秋天，大自然隆重?fù)Q妝，樹(shù)葉從綠色變成了黃色或火焰般的紅色，為什么會(huì)這樣呢,？美國(guó)北卡羅萊納大學(xué)的研究生Emily Habinck試圖回答這個(gè)問(wèn)題：因?yàn)榧t色的樹(shù)葉能夠從周圍的環(huán)境和土壤中汲取盡可能多的營(yíng)養(yǎng)，為樹(shù)木迎接冬天的來(lái)臨作準(zhǔn)備,。
據(jù)《自然》雜志在線新聞報(bào)道,，10月29日，在科羅拉多州丹佛市召開(kāi)的美國(guó)地質(zhì)學(xué)會(huì)年會(huì)上,，Habinck報(bào)告了自己的最新發(fā)現(xiàn),。她說(shuō)，土壤的質(zhì)量越差,，樹(shù)木就要花更多工夫從葉子中獲取營(yíng)養(yǎng),，樹(shù)葉因此就越紅。
在北卡羅萊納州夏洛特的自然保護(hù)區(qū),，Habinck對(duì)楓香樹(shù)和紅槭樹(shù)進(jìn)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(yíng)養(yǎng)越少,，樹(shù)葉越紅。她解釋說(shuō),，丘陵地帶的土壤所含營(yíng)養(yǎng)成分低,，樹(shù)葉因此就紅得更鮮艷，而在漫灘區(qū),，土壤富含營(yíng)養(yǎng),，樹(shù)葉在秋天里變成黃色。“簡(jiǎn)而言之：葉子越紅,，它所循環(huán)傳送的營(yíng)養(yǎng)就越多,。”
在秋天，樹(shù)葉色彩斑斕,，滿山紅葉似彩霞,，但是認(rèn)識(shí)大自然這一變化的機(jī)制并不是一件容易的事，特別是深秋里的樹(shù)葉為什么會(huì)越來(lái)越紅,。當(dāng)秋天來(lái)臨時(shí),，樹(shù)木開(kāi)始分解葉子里的葉綠素，并將其中所含營(yíng)養(yǎng)分配進(jìn)樹(shù)干和樹(shù)根,，這樣就能讓樹(shù)木渡過(guò)嚴(yán)寒的冬天,。部分樹(shù)木的葉子在秋天里變成黃色,，這是因?yàn)槿~綠素的分解讓黃色的類胡蘿卜素暴露出來(lái),。但葉子的紅色卻來(lái)自一種名為花青素的色素,，這種色素在秋天會(huì)變得很活躍。
為什么樹(shù)木在季節(jié)轉(zhuǎn)換時(shí)會(huì)以這種方式汲取能量呢,？科學(xué)家們對(duì)此有不同的觀點(diǎn),。部分研究人員推測(cè)，這些色素就像是抗氧化劑,，幫助樹(shù)木抵御艱難困境,。其他人則說(shuō)，這有助于吸引鳥(niǎo)兒摘取樹(shù)上的果實(shí),，或者是增加葉子的溫度,，幫助樹(shù)木渡過(guò)寒冬。
也有科學(xué)家發(fā)現(xiàn),，如果某個(gè)秋天光照特別好且溫度很低,，那么樹(shù)葉會(huì)變得更紅。美國(guó)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(shù)木的防曬霜,，讓葉子在樹(shù)上停留的時(shí)間更長(zhǎng)一些，這樣樹(shù)木就能從葉子中獲取更多的營(yíng)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(yáng)的灼傷,，受損的葉子很快就從樹(shù)上墜落,，樹(shù)也失去了營(yíng)養(yǎng)供應(yīng)。
Hoch做了一個(gè)實(shí)驗(yàn),，他培育了一種不能生產(chǎn)花青素的變異樹(shù)木,。當(dāng)將這些樹(shù)置入明亮而寒冷的高壓力環(huán)境中后，葉子在還是綠色時(shí)就紛紛墜落了,，表明這種變異的樹(shù)為冬天儲(chǔ)備營(yíng)養(yǎng)的效率更低,。因此他推測(cè)，生活在貧瘠土壤中的樹(shù)木會(huì)產(chǎn)生更多的花青素,，保護(hù)葉子不會(huì)過(guò)早墜落,，從而讓樹(shù)木得以從葉子中吸收更多的營(yíng)養(yǎng)，以備冬天來(lái)臨時(shí)使用,。
專家們認(rèn)為,，Habinck對(duì)樹(shù)木的研究與Hoch的假設(shè)相一致。Habinck說(shuō),，花青素的出現(xiàn)實(shí)際上是一種功能,，因?yàn)闃?shù)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(shuō)：“我們?cè)谇锾炜吹降牟屎绨忝利惖念伾皇谴笞匀粚?duì)人類的取悅,，而是樹(shù)木為生存付出的努力。”
Eppes打算繼續(xù)這項(xiàng)研究,，他計(jì)劃通過(guò)衛(wèi)星數(shù)據(jù),，進(jìn)一步了解在更大范圍內(nèi)樹(shù)葉顏色與土壤類型的廣泛相關(guān)性。（科學(xué)時(shí)報(bào)）
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.
