In this lab, we looked at Phytochrome and Phytohormone effects on the germination of lettuce seeds. We narrowed our experiment down to the effects that red and far-red light have on germination. Phytochromes are proteins that have a chromophore, which can either be active or inactive. Pfr (active) absorbs far red light, and Pr (inactive) absorbs red light. When a plant absorbs red light, Pr is converted to Pfr. By contrast, absorption of far-red light converts Pfr to Pr. Before the experiment, we read that lettuce seeds require light to germinate. Phytochrome was discovered by Borthwick (1952) and isolated by Butler (1959) from etiolated seedlings of maize. It causes germination of seeds, and can inhibit flowering in Short Day Plants and Long Day plants. Lane, H. C., H. WV. Siegelman, W. L. Butler, and E. M. Firer (1963) found that phytochrome is required in seed germination of some plants, in the prevention of photo-oxidation, and for photoperiodism. In this experiment, we saw the effect that light quality can have on lettuce seed germination. Siegelman, H. W and E. M. Firer (1964) ran a similar experiment like we did, except with oat seedlings and not lettuce seeds. They found that red light can cause the seeds to germinate, but if they are subsequently exposed to far-red light, germination will be inhibited. We hypothesized that far-red light would not be able to germinate as well as red-light as it induces phytochrome to become inactive.
The first thing we did was head over to the dark room and located the light-tight box with the Petri dishes that consisted of water-filled lettuce seeds. After labeling, we placed dishes #1 and #2 in the red light chamber and turned on the incandescent light for 5 minutes...
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... to potentially save species from extinction. There may be plants that have had trouble surviving because they were being grown in unfavorable conditions, and perhaps this data could change that. We also better understood the classic case of the shade avoidance response of shade intolerant plants. Foliage, for example, when it absorbs red light induces phytochrome to the Pfr, or active form. But in nearby plants in the shade, there is far-red light that induces phytochrome to the Pr, or inactive form. This is important as Pr ultimately allows shaded plants to elongate to reach the sunlight, as stem elongation is not inhibited. In this lab, we tested the effects different hormones had on seed germination and we found that seeds germinate better when they are exposed to red light as opposed to far-red light by itself.
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