Contrasting Responses to Phosphorus Status by Arachis pintoi (Krapov and W.C. Gregory): A Lesson for Selecting Vegetables for Cultivation in Kenyan Ecozones
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Tropical soils are characterized by low pH with high aluminium saturation and phosphorus (p) fixation. Pinto Peanut (Arachis pintoi (Krapov and W.C. Gregory)) grows in such soils with no P deficiency symptoms. The current experiment dissected an array of mechanisms envisaged to explain growth advantages of Arachis pintoi (AP) in such P-deficient environments. Split root experiment was conducted with three ecotypes (CIAT 17434, CIAT 18744 and CIAT 22172) cultured in hydroponics with Hoagland solutions and P was later withheld from one root compartment. Nutrient solution pH was determined from 20-49 Days after sowing using pH meter, while exudates were collected by overlaying filter papers on root surfaces. Carboxylates were extracted from filter papers and later determined by HPLC (Bio-Rad, Richmond, CA, USA). CIAT 18744 produced significantly higher (p≤0.05) shoot and root (biomass), whereas CIAT 22172 had lowest. CIAT 18744 registered minimal acidification under P-deficiency compared to other two ecotypes and produced highest TCA cycle carboxylates (1.72 and 2.2 nmol/h/cm root length) malate and citrate, respectively whereas CIAT 22172, exuded only 0.50 and 0.52 nmol/h/cm root length] malate and citrate, respectively. There was no correlation between growth medium acidification and carboxylate exudation; probably implying carboxylates lacked contribution to growth medium acidification. Selecting for carboxylate exudation and internal P reallocation would be a better strategy in acid soils (e.g., Kenyan Alfisols) since carboxylates can complex aluminium without acidification of rhizosphere, while protonation that leads to rhizosphere acidification would be a better strategy in high pH soils, favoring availability of precipitated P (e.g., Calcareous Kenyan soils).