Discovery of Phytochrome
Phytochrome is a blue-green pigment involved in the perception of photoperiodic stimuli controlling flowering and other growth phenomenon such as seed germination, etc. The discovery of phytochrome was made during studies by Borthwick, Hendricks and their colleagues. They revealed that a pigment system is involved in the germination of lettuce (Lactuca sativa) seeds. They found that germination of these seeds is stimulated by red light (660 nm). They found that far-red light (730 nm), given immediately after a red-light exposure, inhibited germination, and when the seeds were again treated with red light, germination was promoted. They suggested that there is a reversible pigment system in lettuce seeds. The last light treatment determines the response of the seeds. Thus, the action spectrum of lettuce seeds is similar to that of photoperiodism.

Chemistry & Physiology of Phytochrome
The phytochrome occurs universally among green plants and was isolated from several plants such as tobacco, oat, corn and bean; purified and identified as a large conjugated protein with a coloured prosthetic group, the chromophore. The pigment part resembles open-chain tetrapyrole phycocyanin of cyanobacteria and red algae. The chromophore is linked to the protein at ring III. The protein part of phytochrome contains a high proportion of acidic, basic and sulphur-containing amino acids. Thus, the phytochrome molecule is highly charged and highly reactive. Phytochrome occurs attached to cell membranes.
The phytochrome exists in two forms: the phytochrome red absorbing form (Pr) and the phytochrome far-red absorbing form (Pfr). The Pfr form of the phytochrome is the enzymatically active form. The two forms are photochemically inter-convertible. In darkness, the Pfr form converts into Pr form slowly while in red light Pr form is converted to Pfr form. The photoc-conversion of Pr and Pfr forms involve electron changes in ring I, with either the addition or less of a proton. The Pfr form of phytochrome is unstable and undergoes a process called decay. It loses its photo-reversibility but is not destroyed, perhaps converted to an unknown inactive form.
Mode of Action of Phytochrome – Mechanisms of Photoperiodism
The precise role phytochrome in flower initiation is not known. Phytochrome is not the flowering stimulus, but it is not involved in triggering the formation (synthesis or activation) of flowering stimulus. How phytochrome exerts its control in flowering can be explained ass following:
Short-Day Plants
The effect of phytochrome on the flowering of short-day plants as follows. At the end of light period the concentration of Pfr is high and the ratio of Pfr to Pr is such that formation of the flowering stimulus is prevented. During the long dark period Pfr reverts to Pr or is destroyed and the ratio of Pfr to Pr finally drops to a level in which metabolic processes are triggered that lead to formation of the flowering stimulus. If the dark period is interrupted by red light, Pr is converted to Pfr and the Pfr to Pr level such that the formation of the flower stimulus is prevented.
Long-Day Plants
The possible role of phytochrome in long-day plants might be as follows. Long-day plants require a high ratio of Pfr to Pr, for the formation of the following stimulus. Such a high ratio of Pfr to Pr is attained at the end of a long day. If the night is too long, Pfr reverts to Pr, or is destroyed and the flowering stimulus is prevented from being formed. When the night is interrupted by a flash of red light, Pr is converted to Pfr, thereby raising the ratio of Pfr to Pr, to a level that allows the flower stimulus to form.
The above mentioned scheme help to explain night break effects, but they do not provide information concerning the role of phytochrome in initiating the formation of flowering stimulus.
Role of Gibberellins
It was observed that application of gibberellins to most long-day plants cause them to flower when they do not exposed to correct photo-inductive cycle. However, gibberellin does not cause flowering directly. Exposure to longer periods of photoperiods causes differentiation of floral primordia along with stem elongation while gibberellin application results in elongation of flower bearing stem (bolting) first and initiation of floral primordia occurs some time later. This suggest that gibberellin fulfills the requirement o floral differentiation and development.
Chaila khyan made actual measurements of the gibberellins levels in leaves of both short-day and long-day plants under photo-inductive cycles. He found that the gibberellin content is higher under long-day conditions both in long-day plants. Chaila khyan presented a hypothesis associating gibberellins with the floral hormone in the photoperiodic response of flowering. He suggested that there are two steps involved in the flowering process, the first mediated by gibberellin and the second by one or more flowering factors called anthesin. Gibberellins and anthesin constitute the true florigen. According to his hypothesis, long-day plants on non inductive cycles have a sufficient amount of anthesin butnot enough gibberellin. This situation is reversed in short-plants, the gibberellin is high and anthesin is low.
Role of Plant Growth Substances in Photoperiodism
Since the original concept of a flowering hormone was introduced by Chaila khyan in 1937, ideas concerning the nature of flowering process have undergone considerable change. Many discoveries such as the role gibberellins in promotion of flowering in long-day plants on non inductive cycles; evidence provided by several investigators that indoleacetic acid (IAA) induces flowering in a number of plants and suggestions that growth substances appear to play a role in the initiation of flower primordia, not as the primary initiator of reproductive growth but rather as modifiers of metabolic reactions.
The evidence of participation of growth substances in flowering suggests that florigen alone cannot account flowering of all plants. Although sequences of events occurring in leaves during photoperiodic stimulation is not known , it is reasonable to assume that changes are initiated by photochrome, followed by changes in leaves of growth substances, metabolites, and cofactors (ATP, NAD, FAD, nucleotides, etc). The change in metabolic activity than leads to formation or activation of a substance(s), which is trans-located out of the leaf of apical meristem. The vegetative shoot apex is transformed to floral apex and floral primordia begin to initiate which culminates in development of buds, flowers and fruits.

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