Introduction

The ability of the plant breeder to create new genetic variation was enormously increased in the mid twentieth century by the invention of tissue culture and the use of growth regulators. Attempts at wide crossing, as discussed in the previous chapter, were often frustrated by the incompatibility of genomes from relatively distant species. Embryo rescue could sometimes help, but one of the most crucial advances came with the development of chemically induced chromosome doubling, which has been the key to the success of many crop breeding programmes. As well as making possible much wider genetic crosses, chromosome doubling has enabled the use of powerful methods such as somatic hybridisation and haploid breeding, which have been especially useful in developing countries. In the past few decades, the technique of mass propagation has also been of considerable benefit in breeding programmes for tree crops, most of which are too long lived to be accessible to the sorts of approaches developed for the much shorter lived annual crops.