Most of the present breeding systems are locked into certain germ plasm sources and the chance development of cultivars with greater water-use efficiency is very small. Extensive germ plasm pools containing genes for greater water-use efficiency exist, but heavy selection pressure is necessary to identify these genes. Plant breeders need to determine the plant characteristics contributing to water-use efficiency. Tests for these characteristics need to be simple and rapid.
The four general methods of developing isogenic pairs (lines homozygous for AA and aa) are: through point mutations; by backcrossing A or a into an adapted cultivar; by crossing AA and aa and selfing Aa; and by crossing AA and aa and bulking homozygous F 2 and F 3 lines. The first two methods result in isogenic pairs in a "specific" genetic background and the second two in isogenic pairs in an "average" genetic background. Genes on chromosome segments adjacent to the isogenic pair being tested may greatly influence the results obtained. Care should be used in interpreting data since the worth of a gene may be completely obscured by pleiotropic effects or compensating effects of the background containing the isogenic pairs. The purpose of this paper is to discuss two areas of plant breeding that are related to the general topic of this symposium. One area is concerned with the present status of and approaches to plant breeding and the probabilities of plant breeders detecting genes contributing to water-use efficiency. The second area is concerned with the use of isogenic analysis to detect genes associated with water-use efficiency. This presentation will draw on general information concerning plant breeding and more specifically on our experiences with barley. This is not intended to be a comprehensive review of the literature. Primarily, this discussion is directed to those that are not plant breeders.
THE PLANT BREEDERS DILEMMA
The typical plant breeder, if there is such, develops a breeding program based on probabilities, a bit of science, and considerable art. With each advance, the probabilities of success, particularly for certain characteristics,