Spatial patterns of citrate exudation along the rice root apex in low-phosphorus soil

Root-released citrate can promote phosphorus (P) mobilisation in soils, but its spatial distribution along intact roots remains poorly characterised. This brief research report presents the millimetre-scale mapping of citrate exuded by soil-grown Nerica-4 rice ( Oryza sativa × Oryza glaberrima ) roots using a zirconium-hydroxide (ZrOH) hydrogel sampling method. Plants were grown in rhizoboxes containing a soil with low P content amended with 50 or 250 mg P kg -1 . ZrOH hydrogels were deployed to the apical 3 cm of selected roots for 24 h. After removal, the gels were cut into 0.4×1 cm sections, citrate was eluted with 0.5 mol L -1 NaOH, and quantified by high-performance-ion-chromatography single-quadrupole mass-spectrometric detection. The method resolved clear longitudinal variation in citrate exudation. In all four examined roots, the most apical section showed both the highest citrate quantity per unit root length and the highest citrate concentration in the hydrogel. Total citrate recovered from gel segments ranged from 3.90 to 8.75 ng in soils supplied with high P and from 18.67 to 59.28 ng in soils supplied with low P. Although the limited replication (n = 2 per treatment) precluded inferential statistical analysis, the consistent apical maxima indicate that citrate exudation is concentrated in young root tissues, where it may support local P mobilisation ahead of the growing root tip. ZrOH hydrogels therefore provide a minimally invasive approach for linking spatially resolved carboxylate exudation to root development and nutrient availability directly in soil, offering the potential for mechanistic investigations of rhizosphere P-acquisition strategies under realistic growth conditions.