Phytochrome - Floral Induction
Phytochrome a pigment that exists in two different inter convertible forms was discovered by Hendricks and Borthwrick et. al. and was chemically extracted and purified. Since then exact chemical nature is now established. Phytochrome has two parts the light absorbing portion (chromophore) and the large protein. Action spectra of photoperiodism, germination, and breaking of dormancy etc.-clearly establish phytochrome as the 'photomorphogenesis' pigment as its absorption spectra genuinely overlaps the action spectra of the above phenomenon. Phytochrome was found located mostly in the cell membranes. It exists in red-light absorbing form and far-red light absorbing forms. Phytochrome (R) absorbs red light and is converted into phytochrome (FR), a far-red light absorbing form.
Phytochrome Fr on exposure to far red light is reconverted to phytochrome R. Phytochrome (Fr) can also be converted to phytochrome (R) in prolonged darkness. The action of phytochrome through gene repression and depression production of flowering inducing hormones enzyme activation leading to 'floral apex' from a vegetative apex is roughly understood. But all the steps are not clear yet. Phytochrome regulated morphogenesis results for change in gene 13 transcription. Several phytochrome activated genes have been identified and isolated. One of them is PAL gene.