Physiological Responses to Chronic Salt Stress at the Young Panicle Stage and Agronomic Performance of Rice Genotypes with Contrasting Salt Tolerance

The selection and breeding of salt-tolerant rice and the use of saline–alkali land for rice cultivation are crucial for food security. However, most studies have focused only on the seedling salt tolerance stage, with little research on the salt tolerance mechanisms during the reproductive growth period. This study selected the salt-tolerant rice line SR17, the salt-tolerant variety SR86, and the salt-sensitive variety IR29 as research subjects. Two salt stress gradients of 0% and 0.5% (7.8 dS m−1) were established. Salt stress was applied continuously from rice transplanting to the maturity stage, and the differences in response mechanisms during the young panicle stage under long-term salt stress were analyzed. The results showed that, under salt stress, SR17 exhibited the least lipid peroxidation and membrane damage, followed by SR86, while IR29 suffered the most severe damage. SR17 and SR86 could reduce oxidative damage and maintain membrane system integrity by activating the antioxidant enzyme system and accumulating soluble proteins. In contrast, the antioxidant system in IR29 was insufficiently activated; this indicates that the adaptability of this variety to salt-induced oxidative stress is relatively poor. The chlorophyll content and most photosynthetic parameters in SR17 showed no significant changes, and leaf gas exchange performance and chlorophyll status were the least affected, whereas IR29 suffered severe damage. Agronomic trait investigation revealed that, compared with the control, SR17 exhibited the smallest reductions in plant height, spikelets per panicle, 1000-grain weight, grain yield per plant, and main spikelet number under salt stress, and the decreases in key yield-related indicators—effective panicle number, grain yield per plant, and seed setting rate—were not significant. This study confirms that SR17 possesses superior salt tolerance and holds potential for further breeding and multi-environment trials, while also providing an important basis for elucidating the physiological mechanisms of salt tolerance during the reproductive stage of rice.