The discovery of photoperiodism

The discovery of photoperiodism


 


 

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The Discovery of Photoperiodism

The phenomenon of photoperiodism is a fairly recent discovery. Scientists first linked the onset of flowering with day length in the 1920s, during experiments with soybeans and tobacco. During one experiment, plots of soybeans were planted at two-week intervals throughout the spring and early summer. Surprisingly, all the plants flowered at approximately the same time, no matter what their age. Based on this result, scientists postulated that an environmental factor was triggering the flowering.

Further experiments on tobacco also pointed to this explanation. Most tobacco plants flower during the summer. However, around 1920, a mutant appeared in a field of tobacco growing near Washington, D.C. The plant had unusually large leaves and grew to an enormous height without ever flowering. This new variety was named ‘Maryland Mammoth’, and was the subject of several experiments by two researchers from the USDA, W. W. Garner and H. A. Allard. The researchers took cuttings of this new variety, and grew them in a greenhouse where they would be protected from frost. These cuttings flowered in December—even though at that time they were only half as tall as the field-grown specimen. Plants grown from seed also flowered in the winter.

Based on these and other experiments, scientists concluded that the flowering was related to day length, or the number of hours of light the plants received. They termed this phenomenon photoperiodism, and categorized plants as long-day, short-day, or day-neutral.


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