Changes in organic acid composition, proton efflux and root length in rice genotypes differing in grain zinc accumulation efficiency

Rice genotypes exhibit notable diversity in grain zinc content, with wild types accumulating higher levels than cultivated varieties, even in zinc-deficient soils. This study focuses on the wild rice genotype Karuppunel (Zn tolerant), along with cultivated varieties CO51 (Zn responsive) and ADT39 (Zn sensitive). The research explores the relationship between the composition of organic acids in root exudates, proton efflux and root length at various growth stages under both +Zn and -Zn conditions.

The experiment was conducted by growing rice genotypes Karuppunel, CO51 and ADT39 under two different zinc applied conditions. Non target based metabolomics was done to study the alterations in organic acid composition in root exudates using LCMS/ MS. Proton efflux was studied by the methodology given by Bashan et al. (1989) and root length was measured from the plant's base to the tip of the longest root. The data was analyzed by using Metaboanalyst 5.0 software and Origin Pro 2023b, version 10.0.5.157.

By non-target metabolomics we identified 14 organic acids were identified in rice root exudates across growth stages in response to zinc. Random Forest analysis's variable importance plot consistently highlights nicotinic acid in Karuppunel, regardless of zinc conditions. Karuppunel exhibits higher proton efflux of 4.23 and 5.12 μmole H⁺ g fresh weight^-1 h^-1 followed by CO51 i.e., 3.76 and 4.14 μmole H⁺ g fresh weight^-1 h^-1 under +Zn and -Zn respectively, while ADT39 shows lower proton efflux. Despite varying zinc conditions, root length increases across stages, with Karuppunel consistently having the highest length, particularly at grain-filling of 16.65 cm under +Zn and 15.75 cm under -Zn. Understanding these physiological changes may contribute valuable insights for breeders in designing zinc nutrient acquisition-focused breeding programs.