CHAPTER 39 PLANT RESPONSES TO INTERNAL AND 课件.ppt

The phytochrome system also provides plants with information about the quality of light. During the day, with the mix of both red and far-red radiation, the Pr =Pfr photoreversion reaches a dynamic equilibrium. Plants can use the ratio of these two forms to monitor and adapt to changes in light conditions. Copyright ? 2002 Pearson Education, Inc., publishing as Benjamin Cummings For example, changes in this equilibrium might be used by a tree that requires high light intensity as a way to assess appropriate growth strategies. If other trees shade this tree, its phytochrome ratio will shift in favor of Pr because the canopy screens out more red light than far-red light. The tree could use this information to indicate that it should allocate resources to growing taller. If the target tree is in direct sunlight, then the proportion of Pfr will increase, which stimulates branching and inhibits vertical growth. Copyright ? 2002 Pearson Education, Inc., publishing as Benjamin Cummings Many plant processes, such as transpiration and synthesis of certain enzymes, oscillate during the day. This is often in response to changes in light levels, temperature, and relative humidity that accompany the 24-hour cycle of day and night. Even under constant conditions in a growth chamber, many physiological processes in plants, such as opening and closing stomata and the production of photosynthetic enzymes, continue to oscillate with a frequency of about 24 hours. 3. Biological clocks control circadian rhythms in plants and other eukaryotes Copyright ? 2002 Pearson Education, Inc., publishing as Benjamin Cummings For example, many legumes lower their leaves in the evening and raise them in the morning. These movements will be continued even if plants are kept in constant light or constant darkness. Such physiological cycles with a frequency of about 24 hours and that are not directly paced by any known environmental variable are called circadian rhythms. These rhythms are ubiqu