If a plant is exposed to periods of light and darkness long enough to promote flowering, it is said to be photoperiodically induced, and the minimum duration of light needed to initiate floral primordia is called inductive cycle photoperiod. It was observed that if a short-day plant growing under long-short conditions is temporarily transferred to short day conditions and then returned to long day environment again flowering will initiate. This is photoperiodic induction. The number of 24-hours cycles required to induce floraI primordia is called photo-inductive cycle.
The number of cycles required to induce flowering differs widely among different species. For example, cocklebur (Xanthium pennsylvanictim), a long day plant, needs to receive 25 photo-inductive cycles, it will flower even if it is returned to photo-inductive cycles.
Partial induction has been observed both in short-day plants and long-day plants. For example, short-day plant Impatiens balsamina requires only three photo-inductive cycles for floral bud initiation but for these buds to form flowers eight more photo-inductive cycles are necessary. Similarly, the long-day plant Plantago lanceolata needs 25 photo-inductive cycles for 100 percent inflorescence formation. If the plant is given 10 photo-inductive cycles and then subjected to a non-inductive cycle, it will not flower. However, if the plant is returned to a photo-inductive cycle, only 15 cycles are needed to produce 100 percent inflorescence.
Significance of Photo-inductive Cycles
The concept of photo-inductive cycle suggest that some factor involved in flowering is accumulated during the inductive cycle. In some plants, e.g., Xanthium enough of this factor is accumulated after only one cycle to promote flowering. In other plants, more than one inductive cycle is needed. In long-day plants the non-inductive cycle as in Plantago lanceolata. However, in short-day plants non-inductive cycle appears to be inhibitory. It inhibits the effect of previous inductive cycle.
Role of Dark & Light Periods (Interplay of Light & Dark Period’s)
Plants under normal conditions are subjected to a 24-hour cycle of light and darkness. Therefore, it is logical to known about the role of light and dark period in promoting flower.
Importance of Dark Period
Early workers on photoperiodism used 24-hours cycle but workers recognize soon that better results can be obtained by changing the normal cycle, for example by following an 8-hour light period with an 8-hour dark period or following a 16-hour light period with a 16-hour dark period. When short-day plants were subjected to cycles other than 24-hours, it was found that flowering in short-day plants respond more to dark period than to the light period. In other words, the short-day plants flower when the duration of the dark period is less than a critical value.
Karl C. Hammer and James Bonner (1938) performed experiment with cocklebur (Xanthium), a short-day plant, to find out the relative importance of a day and night to photoperiodic induction. It was observed that if the dark period is interrupted with a brief period of darkness had very little effect. The interruption of dark period by light is referred to as night break. Later, Hammer working on Biloxi soybean, a short-day plant found that flowering could not be induced until the plants received dark periods in excess of 10-hours. It is now known that length of the dark period determines actual initiation of floral primordia.
Importance of Light Period
While the length of the dark period determines initiation of primordia, the length of light period influences the number of floral primordia initiated. The photo-inductive cycle for Biloxi soybean is 16 hours of darkness and 11 hours of light. Photoperiods of more than 11-hours result in the differentiation of a smaller number of floral primordia. Thus, the response to light period is quantitative.
Perception of Photoperiodic Stimulus Ripeness .to Respond
After germination, a young plant grows vegetatively, forming an aerial system of leaves and stem, and a root system below the ground. Vegetative growth continues until a certain minimum size is reached. Only then can the plant switch to reproductive development. This condition is called ripeness to flower. The shoot meristem switch from producing leaves and lateral buds to producing flowering.
Most of the plants flower only at a particular time of the year, suggesting that some seasonal change, triggers the switch from vegetative growth to flower production. It has been observed that day-length triggers the onset of flowering in plants. Kaleb’s termed the condition which a plant must achieve before it flowers in response to the environment as ripeness to respond. For example, some varieties of tomato grow vegetatively until the thirteenth node and then switch to flower formation and many weed species flowers immediately when a certain size has been reached. Similarly, certain bamboo species flower when they are 5 to 40 or more years old.
Center of Photoperiodic Response
In an experiment, Xanthium plants were grown under vegetative condition of 16-hours of light and 8-hours of darkness until four or five fully expanded leaves developed. The plants were divided into three groups:
One group of plants were kept intact, the other was defoliated and plants of third group were defoliated except for one fully expanded leaf. These groups of plants were than provided with photo-inductive cycle of 15-hours of light and 9-hours of dark and then returned to 16-hours of light and 8-hours of dark cycle. Flowering and fruit formation was observed in groups of plants with intact leaves and with a single leaf, but not in defoliated plants. This experiment confirms that the center of photoperiodic response in leaves.
A minimum amount of leaf tissue necessary for flowering to occur. The developmental stage of leaf is also important in regard to sensitivity photoperiodic induction. For example, partially mature Xanthium leaves are much less sensitive to photoperiodic induction. Mature leaves seem to antagonize flowering promoting effect of photoperiodic stimulus.
Transmission of Photoperiodic Stimulus – Flowering Hormone
The leaves of the plant are the organs that detect the exposure to correct photoperiods, but response occur at the growing points. Cajlachjan (Chailakhyan), a Russian physiologist, in 1936 working on floral initiation suggested a hypothesis that flowering in plants is controlled, by a specific phyto-hormone, florigen produced in leaves in response to photo sufficient quantities at the growing points, buds are inducedto produce flowers instead of leaves.
The hormone is hypothetical and yet to be isolated but its existence and proof of its translocation comes from grafting experiments performed with Xanthium. One branch of two branched comas plant was grafted in series to five other Xanthium – plants and the end branch is exposed to correct photo-inductive cycle while the other second of first plant and of other five plants were kept on non photo-inductive cycle, all plants flowered. The experiment demonstrate existence and transmission of photoperiodic stimulus.
In other experiments Zeevaart grafted long-day plants to short-day plants and vice versa. When the long-day plant Sedum spectabile was grafted to the short-day condition. When the latter plant was grafted to the long-day plant, it flowered under long-day conditions. Also experiments by Hodson and Hammer demostrated that extracts of flowering Xanthium could initiate flowering in Lemma (duckweed), but the extracts of vegetative Xanthium does not. These experiments show the florigen is not species specific and has nearly the same properties in both long-day and short-day plants.
Light Quality & Photoperiodism
It is customary practice in the investigation of photo-biological reactions to find the wavelength that are most effective or in other words to develop an action spectrum (effectiveness of wavelength of light) for the process. Parker and colleagues obtained the first action spectra for the control of flowering from two short-day plants, Kanthium and Biloxi soybean. A group of scientists, headed by Borthwick and Hendricks, studies the action spectra and also investigated inhibitory action of light-breaks during the dark period. They found that most efficient wavelengths for inhibition of flowering are between 620 and 660 nm (orange-red) with a maximum at about 640 nm. Therefore, red light is considered to be the most efficient radiation in light-break reactions. Far-red radiation was found to have no effect as light-break factor. However, during experiments with short-day Xanthium and Biloxi soybean plants, it was discovered that if a brief flash of far-red radiation is followed by a brief flash of red light in the middle of a long night of a photo-inductive cycle for short-day plants, flowering will occur. If the red radiation is followed by far-red radiation again, the flowering will again be inhibited. This suggests that radiation used last determines the respond of the plant.