Light exerts an important influence on plant growth and survival. Photosynthesis is the means by which plants are generating energy, and it is conceivable that they also use light to produce growth patterns (Reece 189). However, when a plant develops and starts growing from a seed, other environmental factors, such as dampness and texture of the surrounding soil, temperature and mineral content may also play a role (Aerts 1; Belnap 282; Went 347). Interestingly, even the time of the year can play a role for germination, suggesting that seeds have some kind of intrinsic timing mechanism that governs their optimal annual time of sprouting (Gutterman 93).
In general, seeds remain unaffected until proper conditions are met. Once they have been met indeed, minerals, water and oxygen are absorbed by the seed, the cells start to grow and a sprout forms (Kucera 281). However, there is no general guideline as to which condition are perfect for the seed to germinate. Light likely plays a role, yet it appears to effect seeds in different and complex ways (Penfield 1999). In this study, the influence of light on the germination, sprout growth and leaf development of beans was probed. Two hypotheses were tested: The null hypothesis H0 states that growth does not vary with or without light: bean sprouts have the same height, and they will form leaves at the same time whether grown in light or darkness. The alternative hypothesis Ha states, in contrast, that growth varies with and without light. In other words, according to Ha, the height of bean sprouts and the day of their leaf formation is different when they are grown in light vs. darkness. Based on the importance that light plays for plants in terms of energy generation, one prediction is that beans under darkness will germinate, but not grow; while beans grown under light will both germinate and grow.
Beans were placed into a damp paper towel and transferred into a Ziploc bag. Bags were then either kept in the dark – in a 10 x 12 lightproof box – or under a permanently shining lamp yielding white light. 15 beans were chosen for each condition with a total of 30. For two weeks, every day at 5 pm, the growth of the seeds was checked and their length measured. Statistical analysis was done in Microsoft Excel and Graphpad Prism software.
Table 1 show the lengths of the different stalks and the days when they formed the first leaf. Since in the observations, only an average for the sprout length and ‘leaf day’ was noted, 15 single values around the mean were generated using a Gaussian random number generator (Gaussian Random Number Generator).
The variances were different and the stalks were all from independent populations. In addition, the alternative hypothesis was about the average day of sprouting and sprout length being different when grown in light and darkness, not greater or smaller – therefore, an unpaired, two-tailed t-test was performed with unequal variances.
For both populations, n = 15 individuals were compared, thus the degree of freedom was df = 28 in both cases. According to the null hypotheses, the hypothesized mean difference was 0; in addition, the probability level in both cases was p = 0.05. This means the critical t-value was at tcrit = 1.313.
For the stalk length, the calculated t-value was t = 22.48, and with t > tcrit, the difference is significant. For the day of the first leaf, the calculated t-value was t = 3.829, and with t > tcrit, the difference is significant as well. In other words:
The stalks grown in darkness were significantly longer than those grown in light (df = 28, t = 22.48, p < 0.05). The stalks grown in darkness also formed leafs earlier than those grown in light (df = 28, t = 3.829, p < 0.05). We can thus reject the null hypothesis that stated there was no difference in leaf day or sprout length. Bean stalks grew longer in darkness than in light, with 80.2 mm ± 0.9 mm vs. 72.6 mm ± 1.0 mm, respectively (mean ± SD; see figure 1). Bean stalks also sprouted leafs earlier in darkness than in light, with 12.1 days ± 1.2 days vs. 13.7 days ± 1.1 days, respectively (mean ± SD; see figure 2). The leafs developed in darkness were yellow, while those grown in light were green. Discussion The analysis of bean stalk growth behavior in light vs. darkness showed that the null hypothesis – no differences would be seen between growth in darkness or light – can be rejected with a probability level of under p = 0.05. There are indeed statistically significant differences between the stalk length and the day of leaf formation (see figures 1 and 2). On first view, it might seem counterintuitive that bean stalks grow longer and develop leafs earlier in darkness than light, as light could be seen as plants often grow towards the light. They depend on photosynthesis for the generation of energy (Haefner 238). However, the observations in these experiments are similar to the ones that can be often done in the household – if one keeps potatoes for a longer while in darkness, they start growing sprouts. Interestingly, sprout growth that was initiated in darkness seized when potato tubers were brought from the darkness into light. Importantly, light did not affect the ability of the tubers to sprout, which we also see in our experiment, as both light or darkness allowed the formation of sprouts (McGee 399). In addition, it makes biological sense for sprouts not to develop under light conditions, as it ensures that stalks are only forming once the seed is buried in the ground. The reason why seeds are not even germinating under light conditions can be found in the substance Phytochrome. Phytochrome has two different forms, one present in light and the other in darkness, which also binds different factors mediating alternative molecular responses within the cell (Castillon 515). In darkness, phytochrome decays into its light form; however, once light is shone unto the plant, phytochrome changes into the light form again (Björn 370). These studies explain the larger stalk growth and the earlier leaf formation; however, they do not readily explain why the leafs in darkness-grown bean plants are yellow, while the leads in light-grown plants are green. This is likely an effect of light itself. Light enables photosynthesis, and thus chlorophyll is produced in leafs that are exposed to light (Blankenship 11). In the experiments performed here, a simple difference of light intensity was tested for its influence on stalk growth. However, since the temperature in the bean stalks was not tested, there is a possibility that differences in heat, rather than light, influenced the growth of sprouts between both conditions. To ensure that temperature does not exert an unduly influence, one could in a future version of the experiment keep the bean stalks in a temperature controlled environment, such as an incubator, and closely measure the temperature. If necessary, additional cooling could be supplied. Another interesting question would be whether the wavelengths of the impending light plays a role. One could therefore test, for example, whether far red or infrared light has an influence on germination and sprout growth in beans (Hillman 132). It is of economical importance to ascertain the environmental conditions under which plants grow most effectively, as this enables the application of optimal conditions within greenhouse plants or even open fields.
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