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The analysis presented here shows that improvements in crop yields during the last 80 years of breeding have been achieved mostly through redistribution of dry matter between the vegetative and the reproductive parts. This has helped to increase the harvest index of crop plants. There is litle evidence to show that the biological yield or the dry matter production has seen a significant increase during this period. The reason for this differential response of the two parameters of yields has been analysed in terms of the evolutionary history of our crop plants. It has been suggested that crop plants grown in traditional agriculture in the absence of modern farm inputs like chemical fertilisers have been selected for those characteristics which have helped them to establish and survive in stress environments. Thus, selection pressures in the past 10,000 years of traditional agriculture have been more for vegetative growth and vigour than for high economic yields. in the last 50–70 years, following the increasing availability of chemical fertilisers and other inputs, it has been possible to evolve cultivars with a different kind of plant architecture marked by a high harvest index and response to increased plant populations. This transformation has been accelerated in the last 20 years with the discovery of genes having major effects on plant type. Following this accelerated improvement, a stage would soon be reached when breeders must select for increased dry matter production for there is a limit to which harvest index could be increased. An important objective for increasing crop yield in future must, therefore, be the search for genetic variability giving high rates of photosynthesis at high population density.
