LIMITATIONS TO THE USE OF PHYSIOLOGICAL VARIABILITY IN PLANT BREEDING

The possibility that breeding programs may be reaching a point of limited progress because of the lack of genetic diversity has often been raised. However, there is also an often-expressed feeling that considerable genetic variability may exist in fundamental physiological processes. In this review, an attempt is made to examine whether there is evidence of genetic variability in quantitative physiological characters, and if so, whether such variability is likely to be useful to crop improvement programs. The results, presented from the literature and the author’s laboratory, indicate that physiological characters demonstrate considerable genotypic variability in expression. Moreover, in cases where genotype performance has been studied over a range of environments or where crossing studies have been carried out, physiological characters often have relatively high heritabilities. This suggests that, at least from a genetic standpoint, improvements in the expression of physiological processes may be possible. On the other hand, the problems of environmental sensitivity and its resultant error variability, combined with cumbersome methodology and complex physiological interactions, make it difficult to relate individual physiological characters to agronomic benefits, and make large genetic studies difficult. The exploitation of physiological diversity remains a major challenge to plant scientists.Key words: Genetic heritability, physiology of yield