Question

Describe the mechanism of photoperiodic induction of flowering.

Answer 1

• Mechanism of photoperiodic induction of flowering:

1. The physiological change on flowering due to the relative length of light and darkness (photoperiod) is called Photoperiodism.
2. The photoperiod required to induce flowering is called critical day length. Eg. 12 hours in Maryland Mammoth’s Tobacco Xanthium 15.05 hours.

• 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. Example: Xanthium (SDP) -1 inductive cycle and Plantago (LDP) -25 inductive cycles.

• Site of photoconductive perception:

1. Leaves are the parts that receive photoperiodic stimulus (PPS), again it is only leaves that synthesize floral hormones and translocate them to the apical tip to promote flowering.
2. This can be demonstrated by experiments conducted in the Cocklebur plant. Which is an SD plant. The nature of flower-producing stimulus has been elusive so far. It is believed by physiologists that a hormone is responsible for it, Chailakyan (1936) named it as Florigen It is not possible to isolate it.

Procedure	Observation	Inference
1. Take potted plant A and defoliate the plant subject it to SD – a condition.	There is no induction of flowering.	No leaf to receive stimulus or induction of flowering
2. Take potted plant B – and defoliate all, except one leaf subject it to SD – condition.	There is the induction of flowering.	One leaf is enough to receive stimulus or induction of flowering.
3. Take potted plant C – and defoliate it and subject it to LD condition.	There is no induction of flowering.	no leaf to receive stimulus or induction of flowering.
4. Take potted plant D and subject all leaves to LD but one leaf to SD.	There is the induction of flowering.	One leaf is enough to receive an induction in the SD condition.