Limited effects of the soluble organic phosphorus fraction on the root phosphorus uptake efficiency of upland rice genotypes grown in acid soil

Abstract

ABSTRACT The root phosphorus (P) uptake efficiency (RE), defined as plant P uptake per unit root mass or root area, may contribute to the P efficiency of upland rice grown in acid, P-deficient soils. The identification of root traits conferring RE of rice has been compromised by the lack of attention given to P speciation when evaluating P-mining mechanisms. Here we disentangled the effect of soluble organic P (PO) from that of total soluble P (PT) on the RE in rice seedlings in acid soil. Six rice genotypes were grown for 21 days in P-deficient substrates, i.e. a sample of an acid mineral soil, an acid peat and a mixture thereof. Each of the three substrates were amended with different P doses, partially limed and incubated, yielding substrates with significant differences in total soluble P (0.005–0.41 mg P/L) and in the percentage of organic P in that pool (PO/PT, 0–55%). Plants showed a large growth response to the P addition. There was a significant genotypic variation in P uptake (0.9–1.5mg P/plant) and RE (4.4–8.3 mg P/g root mass) under moderate P deficiency but these traits were unaffected by the PO/PT in the soluble fraction of the substrates. Along the same lines, phosphatase activity in the rhizosphere soil was unaffected by genotypes and did not explain the RE among all data (R2 = 0.17, ns). A multiple regression model showed that the RE of rice seedlings was mainly affected by the inherent genotypic effects, the PO/PT and the total soluble P concentration in soil (R2 = 0.73), while the genotype-PO/PT interaction only marginally improved the RE model (R2 = 0.78). This suggests limited genotypic effects due to the better use of organic P. A root elongation test in the acid mineral soil that was either or not limed suggests that the differences in acid soil tolerance may play a larger role in the genotypic performance of RE than organic P utilization potential.