For nearly 100 years scientists tried to identify the elusive flowering hormone called florigen.
Early in the last century two USDA researchers took a major step toward this by discovering how to induce flowering in plants under controlled conditions. In 1920, these two scientists, W.W. Garner and A.H. Allard, first published (PDF) their work on the effect of photoperiod on flowering in tobacco, soybean, and many other plants. (Their findings are summarized here and nicely described with an historical perspective at a USDA webpage.)
At first, scientists thought that the day-length was the controlling factor in inducing flowering. Hence, plants were divided into three groups with regard to photoperiodic effects on flowering.
We now know that the night-length is more important than the day-length in inducing flowering in responsive plants. So, we can divide flowering plants into three groups – “Short-Night” plants, “Long-Night” plants, and “Night-Neutral” plants. (Unfortunately, most textbooks persist in using the old – and incorrect – nomenclature. Sigh.)
Thus, many plants make the flowering transition from vegetative growth in response to a very dependable environmental cue, namely, the photoperiod.
But What Does This Have To Do With Florigen?
Firstly, by finding a way to induce many plants to flower at will by adjusting the photoperiod in the laboratory, Garner and Allard set the experimental stage for the eventual discovery of florigen.
In other words, this finding allowed other scientists to artificially induce the floral transition in some plants. Thus, by enabling them to initiate flowering at will, scientists began to study the sequence of events in how plants make flowers.
Secondly, it was discovered that plants sense the photoperiod in their leaves. (We’ll see how they do this later on.)
But the flower transition occurs, not in the leaves, but at the shoot apical meristems (SAM).
Therefore, in plants that flower in response to photoperiod, some sort of flower-inducing signal must be sent from the leaves to the shoot apex.
This signal turned out to be florigen.
Are There Other Environmental Cues That Induce Flowering?
Another important environmental cue that regulates flowering time is temperature.
In temperate regions of the world, flowering plants may use temperature as a significant seasonal indicator. Indeed, it’s well known that warmer conditions can accelerate flowering in spring.
And there is some recent information from scientific research that suggests that lower temperatures may actually inhibit the production of florigen. (For more on this see Ref. 1 below.)
But we all know how variable spring-time temperatures can be from year to year, due to fluctuating weather patterns. So, the photoperiod is likely a much more reliable cue for plants with regard to the correct time to flower.
Another effect of temperature on flowering in plants has to do with sensing the passage of winter. Many plants from temperate regions flower only after they experience an extended period of relatively cold weather, or vernalization. Specifically, vernalization results in “…the acquisition or acceleration of the ability to flower by a chilling treatment.” (from Ref. 2 below) This cold exposure does not necessarily cause flowering but rather renders the plant competent to do so. (Please see Ref. 3 below for an excellent review on the subject.)
Some biennial plants, such cabbage and carrots, require a long period (weeks) of “cold” (below 35o to 40o F) to become competent to flower. (Please note that this does not induce flowering but allows flowering to be induced.)
A requirement for vernalization permits biennials to become established during the fall without the risk of flowering as winter begins. During the winter, these plants experience and “remember” a cold treatment, which enables them to flower during the favorable conditions of spring.
Unlike photoperiod, which is perceived in leaves, cells of the SAM directly sense cold and become “vernalized”, that is, competent to flower. This is because a protein that blocks flowering is removed by vernalization, which does so by causing the gene for this protein to be chemically “pad-locked” (DNA methylation) so that is can’t be transcribed.
This, by the way, is how the plant “remembers” it has experienced winter, weeks later on in the spring. The story is a complex one, however. (For more about this, please see here).
Bottom Line: Plants may respond to a combination of photoperiod, temperature, and vernalization to ensure optimal timing of flowering.
But by discovering a way to systematically induce flowering primarily via photoperiod, Garner and Allard took the first major steps toward the identifying a flowering hormone in plants.
Next: Are there endogenous signals, other than florigen, that induce flowering in plants?
1. . Greenup, A., et al. (2009) “The molecular biology of seasonal flowering-responses in Arabidopsis and the cereals.” Annals of Botany, Vol. 103, pp. 1165-1172. (Full Text)
2. Chouard, P. (1960) “Vernalization and its relations to dormancy.” Annual Review of Plant Physiology, Vol. 11, pp 191-238.
3. Amasino, R. (2004) “Vernalization, competence, and the epigenetic memory of winter.” The Plant Cell, Vol. 16, pp. 2553-2559. (Full Text)
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