Root exudation index: screening organic acid exudation and phosphorus acquisition efficiency in soybean genotypes

High-molecular-weight secretory proteins and low-molecular-weight exudates (carboxylates, phenols, free amino acids and sugars) released from roots of soybean (Glycine max (L.) Merr.) differentially influence genotypic phosphorus (P) acquisition efficiency (PAE). We hypothesised that genotypes with higher root exudation potential would exhibit enhanced P acquisition, and screened 116 diverse soybean genotypes by labelling shoots with 14CO2. A root exudation index (REI) derived from total 14C in the root exudate at sufficient (250 μm) and low (4 μm) P levels was used to classify genotypes for PAE. Genotypes with REI >2.25 exhibited significantly higher exudation at low than at sufficient P, which in turn increased PAE. Under low P availability, efficient genotypes exude a greater quantity of organic compounds into the rhizosphere. This increases P availability to meet the crop requirement, enabling the crop to produce consistent biomass and seed yield with reduced fertiliser addition. Such maintenance of growth and yield potential by mining the inherent soil P is a favourable trait in genotypes, reducing dependence on P fertilisers. Measuring REI at seedling stage to select P-efficient plants accelerates the screening process by accommodating large numbers of genotypes.