November 15, 2011
Shelley Schlender | Boulder, Colorado
As days grow shorter and nights grow cooler in Boulder, Colorado, the green leaves of summer have been putting on party colors.
On cottonwood trees and aspens, they’re golden yellow coins. On Mapleton Avenue, the maple trees are sunset shades of orange and red.
The same seasonal phenomenon occurs in many countries in late autumn, yet people don't always know the reason leaves change color when the weather gets cooler.
Some do have their theories about the autumnal transformation.
“I’ve heard that the reason why the leaves change throughout the seasons is because God likes to see different colors all over the world," says one girl, "so that he can tell what season it is.”
“According to legend," says a woman, "three hunters and their dog chased the Great Bear into the sky, slaughtered it and its blood dripped onto the leaves, turning them red.”
Dave Sutherland, a naturalist with Boulder Mountain Parks, recalls the explanation he heard in childhood. “My mom told me it was because at night, when everybody’s asleep the fairies come out and paint them.”
It turns out the scientific explanation for why leaves change color is almost as magical as artistic fairies.
The story begins in summer, when leaves are green thanks to a pigment known as chlorophyll. It captures just the right spectrum of color from the sun’s light, to power the process known as photosynthesis.
VOA - S. Schlender
As the days grow shorter in autumn, leaves stop making chlorophyll, revealing a dazzling secret: green leaves are actually yellow.
“As the chlorophyll is grabbing that sunlight, it’s pulling water from the soil and carbon dioxide from the air and energy from the sunlight and shoving it together to make sugar,” Sutherland says.
That helps the tree blossom and grow. As the days grow shorter in autumn, leaves stop making chlorophyll, revealing a dazzling secret.
“Green leaves are yellow. You just don’t see the yellow," Sutherland says. "The yellow pigments are there all along, hidden beneath the mask of green. They’re just waiting for a chance for the chlorophyll to start breaking down so the yellow color can start shining through.”
That yellow comes from pigments known as carotinoids, which make carrots orange and autumn aspen leaves golden yellow. Some scientists say leaves keep making carotinoids after they’re done with chlorophyll, because the yellow pigments help them capture a little more solar energy.
And when they turn red? That’s another pigment called anthocyanin.
“Everybody knows anthocyanin," Sutherland says. "It makes strawberries red and blackberries black.”
And fall maple leaves red.
But, unlike carotinoids, leaves don’t make this pigment during summer, only in autumn. For decades, scientists believed anthocyanin made a sort of accidental color, caused by sugars trapped in dying autumn leaves.
But Bill Hoch, a plant scientist at Montana State University, is among those who believe leaves make anthocyanin on purpose, as a kind of natural “sunscreen.” As an autumn leaf loses its ability to make energy from photosynthesis, he says, the red anthocyanin pigment protects the leaf from the harmful effects of sunlight - especially the blue spectrum of light.
This helps the leaf’s energy-making machinery stay intact just long enough to send the last bits of nutrients, especially nitrogen and phosphorus, into the tree’s twigs and roots.
“It’s only to recover those nutrients, because those leaves are going to be discarded pretty quickly," Hock says. "So they’re just trying to protect their photo systems long enough to just recover as many nutrients as possible.”
Hoch adds that carotinoids protect the photo systems, which are light-powered energy cells. But instead of being a sunscreen for the final days of a dying leaf, the yellow pigment is an anti-oxidant that protects the leaf throughout its life.
As for why some leaves turn yellow and others turn red, Hoch says plants that grow best in full sun tend to have more robust leaves. So, in the fall, they can get by with the protection of just the yellow carotinoids. Plants that thrive in shade or in nutrient-poor soil tend to have more delicate photo machinery, which need more protection, so their leaves make more anthocyanins and turn dark.
There’s much more that scientists want to learn about how leaves work. In the meantime, people in Boulder - and across the northern hemisphere - can simply enjoy the natural splendor of spectacular autumn colors.
aspen (n) 山楊
shade (n) 陰涼處；(色彩的)濃淡,色度
本文中兩種意思都有出現，”sunset shades of orange and red”是指「黃昏時的橘紅色彩」；"Plants that thrive in shade"則是「在陰涼處成長的植物」。
legend (n) 傳說
文中使用片語結構 “according to legend”，「根據傳說」。由於不是指某個特定的傳說，所以這裡把legend當不可數名詞使用。
slaughter (v) 殺戮,屠殺
fairy (n) 小妖精
到了秋季，本來翠綠的樹葉漸漸換上不同的顏色。三角葉楊(cottonwood tree)與山楊(aspen)的葉子如黃金色硬幣(golden yellow coins)，但楓樹(maple tree)葉子呈現的是如日落的橘紅相間色彩(sunset shades of orange and red)。這同樣的季節現象(seasonal phenomenon)在很多地方會發生，但背後卻會被賦予不一樣的故事。有的神話說(according to legend)，因為獵人把大熊(星座)趕到了天上後殺了它(slaughter)，它的血滲出而沾染到樹葉上(dripped onto the leaves)，所以就把樹葉都變紅了。自然學家(naturalist) Dave Sutherland 則回憶起小時候媽媽的解釋，是小精靈(fairies)在大家睡著的時候把葉子給漆紅的。
pigment (n) 顏料；色素
chlorophyll (n) 葉綠素
dazzling (a) 使目眩的；使讚歎不已的
文裡 “dazzling secret”一詞的使用巧妙的結合了dazzle的兩個含意，指樹葉「光彩奪目而令人讚嘆的秘密」。
shove (v) 亂塞；強使
blossom (v) 開花;生長茂盛
葉子變色後面科學的解釋也如神話一樣充滿魔力。從夏天開始，葉子會呈現綠色是因為含有葉綠素(chlorophyll)這個色素(pigment)，功用是捕捉到太陽光光譜裡對應的顏色(captures just the right spectrum of color form the sun’s light)，進而可以進行光合作用(photosynthesis)。葉綠色抓住了太陽光的能量，連同土壤裡吸取的水，空氣裡的二氧化碳，全都混在一起後就產生了糖(shoved it together to make sugar)。
break down (v phr) 失敗; 崩潰
carotinoid (n) 類胡蘿蔔素 *carotene 胡蘿波素
anthocyanin (n) 花青素
Sutherland說到，「其實綠色葉子本就是黃色的，只是沒被看到而已。黃色色素一直都存在於葉子裡(. . .are there all along)，只是被藏在綠色的掩罩之下(hidden beneath the mask of green)。直到葉綠素開始瓦解了(break down)，黃色才能真的透出光來」。黃色是來自於葉子裡的類胡蘿蔔素(carotinoid)色素。有科學家提出解釋，當葉子停止生產葉綠素後(done with chlorophyll)，類胡蘿蔔素還能再多捕捉一點的太陽能。而讓葉子變紅的則是花青素了(anthocyanin)。草莓呈現紅色，黑莓呈現黑色都是花青素的功勞。
on purpose (prep phr) 故意;有目的地
intact (a) 完整無缺的
discard (v) 拋棄,摒棄
robust (a) 強健的;茁壯的
hemisphere (n) (地球的)半球
葉青素只有在秋天產生，這跟在夏天時就開始被製造的類胡蘿蔔素不同。科學家們多年來認為葉青素其實是種「意外的」顏色(accidental color)，是因為糖份被困在秋天漸漸死去的葉子裡所造成的(caused by sugars trapped in dying autumn leaves)。但有植物學家則認為葉子製造葉青素是有意義的(on purpose)。葉青素是葉子的一種天然「防曬劑」(sunscreen)，當葉子無法再透過光合作用產生能量的時候，葉青素可以保護葉子免受到陽光的傷害，尤其是光線裡的藍色光譜(the blue spectrum of light)。如此一來，葉子的能量製造機制(energy-making machinery)還能保持完整(intact)，可將最後一點養份(nutrients)送回到樹木的枝枒(twigs)和根部去。
科學家們對於葉子到底是如何運作的還有很多需要傷腦筋的地方。但對北半球的人來說(people across the northern hemisphere)，只要單純來享受自然壯麗的秋天景色就好了(the natural splendor of spectacular autumn colors)。