As stated previously, a plant's internal clock (called a circadian rhythm) allows a plant to determine the time of day, but not the time of year. To do this, they rely on the phytochrome system.

The phytochrome system is an important influence on when many species choose to flower.

Measuring the time of year is not just important for some plants to be able to schedule flowering, but also so they know when to produce storage organs (such as tubers) and when to begin dormancy. Temperature is not a reliable indicator of the time of year because it fluctuates too much and no two years are alike. The patterns of day length are constant and so a much more reliable indicator.
Chrysantemum bloom
Figure 1: Flowering in Chrysanthemums (Dendranthema √ómorifolium) is influenced by photoperiod.
The use of day length to measure the time of year and to regulate physiological processes is called photoperiodism.

There are four key concepts which are of importance to this system:
  1. It is the night length and not the day length which is of importance to plants which utilise photoperiodism.
  2. The circadian rhythms are an essential part of the photoperiodic mechanism
  3. Phytochrome is the chemical which receives the light and allows the photoperiod to be measured.
  4. Cytochrome, a blue-light receptor is also important to photoperiodism.
Plants are classified by their flower response systems:
  • Short Day Plants (Qualitative): Plants flower in response to short days.
  • Short Day Plants (Quantitative): Floral initiation is promoted by short days, but not necessarily dependant upon them for flowering.
  • Long Day Plants (Qualitative): Plants flower in response to long days.
  • Long Day Plants (Quantitative): Floral initiation is promoted by long days, but not necessarily dependant upon them for flowering.
  • Intermediate day plants: flower in response to a narrow range of photoperiods.
  • Ambiphotoperiodic: Plants flower in response to long or short days, but not intermediate day lengths.
Generally, the three key categories of importance are long day plants (LDPs), short day plants (SDPs) and day-neutral plants. LDPs require the day length to exceed a certain duration, while SDPs require the day length to be less than a critical time. The critical value differs between species. Wheat is an example of an LDP and flowers in spring or summer while chrysanthemums are an example of a SDP, and flower in Autumn.

So, how do plants actually measure day length? For a start, plants don't directly measure day length but rather they measure the duration of the dark period (night).

Since a day is 24 hours under natural conditions, plants can calculate the day length by measuring the length of the dark period. We know that plants measure the dark period because researchers have experimented with plants by increasing the day length and night length under controlled conditions so that a day might equal 30 hours instead of 24.

Many SDPs will still flower when the days are artificially long, provided that the night is of adequate length also. This demonstrates that in SDPs; it is actually the duration of the dark period that determines whether floral initiation commences. The other evidence comes from experiments using night breaks. A night break is a short exposure of light in the middle of the night, which has been shown to "cancel out" the effectiveness of the dark period. However a short burst of darkness in the middle of the day appears to have no effect.

Plants measure night length using a chemical called phytochrome. Phytochrome has two chemical forms; phytochrome far-red (Pfr) and phytochrome red (Pr).

Phytochrome is located in the leaves; so it is the leaves which are responsible for measuring the photoperiod. Hamner & Bonner (1938) were able to stimulate flowering in parts of the SDP Xanthium by exposing some leaves to short photoperiods while the rest of the plant was exposed to a lengthy photoperiod.

Phytochrome is called a photoreceptor because it is a chemical which absorbs light energy. Pr absorbs red light energy (wavelength (λ)=666 nanometres) and is chemically converted into Pfr. Pfr absorbs far-red light (λ=730nm) and is converted into Pr.

Since the sun produces both red and far-red light, there is constant interconversion between Pr and Pfr and an equilibrium develops. Conversion of one form to another can occur within milliseconds. Pfr is biologically active, while Pr is inactive.
A schematic diagram of the chemical change in phytochrome as a result of exposure to light, and its degradation over time
Figure 2: A schematic diagram of the chemical change in phytochrome as a result of exposure to light, and its degradation over time.
During the night Pfr very slowly degrades to Pr in a process called dark reversion so that in the morning there is a lot more Pr than Pfr. When the level of Pfr falls below a critical point, flowering is triggered.

This works because:
  • When there is a short day (long night), a lot of Pfr will be degraded to Pr.
  • When there is a long day (short night), little Pfr will be degraded to Pr.
As the days become longer (nights shorter), the amount of Pfr left over in the morning increases and indicates to the plant what time of year it is.

A complxt chain of molecular and biochemical signals are used to communicate this between Pfr and the plant's meristems to initiate floral development if it is an LDP and inhibit floral development in a SDP. Vice versa as the days become shorter and nights longer, where LDPs are inhibited and SDPs initiate flowering. The nature of the chemical signal which inhibits or promotes floral initiation remains to be discovered.

The night breaks (referred to earlier) cancel out the effect of the dark period because the flood of red light converts Pr to Pfr within seconds and so the slow degradation from Pfr to Pr has to start again.

In addition to phytochrome, another chemical called cryptochrome has been found to be responsible for initiating flowering as a result of exposure to blue light.