AMELIORATING ROLE OF N.P.K. FERTILIZER ON THE TOXIC EFFECTS OF Ni ON (SORGHUM) ROOT ANTIOXIDANT ENZYMES1

ABSTRACT

This study investigated the activities of superoxide dismutase (SOD), catalase (CAT), glutothione peroxidase (GP) and the level of malondialdehyde (MDA) in the root of sorghum grown in soils contaminated with 30ppm nickel, 30ppm nickel +20ppm fertilizer and 30ppm nickel + 40ppm fertilizer.  Sixty sorghum seeds were germinated in these contaminated soils and  were harvested after 2 weeks, 3 weeks, and 4 weeks of planting.  Treatment of the plants with 30ppm nickel significantly increased (P < 0.05) the activities of SOD and the level of MDA in the roots compared with the controls.  Also, the treatment significantly decreased (P < 0.05) the activities of CAT and GP in the roots compared with controls.

The study also revealed a significant decrease (P < 0.05) in the activities of SOD and the level of MDA in plants grown in 30ppm Ni + 20ppm NPK fertilizer and 30ppm Ni + 40ppm NPK fertilizer respectively compared with those grown in 30ppm Ni concentration.  These results show that 30ppm Nickel is toxic to sorghum roots for it increases significantly the production of reactive oxygen species but decreases significantly the excretion of reactive oxygen species.  This is due to significant increase in the activity of SOD but significant decrease in the activities of CAT and GP.  These results also showed that 30ppm Nickel damaged sorghum roots by significantly increasing lipid peroxidation and the levels of MDA.  In addition, the results revealed that 20ppm and 40ppm NPK fertilizer had ameliorating effect on the toxicity caused by 30ppm nickel.

CHAPTER ONE

INTRODUCTION 

INTRODUCTION

Trace metals are redistributed in environment by fossil fuel combustion.  This release can be expected to increase soil levels of trace elements such as Ni2+ resulting in a concomitant increase in the concentration of Ni2+ in plants and possibly in the food chain (Dominic et al, 1978). 

Nickel (Ni) is an essential micronutrient for plants since it is the active centre of the enzyme urease required for nitrogen metabolism in higher plants (Yan et al, 2008).  Nickel deficiencies lead to reduced urease activity in tissue cultures of sorghum, rice and tobacco and in excessive accumulation of urea and toxic damage to the leaves of leguminous plants such as sorghum (Peter and Andre, 1986).  However, excess Ni is known to be toxic and many studies have been conducted concerning Ni toxicity of various plant species. 

The most common symptoms of nickel toxicity in plants are inhibition of growth, photosynthesis, mineral nutrition, sugar transport and water relations (Seregin and Kozhevnikova, 2006).  Heavy metal affects plants in two ways.  First, it alters reaction rates and influences the kinetic properties of enzymes leading to changes in plant metabolism (Yan et al, 2008).  Second, excessive heavy metals lead to oxidant stress. 

During the period of metal treatment, plants develop different resistance mechanisms to avoid or tolerate metal stress, including the changes of lipid composition, enzyme activity, sugar or amino acid contents, and the level of soluble proteins and gene expressions.  These adaptations entail qualitative and/or quantitative advantage, and affect plant existence (Schutzendubel and Polle, 2002). 

It is known that excessive heavy metal exposure may increase the generation of reactive oxygen species (ROS) in plants, and oxidative stress would arise if the balance between ROS generation and removal were broken.  Oxidative stress is a part of general stress that arises when an organism experiences different external or internal factors changing its homeostasis.  In response, an organism either aims to maintain the previous status by activation of corresponding protective mechanisms or goes to a new stable state (Mittler, 2002). 

In several plants, Ni has been shown to induce changes in the activity of ROS – scavenging enzymes, including SOD catalase and glutathione peroxidase (Yan et al, 2008).

The aim of this study is to investigate the effects of nickel on the activities of sorghum root antioxidant enzymes and also monitor the ameliorating effects of N.P.K. Fertilizer.