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

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 O-2, and by the direct release of H+ from the roots to balance cation-anion intake. Here, are report rates of release of O-2 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 O-2 release were 0.8-3.3 mu mol per plant d(-1), or 22-87 mu mol g(-1) (root dry mass) d(-1). Assuming that the bulk of the O-2 was released from medium and fine roots, the fluxes of O-2 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 Hf 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.