Endophytic bacteria improve physiology, growth, and yield of rice under salinity stress across two growing seasons

Salinity is a major constraint on rice productivity, threatening food security across salt-affected agricultural regions including Thailand. Endophytic bacteria have shown promise in alleviating salinity-induced stress in crops. This study evaluated the effects of four salt-tolerant endophytic bacterial isolates on the physiology, growth, and yield of rice under saline greenhouse conditions across two consecutive growing seasons. A 3 × 5 factorial experiment in a randomized complete block design was established with three salinity levels and five bacterial treatments (uninoculated control, CKPK01, SHPP21, SHPP24, and SKPK29). SPAD chlorophyll meter reading (SCMR), proline, relative water content (RWC), electrolyte leakage (EL), plant height, tiller number, unfilled grain percentage, 1,000-grain weight, grain yield, and visual salt injury scores were recorded. Increasing salinity significantly reduced rice physiological performance, growth, and yield while increasing membrane damage, proline accumulation, unfilled grain, and injury score across both seasons. During the rainy season, plants exposed to the highest salinity level (8 dS/m) for 21 days triggered severe physiological and morphological stress. This treatment resulted in the lowest SCMR (35.9) and RWC (72.9%), while driving EL (30.9%) and proline levels (665.5 µg/g) to their maximums. Morphological degradation was equally pronounced, as evidenced by the shortest plant height (123 cm) and fewest tillers (9.1). These cumulative stresses severely depressed final yield, leading to the lowest 1,000-grain weight (16.8 g), and grain yield (10.0 g/pot), alongside the highest unfilled grain percentage (70.8%) and damage score (7.8), showing a consistent pattern across both seasons. Bacterial inoculation mitigated these adverse effects, improving plant performance compared with the uninoculated control. Among the isolates, SHPP24 consistently had the best results even under 8 dS/m, showing the greatest improvements in physiological traits and overall yield. In the rainy season, it recorded the highest SCMR (43.9), proline content (466.3 µg/g fresh weight), and RWC (84.6%) at 21 days after salt treatment (DAT). Additionally, it achieved the highest 1000-grain weight (23.2 g) and grain yield (21.7 g/pot), while maintaining the lowest EL at 16.0% and unfilled grains at 38.7%. SKPK29 isolate closely followed this performance, displaying a similar trend across both seasons. Both isolates demonstrated robust and consistent potential as bioinoculants for improving rice salt tolerance, growth, and productivity across contrasting growing seasons, and multi-location field trials are recommended to validate these findings under diverse saline field conditions prior to commercial application.