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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