Photoperiodic cues can initiate flowering
The study of how light affects the transition to flowering began with two observations in the early twentieth century:
Normally, tobacco grows to about 1.5 meters tall before flowering in the summer, but a variety called Maryland Mammoth grows to an amazing 5 meters. Farmers in Virginia were frustrated because they could not easily get seeds from this luxuriant plant for successive crops. Instead of flowering, it continued to grow until the late fall frost killed it.
Because of improvements in agricultural techniques, soybean yields became so great that it was hard for farmers to harvest all the plants at once. Hoping to stagger the harvests, farmers tried planting the seeds in groups several weeks apart, but all resulting plants nevertheless formed flowers and seeds at the same time.
The explanation for both of these observations was the same: the signal that set the plants’ shoot apical meristems on the path to flowering was the length of daylight, or photoperiod. When soybeans experience days of a certain length, they flower, regardless of how “old” they are. Maryland Mammoth tobacco can flower, but it doesn’t do so in Virginia because it dies when the weather there gets cold. Maryland Mammoth is now grown commercially in Florida. You saw another example of the role of photoperiod in the opening investigation of this chapter, dealing with poinsettias.
Scientists used greenhouse experiments to measure the day length required for different plant species to flower. Maryland Mammoth tobacco did not flower if exposed to more than 14 hours of light per day; flowering was only initiated once day length became shorter than 14 hours, as it does in December. Other plants (such as soybean and henbane) flowered only when the days were long (Figure 37.8). Control of an organism’s responses by the length of day or night is called photoperiodism.
