Long-Day Plants

Photoperiodism is the phenomenon of physiological changes that occur in plants in response to the relative length of day and night (i.e. photoperiod). 

The response of the plants to the photoperiod, expressed in the form of flowering is also called as photoperiodism. 

The phenomenon of photoperiodism was first discovered by Garner and Allard ( 1920) who observed that Biloxi variety Soybean and Maryland Mammoth variety of tobacco could be made to flower only when the daily exposure to the light was reduced below a certain critical duration and after many complex experiments concluded that the relative length of the day is first importance in the growth and development of plants.

Depending upon the duration of the photoperiod, the plants are classified into three categories. 

1. Short-day plants (SDP) 

2. Long-day plants (LDP) 

3. Day-neutral plants (DNP) 

1.0Short-Day plants 

These plants require a relatively short daylight period (usually 8-10 hours) and a continuous dark period of about 14-16 hours for subsequent flowering. These plants are also known as long-night plants (LNP) E.g. Rice, coffee, soybean, tobacco and chrysanthemum (Maryland Mammoth variety of tobacco, Biloxi variety of Soybean)

In short-day plants, the dark period is critical and must be continuous. If this dark period is interrupted by a brief exposure to red light (660-665 nm wavelength), the short-day plant will not flower. 

• Maximum inhibition of flowering with red light occurs at about the middle of a critical dark period. 
• However, the inhibitory effect of red light can be overcome by a subsequent exposure to far-red light (730-735 mm wavelength) 
• Interruption of the light period with red light does not have an inhibitory effect on flowering in short-day plants. 
• Prolongation of the continuous dark period initiates early flowering.

2.0Long-Day plants (LDP)

These plants require longer day light period (usually 14-16 hours) in a 24 hours cycle for subsequent flowering. These plants are also called as short night plants (SNP). E.g. Wheat, radish, cabbage, sugar beet and spinach. 

• In long-day plants, light period is critical
• A brief exposure of red light in the dark period or the prolongation of light period stimulates flowering in long-day plants.

3.0Differences Between Short-Day and Long-Day Plants

4.0Phytochrome Concept

Sterling Hendricks and Harry Borthwick discovered the phytochrome pigment at the USDA-ARS Beltsville Agricultural Research Center in Maryland from the late 1940s to the early 1960s.

It is observed that a brief exposure to red light during a critical dark period inhibits flowering in a short-day plant, and this inhibitory effect can be reversed by a subsequent exposure to far-red light. 

Similarly, prolongation of the critical light period or the interruption of the dark period stimulates flowering in long-day plants. 

This inhibition of flowering in short-day plants and stimulation of flowering in long-day plants involves the operation of a proteinaceous pigment called phytochrome. 

It is present in the plasma membrane of cells and it has two components, chromophore and protein. Phytochrome is present in roots, coleoptiles, stems, hypocotyls, cotyledons, petioles, leaf blades, vegetative buds, flower tissues, seeds and developing fruits of higher plants. 

The pigment, phytochrome exists in two different forms i.e., red light absorbing form which is designated as Pr and far red light absorbing form which is designated as Pfr. These two forms of the pigment are photo chemically inter convertible. 

When Pr form of the pigment absorbs red light (660-665 nm), it is converted into Pfr form. 

When Pfr form of the pigment absorbs far red light (730-735 nm), it is converted into Pr form. 

The Pfr form of pigment gradually changes into Pr form in dark.

It is considered that during day time, the Pfr form of the pigment is accumulated in the plants which are inhibitory to flowering in short day plants but is stimulatory in long day plants. 

During critical dark period in short day plants, this form gradually changes into Pr form resulting in flowering. 

A brief exposure with red light will convert this form again into Pfr form thus inhibiting flowering. Reversal of the inhibitory effect of red light during critical dark period in SDP by subsequent far-red light exposure is because, the Pfr form after absorbing far-red light (730- 354 nm) will again be converted back into Pr form. 

Prolongation of critical light period or the interruption of the dark period by red- light in long day plants will result in further accumulation of the Pfr form of the pigment, thus stimulating flowering in long-day plants.

5.0Differences Between Pr and Pfr Forms of phytochrome

6.0Significance of Photoperiodism 

• This is a case of physiological preconditioning. The stimulus is applied at one time, but the response is recorded after months.
• E.g. Longer photoperiods expose it to flowering quickly. Ear formation is induced quickly in wheat. Long light exposure converts Pr form to Pfr form and induces flowering.
• If dark period is greater, Pfr is converted into Pfr form that inhibits flowering. 
• The important phytochrome mediated photo responses in plants include photoperiodism, seed germination, sex expression, bud dormancy, rhizome formation, leaf abscission, epinasty, flower induction, protein synthesis, pigment synthesis, auxin catabolism, respiration and stomatal differentiation.