The region of stimulus for flower formation is the leaf. When subjected to necessary photoperiod (day length), it may induce growing points to flower. This stimulus is synthesized in the leaf and translocated to the regions where flower formation takes place at the growing points.
The number of photoperiodic cycles required to induce flowering varies in different species. But once induced, the plant – flowers, even if it is kept under noninductive conditions
The flowering stimulus is not species specific. A photo induced twig of one species when grafted to another species, can induce flowering. Even a long’
day plant grafted to a short day plant can be induced to flower under short day condition and vice versa. This led scientists to believe that the flowering stimulus is a flowering hormone or fiorigen.
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1. General characteristics of Phytochrome
Phytochrome is a plasma membrane bound red-light receptor pigment system involved in photomorphogenic response of plants. Phytochrome generally occurs in all the green plants except in algae. It is a phycobilin bound protein and exists in two interconvertible forms. One form, phytochrome red (Pr), absorbs short red light in around 660 nm.
It is the stable form. The other form, phytochrome far red (Pfr), absorbs in the far red or near infrared region of 73onm. On absorbing red light Pr changes into Pfr.
The Pfr changes to Pr either rapidly by absorbing far-red light or slowly in darkness. Most probably, Pfr and Pr ratio determines the morphogenetic response it initates reactions accompanying morphogenetic changes by activating specific genes.
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2. Phytochrome and flowering
The unique characteristic of the phytochrome system in plants is that this pigment is involved in timing the mechanism of flowering of both short day and long day plants. This has been verified from night interruption experiments. Short day plants remain vegetative when exposed to red night break but flower under fared.
Long day plants when grown in short day conditions, flower under red light but remain vegetative far-red when red (R) and far-red (FR) exposure are given in close succession, the nature of the light in the last exposure determines the response as follows.
Response of short day plant
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Vegetative Flower Vegetative Flower