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

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Abstract
SUMMARY: 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.