Changes in rice root architecture, porosity, and oxygen and proton release under phosphorus deficiency

Abstract

Earlier work has shown that rice plants growing in reduced soil are able to solubilize P and thereby increase their P uptake by inducing an acidification in the rhizosphere; the acidification is caused by H⁺ produced in Fe²⁺ oxidation by root-released O₂, and by the direct release of H⁺ from the roots to balance cation–anion intake. Here, we report rates of release of O₂ and H⁺ from P-stressed and P-sufficient rice plants into sand cultures continuously perfused with deoxygenated nutrient solution. The P stress was sufficient to reduce plant dry mass by roughly half, but root dry mass increased roughly twofold and root surface area 2·5-fold. The proportion of fine roots increased from 11 to 21% of root length under P deficiency; root porosity, averaged over the whole root system, increased from 0·25 to 0·40. Apparent rates of O₂ release were 0·8–3·3 μmol per plant d⁻¹, or 22–87 μmol g⁻¹ (root dry mass) d⁻¹. Assuming that the bulk of the O₂ was released from medium and fine roots, the fluxes of O₂ were 0·02–0·13 nmol dm⁻² (root surface) s⁻¹, which is in the range found for soil-grown plants. The release per plant was twofold greater in the low P treatment, although rates of release per unit root mass were slightly lower. The increased release under P deficiency is consistent with the increased length of fine roots and increased porosity. Rates of H⁺ release were 0·7–1·2 mmol per plant⁻¹ d⁻¹, or 1·4–6·1 mmol g⁻¹ (root dry mass) d⁻¹. The H⁺ release per unit plant dry mass was 60% greater in the low P treatment, but the release per unit root mass was 2·5-fold lower. The increased H⁺ release under P deficiency was associated with increased NH₄⁺ intake and decreased NO₃⁻ intake, and a tenfold increase in plant NO₃-N. This suggests that P deficiency reduced NO₃⁻ assimilation, causing reduced NO₃⁻ influx and/or increased efflux.