PHOTOPERIODISM

                          

 INTRODUCTION 

• 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 plants to the photoperiod expressed in the form of flowering called as PHOTOPERIODISM. 

DISCOVERY

• The phenomenon of discovery was first discovered by GARNER and ALLARD (1920-1922).Who observed that the Biloxi variety of soybeans (Glycine max) and Maryland mammoth variety of tobacco(Nicotina tabaccum) could be made to flower only when the daily exposure to light was reduced below a certain critical duration and after many complex experiments concluded that the "the relative length of day is a factor of the first importance in growth and development of plants ". 

CLASSIFICATION

•  Depending upon the duration of photoperiod the classification of plants is- 

1. Short day plants-These are plants that require a relatively short day light period (8-10hours) and a continuous dark period of about 14-16 hours for subsequent flowers.Also called as Long night plants .  E.g Maryland mammoth and Biloxi . 
2. Long day plants-These plants require a longer day light period(usually 14-16 hours)in a 24 hours cycle for subsequent flowering.Also called as Short night plants. E.g-spinaceae,Beta vulgaris.
3. Day Neutral plants-These plants have photoperiods range between long day and short day plants ,from 5 hours to 24 hours continuous exposure. E.g-Cotton ,tomato,sunflower ,cucumber.

 PHOTOPERIODIC INDUCTION

•  Plants may require one or more inductive cycles for flowering .An appropriate photoperiod in 24 hours cycles constitutes one inductive cycle. 
• If a plant which has received sufficient inductive cycles (Continuous cycles )is subsequently placed under unfavourable photoperiods ,it will still flower. 
• Flowering will also occur if a plant receives inductive cycles after interval of unfavourable photoperiods(i.e discontinuous inductive cycles). 
• This persistence of photographic after effect is called as PHOTOPERIODIC INDUCTION. 

PRESENCE NATURE OF FLORAL HORMONE

• It is now proven through different studies that photoperiodic stimulus is perceived by the leaves.As a result of which floral hormone is produced in leaves which is then translocated to the tip subsequently causing the initiation of floral primordia. Although there are firm evidences for the existence of a floral hormone but it has not been isolated yet . Therefore the nature of hormone known as FLORIGEN is not clear. But  its quite evident that this hormone is a material substance which can be translocation from leaves to the apical tips of the plant resulting in flowering. 

PHYTOCHROME

• It is noticed that in Short day plants a brief exposure with red light during critical dark period inhibits flowering in short day plants and this inhibitory effect can be reversed by exposing the plant with far red light.
•  Similarly in Long day plants prolongation of critical light period or the interruption of the dark period stimulates the flowering . 
• This inhibition of flowering in short day plants and its stimulation of flowering in long day plants involves operation of a proteinaeous pigment called PHYTOCHROME. 

• The pigment phytochrome exist in two different forms- Red light absorbing form which is designated as PR Far red light absorbing form which is designated as PFR 
• These two forms of pigment are photo chemically interconvertible. When PR form of the pigment absorb red light(660-665 micro meter)it is converted into PFR form. 
• When PFR form of the pigment absorbs far red light(730-735 micrometer)it is converted into PFR form. The PFR form gradually changes to PR form in the dark. 

 STRUCTURE OF PHYTOCHROME

•  It was successfully purified in intact native phytochrome (from etiolated seedlings)was first reported by VIESTRA and QUAIL in 1983. It is a solute protein with molecular weight of about 250 KDa. 
• It is a homodimer of two identical poly peptides each with a molecular weight of about 125 KDa.
•  Chromophore of phytochrome is synthesised in plastids while Apoprotein is synthesised in nuclear genome.Assembly of these two is autocatalytic and occurs in cytosol. 
• None of them can absorb light alone. 

PHYTOCHROME MEDIATED RESPONSES

 1.Photoperiodism 
2.Leaf germination 
3.Elongation of leaf ,petiole ,stem.
4.Hypocotyl Hook unfolding. 
5.Unfolding Of grass leaf
6.Sex Expression. 
7.Bud dormancy. 
8.Plastid morphology. 
9.Leaf abscission. 
10.Flower induction 
11.Auxin catabolism 
12.Epinasty etc.