Photoperiodic After-Effects in Six Composites

Abstract

1. Photoperiodic after-effects were observed in two short-day species (Cosmos bipinnatus, C. sulphureus) and four long-day species (Rudbeckia hirta, Matricaria parthenoides, Centaurea cyanus, Coreopsis tinctoria). Twenty-eight days after planting the seed, the plants being kept under photoperiods unfavorable for reproductive development, they were subjected to from 1 to 20 or 30 induction photoperiods of a length which favored initiation of flower primordia. They were then transferred to photoperiods not favoring such development and retained there for the duration of the experiments. 2. In Cosmos bipinnatus anthesis occurred in some plants exposed to 5-11 short photoperiods, and in abundance in all plants exposed to 12 or more short photoperiods. In the Orange Flare variety of C. sulphureus even the plants retained continuously under long photoperiods eventually bloomed sparsely, but the abundance of flowers increased with an increase in the number of short induction photoperiods. Plants of the long-day species Rudbeckia hirta, Matricaria parthenoides, Centaurea cyanus, and Coreopsis tinctoria bloomed sparsely after exposure to minima of 17, 10, 10, and 18 long photoperiods, respectively. 3. Photoperiodic inhibition of stem elongation by the short photoperiods of the transfer environment occurred in R. hirta, even in the plants which bloomed. In the other three long-day species such inhibition of stem elongation was not universal, and only plants with elongated stems bloomed. 4. The short photoperiods inhibited the vegetative growth of both the long- and short-day species. In Matricaria the plants retained continuously under short photoperiods reached a total height of only 1.5 cm. and soon died, while plants exposed to as little as one long photoperiod thrived. 5. Reproductive-vegetative interphases, which may be considered as initial stages in rejuvenation, occurred in two species. A number of "vegetative flowers" with green petals and more or less vegetative stamens and pistils developed on plants of Rudbeckia hirta. In Cosmos bipinnatus there were many elongated bracts, incised and foliaceous bracts, elongated internodes between bracts, and defective flower heads and abortive buds. Some plants of this species actually resumed vegetative growth from young flower buds. One plant exposed to 5 short photoperiods made a double reversal in its terminal bud, changing from vegetative to reproductive growth, back to vegetative growth, and once more to reproductive growth.