Proper evaluation of the very large germplasm collections now assembled for many crop species presents major problems. These arise principally from the effects of the environment on the expression of plant characteristics. For qualitative characteristics there is little difficulty as their expression is usually affected little by environment. Examples are seed coat and flower colour and colour pattern. A single evaluation is all that is required to characterize a set of materials for such characteristics.
It is, however, the quantitative characteristics, and these are of most interest to the breeder, that are especially intransigent as their expressions are always modified by environment to some degree, so that the separation of the contributions of genotype and environment to the phenotype requires special techniques.
Environment and genotype × environment interaction
The modification of plant characteristics by environment takes two forms. First, there is a general reduction or increase in expression of a character across all genotypes. Environmental features such as soil fertility, moisture availability, temperature and pathogens, pests and weeds may all affect plant characters in this way. The result is what is often termed ‘field variability’ and this will always occur in a single evaluation at a single location in a single season. It also occurs across locations and seasons.
Secondly, there is the situation where all genotypes are not affected equally by differences in environment, normally described as ‘genotype × environment (g × e) interaction’.