Root exudation of organic adds and/or acid phosphatase cannot explain phosphorus mobilization by Arachis pintoi

摘要

Forage groundnut, Arachis pintoi (Ap) is known to grow well on highly weathered low P soils of the Amazonia and the objective of this research was to investigate whether arbuscular mycorrhiza (AM) infection, organic acid root exudates or acid phosphatase (APase) activity could explain the P efficiency of Ap. Two genotype of forage groundnut (Ap cv. CIAT 17434 and CIAT 18744) with different P efficiency were grown under controlled environmental conditions on a fossil Oxisol, 50% clay, pH (CaCl_2) 5.2 and P soil solution concentration of 0.12 p.M. There were two treatments: -P (no P added) and +P [1000 mg P kg~(-1) as Ca(H_2PO_4)_2]. Plants were harvested after 30, 60 and 100 days of growth. The dry weight of shoot for both genotypes in the +P treatment was similar. However, in the -P treatment, the genotypes showed significantly different growth, and after 100 days CIAT 18744 was 4 times larger than CIAT 17434. The higher growth was related to a higher P uptake, which was not because of a larger root system but because of a higher P influx. The P influx was significantly different between the genotypes and with age, especially the CIAT 18744, showed a 5 fold increment of the P influx from young to older plants, which was related to a plant induced increase of P in soil solution and to a light increase of pH in soil solution. Neither AM nor the APase activity of the root explained the differences in P influx. Of 14 organic acids analyzed, only lactic and acetic acid were found in root exudates. But they could not be related to the P influx. Phosphorus influx of both genotypes increased with age but the rate of organic acid exudation decreased. From this, neither AM nor organic acid exudation nor APase activity of Ap were the explanation for its high P acquisition.