Greenhouse experiments were carried out to assess the phosphate uptake efficiency (EPa) by the aerial plant parts and associated plant parameters in sorghum (Sorghum bicolor L. Moench) and rice (Oryza sativa) genotypes with differential tolerance to aluminum. The following plant parameters were determined: acid phosphomonoesterase activity (APA), arbuscular mycorrhiza colonized root lenght (%CRL), soil-pH and rhizosphere-pH and P uptake efficiency. Two Ultisols of low pH and low phosphate availability from the savanna of Venezuela were employed. In a greenhouse experiment five sorghum cultivars, i.e. Chaguaramas III (Ch III), Chaguaramas VII (Ch VII), Dekalb-59 (D-59), Criollo-26 and Ismael were studied. In another experiment four acid tolerant rice cultivars of Oryza sativa (CT-81-1, Fonaiap 2000, Fedearroz y CT-102-1) and one cultivar of Oryza glaberrima susceptible to Al toxicity were compared. P fertilization treatments were: control (OP), Riecito rock phosphate (RRP) and triple superphosphate (TSP). Significant sorghum genotypic differences were found in the parameters evaluated enabling them to grow on the acid soils. Their response was differential depending on the P source and the interaction between source and genotype. The use of the Al-tolerant cultivars Ch III, Ch VII is a potential means to obtain high dry matter production for cattle feeding when pasture supply is low. The cultivars Ch VII, Ch III and Ismael showed higher P-uptake efficiency index (EPa) when fertilized with the local Riecito rock phosphate, thus this PR can be considered to increase soil P status and improve agricultural production in the acid soils of the savannah of Venezuela. Cultivars with higher P uptake efficiency, in particular Ch III and Ch VII (tolerant to Al toxicity) expressed mechanisms such as high % of CRL by arbuscular mycorrhiza and increases in rhizospheric pH whereas cultivars with lower P uptake efficiency such as D-59 (susceptible to Al toxicity) expressed higher APA. The acid tolerant rice cultivars (CT-81-1, Fonaiap 2000, Fedearroz 50, CT-102-1) showed increased rhizosphere-pH whereas the acid susceptible genotype (Oryza glaberrima) had lower rhizosphere-pH. All acid tolerant rice cultivars can be considered for cultivation in the savannahs of Venezuela.
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