| Abstract: | We investigated whether concentrations of carboxylates in the rhizosphere of chickpea (Cicer arietinum L.) roots were related to soil phosphorus levels. In a field experiment, cultivar Sona was grown at two P levels on eight soil types at three locations. There were large differences in extractable (0.2 mM CaCl2) rhizosphere carboxylate concentrations amongst the locations. The effect of P fertiliser was variable and carboxylate concentrations depended on soil type. To examine the effect of soil P in more detail, a glasshouse experiment was carried out, in which three cultivars (Heera, Sona and Tyson) were grown at four P levels on one soil type. The biomass of chickpea plants increased with increasing P level of the soil, and the root mass ratio decreased at the highest soil P level. However, rhizosphere concentrations of the carboxylates malonate, malate and citrate did not differ significantly between P treatments. This implied that there was no simple relation between available P and root exudation rates, in contrast to earlier results in studies using hydroponics. Cultivars differed in carboxylate concentration pattern: Sona and Tyson showed a tendency towards increased rhizosphere carboxylate concentrations at the second harvest, whereas the carboxylate concentration of Heera tended to decrease. It is hypothesised that chickpea roots always exude a basal level of carboxylates into the rhizosphere. They only increase carboxylate exudation considerably when the P availability is extremely low, which may occur in soils that strongly bind P. |
