Physiological traits and metabolic profiles of contrasting rice cultivars under mild salinity stress during the seedling stage

This study aimed to investigate the effects of mild salinity stress on some physiological traits and metabolic profiles of rice during the seedling stage. Three rice cultivars differing in salinity stress response, 'KDML105' (salt-sensitive, local Thai jasmine rice), 'IR29' (salt-sensitive rice), and 'Pokkali' (salt-tolerant rice), were subjected to mild salinity stress by flood-irrigated with 40 mM NaCl. Salinity stress significantly reduces the growth and biomass of the sensitive cultivars. Electrolyte leakage in 'KDML105' and 'IR29' also increased significantly while the relative water content (RWC) decreased. However, salinity stress did not affect growth, electrolyte leakage, and RWC in the tolerant cultivar. Salinity significantly increased Na+/K+ ratio in shoot and root of all cultivars. Nevertheless, the increase of Na+/K+ ratio in 'KDML105' and 'IR29' was higher than that of 'Pokkali'. The metabolic profile analysis of 'KDML105' showed that uracil, L-proline, malonate, L-carnitine, L-lactate, and guanine were significantly increased, but the levels of dihydroxyacetone and glycine were significantly decreased under salinity conditions. In 'IR29', L-proline, melatonin, gammaaminobutyrate, and acetate were significantly increased. In 'Pokkali', however, sugar alcohols and sugars were slightly increased when exposed to salinity conditions. The metabolic pathway analysis revealed one involved metabolic pathway of 'KDML105': aminoacyl-tRNA biosynthesis, and three involved those of 'IR29': arginine proline metabolism, sulfur metabolism, and butanoate metabolism. The difference in the metabolic profile of the salt-tolerance salt-sensitive cultivars provides new insights to identify additional novel biomarkers that will help better understand the mechanisms for salt tolerance in rice.