How do plants ‘know’ when to flower and what triggers plants to bloom?

Picture of How do flowers know when to bloom.
What triggers plants to flower?

How do plants know when it’s time to flower? Like animals, plants also have an internal clock that prepares the internal cellular mechanisms in anticipation of upcoming environmental changes. This ensures that plants only perform specific tasks at the most appropriate time. For example, during the day, they perform photosynthesis, extracting energy from sunlight. However, completely different processes occur at night in the absence of sunlight. Many plants grow significantly more than during the day. Therefore, to distinguish the difference between day and night, plants have special receptors on their cells that can detect sunlight and start and stop metabolic processes as needed.

Picture of a vegetative organ that represents the origin of a new vegetable axis, from which leaves, branches and flowers can originate.
Buds form a flower.

One of these processes is what we know as flowering, or blooming. Researchers at Martin Luther University Halle-Wittenberg (MLU) have identified two genes that are key to this process. The ELF3 and GI genes control the plants’ internal clock, which monitors the length of daylight and determines when it is the right time to flower. The findings help to breed a better adapted plants to their environments.

Flowers are involved in the sexual reproduction of plants, but not all plants have flowers. Because of their function, flowers only appear at a certain point in the life cycle of plants we call angiosperms (with seeds contained in fruits).

Through circadian mechanisms, plants can anticipate certain regularities in their environment, such as the alternation of day and night, and adjust accordingly. This also includes flowering at the right time.

Plants orient themselves to the proportion between the hours of sunlight and darkness.

Some plants only flower when the days are particularly long. Others only bloom when the nights exceed a certain period of time. Different species of plants flower at different times of the year, when the days are of different lengths.

An example of this annual adaptation is the arrival of spring. The mechanism responsible for flowers emerging in spring is known as vernalization. And that it occurs at the exact time is essential for pollination to take place. According to a study by scientists at the University of Texas (USA), plants recognize this season because they “remember” that they have just gone through a long cold period thanks to a long RNA molecule called COLDAIR.

  • According to the authors of the research, this molecule creates a cellular memory for the plants when they pass 30 to 40 days of cold.
  • At that time, a gene called FLC, which has been dedicated to suppressing flower production during the fall and winter, is silenced, and the plant prepares to flower.

The amount of light is not the only external source of information for the plants’ circadian clock.

The ambient temperature also changes during the course of the day and the year. In future research, scientists will try to understand how temperature influences the flowering of plants and whether temperature can compensate for the lack of information about light.

Most plants have adapted to their original environment in such a way that they require a specific ratio of sunlight to darkness in order to flower. New findings could allow plants designed to flower elsewhere to produce good yields.

Photosensitivity in plants (How do flowers know when to bloom?)

Besides chlorophyll, plants have three other classes of light-sensitive pigments:

  1. Phytochromes are mainly sensitive to red light, and to a lesser extent to blue light.
  2. Cryptochromes are particularly sensitive to blue light. They are also used as signal molecules when the phytochromes “catch” light.
  3. Phototropins are not involved in the regulation of the daily rhythm. They control the phototropism of plants, i.e. the plant grows towards a light source.
  4. The plant regulates its sensitivity to light by producing phytochromes and cryptochromes, intensified in the morning. During this time the plant is most sensitive to light.

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