In this article we will discuss about:- 1. Meaning of Photoperiod 2. Photoperiodic Response Groups 3. Photoinductive Cycles 4. Genetic Approach 5. Flowering Hormone.
Subject-Matter of Photoperiod:
Two American plant physiologists, W.W. Garner and H.A. Allard (1920) while working at the USA department of Agriculture Research Laboratories at Beltsville, Maryland made observations that tobacco variety, Maryland Mammoth, grew vegetatively in summer and flowered during winter alone.
On the other hand, soybean planted at different times of spring flowered nearly at the same time during summer. Garner and Allard tested several factors which possibly could affect flowering and concluded that the critical factor was the length of the day.
The response of the plants to the relative length, and alternations of light and dark periods with regard to the initiation of flowering is called photoperiodism. It is also defined as a response to the timing of light and darkness. For the period of darkness Photoperiod is used.
Photoperiodic Response Groups:
Based on flowering in response to different length of photoperiod, Garner and Allard divided plants into three groups. Plants which flower in response to days longer than some critical length were classified as long-day plants (LD). These are Spinaceaoleracea (spinach), Beta vulgaris (sugar beet), and Hyoscyamusniger (Black henbane).
Those which flower when the day length is less than the critical length were called short-day (SDP) plants. These are Nicotianatabacum (Maryland Mammoth), Xanthium pennsylvanicum (Cocklebur) and Glycine max (soybean). However, some of the plants were unaffected by the photoperiod conditions. These are called day neutral plants e.g. Lycopersicumesculentum (tomato), Mirabilis (four O’clock plant).
The further subdivisions of the photoperiodic classes can be made into:
(i) Absolute photoperiodic responses where the particular day length becomes essential for flowering e.g. Glycine max, Impatiens balsamina in SD conditions and Avena sativa and Hyocyamusniger in LD category.
(ii) Quantitative photoperiodic responses where the particular day length promotes but is not essential for flowering e.g. Helianthus annus and Daturastramonium in SD category and Brassica compestris and Hordeumvulgare in LD category.
There are certain plants which have dual photoperiodic requirements e.g., the LSDP flower in short days (SD) only after plants have previously received a sufficient number of long days (LD) as in Cestrum nocturnum. Likewise SLDP flower under long day (LD) conditions only when they have previously received sufficient number of short days (SD) as is the case in Scabiosa sp.
The critical photoperiod varies for both LD and SD plants under 24 hour light-dark cycle (Fig. 22-2). Xanthium, an SD plant, with a critical period of 15⅟2hours, flowers only if this period is not exceeded. On the other hand, Hyoscyamus is an LD plant with a critical period of 11 hours and will flower only when this critical period is exceeded.
Both the species will flower under a photoperiod of 13 hours. Subjecting LD and SD plants to light and darkness, other than 24 hours, shows that flowering in plants is more of a response to the dark period than to the light.
The critical night is that period of darkness, which must be exceeded before SD plants would flower. In LD plants, the dark period must be shorter than the critical night if flowering is to occur. Thus Xanthium, a SD plant, will not flower if the long dark period is interrupted by a single brief flash of light (Fig. 22-3).
On the other hand, interrupting the light period for a short period of darkness had no effect. It may be interpreted to mean that the long dark period which is necessary for flowering in Xanthium, consists of two short dark periods. Most LD plants will flower when so treated.
Alternatively, the SD plants may be called long night plants and LD plants as short night plants. While the length of dark period determines initiation of floral primordia, the length of photoperiod determines their number. In Glycine, the optimum Response is obtained with a photo-cycle comprising 16 hours of darkness and 11 hours of light, respectively.
Table 22-1 gives selective list of some important short day (SD) and long day plants (LD):
Photoinductive Cycles:
The number of cycles required to induce flowering varies with different plant species. For instance, Xanthium, requires only one photo-inductive cycle whereas Salvia occidentalis, as SD plant, 17, and Plantagolanceolata, a LD plant as many as 25, to flower.
Once a plant has been subjected to the minimum number of photoinductive cycles, it will flower even if it was returned to non-inductive cycles (Fig. 22-2A). Such a ‘carrying-on’ effect of suitable photo-inductive cycles is called photoperiodic induction. All this shows that some factor(s) for flower formation is produced and accumulated during the inductive cycles.
Perception of Photoperiodic Stimulus:
In spinach, a LD plant, it was shown that the leaves are the receptors of the photoperiodic stimulus. It was assumed that something was produced in the leaves in response to photo-inductive cycle and then it was translocated to the apical bud where it initiated flower primordia. The experiments with flowering clearly illustrate this point.
If a single leaf of Xanthium is exposed to short photoperiod (16 HD + 8 HL) while the rest of the plant is subjected to a long photoperiod there is formation of flowers. Grafting of photo-induced leaves from one plant to the one with non-inductive cycle also promotes flowering on the receptor plant. The receptor plant is defoliated before the initiation of the experiment to eliminate the antagonistic effect of the natural leaves.
Genetic Approach to Photoperiod:
The basic approach is to identify the mutant that influences timing at any level, then the wild type gene is isolated and its gene product is analysed for clues to its role in the timing mechanism. In fact in several plant breeding programmes, flowering genes were also sought after. In crop species it is highly desirable to select early flowering photoperiod insensitive genes.
In pea and wheat, genetics of photoperiodic processes have received much attention. Incidentally, both the species are qualitative long-day plants. In pea several genes which affect photoperiodic timing and the onset of flowering have been identified, fsd (flowering short day) is a recessive mutant and makes the plant behave as a qualitative SD plant. When mutant is grafted to a wild-type stock under LD, the mutant will flower.
Recently approach in flowering genes has been made in Arabidopsis where several genes have been identified. This plant is a quantitative LD plant with a critical photoperiod of 8 to 10 hours. Under LD, it flowers with 4 to 7 leaves in the rosette. Under SD, flowering is delayed until 20 leaves are formed.
Flowering is also enhanced when the plant is exposed to blue or FR light suggesting the role of phytochrome in photoperiod phenomenon. Search is made to locate and isolate early and late flowering mutants. Here the flowering time refers to the number of rosette leaves produced before the flowering stem appears.
Incidentally, the mutants that affect phytochrome also affect flowering, hyl mutant is defective in the synthesis of the phytochromechromophore. Hence such mutants show an elongated hypocotyl but they flower earlier than the wild type under both LD and SD. Since the mutants flower under both conditions, they still show a response to photoperiod.
Several mutants which are insensitive to photoperiod and flower either earlier or later than the wild type plants have been identified, elf-3(early flower (-ing 3)—This is an early flowing type. On the contrary co-constants and gi-gigantea are later flowering, day-length insensitive mutants. They flower under LD conditions but SD has no effect.
Grafting experiments have shown that wild type genes, GI, operates earlier than CO gene in the same pathway and the floral promotion under LD depends on the amount of COmRNA transcribed. This mutant is also of special attention since it appears to interact with endogenous clock. elf3 gene affects leaf movement, CAB gene expression besides advancing flowering.
Through traditional plant physiology techniques photoperiodic signal transduction chain and endogenous clock proved difficult to investigate, the molecular genetic approach has provided insight into mechanisms that have eluded researchers for more than seven decades. There are many other instances where modern approaches have solved several of the plant physiology enigmas.
Flowering Hormone of Photoperiod:
The flowering hormone which was named ‘florigen’ by M.K.R. Chailakhyan is produced in photo-induced leaves and transported in the plant. This is clearly shown when two branched, Xanthium plants are grafted in series. If the end branch of a plant in series is given a photo-inductive cycle, it causes flowering in all the plant in a chainlike reaction (Fig. 22-4).
Translocation of the floral hormone: When LD Sedum spectabilus was grafted to the SD Kalanchoeblossfeldiana, it flowered under SD conditions. When the latter plant was grafted to the LD plant, it flowered under LD conditions. The experiment shows that ‘florigen’ was not specific and was same in both LD and SD plants.
Light Quality and Phytochrome:
If a long light of a photo-inductive cycle for Xanthium is interrupted by a brief flash of light the plant does not flower. The most effective radiation in light break reactions is the red light. Its effect is reversed by far-red light suggesting participation of the phytochrome pigment. The phytochrome exists in two forms, one red absorbing form (Pr) and the other far-red absorbing (Pfr).
According to Borthwick, the far-red absorbing form of phytochrome (Pfr) accumulates in the plant during the day (white light). It is stimulatory to flowering in LD plants and inhibitory to flowering in SD plants.
At the onset of a dark period the Pfr is converted to Pr form which is stimulatory to flowering in SD plants and inhibitory to flowering in LD plants. The interruption of the dark period with red light will return the accumulated Pr form to Pfr form, thus inhibiting flowering in SD plants. If the red light break is followed by a far-red break the red light influence is reversed, since Pfr is converted to pr form, and flowering occurs (Fig 22-5).
The role of red and far-red light on the initiation of flowering in the LD and SD plants is shown in Fig. 22-6. It may be said that the precise role of phytochrome in flower initiation is not clear. However, it may be added that this pigment is not the flowering stimulus but possibly aids the stimulation of flowering stimulus.
The effect of phytochrome on the flowering of SD plants may be explained as follows. At the end of light period Pfr is in high concentration and Pfr and Pr ratio is such that it prevents formation of flowering stimulus. If the dark period is prolonged, then either Pfr is destroyed or it is reverted to Pr state.
At this stage the ratio of Pfr to Pr is such which triggers processes leading to formation of flowering stimulus. Under the situation where dark period is briefly interrupted by red light, there is formation of Pfr from Pr and the ratio of two is altered and therefore formation of flowering stimulus is prevented.
In the LD plant the role of Phytochrome can be explained as follows long day plants need high ratio of Pfr to Pr to trigger flowering stimulus. This ratio is attained at the end of a long day. Once the night is long then Pfr is reversed back to Pr and the formation of flowering stimulus is prevented.
Under the conditions where night is interrupted by a red light, there Pr is changed to Pfr. Here the ratio of the two pigments is high and flowering stimulus is produced. These explanations, however, fail to provide explanation for the initiation of flowering stimulus as mediated by phytochrome.
Figure 22-7 shows a possible hypothesis regarding photoinduction of a short day plant. According to Hess (1975), day light has more red than far-red light and therefore, concentration of P730 in the leaf is high and causes ‘chemical inhibition’ of flowering hormone. In the absence of light, P730 is converted into P660.
The continued absence of light leaves very little of P730 and consequently synthesis of flowering hormone begins. Since antimetabolites inhibit flowering and GA promotes it, a working explanation has been proposed. It is believed that in plants exposed to inductive dark period more of “florigen” is synthesized in the leaf and this is translocate through phloem to the shoot apical flower.
GA and the flowering response. The application of GA to most LD plants will cause them to flower when on a non-inductive cycle but it has no effect on SD plants on a non-insuctive cycle.
Brian has suggested the role of GA in a scheme explaining photoperiodic reactions. According to him, a “GA-like” hormone is produced during the photoperiod as follows:
Red light promotes conversion of the precursor to the GA-like-hormone. During the dark the hormone is reconverted to the precursor. This back reaction is accelerated by far-red light. The GA-like hormone is then converted to florigen.
The steps of conversion are different in LD and SD plants. In LD plant a high level of GA-like hormone is required for the production of florigen. In SD plants, a low level of the GA- like hormone is optimum for a flowering response. However, if enough florigen is produced flowering will occur in both the LD and SD plants.
Chailakhyan Hypothesis:
He gave the concept of ‘florigen’ as a bicomponent complementary flowering hormone complex. According to him, gibberellins are essential for flowering for long day species and anthesins for flowering of short day plants. In the first step, gibberellin is synthesized in the leaves during the light period.
During the dark ensuing period, anthesin is synthesized. Together GA and anthesin constitute the true florigen which migrates to the shoot tip where it initiates flowering. Both these components of florigen are localized in different zones of the apices, the gibberellin mostly in medullary zone which controls stem formation and growth, and the anthesins in the control zone, which controls the flower formation.
In long day plants on non-inductive cycles there is sufficient amount of anthesin but not enough GA. Hence flowering is promoted in LD plants by the application of GA. In SD plants on non-inductive cycle, it is just the reverse. There is sufficient GA but low anthesin. Hence exogenous application of GA has no effect on the flowering of SD plants of non-inductive cycle.
Through the usage of antimetabolites for nucleic acid and protein synthesis, it has been shown that flowering induction of plants is dependent upon the two processes. Once used, these inhibitors cause their effect partially on the leaf as well as shoot meristem.
In the leaf, possibly flowering hormone synthesis is affected whereas in the shoot, meristem fails to differentiate in flower. Alternatively, antimetabolites like 5 FUDR possibly inhibits DNA reduplication in the shoot meristem. In a way the situation is quite comparable with that of vernalization.
Even though there is general acceptance of the hormonal concept of flowering, its chemical identity as well as exact metabolic role in floral induction and differentiation remains to be expanded. In recent years several schools have undertaken cytochemical, morpho-physiological, biochemical and physiological studies to throw light in floral induction and differentiation.
One of these school was led by late Prof. J.J. Chinoy at Ahmedabad who suggested that the transformation of the shoot apex from vegetative to reproductive state may be (Fig. 22-9) “――― brought about by a change in the redox system of the plant, as well as in other properties of the cytoplasmic and nuclear colloid paving the way for the ultimate meiotic division.”
Since then the dynamics of ascorbic acid (AA) production and utilization and also the metabolic drifts of nucleic acids, proteins and other cell constituents have been studied in the shoot apex and differentiating floral organs of several species under different photoperiodic and vernalization treatments.
Based upon these studies the AA, nucleic acid, and protein metabolism concept of growth and development was advanced by Chinoy.
The distinguished author demonstrated that during floral differentiation, there was an increased rate of ascorbic acid, nucleic acid and protein metabolism due to production of the radicals of AA and also due to the formation of a charge transfer complex between macromolecules and AA.
Thus both contributed substantially to the enhanced energy flow during the period of substantially to the enhanced energy flow during the period of reproductive differentiation.
It will be observed that AA undergoes monovalent oxidation in aerobic conditions with the aid of special preoxidase. The activity of the latter increases considerably during the differentiation of reproductive structures and thus produces a powerful reducing agent monodehydro-ascorbic acid (MDHA) which being a free radical serves as an electron donor in the biosynthesis of macromolecules.
The flow of electrons from MDHA, photophosphorylation and oxidative phosphorylation are also energized. Thus ATP pool is increased. The formation of high amount of free radicals in shoot apex causes establishment of a direct flow of electron energy for biosynthesis of cell constituents.
This charge transfer paves the way for the production of different types of RNA and subsequently of structural proteins, enzymes and other cell constituents at greatly accelerated rates during the period of reproductive differentiation.
The increased formation of CTC and free radicals under photo and thermo-inductive conditions results in further increase of orbital energies during reproductive differentiation and bringing about an increase in the biosynthesis mentioned above. Consequently rates of cell division and formation of growth centres are enhanced.
Thus there is floral induction and differentiation. Indeed the beginning of meiosis is the result of a change in the rhythm of nuclear and cytoplasmic processes participating in energy regulation and consequently in the formation of new cells and growth centres at an intensified rate.
Several of the crop plants have attracted the attention of physiologists to investigate their photoperiodic responses.
Some experiments on physiology of flowering in Impatientsbalsamina. Late Prof. K.K. Nanda at Chandigarh did some interesting work on the physiology of flowering in Impatiens plant.
The salient findings from his research group are summarized below:
Impatientsbalsamina is a qualitative short day (SD) plant which requires 3 SD cycles for the initiation of floral buds and at least 8 for them to develop into flowers. However, even 2 SD cycles were found to cause floral induction in later experiments probably affected by temperature. Under continued illumination plants continued to grow vegetatively.
The critical dark period for the initiation of floral buds in this species was 8⅟2hr alternating with 15⅟2hr light at 26 ± 1°C although the daily requirement for floral bud initiation decreased to 8 hr at lower temperature. The critical photoperiod changed with the prevailing temperature conditions.
Later studies by Nanda and his students demonstrated that floral buds could be made to revert to vegetative growth under non- inductive photoperiods. So much so that even apical growing point could be made to flower.
Another interesting point that arose from their studies was that the effect of individual SD cycles which in themselves were not inductive could be summated even when intercalated by as many as 16 LD cycles. Evidently the sub-threshold stimulus of inductive SD cycles could persist through long non-inductive periods and was not destroyed (Fig. 22-10).
Figure 22-10 shows effect of light interruption in the middle of dark period in October and November in flowering of Impatiens balsamina. Floral buds were produced even when the dark period was interrupted by light for 2 hr.
One of the spectacular responses of this qualitative SD plant was that GA was able to cause induction of florar buds under strictly non-inductive photoperiods. Further sub-threshold GA effect could be summated to the sub-threshold photo-inductive effect. So much so that even phenolic compounds (salicylic acid, β-naphthol) could also substitute for the photo-inductive requirements.
Similar effects were seen with purine derivatives. Nanda and his group also demonstrated that auxins in general delayed while triiodobenzoic acid (TIBA) induced floral bud initiation. However, when different vitamins were tested they did not affect flowering. Phosphon D and cycocel affected the SD cycle requirement and also the critical photoperiod in this species.
Studies pertaining to search, total sugars, total nitrogen, free amino acids, nucleic acids contents of stem and leaves under 8 and 24-hr photoperiod revealed metabolic drifts related to flowering. Under inductive conditions protein metabolism and isozymes patterns also changed.
When metabolic inhibitors were tested the results were entirely unpredictable. Cycloheximide increased the number of leaves and the number of flowers as well.