Mutual correlative relationships between young leaves and cyathia in the control of floral development and bract pigmentation in poinsettia (Euphorbia pulcherr1ma willd)

Mutual correlative relationships were found in poinsettia (Euphorbia putcherrima Willd cv. ‘Brilliant Diamond') between the developing inflorescence and the young leaves situated below it. Anthocyanin pigmentation of the bracts appeared after ca. 35 short-day (SD) cycles and gradually developed parallel to the development of the inflorescence. It was found, however, that SD cycles do not induce directly pigmentation of the bracts, which is controlled by the developing cyathia. The results indicate that the floral tissue induces the pigmentation of the subtending primordial leaves. Removal of floral shoot tips completely eliminated bract pigmentation when performed at the transition stage, but not at later stages after initiation of cyathia. The degree of pigmentation was enhanced when the removal of floral organs was delayed. Only primordial leaves were sensitive to the pigmentation stimulus, while expanded leaves did not respond to it. On the other hand, the young leaves designated to develop into colored bracts control the development of the inflorescence: their removal strongly inhibited inflorescence growth.