Why do leaves change color in the fall? asks Patricia Brown, of New York City.
Autumn's cool days are trimmed with deep blue skies and golden light, and brilliant leaves of yellow, orange and red. Leaves changing color in the fall are a tree's way of preparing for long winter, rather like we put up storm windows and pull warm clothes and blankets out of storage.
In summer, the leaves on trees like pin oaks and sugar maples are green because they are chock-full of the green pigment chlorophyll.
Trees need sunlight to produce chlorophyll. In turn, chlorophyll uses sunlight's energy to split water (H2O) into hydrogen and oxygen. Meanwhile, leaves also absorb carbon dioxide gas from the air. The end products of leaf chemistry: carbohydrates (homemade plant food for the tree), and oxygen, released into the air (the gas we need to breathe). The whole process is called photosynthesis.
Along with green chlorophyll, most leaves also contain yellow, orange and red-orange pigments celled carotenoids. Trees don't need light to make carotenoids. Botanists call them "helper pigments," because carotenoids absorb some sunlight and (nicely) pass the energy along to chlorophyll. We don't see much of these deputy pigments (carotene, lycopene, and xanthophyll) in summer, because they are masked by abundant green chlorophyll.
But the ever-shortening days of fall mean less daylight and colder weather. The average tree is rushing to save all the nutrients it can for its winter hibernation. Nitrogen and phosphorus are pulled from leaves for storage in branches. A layer of corky cells grows between the leaves' stems and their branches, reducing the leaves' supply of nutrients and water.
With diminished sunlight, water, and nutrients, chlorophyll synthesis slows. Old, worn-out chlorophyll breaks down at the usual rate--ironically, sunlight destroys it--so each leaf's stock gradually dwindles. And as the green fades, yellow and orange emerge from hiding.
Unlike the green and yellow pigments, red and purple pigments (anthocyanins, part of the flavonoid class) actually form in leaves in the autumn, tinting leaves scarlet and burgundy.
Botanists have long wondered why some trees are genetically programmed to manufacture anthocyanins in the fall. New research indicates that anthocyanins may be a tree's own sunscreen.
Anthocyanins are made in a leaf's sugary sap, with the help of lots of sun and cool temperatures. Botanists think that anthocyanins shield the leaves' fading photosynthesis factories from too much sunlight, rather like the pigment melanin protects our skin from the sun. While the red pigments act as a shield, the tree feverishly breaks down and pulls nutrients out of leaves and into its limbs and trunk before leaves drop or die.
Anthocyanins may also act like Vitamin C or E, scavenging so-called "free radicals" before they can do oxidizing damage to a fall leaf's fragile structure.
Upper and outer leaves tend to be reddest, since they are most exposed to sunlight and cold. In some trees, like sugar maples, the reds of the anthocyanins combined with the yellows of the carotenoids make especially brilliant orange leaves.
為什么樹葉在秋天變顏色呢？問帕特麗夏棕色，紐約市。
秋天的涼爽的天氣里有著深藍(lán)色的天空和金色的陽光，以及燦爛的黃色、橙色和紅色的葉子。樹葉在秋天變顏色是一棵樹的方法，準(zhǔn)備漫長的冬天，而不是像我們提出的風(fēng)暴窗口和拉溫暖的衣服和毯子的存儲。
在夏天，樹上像針櫟樹和糖槭樹的葉子是綠色的因?yàn)樗麄兂錆M綠色的葉綠素。
樹木需要陽光來產(chǎn)生葉綠素。反過來，葉綠素利用陽光的能量把水分解成氫氣和氧氣。同時(shí)，樹葉也從空氣中吸收二氧化碳?xì)怏w。葉化學(xué)的最終產(chǎn)品：碳水化合物（自制的植物為樹的食物），氧氣，釋放到空氣中（我們需要呼吸的氣體）。整個(gè)過程叫做光合作用。
隨著綠色的葉綠素，多數(shù)葉中還含有黃色，橙色和紅色，橙色的類胡蘿卜素色素細(xì)胞。樹不需要光來做類胡蘿卜素。植物學(xué)家稱之為“輔助色素類胡蘿卜素的吸收，因?yàn)橐恍╆柟夂停ê茫┩ㄟ^能量傳遞到葉綠素。我們沒有看到太多的這些副色素（β-胡蘿卜素，番茄紅素，葉黃素）在夏天，因?yàn)樗麄冇胸S富的綠色葉綠素掩蓋
但有過縮短的日子意味著更少的白天和寒冷的天氣。平均樹是為了節(jié)省所有的營養(yǎng)，它可以為它的冬天冬眠。氮和磷是從樹葉中抽出的樹枝。一層木栓細(xì)胞生長的葉子的莖和枝條之間，減少養(yǎng)分和水分供應(yīng)的葉。
隨著陽光的減弱，水和營養(yǎng)素，葉綠素合成減慢。舊的，陳腐的葉綠素分解在平常的速度——具有諷刺意味的是，陽光破壞——所以每個(gè)葉的股票逐漸減少。隨著綠色的漸漸褪去，黃色和橙色的隱藏。
不同的是綠色和黃色的顏料，紅色和紫色的色素（花青素，黃酮類的一部分）實(shí)際上是在秋天時(shí)葉子，葉子和勃艮第紅色著色。
植物學(xué)家一直在想為什么有些樹的遺傳程序在秋季生產(chǎn)花青素。新的研究表明，花色苷可能是一棵樹的防曬霜。
花青素是在一片葉子的含糖的汁液中，有助于大量的陽光和涼爽的溫度。植物學(xué)家認(rèn)為，花青素保護(hù)葉衰落光合作用工廠太多的陽光，而不是像黑色素可以保護(hù)我們的皮膚免受太陽。而紅色顏料作為一個(gè)盾牌，樹興奮地分解和養(yǎng)分的葉子，拉到它的四肢和軀干前葉脫落或死。
花青素還可以起到像維生素C或電子，清除所謂的“自由基”之前，他們可以做氧化損傷的一個(gè)秋天葉的脆弱結(jié)構(gòu)。
上的葉子往往是最紅的，因?yàn)樗麄兪亲畋┞对陉柟夂秃?。在一些樹木，如糖楓樹，花青素的結(jié)合對類胡蘿卜素的黃色紅色特別艷橙葉。
Autumn's cool days are trimmed with deep blue skies and golden light, and brilliant leaves of yellow, orange and red. Leaves changing color in the fall are a tree's way of preparing for long winter, rather like we put up storm windows and pull warm clothes and blankets out of storage.
In summer, the leaves on trees like pin oaks and sugar maples are green because they are chock-full of the green pigment chlorophyll.
Trees need sunlight to produce chlorophyll. In turn, chlorophyll uses sunlight's energy to split water (H2O) into hydrogen and oxygen. Meanwhile, leaves also absorb carbon dioxide gas from the air. The end products of leaf chemistry: carbohydrates (homemade plant food for the tree), and oxygen, released into the air (the gas we need to breathe). The whole process is called photosynthesis.
Along with green chlorophyll, most leaves also contain yellow, orange and red-orange pigments celled carotenoids. Trees don't need light to make carotenoids. Botanists call them "helper pigments," because carotenoids absorb some sunlight and (nicely) pass the energy along to chlorophyll. We don't see much of these deputy pigments (carotene, lycopene, and xanthophyll) in summer, because they are masked by abundant green chlorophyll.
But the ever-shortening days of fall mean less daylight and colder weather. The average tree is rushing to save all the nutrients it can for its winter hibernation. Nitrogen and phosphorus are pulled from leaves for storage in branches. A layer of corky cells grows between the leaves' stems and their branches, reducing the leaves' supply of nutrients and water.
With diminished sunlight, water, and nutrients, chlorophyll synthesis slows. Old, worn-out chlorophyll breaks down at the usual rate--ironically, sunlight destroys it--so each leaf's stock gradually dwindles. And as the green fades, yellow and orange emerge from hiding.
Unlike the green and yellow pigments, red and purple pigments (anthocyanins, part of the flavonoid class) actually form in leaves in the autumn, tinting leaves scarlet and burgundy.
Botanists have long wondered why some trees are genetically programmed to manufacture anthocyanins in the fall. New research indicates that anthocyanins may be a tree's own sunscreen.
Anthocyanins are made in a leaf's sugary sap, with the help of lots of sun and cool temperatures. Botanists think that anthocyanins shield the leaves' fading photosynthesis factories from too much sunlight, rather like the pigment melanin protects our skin from the sun. While the red pigments act as a shield, the tree feverishly breaks down and pulls nutrients out of leaves and into its limbs and trunk before leaves drop or die.
Anthocyanins may also act like Vitamin C or E, scavenging so-called "free radicals" before they can do oxidizing damage to a fall leaf's fragile structure.
Upper and outer leaves tend to be reddest, since they are most exposed to sunlight and cold. In some trees, like sugar maples, the reds of the anthocyanins combined with the yellows of the carotenoids make especially brilliant orange leaves.
為什么樹葉在秋天變顏色呢？問帕特麗夏棕色，紐約市。
秋天的涼爽的天氣里有著深藍(lán)色的天空和金色的陽光，以及燦爛的黃色、橙色和紅色的葉子。樹葉在秋天變顏色是一棵樹的方法，準(zhǔn)備漫長的冬天，而不是像我們提出的風(fēng)暴窗口和拉溫暖的衣服和毯子的存儲。
在夏天，樹上像針櫟樹和糖槭樹的葉子是綠色的因?yàn)樗麄兂錆M綠色的葉綠素。
樹木需要陽光來產(chǎn)生葉綠素。反過來，葉綠素利用陽光的能量把水分解成氫氣和氧氣。同時(shí)，樹葉也從空氣中吸收二氧化碳?xì)怏w。葉化學(xué)的最終產(chǎn)品：碳水化合物（自制的植物為樹的食物），氧氣，釋放到空氣中（我們需要呼吸的氣體）。整個(gè)過程叫做光合作用。
隨著綠色的葉綠素，多數(shù)葉中還含有黃色，橙色和紅色，橙色的類胡蘿卜素色素細(xì)胞。樹不需要光來做類胡蘿卜素。植物學(xué)家稱之為“輔助色素類胡蘿卜素的吸收，因?yàn)橐恍╆柟夂停ê茫┩ㄟ^能量傳遞到葉綠素。我們沒有看到太多的這些副色素（β-胡蘿卜素，番茄紅素，葉黃素）在夏天，因?yàn)樗麄冇胸S富的綠色葉綠素掩蓋
但有過縮短的日子意味著更少的白天和寒冷的天氣。平均樹是為了節(jié)省所有的營養(yǎng)，它可以為它的冬天冬眠。氮和磷是從樹葉中抽出的樹枝。一層木栓細(xì)胞生長的葉子的莖和枝條之間，減少養(yǎng)分和水分供應(yīng)的葉。
隨著陽光的減弱，水和營養(yǎng)素，葉綠素合成減慢。舊的，陳腐的葉綠素分解在平常的速度——具有諷刺意味的是，陽光破壞——所以每個(gè)葉的股票逐漸減少。隨著綠色的漸漸褪去，黃色和橙色的隱藏。
不同的是綠色和黃色的顏料，紅色和紫色的色素（花青素，黃酮類的一部分）實(shí)際上是在秋天時(shí)葉子，葉子和勃艮第紅色著色。
植物學(xué)家一直在想為什么有些樹的遺傳程序在秋季生產(chǎn)花青素。新的研究表明，花色苷可能是一棵樹的防曬霜。
花青素是在一片葉子的含糖的汁液中，有助于大量的陽光和涼爽的溫度。植物學(xué)家認(rèn)為，花青素保護(hù)葉衰落光合作用工廠太多的陽光，而不是像黑色素可以保護(hù)我們的皮膚免受太陽。而紅色顏料作為一個(gè)盾牌，樹興奮地分解和養(yǎng)分的葉子，拉到它的四肢和軀干前葉脫落或死。
花青素還可以起到像維生素C或電子，清除所謂的“自由基”之前，他們可以做氧化損傷的一個(gè)秋天葉的脆弱結(jié)構(gòu)。
上的葉子往往是最紅的，因?yàn)樗麄兪亲畋┞对陉柟夂秃?。在一些樹木，如糖楓樹，花青素的結(jié)合對類胡蘿卜素的黃色紅色特別艷橙葉。