Relative contribution of ion exclusion and tissue tolerance traits govern the differential response of rice towards salt stress at seedling and reproductive stages

Rice shows considerable variability toward salinity stress at the seedling and reproductive stages. Though ion exclusion (IE) is one of the most important tolerance strategies, some good ion-excluders do not necessarily show an expected level of tolerance at the reproductive stage. Here, we aimed to identify what could be the additional factors that help a few genotypes to tolerate salt stress at the reproductive stage more efficiently than others. We took rice genotypes having stage-specific variations in salt-tolerance ability and studied their physiological and molecular responses towards salt stress at both seedling and reproductive stages. We also studied the contribution of the tissue tolerance (TT) trait to understand its stage-specific influence on salinity tolerance. Tissue Na+ and K+ concentrations, and the values of selective transport of K+ over Na+ suggested that FL478 and AC41585 were very good ion excluders, while Rashpanjor and IR29 were moderate and poor ion excluders, respectively. The gene expression profiles of different Na+/H+ transporters; high-affinity K+ transporters, K+ uptake channels/pumps, and H+-pumps indicate very high IE behavior of FL478 and AC41585, which perfectly resembles the phenotypic manifestation of salt tolerance at the seedling stage. But, even with such dominating IE, FL478 failed to show a similar level of tolerance, while AC41585 could. Interestingly, we observed a significant difference in TT (measured in terms of LC50 score) of these genotypes. IR29 showed the highest TT, followed by Rashpanjor and AC41585, while it was the least in FL478. We conclude that only IE, without much contribution of TT, could be enough for seedling stage tolerance, however, may not be sufficient for the reproductive stage tolerance. Rather a fine balance between IE and TT is crucial for prolonged salt tolerance at the reproductive stage.