Abstract

 
 
 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.
 
 
 
 
 Significance: 
 
 
 
 Guaiacol peroxidase is selectively involved in soybean drought resistance at flowering stage.
 The upregulation of ascorbate peroxidase activity at both growth stages in drought-resistant cultivars suggests that this enzyme could be used as a biochemical marker of drought resistance in soybeans.
 In contrast to the literature, activities of both enzymes increased with maturity irrespective of whether the plant is drought susceptible or resistant.
 
 
 
Highlights
Soybean (Glycine max L.) is a small but growing component of the agricultural economy of South Africa[1] and is predicted to become a major crop in Africa because of its high protein content[2]
Drought at flowering or early pod development stages significantly increases the rate of pod abortion and decreases seed yield.[22]
Prolonged drought stress leads to overproduction of ROS23; plants need to respond with a battery of antioxidative enzymes in order to thrive[9]
Summary
Soybean (Glycine max L.) is a small but growing component of the agricultural economy of South Africa[1] and is predicted to become a major crop in Africa because of its high protein content[2]. Plants respond to drought stress with a cascade of biochemical reactions such as production of abscisic acid, which is aimed at facilitating stomatal closure thereby reducing water loss through transpiration This action reduces water loss, it limits carbon dioxide fixation and reduces regeneration of NADP+ by Calvin cycle, which results in increased formation of oxygen radicals or reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), superoxide anion (O2-) and hydroxyl radicals (OH-) through enhanced leakage of electrons to molecular oxygen.[5] Studies show that ROS such as H2O2 can act as signal molecules for induction of defence responses in plants when moderately produced.[6,7] excessive production of these molecules may lead to oxidative stress, which in turn may have damaging effects on the photosynthetic pigments, membrane lipids, proteins and nucleic acids.[8] To avoid oxidative stress and to thrive, plants need to keep ROS production in the cells to a minimum,[9] which can be achieved through employment of different antioxidative mechanisms that may be enzymatic or non-enzymatic in nature. These enzymes may act independently or in conjunction to catalyse conversion of H2O2 to water and O2,11,12 thereby minimising damage within the plant
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