 |
| Flowering regulation |
All flowering is regulated by the integration of environmental cues into an internal sequence of processes. These processes regulate the ability of plant organs to produce and respond to an array of signals. The numerous regulatory switches permit precise control over the time of flowering.
Control over the time of flowering is essential for the survival of flowering plants (angiosperms). Insect pollinators may be present only at certain times; unless an insect-pollinated plant is flowering at that time, pollination and the production of the next generation cannot occur.
Embryo and seed development may be successful only under certain climatic conditions. The ability to respond to environmental cues is an essential factor in the regulation of flowering.
    |
While the basic biochemical sequence of events may be common to all angiosperms, the specific regulatory steps vary greatly among species. A floral promoter is produced by the leaves and is transported to the shoot apex, which results in the initiation and, ultimately, the production of flowers.
To analyze control points in this sequence, it is helpful to focus separately on environmental signals such as temperature and photoperiod and the way organs perceive and respond to these signals.
Chemical Communication
The regulation of flowering requires interactions between the shoot apex and other organs and thus depends heavily on chemical signals. There is strong evidence for the existence of a floral promoter called florigen, which may be produced in the leaves.
The existence of florigen was first proposed by M. Kh. Chailakhyan, a Soviet plant physiologist, in 1937. Florigen was believed to be produced in leaves, because if leaves were removed before the photoperiod was right for flowering (a process called photoinduction), no flowering occurred.
Laterwork by Anton Lang showed that the plant hormone gibberellin could induce flowering in certain plants, even without appropriate photoinduction. This prompted Chailakhyan to consider the possibility that florigen was actually composed of two different substances, gibberellin and a new substance he called anthesin.
In the late 1970’s Lang, Chailakhyan, and I. A. Frolova, working with tobacco plants, discovered that there was also a floral inhibitor they called antiflorigen. Later, several genes controlling the production of an inhibitor in pea cotyledons and leaves were identified in other laboratories.
In addition to leaf-derived inhibitors, root-derived inhibitors have been shown to regulate flowering in black currant and tobacco plants. Aside from the clear role of gibberellin in flowering, none of the other promoters and inhibitors has been identified. Nutrient levels and allocation throughout the plant may also control the time of flowering.
Photoperiod
 |
| Japanese morning glories |
One major role of environmental signals is to control the timing of the production of florigen and antiflorigen. This link between environmental and internal signals has been most clearly established for photoperiod. The role of day length in the regulation of flowering had been recognized by 1913. The impact of photoperiod on flowering in numerous species soon became apparent.
In the 1930’s W. W. Garner and H. A. Allard found an unusually large tobacco plant growing in a field. The plant stood out because it failed to flower; they named it the Maryland Mammoth.
Maryland Mammoth cuttings flowered in a greenhouse that December, and subsequent experimentation demonstrated that flowering would occur only when days were short and nights long. The Maryland Mammoth is an example of a short-day plant. Short-day plants generally flower in the spring or fall, when day lengths are shorter.
Other examples of short-day plants are poinsettias, cockleburs, Japanese morning glories, and chrysanthemums. Plants such as spinach, lettuce, and henbane will flower only if a critical day length is exceeded; they are categorized as long-day plants and generally flower during long summer days.
      |
Photoperiodic control mechanisms may be more complex, as in the case of ivy, a short-and-long-day plant, which requires at least a twelve-hour photoperiod followed by a photoperiod of at least sixteen hours. Still other plants, including sunflowers and maize, are day-neutral: They flower independent of photoperiod.
By the 1940’s it was established that night length, not day length, is critical in the photoperiodic control of flowering. For example, flowering in the short-day Japanese morning glory can be prevented by a brief flash of light during the critical long night.
In comparing short-day and long-day plants, the distinguishing factor is not the absolute length of night required; rather, the difference is whether that night length provides the minimum (short-day plants) or maximum (long-day plants) period of darkness required to permit flowering.
How a plant perceives night length and translates this into the appropriate response in terms of flowering is not fully understood. A pigment known as phytochrome, however, plays a critical role. Phytochrome exists in two forms (Pr and Pfr) that are interconvertible. Pr absorbs red light and is converted to Pfr, which absorbs far-red light and is subsequently converted back to the Pr form of the pigment.
Sunlight contains both red and far-red light, and thus an equilibrium between the two forms is achieved. At noon, about 60 percent of the phytochrome is in the Pfr form. In the dark, some Pfr reverts to Pr, and some breaks down. Because of the absence of red light, no new Pfr is generated.
The relationship between phytochrome and photoperiodic control of flowering has been established using night-break experiments with red and far-red light. (In these experiments, darkness is interrupted by momentary exposure to light.) Flowering in the Japanese morning glory, a short-day plant, can be inhibited by a flash of red light (as well as light equivalent to sunlight) in the middle of a long night.
Far-red light has no effect. A flash of far-red light following a flash of red negates the inhibitory effect of the red light. In long-day plants, flowering can be induced when the dark period exceeds the critical night length with a red-light night break.
Far-red light flashes do not result in flowering. The effect of the red flash can be negated by a subsequent far-red flash. In these experiments, the light flashes alter the relative amounts of Pr and Pfr.
Circadian Rhythms
How the perception of light by phytochrome is linked to the production of gibberellin and anthesin in long-day and short-day plants is not clear. One idea is that plants measure the amount of Pfr present. Flowering in short-day plants would be inhibited by Pfr, and these plants would not flower until very little or no Pfr remained after a long night.
To flower, long-day plants would require some minimum level of Pfr, which would not be available if the nights were too long—but this explanation is not viable because Pfr vanishes within a few hours after the dark period begins.
 |
| Tobacco flower |
Alternately, levels of phytochrome may influence an internal biological clock that keeps track of time. The clock establishes a free-running circadian rhythm of about twenty-four hours; this clock needs to be constantly reset to parallel the natural changes in photoperiod as the seasons change.
Phytochrome interacts with the clock to synchronize the rhythm with the environment, a prospect that is strengthened by night-break experiments, where the time of the light flash during the night is critical.
In the case of the Japanese morning glory, there are times during the night that a red light flash completely inhibits flowering and other times when it has no effect. In these experiments, the phase of the rhythm of the clock defines the nature of the interaction with phytochrome.
Studies on the relationship between flowering and day length have focused on the production of gibberellin and anthesin. There is evidence that the production of inhibitors by leaves is also under photoperiodic control.
This has been demonstrated for photoperiodic tobacco plants and for some peas. In the case of the pea, the inhibitory effect is most obvious for short days, but lower levels of inhibitors continue to be produced as the days grow longer.
Temperature
Plants also use temperature as an environmental clue to ensure flowering. Assessing two environmental factors provides added protection. Some plants have a vernalization requirement—a chill that promotes or is essential for flowering.
The control point regulated by vernalization may be different for different species. Vernalization has been shown to affect the sensitivity of leaves to respond to photoperiod. In some plants, only leaves initiated after the shoot apex has been chilled will induce flowering after the appropriate photoperiod.
Clearly, the competence of the leaves and shoot apex to respond to environmental and internal signals is crucial to the regulation of flowering. Pea mutants have been identified that have shoot apexes with differential sensitivity to floral induction or inhibition.
Another pea mutant has an apex that is not competent to initiate flowers and remains perpetually vegetative. The competence of a day-neutral tobacco apex changes with age. In another species, the apexes of shoots cannot respond to vernalization early in development; this period of time is considered to be the juvenile phase.
A juvenile phase of development is most common in woody perennials. During this time, flowering cannot occur even under optimal environmental conditions. Maturation occurs gradually and may be accompanied by changes in leaf morphology and the ability of cuttings to root. The most significant occurrence is that the plant becomes competent to flower.
Genetic Control of Flowering
The Maryland Mammoth, discussed above, is an example of a short-day plant resulting from a mutation. Mutations that affect flowering time can lead to delayed flowering or to rapid flowering, regardless of photoperiod.
Harmful effects of such mutations may include inadequate photosynthetic capability to sustain the crop (when flowering occurs too soon) or susceptibility to pests or cold temperatures at the end of the season (because of flowering too late). A focus of work on genetic modification of plants is the achievement of optimal flowering times.
