The control of flowering

Abstract

A control of flowering is important in the growing of any crop plant. The juvenile phase is the period of vegetative growth during which flower formation is impos sible. Its duration may vary from a few weeks to several years among different plants. In her baceous plants the duration of the juvenile phase is modifiable and high light intensity reduces it. During the juvenile phase the necessary substrate for the formation of flowers is lacking. In "neutral" plants flower formation takes place without the action of specific factors. In several cases an optimal vegetative growth is accompanied by optimal flowering. Thermoperiodicity may play a part, but this factor is not specific. A prerequisite for vernalization is the presence of dividing cells during the cold treatment. After vernalization a transmissable flowering-stimulus may be formed, but this has been demonstrated in few cases only. In short-day plants both a transmissable flowering-stimulus and a flowering-inhibition have been established. The inhibition by light is not formed at low temperature or at low light intensity, which makes it possible to induce flowering of short-day plants in long day and even in con tinuous light. From a discussion of the mutual relationships between vernalization and photoperiodism, it is concluded that in one and the same plant -Campanula medium -at least two different mecha nisms of flower formation occur. Gibberellins cannot be considered to represent a universal flowering hormone. The direct action of gibberellins seems to be stem elongation, followed by flowering in those plants where stem elongation precedes flowering. The hypothesis of one universal flowering hormone is rejected. The flowering of each plant should be considered as a separate case. A general directive in explaining flowering is that both flowering stimuli and flowering inhibitors may occur, while sufficient substrate must be present. Additional remarks with respect to control of flowering in practical plant cultivation are as follows. A too rapid flower formation should be avoided. In some cases flowering "the year round" has been achieved. Such a complete control of flowering is important for research purposes, for the growth of valuable horticultural plants and for breeding-projects of all plants.