Photoperiodism in Crop Plants

CHAPTER: 14(D)

Photoperiodism in Crop Plants

Photoperiodism:

•      Photoperiodism is the phenomenon of physiological change occurring in plants in response to relative length of day and night (i.e photoperiods).

•      The plants in order to flower require a certain day length i.e, the relative length of day and night which is called as photoperiod. The response of plant to the photoperiod expressed in the form of flowering is called as Photoperiodism.

•      Garner and Allard (1920) were the first to use the term photoperiodism.

•      They observed that Maryland Mammoth variety of Tobacco failed to produce flower profusely. They further subjected Maryland Mammoth tobacco plant to short day length during summer by placing the plant in darkness after exposure to a day length that would be equivalent to a winter day. Plant treated in this way produced flowers.

•      Furthermore, they found that the plant can be kept in vegetative state during winter months by lengthening the days with artificial additional light.

•      This variety of tobacco is called a short day plant because it flowers only under short days.

•      So, the phenomenon of physiological changes occurring in plants in response to relative length of day and night is called photoperiodism.

Importance of Photoperiodism:

1. It has great practical importance in hybridization program.
2. Useful in successive cultivation of agricultural crops including horticultural crops.
3. The yield of tubers, bulbs, rhizomes, and corns can be increased by using the knowledge of photoperiodism.
4. Annual crops may be grown twice or thrice in a year.
5. Long day photoperiodic induction influences stolon formation in strawberry.
6. Short day photoperiodic induction may help in the production of winter buds in a several crop plants, root thickening, in radish etc.
7. Winter dormancy and autumn leaf falls of plants can be prevented by increasing photoperiodic hours.
8. It may serves to stimulate the development of fruit and flowers throughout the year.

Photoperiodic Response:

Ø  Any response by a plant to the duration and order of sequence of light and dark period may be called as photoperiodic response.

Critical Day Length (CDL):

Ø  Plants are sensitive to photoperiods. The flowering patterns are determined by a particular set of photoperiod conditions. The photoperiod required to induce flowering is called critical day length (CDL). Short day plant will produce flower when the photoperiod is shorter than CDL i.e. less than 12 hours light. Similarly. Long day plant will flower under photoperiod longer than CDL i.e. more than 12 hours light.

Photoperiodic Induction:

•      In short day plant as well as long day plants only a few days exposure to the appropriate photoperiod is enough for inducing flowering, even if the treated plant is than kept in unfavorable photoperiods.

•      This initial important effect on the flowering of a plant is known as photoperiodic induction.

•      Plant need the appropriate photoperiodic treatment for flowering.

•      Plants may require one or more inductive cycle for flowering. After receiving sufficient inductive cycle, plant may flower though it is subjected to unfavorable photoperiods.

•      So, the persistence of photoeriodic effect on the flowering of plant is known as photoperiodic induction.

•      It is a progressive process which produces a quantitative response.

•      Certain plants require continuous photoperiodic treatment. The process is known as steady state induction and some require partial or fractional induction.

Plant types based on photoperiodic response:

Ø  According to the photoperiodic response of the plants , they are of the following types:

1. Short Day Plant (SDP):

•      Short day plant is those that flower only within a range of relatively short photoperiods.

•      Rice, millet, coffee, Dahlia, potato, Strawberry, etc are among short day plants.

•      SDP are those plants which require a relatively short day light period or below the critical day length for flowering.

•      SDP requires 8-10 hrs of light period in a 24 hours for their flowering or continuous dark period of about 14-18 hrs for subsequent flowering.

•      SDP are also known as long night plant, since they require an uninterrupted dark period for flowering.

2. Long Day Plants (LDP):

•      Long day plants are those that flower only within a range of relatively long photoperiods.

•      These are plants which require comparatively long photoperiod or above a critical length of light period for flowering.

•      The critical length of light and dark period varies from plant to plant.

•      This plant requires 14-16 hrs of light period in a 24 hrs cycle for subsequent flowering.

•      LDP are also called as Short night plants.

•      Even continuous illumination will not inhibit their flowering.

•      A brief exposure during dark period or providing light for longer period or alternate short light and still shorter dark period stimulate flowering.

•      Examples: Wheat, Barley, spinach, radish, Grains etc.

3. Interminate  Plants or Day Neutral Plants:

•      These are plants in which there is no effect of length of day and night on subsequent flowering.

•      These plants flower in all photoperiods ranging from few hours to 24 hours continuous exposure. e.g., tomato, tobacco, cotton etc.

4. Intermediate Plants (Stereophotoperiodism):

•      Intermediate plants bloom only under day lengths within a certain range: above and below this range, they fail to flower. E.g., Sugarcane, been etc.

5. Short Long Day Plants (SLDP):

•      These are long day plants but must be exposed to short day during early period of growth for subsequent flowering. E.g., certain varieties of wheat, rye etc.

6. Long Short Day Plants (LSDP):

•      These are short day plants but must be exposed to long day during early period of growth for subsequent flowering. E.g. Certain species of Bryophyllum, Night Jasmine etc.