Effect of benzoic acid hydroxy and methoxy ring substituents during cucumber (Cucumis sativas L) germination. I. Isocitrate lyase and catalase activity

Benzoic acids (BAs) are released by plants through leaching of leaf exudate s and rhizodeposition. Most of them exert allelopathic effects. BA-OH ring substituents generally caused an increase in germination percentage, wherea s -OCH3 ring substituents decreased the percentage of germination. Hydroxy substituents increased isocitrate lyase (ICL) activity, whereas -OCH3 ring substituents decreased enzyme activity indicating a direct effect on enzyme catalysis. The addition of BAs to the reaction mixture containing extracts from seedlings incubated with the related BA derivatives and to extracts f rom controls confirmed the direct stimulating (-OH) and inhibiting (-OCH3) effect of BA substituents on ICL catalysis. Catalase (CAT) activity was dra matically increased by hydroxylated BAs, even though some methoxylated BAs exerted a strong activating effect. Native and SDS-PAGE analyses indicate t he presence of two bands of 236 and 66 kDa, which correspond to ICL enzyme activity levels. The enhancing of silver staining and the disappearance of these bands are related to ICL enzyme activity suggesting a possible effect of BAs on synthesis of ICL. The physiological effect of BAs on cucumber ge rmination is also discussed. (C) 1999 Elsevier Science Ireland Ltd. All rig hts reserved.