CHAPTER ONE

1.0      INTRODUCTION AND LITERATURE REVIEW

1.1   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.

1.2      LITERATURE REVIEW

1.2.1        HEAVY METALS

        Heavy metal is any of a number of higher atomic weight elements, which has the properties of a metallic substance at room temperature.  There are several definition concerning which elements fall in this class designation.  One school of thought classifies metals having density greater than 5g/cm3 as heavy metals.  This classification includes most transition metals and higher atomic weight metals of Group III to V of the periodic Table. Examples of these heavy metals are zinc, cadmium, chromium, and Nickel.

1.2.2 CHARACTERISTICS OF NICKEL

        As with other metals, the biological significance, of Ni is related to its physicochemical properties some physical properties of Ni are shown in Table 1 below.  The preferred oxidation states of Nickel are O and +2 but in complexes +3 and +4 states can also occur (Sengar et al, 2008).  Nickel forms stable octahedral complexes with, for example, EDTA nitrolotriacetate, cysteine and citrate (Bagati and Shorthours, 1999).