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  3. WHAT CONTROLS FLOWERING (1) The timing of flowering and seed production is precisely tuned to a plant’s physiology and the r

WHAT CONTROLS FLOWERING (1) The timing of flowering and seed production is precisely tuned to a plant’s physiology and the r

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问题                                         WHAT CONTROLS FLOWERING
    (1) The timing of flowering and seed production is precisely tuned to a plant’s physiology and the rigors of its environment. In temperate climates, plants lost flowers early enough so that their seeds can mature before the deadly winds of fall. Depending on how quickly the seeds and food develop, flowering may occur in spring as it does in oaks; in summer as in lettuces; or even in fall as in asters.
    (2) What environmental cues do plants use to determine the seasons? Most cues such as temperature or water availability are quite variable: Fall can be warm; a late snow could fall in spring; also summer might be unusually cool and wet. So the only reliable cue is day length: longer days always mean that spring and summer are coming; shorter days foretell the onset of fall and winter.
    (3) With respect to flowering, botanists classify plants as day-neutral, long-day or short-day. A day-neutral plant flowers as soon as it has sufficiently grown and developed regardless of the length of day. The neutral plants include tomatoes, corn, snapdragons, and roses. Although the naming is traditional, long-day and short-day plants are better described as short-night and long-night plants because their flowering actually depends on the duration of continuous darkness rather than on day length. Short-night plants (which include lettuces, spinach, irises, clover and petunias) flower when the length of darkness is shorter than a species’ specific critical dark period. Long-night plants (including asters, potatoes, soybeans, goldenrod, and cockleburs) flower when the length of uninterrupted darkness is longer than the species’ specific critical dark period. Thus spinach is classified as a short-night plant because it flowers only if the night is shorter than 1I hours (its critical dark period), and the cocklebur is a long-night plant because it flowers only if an uninterrupted darkness lasts more than 8.5 hours. Both of these plants will flower with 10-hour nights.
    (4) Plant scientists can induce flowering in the cocklebur by exposing leaves to long nights (longer than its 8.5-hour critical dark period) in a special chamber, while the rest of the plant continues to experience short nights. Clearly, a signal that induces flowering transmitted from the leave to the flowering bud. Plant physiologists have been attempting for decades to isolate these elusive signaling molecules often called florigen (literally, flowering maker). Some researchers believe they are close to demonstrating a flower’s stimulating substance for specific type of plant. Using genetic manipulation, it is likely, however, that interactions among multiple and yet unidentified plant hormones stimulate or inhibit flowering, and that these chemicals may differ among plant species. Researchers have had more success in determining how plants measure the length of uninterrupted darkness, which is a crucial stimulus for producing whatever substance controls flowering.
    (5) To measure continuous darkness, a plant needs two things: some sort of metabolic clock to measure time (the duration of darkness) and a light detecting system to set the clock. Virtually all organisms have an internal biological clock that measures the time even without environmental cues. In most organisms including plants, the biological clock is poorly understood, but we know that the environmental cues, particularly light, can reset the clock. How do plants detect light? The light detecting system of plants is a pigment in leaves called phytochrome (literally, plant color).
    (6) Plants seem to use the phytochrome system in combination with their internal biological clocks to detect the duration of continuous darkness. Cockleburs, for example, flower under the schedule of 16 hours of darkness and 8 hours of light. However, interrupting the middle of the dark period with just a minute or two of light prevents flowering. [A] Thus their flowering is controlled by the length of continuous darkness. [B] It is evident that even brief exposure to sunlight or white light will reset their biological clocks. [C] The color of the light used for the light exposure is also important. A nighttime flash of pure red light inhibits flowering, while a flash of light at the far-red end of the spectrum has no effect on flowering, as if no light were detected. [D]
According to paragraph 6, which of the following types of light will not reset a cocklebur’s biological clock?
选项 A、White light
B、Sunlight
C、Pure red light
D、Far-red light
答案D
解析 本题询问根据第6段，哪种光不会重置苍耳的生物钟，属于事实信息题。第6段最后一句提到，远红外光对植物开花没有影响。由此可判断，D项“远红外光”不会重置苍耳的生物钟。第6段倒数第3句提到“即使是很短的日照或向光照射都能重置它们的生物钟”，末句提到“夜间发出的纯红光会抑制开花”。因此，A项“白光”、B项“日光”和C项“纯红光”均可排除。
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