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