Not just the presence of light, but direction, intensity, wavelength as well.

Transcription

1 Not just the presence of light, but direction, intensity, wavelength as well. Need this to measure the passage of days and seasons ED and BLUE light are the most important colors in regulating this in plants

2 The Plot Thickens Not all wavelengths of light are created equal in plant responses How can this be explained?

3 Phytochrome: a molecular switching mechanism Synthesis P r ed light Far-red light P fr esponses: seed germination, control of flowering, etc. Slow conversion in darkness (some plants) Enzymatic destruction

4 How does the order of red and far-red illumination affect seed germination? EXPEIMENT During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or farred light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. ESULTS The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right). Dark (control) ed Dark ed Far-red Dark ed Far-red ed Dark ed Far-red ed Far-red CONCLUSION ed light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible.

5 Circadian hythms in plants Plants need to know the season to live at maximum efficiency. - flower at the wrong time, no Waste pollinators of energy - produce leaves in the middle Waste of energy winter Plants use the lengths of night and day to determine the time of year

6 Photoperiodism and control of flowering Short day plant/long night plants requires a dark period longer than a critical length to flower Fall/winter Long day plant/short night plants requires a dark period shorter than a critical length to flower Spring/summer

7 How does interrupting the dark period with a brief exposure to light affect flowering? EXPEIMENT During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in short-day and long-day plants. ESULTS Darkness Flash of light Critical dark period Light (a) Short-day plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) Long-day plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example). CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness ( critical night length ) for that species, not by a specific period of light. Therefore, short-day plants are more properly called long-night plants, and long-day plants are really short-night plants.

8 Is phytochrome the pigment that measures the interruption of dark periods in photoperiodic response? EXPEIMENT A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in short-day and long-day plants. ESULTS F F F F Short-day (long-night) plant Long-day (short-night) plant CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods.