For hundreds of years, it has been known that certain plants were triggered to begin flowering by a variety of factors. Some can be triggered by warm weather, rain, or even by the amount of light a plant receives. Because of the flowering responses that occur to plants, photoperiodism could be defined as a physiological reaction that plants can have in response to changes in the length of day. The duration of day and night can signal for plants to grow, reproduce, or even to come out of dormancy. Soybeans were one of the first plants through experimentation to show many diverse effects with changes in light. Even though many plants can be affected by photoperiodism, soybeans are considered a vital crop for farmers worldwide.
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In the early 1900’s, Mr. Wightman Garner and Mr. Henry Allard began researching the concept of photoperiodism. At first, they were experimenting on tobacco, barley, as well as other crops, and found that the length of day affected the growth and development of the plants. Later on, Garner and Allard decided to research the effects of photoperiodism on the soybean crop.
The first study research was titled “Photoperiodic Response of Soybeans in Relation to Temperature and Other Environmental Factors.” In experiments prior to this study, they noted that the Biloxi variety was one to behave as a late variety and would not flower until the beginning of September. With the late flowering, the early study showed that this variety failed to make mature seed. The other varieties behaved as earlier varieties and successfully matured. When Garner and Allard exposed the four varieties to an artificially shortened daylight of around ten to twelve hours, all varieties were shown to flower around the same time. The final conclusion of the prior experiments displayed that the differences in soybean behavior were primarily due to the length of day. Because of these results, researchers wanted to have another research trial. For this experiment mentioned in 1930, there were three series that were carried out in this research trial. This study took place from the years 1920 to 1927, except for 1921. The first series was described as four varieties of soybeans that were planted at regular intervals of four/five days during the growing season (years 1920, 1922, 1927) in Washington. This series determined the basis of what was considered normal behavior for these different varieties of soybeans. The second series of soybeans were in a greenhouse and were planted at regular intervals with a mean temperature at a uniform level. The third series was first planted in the greenhouse. After germination, these plants were at a fixed day length of ten hours under proper outdoor conditions. Because of this, the first series had normal conditions of day length and temperature in that specific area. The second series had normal conditions of day length but had a fixed temperature. The third series was exposed to a specific short duration of day length of about ten hours. All of the plantings occurred in intervals of three to five days. All three series had seed varieties of Mandarin, Peking, Tokyo, and Biloxi. For planting, the second series only had the varieties Mandarin, Peking, and Biloxi planted in the greenhouse. After many years, all of the results were shown. As mentioned earlier, all of the varieties tested in this study had different behaviors and had varying flowering types. The results showed that when the soybeans were exposed to an artificially shortened day length, they all flowered within twenty to twenty-five days after germination. Because of this, they all behaved as early varieties. As the length of day started increasing, the Biloxi was the first variety to start lengthening the period of vegetative growth until around 145 days or until there was the return of shortened days. About six weeks after the Biloxi started lengthening the period of vegetative growth, the Peking experienced the same problem, but it was less pronounced. Although the Biloxi and Peking varieties had major changes, the Mandarin showed no change of vegetative growth. This shows how different varieties respond to changes in length of day light. With the third series, the results displayed that there was a delay in flowering. This may be due to the plants having fixed day lengths of ten to twelve hours. Overall, the variation of growth and flowering rates can be due to temperature fluctuations at the study site, but day length is primary external factor of why one variety is early or late to get to the reproductive stage (Garner, 1930).
The second study of research analyzed was titled “Photoperiodic Flowering Response of Biloxi Soybean in 72-Hour Cycles. It was very evident that plants and animals have the ability to measure time with great accuracy. Because of this, many behaviors or activities follow the biological clock or circadian rhythms. In the case of soybeans, flowering is a response of the changing photoperiods. There has been evidence throughout the years suggesting that the photoperiodic control over floral responses in soybeans was mediated with an endogenous rhythm with periods of about 24 hours. Because of this, researchers decided to test a variety of photoperiods and see how the floral response was affected. To start the experiment, Biloxi soybean variety was planted in four inch pots in a greenhouse at the University of California, Los Angeles. Seeds were sown into a mixture of two parts soil (sandy loam) and one part vermiculite. Three to five seeds were then placed in the soil approximately three centimeters deep. The soil was also inoculated with root nodule bacteria. Over the course of a few years, Experiment II took place from June to August 1963 while the other experiments took place from February-August 1962. Even though the experiments took place in different years, all of the studies were under similar conditions. The seedlings were grown under long-day conditions at a temperature of 20-30°C. In the greenhouse, Mazda lambs were used to extend the day length to 20 hours of light. After the first primary leaves appeared, researchers chose two uniform plants in each pot and thinned them out. This would give the crops more room to grow. After the third trifoliate had appeared approximately a month after planting, the plants were moved out of the greenhouse and into the experimental area. In this experimental area, the plants were prepared for treatment, but 20-30 control plants from each lot were left in the greenhouse. There were control plants in order to compare the various treatments that were used in the experiments. After the first phase was completed, the second portion of the experiment began. In this second portion of the experiment, fluorescent lighting was placed above the plants where the temperature ranged from 27-30°C. The temperature experiment was completed in a refrigerated room of about 12°C, and the photoperiod was decreased. Dark treatments maintained a temperature of about 22°C. Plants remained in the second part of the experiment for a total of seven periodic cycles and then returned to the greenhouse. Six weeks after plants returned to the greenhouse, the plants were removed for dissection. To compare with the results, the control in this experiment had seven consecutive photoperiodic cycles of eight-hour photoperiods followed by 64 consecutive hours of complete darkness. Experiment I had four hours of light with light interruptions (every hour) for 64 hours. This took place for the seven photoperiodic cycles. Additional treatments of 24 and 48 hours, both having eight-hour photoperiods, produced the same amount of flowering as experiment I. The results of this experiment as a whole displayed that a complete alternation of phases occurs every 24 hours. The floral response to the photophil phases during the 24 and 48 hour points were shown to be highly variable. Experiment II had the photophil light stimulation throughout the second 24 hours of the tridiurnal cycle. At the 30-hour point of this experiment, there was a major decrease in floral stimulation. Researchers have no explanation for this happening except that there may be a secondary interaction occurring. The first two experiments in this study supports previous findings that an endogenous rhythm plays an important role in the photoperiodic flowering response. In experiment III and experiment IV, different lengths of photoperiods were tested within a 72-hour cycle. All of the plants maintained a normal temperature of 28°C following a dark period at 22°C. After the 8-12-hour point, the photoperiodic response started declining. Because this occurred in all of the plants, these results may be in response to the temperature decline. Overall, researchers found that there may be the involvement of phytochrome in the photoperiodic response. In Experiment I and Experiment II, the research showed that the light break is more than sufficient in order to covert P(tr) to P(r). Experiment III and Experiment IV shows similar responses. Although researchers did not know the different roles of phytochrome in the 1960’s, it was interesting to see how researchers discovered these reactions. In addition, it was evident that the endogenous circadian rhythm regulates photoperiodic flowering responses (Coulter, 1964).
After researching both studies, it was interesting to see how both temperature and length of day affected flowering. Without photoperiods, the soybeans would not even have a chance to start flowering. On the other hand, even if soybeans were receiving plenty of light, the floral response could begin to decline if the temperature is not fit for plants to grow. The studies by Coulter, Hamner, Garner, and Allard were just the start of researching the effects of photoperiodism. With the modern technology in the 21st century, it will become much easier the understand what triggers floral responses in soybeans as well as other plants.
- Coulter, M.W. & Hamner, K.C. (Sept. 1964). Photoperiodic Flowering Response of Biloxi Soybean in 72-Hour Cycles. Plant Physiology American Society of Plant Biologists, vol. 39, no. 5, pp. 848-856. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC550178/pdf/plntphys00414-0144.pdf
- Garner, W. W. & Allard, H. A. (15 Nov. 1930). Photoperiodic Response of Soybeans in Relation to Temperature and Other Environmental Factors. Journal of Agricultural Research, vol. 41, no. 10, pp. 719-735. Retrieved from: https://naldc.nal.usda.gov/download/IND43967879/PDF
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