Introduction

In field crops grown from seed the root system develops, in general, in an uncomplicated way so as fully to exploit the soil to a depth characteristic of the crop. In crop physiological analysis, and for irrigation scheduling, the root system is considered to be defined fully in terms of rooting depth and density, the roots usually being considered to be evenly distributed.

Root development and distribution is much more complex in apple and pear trees as grown commercially. It involves, and relates to, very distinctive technologies of propagation, tree establishment, and nursery and orchard soil management and irrigation. Prior to consideration of the biological basis of these technologies it is best to consider the general anatomy and structure of the roots of the orchard tree.

The essential root system of the mature tree consists, as a rule, of an underground rootstock stem from which arises a system of permanent, thickened, scaffold roots spreading almost horizontally, usually less than 50 cm from the surface, and numerous more or less vertical ‘sinkers’ descending as a rule to either an impermeable layer or a water table (Rogers and Head, 1966; Atkinson, 1980). These woody ‘skeletal’ roots are long-lived, provide anchorage and form the framework which bears the fine (fibrous) roots. New roots can emerge from the underground rootstock stem, from coarse roots (>2 mm diameter) or from fine roots. They are initially white and most are ephemeral with only a proportion thickening and becoming perennial roots.