Growth Hormone Changes in Chrysanthemum morifolium

Simultaneous quantitative analyses have been made of the endogenous levels of auxin- and gibberellin like substances, growth inhibitors, and auxin-oxidizing enzyme activity in the cold-requiring Chrysanthemum morifolium cv. Sunbeam subjected to different daylength, light intensity and temperature regimes known to affect flowering. While little hormone or enzyme activity was found in extracts from unvernalized plants, a striking rise in auxin-oxidizing enzyme activity occurred rapidly after the end of cold treatment. Increased auxin activity was also recorded shortly after vernalization. At 28 °C both enzyme and auxin activity declined over a period of 3–4 weeks; at 20 °C this response was delayed. Gibberellin activity at 28 °C rose steeply about 2 weeks from vernalization and declined several weeks later; at 20 °C a similar response was less marked. Low light intensity treatment, which may have increased endogenous auxin levels, or exogenous auxin application reduced gibberellin-like substance levels and cause d devernalization.Phosphon D treatment also lowered gibberellin levels and prevented flowering. An extract from vernalized plants containing gibberellin-like substances intensified the flowering of partially vernalized test plants. Persistence of high auxin activity in vernalized plants on long days was associated with failure to form normal flower buds. Stem elongation rates correlated in general with levels of endogenous auxin- and gibberellin-like substances. Significant amounts of an abscisin-like inhibitor were found in extracts of flower buds. The mechanism of natural devernalization is discussed in relation to these observations.