Induction of oxidative stress and antioxidative mechanisms in hyacinth bean under zinc stress

Abstract

Zinc toxicity to plants occurs in soils contaminated by mining and smelting activities, in agricultural soils treated with sewage sludge, and in urban and peri-urban soils enriched by anthropogenic inputs of Zn. Zinc-induced stress in Hyacinth bean (Lablab purpureus) was investigated by growing seedlings in hydroponics, supplemented with Zinc (0-600 μM), under controlled growth conditions. Changes in growth parameters, enzyme activities and other stress response factors, as a function of Zn uptake are directly or indirectly related to the cellular free radical scavenging systems. These parameters were studied in 10-day old Lablab seedlings over 72 h of exposure. Relative to leaves, roots accumulated ~12-fold higher amounts of Zn. Oxidative stress markers, H2O2, malondialdehyde, ascorbate, proline, soluble sugars, putrescine and spermidine were elevated in both tissues, whereas, glutathione and spermine levels showed a steady decline. Total phenol increased in roots, but decreased in leaves. Zn-stress enhanced antioxidant enzymes, guaiacol peroxidase, glutathione reductase, and polyphenol oxidase in concentration and time dependent manner in leaves, but showed an inverse relationship in roots. Catalase decreased in both leaves and roots. Metabolic enzymes β-amylase and acid phosphatase increased in both tissues. Invertase increased in leaves but declined drastically in roots. The results suggest that primary antioxidative response originates in leaves of Lablab, while roots are involved in direct uptake of heavy metals and are not adept in extrusion. Also, lower concentrations of Zn (up to 100 μM) stimulate growth of Lablab, but higher concentrations proved detrimental to the plant.

Keywords: Lablab purpureus, Malondialdehyde, Polyamines

African Crop Science Journal, Vol. 20, No. 1, pp. 17 - 29