PHOTOPERIODISM Flowering is an important phase in the life cycle of a plant. Transition from vegetative phase to reproductive phase. Physiological mechanism responsible flowering is controlled by- 1) Photoperiodism (Periodicity of light) 2) Vernalization (Temperature) Length of day period has marked effect on the flowering. Garner & Allard (1920) discovered photoperiodism. Definition : “Response of plants to relative to relative length of day & night”

Classification of plants on the basis of requirement of length of photoperiod Short Day Plants (SDP) a) SDP flower when the day length is less than a certain critical length. b) SDP will not flower even if a flash of light is provided during continuous dark period. c) They can not flower if short dark & light periods are provided alternately. d) SDP flower when kept continuously in dark & provided with sucrose e) In SDP flowering can induced even during long days by increasing the dark period by putting them in dark for some time. f) Examples : Tobacco, Potato, Sugarcane, Strawberry, Crysanthemum, Cocklebur etc.

II) Long Day Plants (LDP) LDP require a photoperiod of more than a critical length. In LDP, period of darkness has an inhibotory effect on the flowering. A LDP requiring 16 h of light period in 24 h can be made to flower if provided with a cycle of 8 h of light period & 4 h of dark period. Flowering in LDP is inhibited because of too long dark periods, so called short night plants. Examples : Spinach, Radish, Sugar beet, Wheat etc.

PHOTOPERIODIC CONTROL OF FLOWERING

Day Neutral Plants (DNP) They flower in all possible photoperiods. Their flowering is not affected by length of the day. They are photoneutral plants. Examples : Tomato, Cotton, Sunflower, Cucumbers etc.

Photoperiodic induction: Few days exposure to appropriate photoperiod is enough for inducing flowering, this initial important effect on flowering is called photoperiodic induction. An appropriate photoperiod in 24 h cycle constitute one inductive cycle. Plants can flower if they receives discontinuous inductive cycles. Increase in the number of inductive cycles promote early flowering. The following plants require more than two inductive cycles for flowering: Soybean var. Biloxy - 2 Inductive cycles Salvia occidentalis - 17 Inductive cycles

Discovery & Role of Phytochromes Flowering in SDP is inhibited if dark period is broken by a flash of light. Red light (660 nm) is most effective in inhibiting flowering. Far-red light (740 nm) reverse the effect of red light and flowering occurs. The radiations used last in the sequence will determine flowering. Butler (1959) isolated a pigment called it as Phytochrome.

Phytochrome It is a protein with chromatophore. It occurs in two forms – Red light (660 nm) absorbing form designated as PR form. Far-red light (740 nm) absorbing form designated as PFR form. 3) These two forms are interconversible. PR form stimulates flowering & PFR form inhibits it. PR Far red light (740 nm) PFR Red Light (660 nm) D a r k

Endogenous rhythms Endogenous rhythms means every day internal activities, which has two phases : a) Photophilous phase : Occurs in light, characterized by intensive photosynthesis & weak respiration, synthetic (Anabolic) processes dominant. b) Skotophilous phase : Occurs in dark, characterized by intensive respiration. Hydrolytic (Catabolic) activities increased & decomposition of starch in to sugars takes place.

Applications of Photoperiodism 1) From the knowledge of photoperiodism, it becomes easy to control daily period of illumination, which helps to obtain vegetative growth & flowering at will. 2) The knowledge of photoperiodism has been of great practical importance in hybridization experiments. 3) The photoperiodic control of flowering can give flowers at the time of your festivals. 4) In sugarcane (SDP), flowering is inhibited by giving a flash of light at night during the short days of the year, so that the yield is increased.

VERNALIZATION Definition : “Method of inducing early flowering in plants by pretreatment of their seeds at very low temperatures.” Example : Winter variety of wheat can be converted in to spring variety if seeds are kept at low temp. (0-5 0C). Applications : 1) Shortens vegetative period. 2) Increases yield, flowering out of season. 3) Increases cold resistance. 4) Increases resistance of plants to fungal diseases.