Effects of the Photoperiod Genes Ppd1 and Ppd2 on Growth and Development of the Shoot Apex in Wheat

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Abstract
Two major genes influencing the photoperiod response in wheat, Ppd1 and Ppd2, have been identified on the group 2 chromosomes. Substitution lines, which had been characterized on the basis of time of ear emergence as carrying either the insensitive or sensitive alleles of the two Ppd genes, were used to investigate the effect of these genes on development. They were grown under short photoperiods, and growth and development of the shoot apex was measured. The primary influence of the Ppd genes was on ear growth. In the plants carrying the insensitive alleles, Ppd1 and Ppd2, the relative growth rate of the floral apex was faster than that of plants with the sensitive alleles, ppd1 and ppd2. There were no differences in the rate of spikelet initiation, but the spikelets of the ppd lines grew and developed more slowly. The Ppd2 material segregated for another gene located on chromosome 2B affecting duration of the life cycle. This gene also affected the relative growth rate of the ear. It was considered that the major effect of the Ppd1, Ppd2 and the second genetic factor on chromosome 2B is on floral growth rate. Differences in apex morphology, stem growth and ear emergence are thought to be due to the differences in floral apex growth and size.