Photoperiodism can merely define as the potential of the plant to incite flowering relative to the changes in the photoperiod. Thus, photoperiod is a light duration or the length of day and night, while the photoperiodism is the effect of light duration in the growth of a plant. Depending upon the photoperiodic effect on flowering, the plants classify into three groups, namely long day, short day and day-neutral plants.
Garner and Allard were the two scientists who have first introduced the phenomena of photoperiodism in the year 1920. Plants experience some physiological changes as a developmental response (like flowering) relative to the length of photoperiod.
Definition of Photoperiodism
Photoperiodism can define as one of the plant’s mechanisms where it can sense the alternations in the day and night length through the photoreceptor proteins and decides when to induce flowering. That’s why different plant species develop flowers on different seasons, which is only due to the difference in the length of photoperiod.
The photoreceptors like phytochromes and cryptochromes can perceive the light stimulus and can produce signals to induce flowering in a plant with respect to the critical length of photoperiod. Critical photoperiod can define as the minimum duration of light required to induce flowering.
Types of Plants
Based on photoperiodic effect, the plant categorizes into three major classes:
Long day plants: These plants undergo flowering in long day length condition and also refer as “Short night plants”. LDP produce flowers mainly in summer and includes plants like spinach, radish, lettuce etc.
Short day plants: These plants undergo flowering in long night length condition and also refer as “Long night plants”. SDP produce flowers mainly in winter and includes plants like Dahlia, soybean etc.
Day-neutral plants: These plants undergo flowering independently, as the length of photoperiod does not affect the process of flowering. DNP can produce flowers throughout the year and includes plants like cucumber, tomato, rose etc.
Mechanism of Photoperiodism
If you are wondering that sunlight is the only factor inducing flower growth, then the answer is no. Sunlight alone cannot cause flowering in a plant, and a plant must contain certain chemicals that can respond to the action of the light stimulus. In the year 1935, a scientist named Cajlachjan introduced one hypothetical hormone called “Florigen” whose existence and origination is still unclear.
Florigen is a phytohormone that is supposed to induce flowering in plants in response to the changes in photoperiod length. Depending upon the length of photoperiod, plants incite flowering in different ways and categorize into three types:
Long day plants
If the day length surpasses the critical photoperiod, then a plant will induce flowering. Oppositely, if the day length is below the threshold period than the plant will not induce flowering. The third condition is quite interesting as if the day length is below the threshold and there is an interruption in the night length, in such case also a plant will incite flowering.
Short day plants
When the day length is below the threshold period, then only the short-day plants will induce flowering. If the day length surpasses the critical photoperiod, and if there will be any interruption in the night length due to a flash of light, then in both the cases the plant will not induce flowering.
It is a process of inciting a floral stimulus in the leaf. Leaves are the region perceiving a light stimulus, which then releases photopigment like phytochromes. A phytochrome changes its conformation depending upon the type of plants. In LDP, PR transforms into PFR during day time. In contrast, there is a reversion of PFR into PR during night time in SDP and the phenomena refer as a dark reversion.
A change in phytochrome configuration will induce the synthesis of flowering stimulus or florigen. Leaf translocate the flowering stimulus into the shoot apex under particular photoperiod. When a flower stimulus reaches an active region of the shoot apex, it incites the growth of flower primordia.
In long-day plants, the flower induction occurs under daylength longer than their critical photoperiod (more than 12 hours). Oppositely in short-day plants, the flower induction occurs under daylength shorter than their critical photoperiod (less than 12 hours). The induction of flowering is thus dependent on the photopigments and phytohormones present in the plant that responds to the light stimulus in different ways or produces flowers in different seasons.
It can define as a photopigment that is photosensitive in nature. Depending upon the day and night light, the phytochrome exists in two disparate conformations like PR and PFR. Phytochrome exists as a dimer that consists of two analogous polypeptide chains, each of molecular weight 125 kDa. The two polypeptide chains attach via a covalent bond.
These are generally the protein pigments that appears as a linear tetrapyrrole. Structure of phytochrome comprises prosthetic chromophore group, and protein part refers to as “Apoprotein” which in combination with chromophore forms holoprotein. The presence of phytochromes has been detected in the number of plants like angiosperms, gymnosperms, bryophytes etc.
A phytochrome absorbs light at a different wavelength and changes its structural configuration either into PR and PFR forms, relative to the light absorbed. Both PR and PFR forms are interconvertible, and their concentration in different plants may vary. When plant’s leave absorbs red-light of wavelength around 670 nm, then the PR will change into an active PFR form. In contrast, the far red-light of wavelength around 730 nm transforms an active PFR into inactive PR conformation.
When the phytochrome exists as an active PFR form, then the plant will produce a response that further induce flowering. In a PR state, a plant will not incite flowering. Phytochromes can also define as the photoreceptors, which respond to the environmental light conditions and controls a variety of photomorphogenic responses. Its concentration is highest near the shoot apex, where it induces flowering.
The photoperiodism has a significant role in the study of cross-breeding experiments and governs the season when a plant shall bloom its flower. It is an example of “Physiological preconditioning” which means a plant receives a photoresponse once and then undergoes physiological changes like seed germination, flowering etc. in a particular season. Exposure of the plants to longer photoperiods transforms inactive PR to an active form PFR that initiates flowering. Oppositely, an exposure of the plant to dark period transforms PFR to PR that inhibits flowering.