秋天,，大自然隆重?fù)Q妝,，樹葉從綠色變成了黃色或火焰般的紅色,，為什么會這樣呢,？美國北卡羅萊納大學(xué)的研究生Emily Habinck試圖回答這個問題：因為紅色的樹葉能夠從周圍的環(huán)境和土壤中汲取盡可能多的營養(yǎng)，為樹木迎接冬天的來臨作準(zhǔn)備,。
據(jù)《自然》雜志在線新聞報道,，10月29日，在科羅拉多州丹佛市召開的美國地質(zhì)學(xué)會年會上,，Habinck報告了自己的最新發(fā)現(xiàn),。她說，土壤的質(zhì)量越差,，樹木就要花更多工夫從葉子中獲取營養(yǎng),，樹葉因此就越紅。
在北卡羅萊納州夏洛特的自然保護區(qū),，Habinck對楓香樹和紅槭樹進行了仔細(xì)觀察。她發(fā)現(xiàn)與漫灘區(qū)的環(huán)境相比,，在丘陵地帶,，土壤營養(yǎng)越少，樹葉越紅,。她解釋說,，丘陵地帶的土壤所含營養(yǎng)成分低，樹葉因此就紅得更鮮艷,，而在漫灘區(qū),，土壤富含營養(yǎng)，樹葉在秋天里變成黃色,。“簡而言之：葉子越紅,，它所循環(huán)傳送的營養(yǎng)就越多。”
在秋天,，樹葉色彩斑斕,，滿山紅葉似彩霞，但是認(rèn)識大自然這一變化的機制并不是一件容易的事,，特別是深秋里的樹葉為什么會越來越紅,。當(dāng)秋天來臨時,，樹木開始分解葉子里的葉綠素，并將其中所含營養(yǎng)分配進樹干和樹根,，這樣就能讓樹木渡過嚴(yán)寒的冬天,。部分樹木的葉子在秋天里變成黃色，這是因為葉綠素的分解讓黃色的類胡蘿卜素暴露出來,。但葉子的紅色卻來自一種名為花青素的色素,，這種色素在秋天會變得很活躍。
為什么樹木在季節(jié)轉(zhuǎn)換時會以這種方式汲取能量呢,？科學(xué)家們對此有不同的觀點,。部分研究人員推測，這些色素就像是抗氧化劑,，幫助樹木抵御艱難困境,。其他人則說，這有助于吸引鳥兒摘取樹上的果實,，或者是增加葉子的溫度,，幫助樹木渡過寒冬。
也有科學(xué)家發(fā)現(xiàn),，如果某個秋天光照特別好且溫度很低,，那么樹葉會變得更紅。美國蒙大拿州立大學(xué)的植物生理學(xué)家William Hoch在2001年就曾提出,，色素相當(dāng)于樹木的防曬霜,，讓葉子在樹上停留的時間更長一些，這樣樹木就能從葉子中獲取更多的營養(yǎng),。當(dāng)葉綠素被分解后,，光合作用就變得更為困難，葉子更容易受到太陽的灼傷,，受損的葉子很快就從樹上墜落,，樹也失去了營養(yǎng)供應(yīng)。
Hoch做了一個實驗,，他培育了一種不能生產(chǎn)花青素的變異樹木,。當(dāng)將這些樹置入明亮而寒冷的高壓力環(huán)境中后，葉子在還是綠色時就紛紛墜落了,，表明這種變異的樹為冬天儲備營養(yǎng)的效率更低,。因此他推測，生活在貧瘠土壤中的樹木會產(chǎn)生更多的花青素,，保護葉子不會過早墜落,，從而讓樹木得以從葉子中吸收更多的營養(yǎng)，以備冬天來臨時使用。
專家們認(rèn)為,，Habinck對樹木的研究與Hoch的假設(shè)相一致,。Habinck說，花青素的出現(xiàn)實際上是一種功能,，因為樹需要更多的能量,。北卡羅萊納大學(xué)地球科學(xué)系的助理教授、土壤學(xué)家Martha Eppes是Habinck的導(dǎo)師,，他說：“我們在秋天看到的彩虹般美麗的顏色不是大自然對人類的取悅,，而是樹木為生存付出的努力。”
Eppes打算繼續(xù)這項研究,，他計劃通過衛(wèi)星數(shù)據(jù),，進一步了解在更大范圍內(nèi)樹葉顏色與土壤類型的廣泛相關(guān)性。（科學(xué)時報）
英文原文:
Published online 29 October 2007 | Nature | doi:10.1038/news.2007.202
Corrected online: 30 October 2007
News
Why autumn leaves turn red
Colour may help trees to store up nutrients before winter.
Autumn leaves turn fiery-red in an attempt to store up as much goodness as possible from leaves and soil before a tree settles down for the winter. The worse the quality of soil, the more effort a tree will put in to recovering nutrients from its leaves, and the redder they get.
That's the conclusion that Emily Habinck from the University of North Carolina, Charlotte, came to after looking at trees in a flood plain and in an adjacent upland area. The soil in the upland area was low in nutrients, and the leaves there were bright red. In the floodplain, where the soil was packed full of goodness, the autumn leaves remained yellow.
"In a nutshell: the redder a leaf is, the more nutrients it is going to recycle," explains Habinck, who presents her findings at the Geological Society of America’s annual meeting in Denver, Colorado, today.
It's not easy being red
Unlikely as it may seem, colour changes in leaves are not fully understood — at least not when it comes to the redder hues.
As autumn approaches, trees begin to break down the green chlorophyll in their leaves and redistribute the nutrients contained there to their trunk and roots. This keeps them going throughout the winter, when sunlight is sparse.
The yellow colour seen in some autumn trees results from the loss of chlorophyll simply unmasking the yellow carotinoids that were there all along. But red coloration comes from a pigment called anthocyanin, which has to be made afresh as autumn takes hold.
Why trees would bother to spend energy doing this as things are winding down for the winter has been widely debated. Some researchers have suggested that these pigments act as antioxidants, which help a tree combat harsh conditions. Others say it helps to attract birds that can then disperse fruits. Or it might increase leaf temperature, helping to protect from the cold.
Sunscreen
Some people have observed that trees tend to turn redder when an autumn is particularly bright and cold. In 2001, William Hoch, now at Montana State University, Bozeman, suggested that the pigment acts as a protective sunscreen, helping to keep leaves on the trees for longer so that more nutrients can be harvested from them. Photosynthesis becomes more difficult as chlorophyll is broken down, and leaves become more susceptible to damage from the Sun. Damaged leaves will fall more quickly, and rid the tree of a nutrient supply.
Hoch did a study in which he made mutant trees that couldn't produce anthocyanins. These dropped their leaves while they were still green when exposed to the high-stress environment of bright light and cold temperatures. The mutant trees were much less efficient at storing up nitrogen for the winter.
Habinck's study of natural sweetgum and red maple trees in a nature preserve in Charlotte supports this notion. Trees in the upland areas, where soils don't have much nitrogen, had much redder leaves than the trees in the flood-plain environment.
"A plant on a nutrient-poor soil is going to be more concerned about keeping the nutrients it has," says Hoch. So it will turn red to stop its leaves dropping prematurely.
Habinck's supervisor, Martha Eppes, now wants to look at satellite data to see whether there is a wider correlation between tree colour and soil type over large areas.
Corrected:
The pigment anthocyanin does not contain nitrogen as this story previously stated. This has been corrected.
