Role of polygalacturonic acid and the cooperative effect of caffeic andmalic acids on the toxicity of Cu(II) towards triticale plants (× Triticosecale Wittm)

In this study, we investigated the cooperative effect of caffeic (CAF) and malic (MAL) acids as well as the role of polygalacturonic acid (PGA) on the toxicity of copper (Cu)(II) towards triticale plants (× Triticosecale Wittm.). Plant growth experiments carried out in hydroponic solutions at pH 6.0 revealed that CAF and PGA, but not MAL, were quite effective at reducing Cu(II) phytotoxicity and that MAL addition abolished the positive effects of CAF. The study of the CAFCu(II) system in aqueous solution clearly indicated that the lower toxicity of Cu(II) in the presence of CAF was due to the occurrence of a redox reaction between CAF and Cu(II), while the phytotoxicity recorded in the Cu(II)-CAF-MAL system was attributed to a competitive effect of MAL in the Cu(II) complexation by CAF, which ultimately reduced the redox activity. The lowest Cu(II) phytotoxicity recorded in the presence of PGA was attributed to the high affinity of the polysaccharidic matrix towards the metal ion. The results from this study shed new light on the Cu(II)-detoxifying action of selected root exudates and highlight the negative cooperation of MAL on the Cu(II)-detoxifying activity by CAF.