Describe the mechanism of photoperiodic induction of flowering.

Explanation

Mechanism of photoperiodic induction of flowering:
(1) The physiological change on flowering due to relative length of light and darkness (photoperiod) is called photoperiodism.
(2) The photoperiod required to induce flowering is called critical day lenght.
Photoperiodic induction:
(1) An appropriate photoperiod in 24 hours cycle constitutes one inductive cycle. Plants may require one or more inductive cycles for flowering.
(2) The phenomenon of conversion of leaf primordia into flower primordia under the influence of suitable inductive cycles is called photoperiodic induction. Plantoago (LDP) - 25 inductive cycles.
Example: Xanthium (SPD) - 1 inductive cycle.
Site of photoinductive perception:
(1) Photoperiodic stimulus is perceived by the leaves.
(2) Floral hormone is synthesised in leaves and translocated to the apical tip to promote flowering.
(3) This can be expained by a simple experiment on Cocklebur (Xanthium pensylvanicum), a short day plant. ,
(4) Usually Xanthium will flower under short day conditions. If the plant is defoliated and kept under short day conditions it will not flower.
(5) Flowering will occur even when all leaves are removed except one leaf.
(6) If a cocklebur plant is defoliated and kept under long day conditions, it will not flower. If one of its leaves is exposed to short day condition and rest are in long day condition, flowering wil occur.
(7) The nature of flower producing stimulus has been elusive so far. It is belived by many physiologists that it is a hormone called florigen. The term florigen was coined by Chailakyan (1936) but is not possible to isolate.