In addition to foliar manganese concentration, both iron and zinc provide proxies for rhizosheath carboxylates in chickpea under low phosphorus supply

Aims Plants deploying a phosphorus (P)-mobilising strategy via carboxylate release have relatively high leaf manganese concentrations ([Mn]). Thus, leaf [Mn] is a proxy for the amount of rhizosheath carboxylates. Whether the concentrations of other leaf micronutrient, such as iron ([Fe]), zinc ([Zn]) and copper ([Cu]), show a similar signal for rhizosheath carboxylates is unclear. Methods We grew a large number of chickpea genotypes in two glasshouse studies with different growth media, P sources and P levels. Seven weeks after sowing, we determined concentrations of micronutrients in mature leaves, and the quantity and composition of rhizosheath carboxylates. Results For 100 genotypes grown in river sand with low P supply, leaf [Fe] (R-2 = 0.36) and [Zn] (R-2 = 0.22), like leaf [Mn] (R-2 = 0.38), were positively correlated with the total amount of rhizosheath carboxylates. For 20 genotypes grown in a soil mixture, leaf [Fe], [Zn], [Cu] and [Mn] showed positive correlations with total rhizosheath carboxylates that were stronger under moderately low P (R-2 = 0.59, 0.59, 0.54, 0.72) than severely low P (R-2 = 0.39, 0.28, 0.20, 0.36) or sufficient P (R-2 = 0.36, 0.00, 0.01, 0.50) supply. Malonate was the predominant carboxylate in the rhizosheath and was significantly correlated with leaf micronutrient concentrations in both experiments. Conclusions In addition to leaf [Mn], leaf [Fe] and [Zn] can be used as alternative and easily measurable proxies for belowground carboxylate-releasing processes in chickpea under low-P supply, particularly on moderately low-P soils.