Plants are well coordinated to the light and many of their actions are light dependent. The highlight, food and air yielding process is light dependent, in the absence produce CO2 that will threaten the living world.
Day and Night changes and climatic changes are well adapted by plants as these changes are expressed in terms of light and in absence of light.
Plants are well sensitized and adapted for light. The change in the habit of plant according to the availability and exposure of light is Photoperiodism.
Photoperiodism can be defined as light dependent response for light stimuli perceived from external environment.
The plants are built in with light detecting changes and are not highly sensitized as small light changes causes drastic response which is undesirable.
The detection is precise and mechanism is intricate where the time is detected continuously. Time measurement – keeps the plant well informed about the season and other climatic changes in the plant body.
The time maintenance becomes essential when plant’s function is dependent on them such as shift from vegetative to reproductive phase where flowering is the indicator to assess plants entering reproductive phase, seed dormancy, tuber formation etc., is well used to detect the climate the Photoperiodism is species specific changing from species to species.
In early 20th century, diurnal response to light by plant was put forth initially by Julein Turnois and Hans Klebs. The Concept of photoperiodism took a clear stand when Garner and Allard, American Physiologist worked at U. S Department of Agriculture, in 1920.
They studied Glycine max and Nicotiana tabacum response to sunlight. Glycine max flowers at the same time when the saplings were planted at different time period and Nicotiana tabacum failed to flower in Summer.
They conducted experiments using light – tight boxes to induce short day effects on Nicotiana tabacum. These 2 observations made Garner and Allard to conclude the plants are light dependent to initiate reproductive phases.
Later it is found that not only day duration is determined to assess the photoperiodism, relative daylength is essential to understand the response of plants to the light. The mechanism was termed as Photoperiodism.
Based on plants response to light, plants are mainly categorized as;
Short Day Plants requires short exposure period to sunlight. The shorter light period induces a rapid flowering. The light requirement for short day plants is below or up to their critical period.
Example: Xanthium, Nicotiana tabacum.
Long Day Plants requires long light exposure period. Plants when subjected to longer sun light exposure induces flowering in plants. The light requirement for LDP is above critical period.
Example: Spinacea oleracea.
Day Neutral Plants are the plants are neither influenced by long term light exposure or short-term light exposure of 24 – hour day – night cycle. The shift from vegetative phase to reproductive phase is controlled or regulated by internal factors – hormones and genetic makeup of the plant.
Example: Pisum sativum
Short – Long Day Plants are the photoperiod and light exposure varies in these plants. The plants are subjected both short and long period exposure. The short and long period exposure are in respective sequence which induces reproductive phase.
Example: Trifolium repens, Campanula medium.
Long – Short Day Plants are subjected to long term photoperiod followed by short – term exposure to induce reproductive stage.
Example: Cestrum nocturnum, Bryophyllum daigremontianum.
Further, plants are also divided into obligate and facultative based on the day length.
• Obligate: It is the qualitative type where daylight and time period are essential for producing flower (i.e.) to transform into reproductive phase.
• Facultative: It is the quantitative type where the presence of daylight is not the governing phase but can accelerate the reproductive phase shift by flowering. The plants might flower even when they are not exposed to light. Light here is an accelerating factor which improves the intensity of flowering.
Both LDP and SDP has crop species which can be either Facultative or obligative.
Critical photoperiod; demarcates SDP and LDP; limits the level of photoperiodic exposure of a plant.
The critical period is a threshold factor determining and differentiating SDP and LDP. In other words, critical periods set an upper limit for SDP’s flowering, when the exposure exceeds the critical period, the SDP’s enter vegetative phase without flowering.
In contrast, LDP’s require photoperiodic exposure above critical period to become reproductively successful.
A main drawback in fixing a critical photoperiod is that, the critical photoperiod differs from species to species.
There are chances the critical photoperiod can be same for both short day and long day plants.
For Example: Xanthium is SDP and Hyoscyamus is LDP. But both flower when they have 12 – hour light exposure. A 12 – hour is minimal light for Xanthium and the same time is above the critical photoperiod for Hyoscyamus.
Photoperiodic induction is a process where the exposure to light initiates reproductive phase.
The initiation of reproductive phase involves the conversion of leaf primordia to flower primordia. Induction takes place in cycles, number of turns in which a plant is induced is species specific.
For Example: Glycine max requires 2- 4 cycles, Xanthium requires only 1 cycle, 25 cycles are needed for Plantago lanceolata, etc.,
When sufficient cycles are obtained even with breaks induced by darkness can initiate flower bearing capacity of the plant.
The plants which flowers after the induction are said to inductive photoperiod and plants which does not flower with minimal photoperiod, these are said to be non – inductive photoperiod.
Hamner and Bonner studied photoperiodic induction in SDP Xanthium by experimenting with light exposure. Xanthium needs 15 and a half day of light and 8 and a half of dark period to get induced.
When the dark period was interrupted by light even for a small duration the plant retains vegetative phase and not reproductive.
From this it was concluded that Light is required for amount of flowering (i.e.) quantitative and dark phase is required for Photoperiodic induction to begin flowering.
The night break experiment was also performed in Long – day plants and was found that the night breaks and when a day extension in LDP is effective for flowering and limits SDP from flowering.
Shoot apex was believed to be a receiver of Photoperiodic stimulus until Knott in 1934 proved that leaves are the stimulus for inducing flowering in Xanthium.
The experiment proved that when all leaves are removed the plant remained vegetative and when even a part of leaf or a leaf when left the flowering was supported.
In leaves, photoreceptor molecules are present to detect the light stimulus.
The light detected are red and blue. The receptor molecules detect red and far- red lights are Phytochromes. Phytochromes are 5 different types they are:
The receptor molecules that detect blue light are Cryptochromes.
They are divided into: CRY1 and CRY2
The role of photoreceptors in LDP and SDP are given in Table below.
| Photoreceptor | LDP | SDP |
| Phytochrome A | Promotes | ? |
| Phytochrome B | Inhibits | Inhibits |
| Phytochrome C | Neutral | ? |
| Phytochrome D | Inhibits | ? |
| Phytochrome E | Inhibits | ? |
| Cytochrome 1 | Promotes (Crucifers) | None |
| Cytochrome 2 | Promotes (Crucifers) | None |