6. Why do plants grow better under certain lighting conditions than others?
So here is the problem with which you are faced. A company marketing a cellophane wrap that alters the light that shines through it claims that it works as well as halide lamps. You are charged with finding our how photsynthetic rate is influenced by varying wavelengths of light and the effect of the filters. So first you have to decide what parameters can you truly manipulate or measure. What do we mean by "different types of light"? Well incandescent and flouresent lights are different, but how? Different heat output. But that doesn't fit the question at the top of the investigation. Aha, so maybe they differ in the colors of the light they emit. What does that mean? Well what does the background information in your lab manual mention? What about what you have learned in lecture (Under the Sea) about light?
OK, so let's say you think that incandecent light is something to test (Why?). Do you shine it on the plant and measure its growth? Would you really see a measurable difference in plant growth in 2 hours? (Hint - No!). Would this experiment answer the questions asked by the student in the background material? He already has evidence that the plants don't grow as well, he wants to know WHY! Showing that plants grow better under different colors of light will not answer his questions anyway.
Is there a parameter that is correlated with growth that you could measure? Growth requires energy. So this should make you think about metabolic rate and cellular respiration (Out of the Rainforest), concepts that involve the production and use of ATP. But is this type of ATP production dependent on light in any way that we have discussed? What process that is dependent on light and that only plants (and algae and cyanobacteria) do provides "food" and energy? (Hint - look at the question for this investigation!).
You have to come up with a hypothesis in the truest sense. What do we mean by that? Hypotheses are explanations for phenomena - What is the mechanism or cause for what is observed. Look at the concept map in the investigation. It shows a variety of relationships and posses questions about the process you should be investigating. Have you looked over the related chapter in your text (8/8 ) have you reviewed the CD-ROM (Check the scenario study guide). You should be testing some idea of why the plants do not grow well under different color lights not whether they grow better under different color light and you can't test growth but must measure something that contributes to growth.
| Absorbance | Chlorophyll a | Transmittance |
| Absorption Spectrum | Chromatography | Visible Light Spectrum |
| Action Spectrum | Phycobilin | Wavelength |
| Carotene | Solvent/Solute | Xanthophyll |
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