Chaney Cranfill

October 30, 2007

Student Project Abstract


            Whether it be antioxidants, “natural sunscreen,” or modifications to metabolism such as C4 and CAM processes, evolution has engendered many different mechanisms for reducing free radical production and/or subsequent damage. The mechanism of interest for this experiment is the natural sunscreen Blue Spruces have invented and implemented – the blue powdery substance which coats many Blue Spruce needles, distinguishing them from most other high altitude needle trees and giving the spruce its name. By reflecting one of the highest energy lights (blue), the Blue Spruce initially reduces light energy absorption, which lessens the amount of energy available to create free radicals, especially when the leaves’ stomata are closed during peak hours of sunlight intensity in which the free radical prone oxygen is present in abundance and photosynthesis slows due to diminished CO2  concentration and subsequently increased bondage of O2 to Rubisco, leaving extra energetic electrons (from the slowed rate of light dependent reactions) to reduce oxygen, creating carcinogenic molecules dangerous to the plant (free radicals). Although this innovation may help to protect the plant from damage, my question is if there is a tradeoff – does the reduced absorption of light energy affect the rate of photosynthesis?

            Since photosynthesis is dependent on the absorption of light energy, my hypothesis is that decreased absorption of light energy due to the reflectance of blue wavelengths will decrease the rate of photosynthesis. If this is true, blue coated Blue spruce needles will have a slower rate of photosynthesis than non-blue coated (i.e. green) Blue Spruce needles. We proceeded to test this hypothesis by measuring and comparing the rates of photosynthesis between Blue Spruce sprigs with green needles and those with blue coated needles. In three trials, sprigs of each were placed separately in a plastic chamber, and CO2 change was measured by a CO2 probe connected to a computer for 5 minutes of full spectrum light implementation, and 5 minutes of complete darkness (obtained by covering the chamber with aluminum foil). While the chambers were introduced to light, a container holding water was kept between the light source and the needle chamber to minimize temperature increase due to light energy, in an attempt to eliminate temperature as a variable. CO2 changes were recorded and total rate of photosynthesis per gram of plant specimen was obtained by subtracting the CO2 change during darkness (rate of cellular respiration), from the CO2 change during light implementation (rate of cellular respiration and photosynthesis), then dividing the figure by the weight in grams of the specimen.

The main variables in this experiment were light, darkness, duration of light and darkness, CO2 concentration, amount of specimen used, and temperature. Temperature gain was minimized by the container of water; duration of light and darkness was measured and controlled; the amount of specimen used was accounted for by calculating the rate of photosynthesis per gram; and the CO2 concentration was measured and recorded. No experimental controls were used since this was a comparison experiment and the only variable which was altered was the color of the needles, the other variables remained fairly constant.

The results obtained were strange. The differences between the rates of photosynthesis in the green needles versus the blue needles were so great that CO2 probe failure is probable. The CO2 change had the tendency to level out, and the results recorded had little comparative consistency. The P Value obtained from the results further emphasizes this inconsistency: P = 0.489. In order for the data to be reliable the P Value must be < 0.05., a huge difference from our P Value. The results contradict my hypothesis and predictions, demonstrating the direct opposite: the more negative the rate of photosynthesis (which represents the amount of CO2 in ppm of each gram of plant specimen per minute), the faster the rate of photosynthesis. The blue Blue Spruce needles have a much more negative average rate of photosynthesis (and thus a much faster rate of photosynthesis) compared to the green Blue Spruce needles. The average rate of photosynthesis of blue needles is -7.34 (ppm CO2/min/g), compared to -0.420 in green needles. This is an incredible difference, as demonstrated visually in this graph:

Although the rate of photosynthesis per gram of plant specimen was obtained, the ratio of needle mass to stalk mass in each specimen could affect the accuracy of comparison, since the needles contain most, if not all, of each specimen’s chloroplasts. Another variable unaccounted for is CO2 concentration in the beginning of each experiment; this may play a role in photosynthesis rate (perhaps the more CO2 available, the more CO2 the plant can use in photosynthesis and thus the faster the rate of photosynthesis). This variable was not consistent at the beginning of each experiment, and making it so – making the concentration of CO2 in ppm constant at time 0 for each experiment – could eliminate one source of doubt. In addition, larger sprigs could be used to increase the CO2 change during 5 minutes because there would be much more needle mass to conduct photosynthesis, strengthening the results since the time intervals used were fairly small. Larger time intervals could also be used to increase the range of measurement and hopefully the accuracy of results – perhaps light and darkness periods of 20 minutes each, instead of 5.

Although due to technological difficulties the data is unreliable and the results are therefore inconclusive, it would not be unlikely for the results, if the experiment is properly conducted, to reject my hypothesis. Since the main element of the photosynthetic reaction center of many photosystems which play a significant role in the light dependent reactions of many plants is generally chlorophyll a, which commonly has a peak wavelength absorption of 680-700nm depending on the photosystem, this essential part of photosynthesis may be unaffected by lessened absorption of blue wavelength light (450-500nm). It is also unclear what wavelengths of light the blue coating actually reflects, it could be a very small range of wavelengths – this could be determined by isolating the blue coating and determining its wavelength absorption and subsequent reflection via a spectrophotometer.

Since one of the main functions of the accessory pigments in the photosystems, especially in the light-harvesting complexes, is to mitigate excess energy, the blue reflection may facilitate this process, making photosynthesis more efficient and thus expediting its rate. Further experiments to better clarify the results and the implications of this lab (other than re-conducting the entire experiment to obtain results of satisfactory accuracy) would be to test the reflection spectrum of the blue coating, compare the free radical production in blue and green needles to determine the effectiveness of blue light reflection in reducing free radical production, and further investigate the effects of different wavelength lights on photosystem (i.e. light dependent reactions) efficiency.

Depending on the results, real world applications could also be derived. Perhaps with more knowledge of nature’s sunscreens, human sunscreens could be better developed to become more effective in reducing sun damage; roof shingles could be engineered to further reduce energy absorption and subsequent heat gain in areas of high temperature and sunlight intensity; contacts could be developed to contain pigments which reflect damaging light wavelengths and eliminate the need for sunglasses (other than, of course, to look cool); the applications and extensions could be endless. Nature is the master of invention and it is time we as a population reduce our emphasis on improving upon her, and shift our priorities toward learning from her own research and innovations which have taken billions of years to conduct.