate iron to nodules, thereby ensuring nodule growth and a higher SNF activity, besides showing higher iron use efficiency for SNF. In both lines, superoxide dismutase (SOD, EC.15.1.1), catalase (EC.1.11.1.6), and peroxidase (POX, EC 1.11.1.7) activities were increased significantly when they were grown on a calcareous soil. Yet, this tendency was more pronounced in the tolerant line, TN8.20. Overall, our results strongly suggest that on calcareous soils, the successful adaptation of TN8.20 would be related to its ability to induce an antioxidant response in order to preserve important metabolic paths such as photosynthetic activity and symbiotic nitrogen fixation.