The theory that flowering in plants might be triggered by day length was first published in 1852. The first experimental results came from J. Tournois, 1910, who observed that Cannabis sativa flowered under short-day conditions. Perhaps the results by Kleba, 1918, seemed contradictory when he found that leeks would flower under continued light. At last, Garner et al, in 1920, established that photoperiods determined the onset of flowering in most plants. They planted seeds at regular intervals but found that flowering from different batches occurred close to the same dates in Spring or Autumn - at about the same day length.
It was also found that to induce flowering, some plants require a maximum and others a minimum day length. Some plants are day length independent and flowering is brought about by other factors, e.g. maturity, breaking of drought. Many tropical plants respond to the slight day length variation at 5 - 15° latitude. Some plants will flower only in a 50-mile belt of latitude. It seems that it is the dark cycle which is important. Short interruption of the dark period can prevent flowering, whereas interruption of the day period is inconsequential.
Our visible light has wave characteristics midway between radio waves (long) and X-rays (short). Plants respond to our visible light range, especially to the red and the far-red portion of the spectrum. Artificially, a mixture of fluorescent tubes and incandescent lamps can provide a fair balance of the important red and far-red portion of the spectrum.
More recent work has shown that plants produce a substance called phytochrome which has two interconvertible forms. Irradiation with red light converts it to the far-red absorbing form (Pfr) and irradiation with far-red light converts it to the red-absorbing form (Pr). In the dark, the Pfr form is slowly converted to the Pr form or broken down.
At nightfall, most of the phytochrome is in the Pfr form and the time required for this to disappear may act as a timing device or 'hour glass'. Flowering in short-day plants requires the Pfr to be at a low concentration, achieved only after a certain minimum period of darkness. On the other hand, flowering in long-day plants occurs when Pfr is at a high concentration, a condition that occurs only up to a certain maximum period of darkness. The balance between Pr and Pfr is very sensitive to low light intensities and may be affected by full-moon light.
Phytochrome production takes place in the plant leaves. Experiments exposing only one leaf to light have shown that the plant will still flower. More amazingly, this experiment permits measurements of the time required to start the formation of an inflorescence. The message to flower moves at 50 cm/hr in short-day plants and 2 cm/hr in long-day plants. However, phytochrome is only a timing device and remains in the leaf - the messenger presumably is a hormone. Unfortunately, this messenger has yet to be found. Plants might be willing subjects - but they don't yield their secrets easily.
DATE: November 1981
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