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Photoperiodism in Plants

a)     Flowering is crucially important for the plant to complete its life cycle. Although people had long known that plants such as tulips flower in the spring and chrysanthemums flower in the fall, until the early 1900s little was known about what actually caused flowering. Yet beginning in 1910, Wightman Garner and Henry Allard conducted experiments to test the effect of day length on flowering, by growing plants in growth chambers, in which timers are used to control the length of the light and dark periods. They discovered that plants such as barley flowered when the day length was longer than a certain critical length. These plants, which they named long-day plants (LDPs), flower mainly in the summer as the days are getting longer. Others, such as soybeans, flower when the day length is shorter than a certain critical length. These short-day plants (SDPs) flower in the fall as the days are getting shorter. Still others such as broad beans and buck wheat flower almost at the same time regardless of day length and are called day-neutral plants. Based on these discoveries, they coined the term “photoperiodism” in 1920 to describe a plant’s ability to flower in response to changes in the photoperiod, the relative lengths of day and night.

【vocabulary】

Crucially: important, key; 重要的， 關鍵的

Photoperiod：【昆】【植】光周期

Photoperiodism：【農】光周期性/現象

Coin：If you coin a word or a phrase, you are the first person to say it. 創造 (新詞語)

Critical length: 臨界長度

【文章中出現的各種植物】

Tulips：郁金香

Chrysanthemum：菊花

Growth chamber：人工生長箱

Timer：計時器

Barley：大麥

Broad beans：蠶豆

Buck wheat：蕎麥

Day-neutral plants：【植】日中性植物

【main idea】

Experiments have been conducted on the effect day of length on flowering in terms of different plants. The term “photoperiodism” has been coined to describe the effect.

b)     All flowering plants have been placed in one of three categories with respect to photoperiodism. Despite their names, however, scientists have discovered that it is the uninterrupted length of night rather than length of day that is the most important factor in determining when and whether plants will bloom. For example, if SDPs are grown under short-day conditions but the dark period is interrupted by a flash of light, the SDPs will not flower. However, an interruption of the light period with dark has no effect. Thus, SDPs should more accurately be called long-night plants; and LDPs should be called short-night plants to emphasize the key role played by darkness in photoperiodism. Most plants require several weeks of the appropriate long-night or short-night cycle before they will flower.

【vocabulary】

With respect to: 就…而言

Uninterrupted: 不間斷的，連續的

Bloom：=flower, 開花

Accurately：準確的

【main idea】

Scientists have discovered that it is the uninterrupted length of night rather than length of day that is the most important factor in determining when and whether plants will bloom.

c)     The late 1920s also saw the discovery of a biological clock in living organisms. It was shown that the movement of leaves on a bean plant, from horizontal at noon to vertical at midnight, continued uninterruptedly for several days, even when plants were placed in total darkness and at a constant temperature, and that the time between given points in the cycle, such as the most vertical leaf position, was almost but not exactly 24 hours. In the case of bean leaves, it was about 25.4 hours. Many other cycles have now been found with similar characteristics in virtually all groups of plants and animals. There is strong evidence that the clocks are internal and not driven by some daily change in the environment.

【vocabulary】

Biological clock: 生物鐘

Living organisms：活物

Horizontal：水平的

Vertical：垂直的

Constant：不變的，恒定的

Virtually：事實上

Internal：內部的

【main idea】

There is strong evidence that the clocks are internal and not driven by some daily change in the environment.

d)     Photoperiodism takes place in specialized pigment materials called phytochrome receptors. Phytochrome receptors respond most to the energy contained in red and far-infrared light waves. Two types of receptors, called P(red) and P(far-red),intercept these waves and coordinate plant metabolism processes accordingly. P(red) receptors absorb red light waves during the day, while P(far-red) receptors absorb far-red waves during dark periods. As each plant type requires only a certain amount of light wave, too much of one or not enough of the other prevents flowering from taking place. Not only can phytochrome receptors absorb available light energy, but they act as light sensors that integrate with a plant’s biological clock to provide a means for a plant to adapt to its lighting conditions. It is suggested that under appropriate conditions, these interactions between phytochrome and the plant’s biological clock are thought to activate the genes for flowering.

【vocabulary】

Pigment:【生化】色素，顏料

Phytochrome receptor：【生化】光敏色素，植物色素

Far-infrared light：遠紅外線

Metabolism：新陳代謝

Intercept：攔截

Available light：自然光線

Light sensors：光敏感/光傳感器

Integrate with：與…結合

Adapt to：使適應于

Lighting conditions：光照條件

Activate：激活

【main idea】

The phytochrome receptor plays an important role in integrating with plants’ biological clock to activate the genes to flower.

e)     The photoperiodic requirements of plants depend on geographic origin and distribution: short-day plants grow in tropical and subtropical regions, and long-day plants occur mainly in temperate and northern latitudes. This factor indicates the adaptability of the photoperiodic process not only to day length as an ecological factor but also to the entire complex of external factors. Photoperiodism is a unique clock that synchronizes the rhythm of ontogenesis with the seasonal rhythm. For example, short-day plants have adapted both to the hot, dry summer of the subtropics and to periodic downpours; they do not flower or bear fruit during the longer days of these seasons.

【vocabulary】

Geographic: 地理性的

Subtropical：熱帶的

Northern latitudes：北緯

Adaptability：適應能力

Synchronize：同時發生

Ontogenesis：個體發育

Seasonal rhythm：季節性規律

Downpours：傾盆大雨

Bear fruit：長出果實