Flag leaf metabolomics and agronomic performance in rice cultivars under nitrogen-potassium fertilization

Rice is a substantial cereal crop and staple food in several world regions. Nitrogen (N) and potassium (K) are key to increasing rice growth and development, ultimately increasing the farmer's net profit. Environmental pollution also results from the careless application of nitrogenous fertilizers for commercial agricultural cultivation. Understanding the metabolic profiling underlying rice nitrogen use efficiency (NUE) is still limited. Therefore screening these two cultivars on a commercial and economic basis is essential, as this would be beneficial in revealing new insights. The flag leaf metabolic expression profiles of two rice cultivars, IRRI 6 (V1) and ksk 434 (V2), collected from low and high NK treatments at anthesis were examined. The optimal doses were applied to 45-day-old transplanted seedlings. Our findings revealed that in response to the NK application, ksk 434 (V2) yielded higher values for morphological traits such as total dry weight, plant height, total number of tillers, rice flag leaf weight, total fresh weight and rice flag leaf area than basmati 385 (V1). Furthermore, N2K2 (114:104kg/ha) application significantly increased NUE, rice grain yield, chlorophyll content and metabolic expression compared to plants treated with N1K1, N3K3 and the control. Twenty-four metabolites related to photosynthetic synthesis were annotated, among which 8-Acetylegelolide, citric acid, methionine, chlorophyll a/b and (S)-2-Aceto-2-hydroxybutanoate were positively correlated with the photosynthetic cycling process. Meanwhile, UDP-glucose, 4-methylcellulose, galactosamine, L-glutamic acid and C5-branched dicarboxylic acid metabolism were positively associated with yield. Furfural, L-piperidine and (S)-2-acetone-2-hydroxybutyric acid were downregulated after nitrogen application in both cultivars compared to control. The optimum dose of fertilizer application also upregulated the expression of NAPDH, ndhA, ndhD, ATP1, psAc, ndhB and rpoB genes in the flag leaf of rice at the heading stage as compared to control plants. In future, multiomics techniques will be performed to identify key genes/pathways involved in N metabolism, that may potentially improve root architecture and increase NUE.