Effect Of Sunlight On Head Size Of Sunflowers Biology Essay


Helianthus annuus, also known as the common sunflower, can be found throughout most of North America (including Mexico) and many European countries, especially Russia (Thornburg 1982:88). Annually flowering from mid-summer to late summer, the Helianthus annuus is heavily dependent on sun exposure, requiring at the minimum 7 to 8 hours of sunlight per day. Consequently, the flower is shade intolerant. (Mitchell, 2008). The Helianthus annuus displays the behavioral adaptation of heliotropism in which both the head and the leaves of the plant move to face the direction of the sun. Each morning, the flower begins the day facing the east as the sun rises, and tracks the sun across the horizon until the end of the day when it faces the west (Mitchell, 2008).

In the northern hemisphere, sunlight hits the earth at a lower angle than closer to that equator, and thus light spreads over a wider area (Ricklefs, 2008:62). Since the flower is native to the more northern parts of the northern hemisphere (going as far as 67.8°N, Norway), heliotropism allows optimum sunlight exposure (GBIF, 2007). It has been suggested that the temperature conditions during seed growth affect seed size (Mohamed and Clark, 1985). Therefore, for a plant found in northern regions, heliotropism would be beneficial in terms of increasing the temperature of the flower head where the seeds develop (Totland, 1996:452). Consequently, the null hypothesis states that heliotropism does not help to warm the head of the sunflower to improve seed production and size. Conversely, the alternative hypothesis states that heliotropism does help increase the temperature of the plant head in order to improve seed development in terms of weight, size and rate of production.


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To investigate the hypothesis, two facts must be determined; firstly, whether or not the amount of sunlight does affect the temperature of the flower head, and secondly, how this change in temperature affects the seed mass and production rates. For this experiment, I will set up two groups of sunflowers, one treated and one control group. Planting will occur simultaneously, and during the growing period there will be no treatments. Once the flowering stage is reached, the control group will be allowed to produce seeds naturally (with heliotropism). Meanwhile, the treatment group will be tethered down with wire to a metal stake (to prevent curving) and bent south in order to further minimize direct sunlight. (Totland, 1996:452). Data about the seed mass and size, time of seed production, and internal flower head temperature will be collected. This data will be used to analyze the relationship between the flower head temperature and the seed size/mass, and the relationship between the rate of seed production and flower head temperature. The constants will include the watering schedule, the soil quality and composition, the type of climate, time of exposure to sunlight (8 hours) and time of planting. The independent variables will include the amount of sunlight exposure (a flower either being free to be heliotropic or being tethered down). The dependent variables will be the weight of the seeds produced by individuals, the physical size of the seeds and the temperature of the flower head.


The null hypothesis is that heliotropism does not affect the temperature of the flower head and does not lead to the faster production of larger seeds. In order for the null hypothesis to be accepted, there should be no difference in temperatures of the flower heads between both groups. Also, both groups would have to produce generally equal sized seeds in the same amount of time. Conversely, the alternative hypothesis is that heliotropism does in fact affect the temperature of the flower head and does lead to the faster production of larger seeds. In this case, data should show that there is a difference in the temperature of the flower heads between the two groups. The control group would be observed to have a higher average internal temperature than the treated group. Consequently, the control group should produce larger seeds (Kjellberg et al. 1982) and in a shorter time period than the treatment group (Totland 456, 1996).


If heliotropism does in fact raise the temperature of the flower head of the Helianthus annuus then it can be important in the production of seeds and the rate of production. Since the Helianthus annuus is found frequently in northern regions, the summer temperature may be cooler than the optimum temperature. As such, the rate of production of seeds will decrease. Similarly, a lower temperature will affect the size of the seed which would lead to less food resources for the offspring (Metz et al. 2010). This experiment directly tests fitness as the size of seeds produced will affect the probability of a seed surviving and being able to successfully germinate.

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The next experiment to clarify if this trait is adaptive will have to test whether or not a larger seed is more beneficial to the sunflower. The Helianthus annuus is pollinated by bees or by self-pollination and then depends on the air for seed dispersal. At the end of each seed, there is a small hair-like tuff called a pappus, which helps the seed become airborne (Utah State University, 2002). A larger seed may be better, but if it is carried by the wind then there must be a limit to seed size and weight.

Another experiment would also be beneficial to determine the primary purpose of heliotropism in the Helianthus annuus. Heliotropism would also greatly improve sunlight absorption and photosynthesis of a plant. Therefore, the effect of heliotropism on photosynthesis should also be tested to see whether there is a benefit to the plant to move in the direction of the sun. Beyond this point, an experimenter may try reducing sunlight exposure to see whether photosynthesis or seed production are affected more.

Literature Cited

Bean, E. W. 1971. Temperature Effects upon Inflorescence and Seed Development in Tall Fescue (Festuca Arundinacea Schreb.). American Journal of Botany (1971): 891-97.

GBIF. "GBIF Portal." Global Biodiversity Information Facility. 2007. Web. 20 Nov. 2010. <http://data.gbif.org/species/13193098?extent=14+67+16+68&zoom=6&minMapLong=14&minMapLat=67&maxMapLong=16&maxMapLat=68&c[0].s=20&c[0].p=0&c[0].o=13193098>.

Kjellberg, B., Karlsson, S., and Kerstensson, I. 1982. Effects of Heliotropic Movements of Flowers of Dryas octopetala on Gynoecium Temperature and Seed Development. Oecologia 54: 10 -13.

Lang, A.R G., and J. E. Begg. 1979. Movements of Helianthus Annuus Leaves and Heads. Journal of Applied Ecology 16.1: 299-305.

Metz, J., Liancourt, P., Kigel, J., Harel, D., Sternberg, M., Tielbörger, K. 2010. Plant Survival In Relation to Seed Size along Environmental Gradients: A Long-term Study from Semi-Arid and Mediterranean Annual Plant Communities. Journal of Ecology 98: 697-704.

Mitchell, Sarah. "Helianthus Annuus." BioWeb Home. 2008. Web. 23 Oct. 2010. <http://bioweb.uwlax.edu/bio203/s2008/mitchell_sara/Index.htm>.

Mohamed, H. A., J. A. Clark, and C. K. Ong. 1985. "The Influence of Temperature during Seed Development on the Germination Characteristics of Millet Seeds." Plant, Cell and Environment 8.5: 361-62.

Ricklefs, R.E. 2008. The Economy of Nature. W.H Freeman and Company, New York. 620 pages. Print.

Thornburg. 1982. Helianthus annuus. Pages 2123-2126, in Mansfeld's Encyclopedia of Agricultural and Horticultural Crops (Vol.5). Springer, 2001. Print

Totland, Orjan. "Flower Heliotropism in an Alpine Population of Ranunculus Acris (Ranunculaceae): Effects on Flower Temperature, Insect Visitation, and Seed Production." American Journal of Botany 83.4 (1996): 452-58. Print.

Utah State University. "Asteraceae." Utah State University: Intermountain Herbarium. 2002. Web. 23 Oct. 2010. <http://herbarium.usu.edu/taxa/asteraceae.htm>.