Photoperiodism in Relation to Hormones as Factors in Floral Initiation and Development

Abstract

1. A description is given of a simple method whereby one portion of a plant may be subjected to one photoperiod while another portion of the same plant is being subjected to another photoperiod. 2. Floral initiation in Xanthium pennsylvanicum results if plants are subjected to photoperiods shorter than 15 hours with accompanying dark periods of longer than 8 hours. If Xanthium plants are subjected continuously to photoperiods longer than 16 hours with accompanying dark periods shorter than 8 hours they remain strictly vegetative. 3. The initial effect of the photoperiodic stimulus is perceived by the leaves which are subjected to short photoperiod. However, this stimulus, resulting in floral initiation, may be transported from these leaves to other portions of the same plant which are maintained under conditions of long photoperiod and may also move across a diffusion contact from a plant subjected to short photoperiod to a plant subjected to long photoperiod. The stimulus to floral initiation may therefore be attributed to a substance or substances manufactured in leaves subjected to short photoperiod. 4. The response of Xanthium to photoperiod is primarily a response to length of dark period rather than to duration of photoperiod. Thus reactions resulting in the formation of floral initiation substances may take place during the dark period. These reactions are adversely affected by light and by low temperature. 5. Fully expanded leaves on receptor branches subjected to long photoperiod may exert some influence inhibitory to floral initiation; under similar circumstances young expanding leaves exert a promotive effect on floral initiation and flower development. 6. In Xanthium the development of mature flowers and fruits from floral primordia is also promoted by a substance or substances formed in portions of the plant which are exposed to short photoperiod and which may move across a diffusion contact. Whether or not this substance or substances is identical with the floral initiation substance has not as yet been determined. 7. A portion of a plant maintained under long photoperiod may be influenced by a portion of the same plant subjected to short photoperiods in such a way that it may behave as though it has been photoperiodically induced by direct exposure to short photoperiod. Flowers and fruits continue to develop on such portions of a branch which has never itself been subjected to short photoperiods. 8. Evidence is presented that the floral initiation substance is not identical with any of the following known plant growth factors: vitamins B₁, B₂, and B₆, ascorbic acid, nicotinic acid, pantothenic acid, theelin, theelol, inositol, or indoleacetic acid.

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