Introduction

A garden can be a source of pleasure. The juxtaposition of different colours and floral scents together with the opportunity of eating home grown vegetables combine to provide gardeners with considerable satisfaction. This book is about the chemistry that is found in the garden and the chemical ecology involved in the interactions between the plants, micro-organisms and insects that live there.

The chemistry of the garden begins with the chemistry of the soil. Next to climatic conditions, what is present in the soil can have the biggest effect on the garden. The structure and chemistry of the soil determines the availability of nutrients, water and air to the roots. The pH of the soil and the mobility of metal ions, such as iron, affects the plants that can be grown. Part of the chemistry of gardening lies in achieving an appropriate mineral balance in the soil.

Soil and climatic conditions can have a marked impact on the chemistry of plants and the natural products that they form. For example plants that grow under arid conditions can produce a resinous or waxy covering on the leaves to reduce their water loss. The mineral content of the soil can vary the colour of flowers such as the hydrangea. Plants have the ability to take up metal ions and this can be used in the bioremediation of 'brown field' sites. It is also a warning to the vegetable gardener who may unwittingly ingest toxic metals such as cadmium.

The very act of gardening changes the chemistry of the soil. Digging, together with the distribution of fertilizer and compost, not only changes the air, mineral, water and organic content of the soil but also affects the presence of many chemical messengers within the soil. The smell of newly dug soil is an indication of the release of volatile chemicals produced by soil micro-organisms. More subtle effects involve the dispersion of insect trail substances and the redistribution of germination inhibitors produced by plants.

1.1 CHEMICAL DIVERSITY IN PLANTS

The chemistry that occurs within a plant is complex and highly organized. The organic compounds that occur in plants fall into three big groups. Firstly there are those compounds that occur in all cells and play a central role in the metabolism and reproduction of the cell. These are known as the primary metabolites and include the common sugars, the amino acids that are constituents of proteins, and the nucleic acids. The second group are the high molecular weight polymeric materials such as cellulose, lignin and the various proteins which form the structural and enzymatic components of the cell. The third group of naturally occurring compounds are those of relatively low molecular weight which are characteristic of a limited range of species. These are the secondary metabolites and they include the polyketides, the phenylpropanoids, the alkaloids and the terpenoids from which the colouring matters, the scent of flowers and the fla