X r P fr P fr Day Night Phytochrome is a bluish biliprotein pigment responsible for the perception of light in photo physiological process. Butler et al., The physiological mechanisms in relation to flowering are controlled by (i) light period (Photoperiodism) and (ii) temperature (Vernalization). The physiological change on flowering due to relative length of light and darkness (photoperiod) is called Photoperiodism . The term photoperiodism was coined by Garner and Allard ( ) when they observed this in ‘Biloxi’ variety of soybean ( Glycine max ) and ‘Maryland mammoth’ variety of tobacco ( Nicotiana tabacum ).
The photoperiod required to induce flowering is called critical day length . Maryland mammoth (tobacco variety) requires hours of light and cocklebur ( Xanthium pensylvanicum ) requires . hours of light for flowering. .
Classification of plants based on Photoperiodism i. Long day plants : The plants that require long critical day length for flowering are called long day plants or short night plants. Example: Pea, Barley and Oats. ii.
Short day plants : The plants that require a short critical day length for flowering are called short day plants or long night plants. Example: Tobacco, Cocklebur, Soybean, Rice and Chrysanthemum . iii. Day neutral plants : There are a number of plants which can flower in all possible photoperiods.
They are also called photo neutrals or indeterminate plants . Example: Potato, Rhododendron , Tomato and Cotton. . Photoperiodic induction An appropriate photoperiod in hours’ cycle constitutes one inductive cycle.
Plants may require one or more inductive cycles for flowering. The phenomenon of conversion of leaf primordia into flower primordia under the influence of suitable inductive cycles is called photoperiodic induction . Example: Xanthium (SDP) – inductive cycle and Plantago (LDP) – inductive cycles. a role in seed germination and changes in membrane conformation.
. Vernalization ( Vernal – Spring Like) Besides photoperiod certain plants require a low temperature exposure in their earlier stages for flowering. Many species of biennials and perennials are induced to flower by low temperature exposure ( o C to