Differential involvement of ascorbate and guaiacol peroxidases in soybean drought resistance

Soybean (Glycine max L.) is a small but growing component of the agricultural economy of South Africa and is predicted to become a major crop in Africa because of its high protein content. Drought induction at flowering or early stages of pod development has detrimental effects on soybean yield. As antioxidative enzymes play a protective role in plants during various abiotic stress conditions, this study was conducted to investigate how ascorbate (Enzyme Commission (EC) number 1.11.1.1) and guaiacol (EC: 1.11.1.7) peroxidases are involved in soybean drought resistance at different maturity stages (flowering and pod development). We also investigated whether the levels of these enzymes decline with plant maturity. Three tolerant soybean genotypes (G1, G2, G3) and a susceptible genotype (G4*) were used. These cultivars were categorised according to their sensitivity to drought stress in previous studies. The activity of ascorbate peroxidase was significantly induced by drought stress at both growth stages with higher activity in the resistant than susceptible plants, strongly supporting the protective role of this enzyme against drought stress at both developmental stages. The guaiacol peroxidase activity was induced to higher levels in the resistant than in the susceptible plants at flowering only, with no significant increase observed at pod development stage, indicating its selective protective involvement against drought stress. Interestingly, the levels of these enzyme activities were induced in all cultivars at both developmental stages, irrespective of drought stress, indicating that their activities increased with maturity.