Phytochrome Affects Flowering

In Short Day Plants

The active form of phytochrome PFR inhibits flowering in short day plant. In order to flower, these plants need long night. The long period of darkness allows the PFR to completely revert back to PR so the plant has some minimum time during the 24-hour period with no PFR present. This initiates flowering.

Short-day plants were grown under a short-day/long-night condition. The night was interrupted with a short burst of red light. Exposure to red light for a brief period as 10 minutes in the middle of the night prevents flowering in short day plants. This effect occurs because the brief exposure to red light converts some of the Phytochrome from the PR (P660) form to the PFR (P730) form.

Therefore the plant does not have a sufficient period of time at night for the conversion of PFR (P73Q). Short day plants need long nights to allow complete dark for reversion of PFR (P730) to PR (P660) to initiate flowering. A brief flash of red Sight in the middle of the night converts PR (P660) to PR (P730) However, if this is followed by a brief period of far red light, the PR(P730) is converted back to PR(P660). Therefore, flowering occurs.

In Long-day Plants

The active form of Phytochrome PFR (P730) induces flowering in long-day plants. Long-day plants exposed to a long-day/short-night condition flower. The long days cause these plants to produce predominantly PR (P730). During the short night some PFR (P730) is slowly changed to PR but sufficient PFR (P730) remains to induce flowering. Plant biologists are puzzled by the observation that PFR (P730) the active form of phytochrome inhibits flowering in short-day plants and induces flowering in long-day plants. Why different plants respond to opposite way to PFR (P730) is not known at this time.

The other factors that play an important role in flowering are:

Presence or Absence of Light

Length of Dark or Light Period

Phytochromes seem to be responsible for detection of either light or darkness. The biological clock once stimulated causes production of florigen hormone in leaves, which travel through phloem to floral buds, initiating flowering.

Plants are incredibly complex organisms that are capable of adapting to various environmental changes in order to survive. One of the key mechanisms by which plants adapt to their environment is through their ability to sense and respond to light. In particular, a group of light-sensitive proteins called phytochromes play a critical role in regulating the plant's response to light, including their ability to flower.

Phytochromes are pigment molecules found in plant cells that absorb specific wavelengths of light. They exist in two interconvertible forms, Pr (inactive form) and Pfr (active form). The conversion of phytochromes from Pr to Pfr occurs when they absorb red light, while the reverse conversion occurs when they absorb far-red light. This process is known as photoreception.

Once phytochromes have been converted into their active form, they are capable of triggering a complex signaling pathway that results in changes to the plant's growth and development. In the case of flowering, phytochromes play a critical role in regulating the timing of the flowering process.

In order to understand how phytochromes affect flowering, it is important to first understand the basics of the flowering process. Flowering in plants is a complex process that involves the activation of various genes and hormonal pathways. In order for a plant to flower, it must first receive certain environmental cues, such as changes in day length or temperature.

Once the plant has received these cues, it begins to produce a hormone called florigen, which triggers the flowering process. Florigen moves through the plant's vascular system to the growing tip, where it triggers the formation of the floral meristem, which is a group of undifferentiated cells that will eventually give rise to the plant's flowers.

Phytochromes play a critical role in regulating the timing of the flowering process by sensing changes in day length. Specifically, phytochromes are capable of detecting the ratio of red to far-red light, which changes throughout the day as the sun moves across the sky.

Plants use this information to determine the length of the day, which in turn triggers the production of florigen. In short-day plants, which require long nights in order to flower, phytochromes are more sensitive to red light than far-red light. This means that when the days are short, and the nights are long, the ratio of red to far-red light is low, and the plant produces more Pfr. This triggers the production of florigen, which in turn triggers flowering.

In long-day plants, which require short nights in order to flower, phytochromes are more sensitive to far-red light than red light. This means that when the days are long, and the nights are short, the ratio of red to far-red light is high, and the plant produces more Pr. This delays the production of florigen, which delays the onset of flowering.

In addition to their role in regulating the timing of flowering, phytochromes also play a role in regulating the quality of the light that the plant receives. For example, phytochromes are capable of detecting changes in the quality of light that occur when the sun passes through clouds or when the plant is shaded by neighboring plants.

When the plant is shaded, phytochromes detect an increase in the ratio of far-red to red light, which triggers a response known as shade avoidance. This response causes the plant to grow taller and to produce more leaves in an effort to reach the sunlight. It also triggers the production of a hormone called auxin, which causes the plant to bend towards the light source.