The phytochromes are present in the leaf. But the flowers are produced at the stein tip and its branches. The apical or lateral buds express the stimulus of flowering. It was believed that some substance or stimulus mines from the leaf to the buds. This substance is a hormone. A Russian plant physiologist Chailakhyan namedit florigen. Florigen is a hypothetical flowering hormone. It has following properties:
(a) Translocation of florigen:
I. Translocation with same plant:This hormone is formed in the leaves. Then this hormone multiplies itself. It is the unique property of this hormone. Then it is translocated through the phloem to the apical and lateral buds. It causes flowering in them.
- Translocation between different plants of same •specie: Florigen can move from one plant to the other by grail union. It can he proved by a simple experiment. A number of xanthitim seedlings grown under long days.. The adjacent branches of a number of plants are grafted together in a series. Only the terminal branches are exposed to short days. It causes flowering of all the plants. Therefore: florigen moves from one plant to the other through the grail union. It can also. move both in the upward and downward directions.
- Translocation between different plants of different species: Two different taxonomically related plants can also be grafted together. Tobacco is a short day plant. Hyoscyamus is a long day plant. Both belong to the family Solanaceae. Now branch of tobacco is grafted to . Flyoscyamus. They are kept under short day conditions. Tobacco starts flowering. Interestingly I lyoseyamus plants also start flowering. Thus norigen formed in tobacco is translocated to Flyoscyamus. It causes flowering in it. Similarly, when they were placed in long day condition again both produces flower. It indicates florigen is universal flower forming substance. It found in all the plats like LDP. SDP. DNP. SDI .P1′ and WSW’. Furthermore, florigen of monocots can cause flowering in dicot and vice-versa.
(a) Chemical nature of florigen
A number of attempts have been made to extract florigen hormone. Hamner and others (1964) extracted a substance from the inflorescence of Xanthium. But this substance was not effective in stimulating flowering of even Xanthium plants. There are live plant growth hormones: gibberellins. cytokinins. abscisic acid and ethylene. Some plant physiologist suggested that some of these hormones might be acting as florigen. Experiments were performed on these hormones.
- Application of auxin causes flowering only in one of the short day plants pineapple.
- Gibberellin causes ffowering only in rosette long day plant like Hyoscyamus and vernalizable plant like carrot. GA causes flowering in only one short day plant.
- Similarly. abscrisic acid. 0.4,0Lip in and ethylene also cause flowering in few species.4. Chailakhyan suggested that two thctors are involved in the flowering stimulus. First factor is gibberellins and the second thctor is anthesins. Both gibberellin and anthesins collectively acts a florigen. But it cannot be prose(‘ experimentally.Theretbre, no one hormone is taken as florigen. It is possible that the flowering stimulus is transmitted by mixture of hormones.