Growth and Development of Horticultural Crops (1+1)

Phytochrome

It has already been seen that a brief exposure with red light during critical dark period inhibits flowering in short-day plants and this inhibitory effect can be reversed by a subsequent exposure with far-red light. Similarly, the prolongation of the critical light period or the interruption of the dark period stimulates flowering in long-day plants involves the operation of a proteinaceous pigment called as phytochrome.

• The pigment phytochrome exists in two different forms,
  
(i) red light absorbing form which is designated as PR and
(ii) far-red absorbing form which is designated as PFR.
• When PR form of the pigment absorbs red light (660-665 nm, it is converted into PFR form).
• When PFR form of the pigment absorbs far-red light (730-735 nm) converted in to PR form.
• The PFR form of the pigment gradually changes into PR form in dark.
• It is considered that during the day the PFR form of the pigments is accumulated in the plant which is inhibitory to flowering in short-day plants but is stimulatory in long-day plants. During critical dark period in short-day plants, this form gradually changes into PR form resulting in flowering. A brief exposure with red light will convert this form again into PFR form thus inhibiting flowering. Reversal of the inhibitory effect of red light during critical dark period in SDP by subsequent far-red light exposure is because the PFR form after absorbing far-red light (730-735 nm) will again be converted back into PR form.
• Prolongation of the critical light period or the interruption of the dark period by red light in long-day plants will result in further accumulation of the PFR form of the pigment, thus stimulating flowering in long-day plants.
• The phytochrome is a soluble protein with a molecular weight of about 250 kDa. It’s a homodimer of two identical polypeptides each with a molecular weight of about 125 kDa. Each polypeptide has a prosthetic group called as chromophore which is covalently linked to the polypeptide via a sulphur atom in the cystine residue of the polypeptide. The protein part of the phytochrome is called as apoprotein.
• Apart from absorbing red and far-red light, the phytochrome also absorbs blue light. The PR form of phytochrome is blue while PFR from is olive- green in colour. But owing to very low concentration, this pigment is not visible in plant tissues. Phytochrome accounts for less than 0.2% of the total extractable protein in etiolated seedlings. None of the two components of phytochrome, i.e., apoprotein and chromophore, can absorb light alone. Phytochromes have been detected in wide range of plants in angiosperms, gymnosperms, byrophytes and algae. Dark grown etiolated seedlings are richest sources of phytochrome where this pigment is especially concentrated in apical meristems.
• Phytochromes have directly been detected in different part of seedlings, in roots, cotyledons, hypocotyle, epicotyls, coleoptiles, stems, petioles, leaf blades, vegetative buds, floral receptacles, inflorescences, developing fruits and seeds. Presence of phytochrome has also been shown indirectly in other plant materials.
• Within the cells, phytochrome exists in nucleus and throughout the cytosol. The chromophore of phytochrome is synthesized in plastids while apoprotein is synthesized on nuclear genome. Assembly of these two components of phytochrome is autocatalytic and occurs in cytosol.