Growth and development of plants are influenced by several environmental factors including light. However, light causes several responses other than photosynthesis. These responses greatly influence the course of plant development and the final plant appearance. They are photomorphogenetic responses.
For example, the seeds of many plants do not germinate unless they are exposed to light. Germination of seeds in light shows that the seedlings require light to grow. Phototropic responses of seedlings and of leaves of mature plants are also beneficial photomorphogenetic processes.
Photomorphogenetic responses are also important to older plants. Many such responses respond to the relative lengths of day and night by forming reproductive structures or by forming dormant buds that can resist a cold winter (i.e., phenomenon of photoperiodism and vernalization.)
Photoperiodism:
The discovery of photoperiodism, i.e., the growth response to the length of light and dark periods by Garner and Allard, completely changed the concept of flower initiation. It was demonstrated by experiments that the length of the dark period rather than the light period is the critical factor in the photoperiod response.
As per the result of investigations, It became apparent that a majority of plants fell into one of three categories:
(i) Short day,
(ii) Long day, and
(iii) Day-neutral plants.
Short-day plants require a dark period exceeding some critical length to flower, and cannot flower under continuous illumination (light). Long-day plants are inhibited from flowering when the dark period exceeds some critical length, and they can flower under continuous illumination.
Day-neutral plants can flower under any night length. In some plants, the leaves need only to be exposed to one light-dark cycle of the proper day length to cause flower initiation, whereas most plants require several or many such cycles.
It has been shown that photoperiodic responses may be altered by brief exposure to low light intensity. For example, interruption of the dark period in short-day plants has shown that it is red light (wave length 660 mµ) which is effective.
Similarly it has been proved experimentally that, far-red light (wave length 730 mµ) reverses the red light effect, i.e., if exposure to red light is followed by exposure to far-red, the result is as if there had been no exposure at all. If repeated alternating exposures to red and far-red are given, the final exposure determines the response.
Phytochrome:
The above given observations led to the discovery of the pigment system isolated from the leaf called phytochrome. This pigment system specifically receives the external photoperiodic message. The phytochrome was discovered by H.A Borthwick and Sterling B. Hendricks for the first time in 1959.
Phytochrome is a pigment present in small amounts in all plants. Under short- day conditions, an interruption of the dark period with light (specifically red light) causes an intramolecular shift in phytochrome that brings about flowering in long-day plants and prevents flowering in short-day plants.
Characteristics of Phytochrome:
The cytoplasmic pigment system phytochrome now identified as a protein and partially purified, which is sensitive to red light.
This pigment can exist in two forms, one that absorbs red light (Pr) and one that absorbs far- red light (Pfr). Red light converts Pr to Pfr, but this form is unstable and is gradually converted back to the Pr form.
Phytochrome is active in the Pfr form and thus the responses are produced by red light. In relation to flowering, reactions promoted by Pfr interfere with the dark reaction that promotes flowering in short-day plants and inhibits the flowering of long-day plants.
Since it is the length of the dark period that is critical, and therefore, it is supposed that the reversion of Pfr to Pr determines the critical length of the dark period. The reversion takes place only about four hours in the dark.
Apart from photoperiodic phenomena, many other light effects upon plants are exerted through the phytochrome system. The germination of lettuce seeds for example, required a light stimulus, and this is effected by red light and reversed by far-red light. Stem elongation and leaf expansion could be influenced by red light, and the effect of etiolation could be checked by exposure to red light.
Phytochrome is a protein with chromophoric (pigment) groups. The molecular weight of phytochrome is about 60,000 with three chromophores per molecule. The chromophore pigment is closely similar but not identical to the chromophore of c-phycocyanin, an algal (blue alga) chromoprotein.
It is believed that phytochrome is also involved in other photo-periodically controlled processes such as tuber and bulk formation, and dormancy.
About the functioning of the phytochrome, it has been suggested that it may operate by influencing membrane permeability in the cells, not only of the plasma membrane but also of those of the nucleus and the mitochondria, thus influencing the metabolism of cells.
Though the discovery of phytochrome has made a very significant contribution towards explaining the first step in the mechanism of flowering, yet much experimentation is required, so that the substances specifically causing the transition of the apical meristems from the vegetative to the flowering conditions could be isolated and their mode of action determined.