Flowering Hormone

After photoperiodism was discovered, many scientists, including M. H. Chailakhyan, a Russian botanist, conducted experiments to determine which part of a plant was sensitive to day length. They soon found it was mainly the leaf, although they also noted that buds of long-day plants exhibited similar responses. If they completely enclosed the leaf of a short-day plant in black paper for all but 8 hours of a 24hour day while exposing the rest of the plant to long days, flowering was initiated, but if they treated a long-day plant the same way, flowering did not occur (Fig. 11.22). This suggested that something in the leaf was being carried to an area where flowers were initiated. In the 1930s, Chailakhyan gave support for this theory by showing that when part of a plant was exposed to the appropriate day length to initiate flowering and then grafted to a plant that had not been so exposed, something would cross the graft so that both plants would flower. Chailakhyan suggested the name florigen for the "something."

In the 1950s, further evidence was obtained through experiments in which the leaves of some plants were removed immediately after exposure to critical photoperiods, while the leaves of similar plants were removed several hours after exposure. The plants whose leaves were removed immediately after exposure did not flower, but those whose leaves were removed later flowered almost as well as others whose leaves were not removed. This indicated something initiating flowering moved out of the leaves, but its departure was prevented if the leaves were removed before it had a chance to do so.

Cocklebur Photoperiodism Graft

Figure 11.22 An experiment illustrating the effect of subjecting one leaf of a short-day plant (cocklebur) to short days while the rest of the plant is exposed to long days. A. The short-day plant exposed to long days. No flowers were produced. B. The same plant exposed to long days while one leaf was covered with black paper 16 hours a day for a few weeks. The plant flowered, presumably because some substance that initiates flowering was produced in the shaded leaf and then diffused or was carried to the stem tip where flower buds are produced.

Figure 11.22 An experiment illustrating the effect of subjecting one leaf of a short-day plant (cocklebur) to short days while the rest of the plant is exposed to long days. A. The short-day plant exposed to long days. No flowers were produced. B. The same plant exposed to long days while one leaf was covered with black paper 16 hours a day for a few weeks. The plant flowered, presumably because some substance that initiates flowering was produced in the shaded leaf and then diffused or was carried to the stem tip where flower buds are produced.

After the existence of phytochrome had been demonstrated, it was theorized that it might be involved in photoperi-odism, with Pft inhibiting flowering in short-day plants and promoting flowering in long-day plants. Presumably the Pf would accumulate in the light in short-day plants and revert back to Pr or be broken down during dark periods. After the length of the night increased sufficiently, all or most of the Pfr would disappear, and flowering then would cease to be inhibited, with the converse taking place for long-day plants. Pfr has been demonstrated to disappear in many plants in as little as 3 or 4 hours of darkness, however, indicating that phytochrome interconversions alone cannot explain photoperiodism.

On the basis of the evidence, it was theorized that plants produce one or more flowering hormones ("florigen"), which may then be transported to the apical meristems where flower buds are initiated. Despite many years of research and all the circumstantial evidence, however, a flowering hormone has never been isolated from a plant, nor has it otherwise been proved to exist. This then led to speculation that photoperiods may bring about a shunting of nutrients that initiate flowering or that flowering is triggered by changes in relative proportions of other hormones. The theory that a flowering hormone exists has now been discarded, and a simple explanation involving hormones for the phenomenon of photoperiodism has proved to be nonexistent. Instead, it is now believed that no hormones are involved in flowering and that flowering is triggered by the direct effects of phytochrome on gene expression. This leads to the accumulation of and/or disappearance of specific RNA's, which, in turn, is thought to initiate flowering.

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  • VIRGINIA
    When a longday plant produces flowers?
    8 years ago

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