Category: Science

Scientific Experiment: The effect of varied intensities of light on the growth of a sunflower plant

Introduction  

Purpose   

The purpose of this scientific experiment was to conduct an investigation in order to determine the effect of varied intensities of light on the growth of a sunflower plant. This is of significant relevance because it would enable the determination of the appropriate light intensity exposure for specific plant species, which is imperative for optimal plant growth to be achieved.

Background

Sunflowers are seed-producing herbaceous plants. The sunflower plants are dichotomous angiosperms; therefore, this means that they produced both flowers as well as seeds, which are attached or carried in the flower part of the sunflower plant. The basis of sunflower reproduction is through its seeds (National Sunflower Association, 2007).

When planted, these sunflower seeds under necessary conditions usually grow to form other sunflower plants. According to National Sunflower Association (2007), sunflower plants have composite flowers, which are composed of numerous smaller florets; which means in a sunflower each petal is actually a distinct floret.

Literature Review

Sunflower plants are native to South and North America, and have their cultivation has been practiced since around 3000 BC (National Sunflower Association, 2007). According to Masefield et al. (1999), oil extracted from sunflower seeds is the chief reason for the cultivation of sunflowers plants today, and the extracted oil is used for cooking as well as in the manufacturing of soaps. National Sunflower Association (2007) reported that the name of sunflowers was coined from the ability of unopened flowers of the sunflower plants to turn and face the sun throughout the day from the time it arises to the time it sets, which increases the number of daylight hours that sunflowers receive.

Like other plants, sunflower plants photosynthesize to obtain energy and food from the sun as well as carbon dioxide from the atmosphere, while releasing oxygen to the atmosphere as the reaction’s waste product. The reaction shown below uses energy and carbon dioxide from ultraviolet radiation and atmosphere respectively to synthesize carbohydrates, which are useful for the growth of the plants (Vendrame, Moore & Broschat, 2014). Therefore, since the atmosphere has abundant carbon dioxide, the amount or intensity of light exposure per day could be the determinant factor that limits the growth of plants.   

CO2 + energy → O2 + starch

Light intensity has the possibility of affecting plant form in terms of plant growth, flowering, leaf color and size in both woody and herbaceous species. Shade tolerant plants have both physiological and morphological adaptations that are essential in allowing them towards adapting to low-light conditions (Schwartz, 2007). However, phenotypic responses to light intensities can be varied within a plant species, which suggests that appropriate selection of the plant species may allow for cultivars to develop that have enhanced tolerance to shade or low-light conditions.

Furthermore, plant response to different light intensities can also be varied among genotypes within a plant species (Smith, 2012). Therefore, this experiment is an important scientific inquiry which is greatly essential in determining how varied light intensity conditions can affect the growth of plant species, including sunflower plants selected for this experiment.

However, an increase in the amount or intensity of light that is received by a plant will not necessarily increase its growth for a long time. This is mainly because at excessive levels of light exposure, the plant leaves often begin to shrivel and wilt, causing the plant distress which hinders continued growth (Squire & Sutherland, 2013). Therefore, most plants usually show an increased rate of growth as light exposure is also increased, but an abrupt decrease in their growth will be observed past a certain light exposure threshold.

Hypothesis

If sunflower plants are exposed to 4 hours, 6 hours, 8 hours, 10 hours or 20 hours of ultra violet light per day, it is hypothesized that their tallest growth will be observed in the 10 hours per day experimental condition since the extra hours of daily exposure to ultra violet light will allow photosynthesis of more carbohydrates by the plants; therefore, this will enable them to have the tallest growth when the heights of the seedlings is measured in centimeters.

Alternatively, if the sunflower plants are exposed to too much ultra violet light, such as 20 hours daily exposure to light sample, then their growth will be shorter compared to other plants, because prolonged exposure to ultra violet is damaging.   

Materials and Method

According to Einstein, Newton and Hawking (2006) and Squire and Sutherland (2013), it is very important for all the steps stipulated in the lab manual for the scientific experiment to be stringently followed and adhered to in order to ensure that credible, reliable, valid and reproducible results are obtained. The materials needed for this scientific experiment included: 100 grams of sunflower seeds; 5 plant pots; soil thoroughly mixed with manure and water.

In this experiment, sunflower seeds are planted in 5 separate plant pots filled with soil that is thoroughly mixed with manure and watered frequently. Upon germination, the experimental conditions of varied light intensity were introduced to the 5 different plant pots already with young sunflower plants including 4 hours, 6 hours, 8 hours, 10 hours, and 20 hours of light exposure per day respectively. The heights of the sunflower seedlings were taken from the 5 plant pots after week 1, week 2, week 3 and week 4; and the measured heights were recorded.

Results

Table 1: A Table of Seedling Heights

A line chart was plotted for the results obtained in the scientific experiment to visually represent the heights of the sunflower seedlings in centimeters after week 1, week 2, week 3 and week 4 with regards to hours of light exposure per day which are 4 hours, 6 hours, 8 hours, 10 hours and 20 hours respectively. The plotted line chart is illustrated in Figure 1 shown below:

Figure 1: A Line Chart of Seedlings Heights

The results illustrated in Table 1 and Figure 1 show that as the amount of or exposure to light is increased through prolonged hours of light per day, there was an increase in the growth of the sunflower plants, with the exception of the sunflower plants that were exposed to 20 hours of light per day, in which less growth was observed. With increasing light exposure from 4 hours to 10 hours per day, there was an exponential increase in the plant height, but the plant height was the least at 20 hours of light exposure per day.

Discussion

The experiment results for between 4 and 10 hours of light exposure per day affirmed my proposed hypothesis that the growth of plant increases with increasing exposure to light. In the lowest light exposure or intensity experimental condition, there were only 4 hours per day in which the sunflower plants received light; and as shown in Table 1, the plant height growth was 7 cm. Moreover, the height of sunflower plants that received 6 and 8 hours of light exposure per day grew by 11 and 10 cm respectively. However, the optimal growth of the sunflower plants’ heights was observed in those that received 10 hours of light exposure per day experimental condition, which support the proposed hypothesis.

Alternatively, shortest growth in height was observed in the sunflower plants that were exposed to 20 hours of ultra violet light per day experimental condition, with a height of 6 cm. There was also a yellowish color observed in the leaves of these sunflower plants compared to their counterparts that were exposed to less light, which had bright green leaves suggesting that the extra hours of light exposure have a damaging effect to the leaves of the sunflower plants subsequently preventing them from thriving. This affirms with the known characteristics and behaviors portrayed by plants when subjected to excessive exposure of light (Squire & Sutherland, 2013).  

Conclusion

During the scientific experiment that was performed to investigate how the amount or intensity of light exposure the sunflower plants received is related to their heights of growth for varied hours of light exposure per day over a period of four weeks. Sunlight exposure of the sunflower plants to between 4 and 10 hours per day made the height of the plants to grow taller as the hours of light exposure were increased. This is in concurrence with the proposed hypothesis that increased light exposure encourages plant growth, but not beyond the threshold level after which further increase in light exposure damages the plants.         

References

Einstein, A., Newton, I., & Hawking, S. (2006). Biology 083 Lab Manual. Vancouver, BC: Vancouver Community College.

Masefield, G. B., Nicholson, B. E., Harrison, S. G., & Wallis, M. (1999). The Oxford Book of Food Plants. London, UK: Oxford University Press.

National Sunflower Association (2007). All about Sunflower. Retrieved from: http://www.sunflowernsa.com/all-about/default.asp?contentID=41

Schwartz, B.  (2007). Filling the shadows with light. American Nursery Management, 185(8), 44-51.

Smith, H. (2012). Light quality, photo-perception, and plant strategy. Annual Review Plant Physiology, 33(2), 481-518.

Squire, D. & Sutherland, N. (2013). The Step by Step Guide to Houseplant Care. Vancouver, BC: Whitecap Books Ltd.

Vendrame, W., Moore, K. K., & Broschat, T. K.  (2014). Interaction of light intensity and controlled-release fertilization rate on growth and flowering of two New Guinea impatiens cultivars. Horticultural Technology, 14(3), 491-495.

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