Unveiling the Impact of Organic Fertilizer on Rice (Oryza sativa L.) Salinity Tolerance: Insights from the Integration of NDVI and Metabolomics

Soil salinization threatens global agriculture, reducing crop productivity and food security. Developing strategies to improve salt tolerance is crucial for sustainable agriculture. This study examines the role of organic fertilizer in mitigating salt stress in rice (Oryza sativa L.) by integrating NDVI and metabolomics. Using salt-sensitive (19X) and salt-tolerant (HHZ) cultivars, we aimed to (1) evaluate changes in NDVI and metabolite content under salt stress, (2) assess the regulatory effects of organic fertilizer, and (3) identify key metabolites involved in stress response and fertilizer-induced regulation. Under salt stress, survival rate of the 19X plants dropped to 6%, while HHZ maintained 38%, with organic fertilizer increasing survival rate to 25% in 19X and 66% in HHZ. NDVI values declined sharply in 19X (from 0.56 to <0.25) but remained stable in HHZ (similar to 0.56), showing a strong correlation with survival rate (R-2 = 0.87, p < 0.01). NDVI provided a dynamic, non-destructive assessment of rice health, offering a faster and more precise evaluation of salt tolerance than survival rate analysis. Metabolomic analysis identified 12 key salt-tolerant metabolites, including citric acid, which is well recognized for regulating salt tolerance. HTPA, pipecolic acid, maleamic acid, and myristoleic acid have previously been reported but require further study. Additionally, seven novel salt-tolerant metabolites-tridecylic acid, propentofylline, octadeca penten-3-one, 14,16-dihydroxy-benzoxacyclotetradecine-dione, cyclopentadecanolide, HpODE, and (+/-)8,9-DiHETE-were discovered, warranting further investigation. Organic fertilizer alleviated salt stress through distinct metabolic mechanisms in each cultivar. In 19X, it enhanced antioxidant defenses and energy metabolism, mitigating oxidative damage and improving fatty acid metabolism. In contrast, HHZ primarily benefitted from improved membrane stability and ion homeostasis, reducing lipid peroxidation and oxidative stress. These findings primarily support the identification and screening of salt-tolerant rice cultivars while also highlighting the need for cultivar-specific fertilization strategies to optimize stress resilience and crop performance. Based on the correlation analysis, 26 out of 53 differential metabolites were significantly correlated with NDVI, confirming a strong association between NDVI shifts and key metabolic changes in response to salt stress and organic fertilizer application. By integrating NDVI and metabolomics, this study provides a refined method for evaluating salt stress responses, capturing early NDVI changes and key salinity stress biomarkers. This approach may prove valuable for application in salt-tolerant variety screening, precision agriculture, and sustainable farming, contributing to scientific strategies for future crop improvement and agricultural resilience.