The effect of photosynthetically active radiation on redness of Apocynum cannabinum over a three day period

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The effect of photosynthetically active radiation on redness of Apocynum cannabinum over a three day period
Erica J. Ross, Ryan Edelen, Matthew Howes and Dana A. Dudle

Department of Biology, DePauw University, Greencastle, IN 46135

We have confirmed that Apocynum cannabinum turn red, but we wanted to know the rate at which the plants turn red over a short period of time. We are also interested in the variation among individuals over time. The objective of this experiment was to observe how the redness of Apocynum cannabinum is affected by sunlight over a short period of time. We predicted that we would see differences among individuals and see a definite change in the redness of the plants over a short period of time.
To determine a stem’s color, we used a reflectance spectrometer. We were concerned about this method because of the difference in scale between the instrument and the subject, in this case an entire plant stem. The spectrometer takes a reading with the fiber optic light receiver which covers a very small area of the plant, and it is difficult to place the instrument on the same spot each time. This means our data may have been collected on different places on the same internode. Another potential problem with the spectrometer is that the instrument is a flat instrument being used to measure a round stem, so excess light may slightly alter the reading. We did the best we could to make sure the spectrometer is taking accurate readings, but this may not have always been the case.
1. We chose 12 healthy green-stemmed Apocynum cannabinum plants. The experiment took place over 3 clear, sunny consecutive days
2. We assessed the initial stem color of the 7th internode with the spectrophotometer, an Ocean Optics USB2000. We marked the specific part of the stem so that we could repeatedly measure the same spot.
3. The plants were placed outside at 6:00 a.m. on June 17th. All 12 potted plants were randomly placed in a line that ran from north to south. Focal internodes were all facing east.
4. Four times each day, we brought the plants inside to take spectral data on the 7th internode on the east-facing side. After the last reading each day at 18:00, we brought the plants inside until 9:00 a.m. the next day when they were returned to the garden and placed randomly. Spectral data were taken every 3 hours starting at 9:00 and ending at 18:00 from June 17th to June 20th. On June 20th the plants were taken in at 15:00 for their final reading.


This graph shows the change in redness of the 12 individual stems over a 3-day period. All plants got redder because of the upward trend in every plant. Over the 3-day time period we also observed occasional reductions in the redness index of the stems, which was unexpected and suggests further study is needed.

This graph shows the amount of light vs. the change in redness over the 3-day time period. The redness of the plant was not immediately affected by the amount of photosynthetically active radiation (PAR). Redness even increased overnight. Instead, there was a time lag between the amount of PAR and the redness.

Past and current experiments have shown us that Apocynum cannabinum turn red when left in direct sunlight. Here we have taken a closer look at how individual plants change over the short term. We hypothesized that we would see differences among individuals as their stems turned red.
During our experiment, we found that Apocynum cannabinum changed color very rapidly. Redness increased about 41.8% over three days. We also saw substantial differences among individuals. One plant’s redness index increased by 84% while another plant changed by only 5%
Future Experiments
The experiment should be repeated on a larger scale, with a larger sample size and more frequent data collection, particularly during nighttime hours where our data showed changes in color. It would be interesting to observe genetically similar plants to focus purely on the physical effect that light has on the plants. Future work should scrutinize the apparent fluctuations in the redness index seen in some individual stems in this study. Environmental effects such as degradation of chlorophyll or a decrease in anthocyanins as a response to decreased PAR might explain this unexpected pattern. Experimental error, specifically our methods of spectroscopy, might also explain the reductions in redness over time.

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