Unveiling the differential salt tolerance strategies in different rice genotypes at both seedling and reproductive stage
Authors: Mohanty Ankita , Jena Priyanka , Sahu Nandini , Palei Suvalaxmi , Kabi Mandakinin , Mishra Udit Nandan , Kesawat Mahipal Singh , Mishra Ritesh , Pattnaik Rasmirekha 
Published: November 2024

Background Salt stress is a major production constraint for rice cultivation, particularly in the coastal agro ecologies. As a glycophyte rice can tolerate a salinity level of 5-6 dS m-1 and specifically susceptible in its seedling and reproductive stages. Rice shows considerable variability for salt tolerance at both these stages. To date, salt-tolerant rice development programmes primarily focussed on imparting salt tolerance strategies for early seedling stage (through introgression of Saltol/SKC1 QTL). But very little is known about the salt tolerance mechanism associated in reproductive stage. To dissect the differential mechanism at both these stages, we took 10 genotypes from different coastal belts of India and imposed the salt stress (12 dS m-1 and 8 dS m-1 respectively) at both early seedling and reproductive stages.
Results Study of different morophological and physiological trait unveiled significant stage specific salt tolerance mechanism employed during salt stress. Genotypes like FL478, AC41585 and AC39416A were able tominimize the overall loading of Na+ into the xylem tissue and hamper the transportation of Na+ to the greener metabolically active tissues in early seedling stage. We found Excellent ion excluders like FL478 excludes excess noxious ions and synthesize higher amounts of organic osmolytes to maintain the tissue hydration process under increased Na+ load around the root zone, while failed to show satisfactory level of tolerance at the reproductive stage. While some genotypes likeRashpanjor and CSR27 were found to possess a higher leaf Na+/K+ ratio than FL478 under similar stress conditions. This encouraged us to understand the balancing strategy of salt stress, which helps to maintain growth and development in early seedling as well as reproductive stages. The ion accumulator genotype (Rashpanjor) generally follows two different mechanisms to achieve salt tolerance (ionic selectivity and tissue tolerance) and therefore, the overall Na+/K+ ratio varies in the mesophyll tissues. High Na+ ion accumulation not only helped them to retain the chlorophyll pigments of mesophyll tissue but also helped these genotypes to use inorganic Na+ as cheap osmoticum against a high external pressure of salt stress. Based on our study, we conclude that salt tolerance varies differentially with respect to different growth stages in rice. Minimising the ion exclusion energy cost along with sequestering excess Na+ in the older tissue helps to achieve sustainable salt tolerance in rice which can satisfactorily with salt stress at both seedling and reproductive stages.
Conclusion Salt tolerance varies differentially with respect to different growth stages in rice. Where, seedling stage tolerant genotype FL478 failed to sustain under prolong salt stress because it might spend high energy in the exclusion of Na+ from the very beginning in both the stages, which is very much energy consuming and that might hamper the grain filling under prolong period of stress in reproductive stage. On the contrary some genotypes like AC41585, AC39416A and Rashpanjor might have the capability to balance the Na+ exclusion more effectively at the root zone or could partitioned/sequestered it in the old leaf or leaf sheath to maintain a low Na+/K+ ratio in the reproductive part.