A comparative study on the aluminium‐ and copper‐induced organic acid exudation from wheat roots

Abstract

It is well established that aluminium (Al) and some heavy metals can elicit organic acid exudation from a range of species. In the present research we found that copper (Cu) can also induce organic acid exudation from the roots of wheat, rye, triticale, maize and soybean. Using intact wheat plants, we made a comparative study of Al‐ and Cu‐ induced organic acid exudation. In 5‐day‐old wheat seedlings, severe Cu stress (40 µM CuCl2) mainly induced the exudation of malate and citrate, and Al‐tolerant genotypes could release significantly greater amounts of malate than Al‐sensitive genotypes. The time course of the exudation of malate and citrate from the roots of 5‐day‐old seedlings of wheat (cv. Atlas) in 200 µM AlCl3 was similar to that in 40 µM CuCl2. In older wheat plants (15‐day‐old), moderate Cu stress (12 µM CuCl2) induced the exudation of large amounts of citrate and addition of Al or La sharply reduced Cu‐induced citrate exudation, while Cu or La did not affect Al‐induced malate efflux. When half of the root system of Atlas wheat was immersed in Al‐ or Cu‐containing solution and the remaining half in Al‐ or Cu‐free solution, organic acids were only exuded into the solution containing Al or Cu. This suggests that no long distance signal transport is involved in organic acid exudation induced by Al or Cu, and that direct contact of Al or Cu with plant roots is a prerequisite for the induction of organic acid exudation. The anion‐channel inhibitor niflumic acid (NIF) significantly stimulated the exudation of both citrate and malate from 5‐day‐old wheat seedlings under severe Al or Cu stress. Our results suggest that Cu‐induced organic acid efflux may be a common response, which may play a role in alleviating Cu toxicity in plants.