MALATE EFFLUX FROM ROOT APICES AND TOLERANCE TO ALUMINUM ARE HIGHLY CORRELATED IN WHEAT

Aluminium (Al) can stimulate the efflux of malate and other organic ac ids from root apices of wheat (Triticum aestivum L.) seedlings. This r esponse has been implicated in a mechanism of Al tolerance since the a mount of malate released from an Al-tolerant genotype was 5-10-fold gr eater than the amount released from a near-isogenic, but Al sensitive, genotype. In the present study, 36 wheat cultivars were screened for Al tolerance and for the amount of malate released from their root api ces with a standard Al treatment. Excised root apices (3.0 mm) were us ed to measure malate efflux, and the relative tolerance to Al was dete rmined from root growth measurements in 3 and 10 mu M AlCl3 with 200 m u M CaCl2, pH 4.3. There was a significant correlation between relativ e tolerance of the genotypes to Al and the amount of malate released f rom their root apices. Growth measurements were also used to investiga te the amelioration of Al toxicity by exogenous malate. In the presenc e of 3 mu M Al alone, relative root growth of an Al-sensitive genotype was reduced to 13% of the control. Addition of 10 mu M malate to the solution increased relative root growth to 50%, and 20 mu M malate com pletely alleviated the Al-induced inhibition of root growth. The resul ts support the hypothesis that the Al-stimulated efflux of malate from root apices is involved in a general mechanism for Al tolerance in wh eat.