Plant phosphorus status has a limited influence on the concentration of phosphorus-mobilising carboxylates in the rhizosphere of chickpea

Two experiments were conducted to investigate whether carboxylate exudation by chickpea ( Cicer arietinum L.) is a response to phosphorus ( P) deficiency or a constitutive trait. The effect of P supply on carboxylate concentrations in the plant and in the rhizosphere of chickpea cultivar Heera was studied in a sand culture. Plants were grown in pots supplied with 200 mL of solution containing 0 - 500 muM P every 3 d. Malonate was the main carboxylate exuded, and the main carboxylate in roots; shoots contained mainly citrate and malate. Contrary to what has been reported for other species, carboxylate concentrations in the rhizosphere decreased only slightly at high P supply, but they were still substantial. The effect of P supply on the rate of exudation was studied in a split-root sand culture. Root systems were split into two pots, one root half received no P and the other half received 200mL of solution containing 0 - 500 muM P. The rhizosphere of both root halves contained similar concentrations of carboxylates, even when the plants received a different supply of P. Our results indicate that carboxylate exudation is determined by internal P rather than external factors. The fact that chickpea roots always exude carboxylates indicates that exudation in this species is largely constitutive.