秋天,大自然隆重换妆,树叶从绿色变成了黄色或火焰般的红色,为什么会这样呢?美国北卡罗莱纳大学的研究生Emily Habinck试图回答这个问题:因为红色的树叶能够从周围的环境和土壤中汲取尽可能多的营养,为树木迎接冬天的来临作准备。
据《自然》杂志在线新闻报道,10月29日,在科罗拉多州丹佛市召开的美国地质学会年会上,Habinck报告了自己的最新发现。她说,土壤的质量越差,树木就要花更多工夫从叶子中获取营养,树叶因此就越红。
在北卡罗莱纳州夏洛特的自然保护区,Habinck对枫香树和红槭树进行了仔细观察。她发现与漫滩区的环境相比,在丘陵地带,土壤营养越少,树叶越红。她解释说,丘陵地带的土壤所含营养成分低,树叶因此就红得更鲜艳,而在漫滩区,土壤富含营养,树叶在秋天里变成黄色。“简而言之:叶子越红,它所循环传送的营养就越多。”
在秋天,树叶色彩斑斓,满山红叶似彩霞,但是认识大自然这一变化的机制并不是一件容易的事,特别是深秋里的树叶为什么会越来越红。当秋天来临时,树木开始分解叶子里的叶绿素,并将其中所含营养分配进树干和树根,这样就能让树木渡过严寒的冬天。部分树木的叶子在秋天里变成黄色,这是因为叶绿素的分解让黄色的类胡萝卜素暴露出来。但叶子的红色却来自一种名为花青素的色素,这种色素在秋天会变得很活跃。
为什么树木在季节转换时会以这种方式汲取能量呢?科学家们对此有不同的观点。部分研究人员推测,这些色素就像是抗氧化剂,帮助树木抵御艰难困境。其他人则说,这有助于吸引鸟儿摘取树上的果实,或者是增加叶子的温度,帮助树木渡过寒冬。
也有科学家发现,如果某个秋天光照特别好且温度很低,那么树叶会变得更红。美国蒙大拿州立大学的植物生理学家William Hoch在2001年就曾提出,色素相当于树木的防晒霜,让叶子在树上停留的时间更长一些,这样树木就能从叶子中获取更多的营养。当叶绿素被分解后,光合作用就变得更为困难,叶子更容易受到太阳的灼伤,受损的叶子很快就从树上坠落,树也失去了营养供应。
Hoch做了一个实验,他培育了一种不能生产花青素的变异树木。当将这些树置入明亮而寒冷的高压力环境中后,叶子在还是绿色时就纷纷坠落了,表明这种变异的树为冬天储备营养的效率更低。因此他推测,生活在贫瘠土壤中的树木会产生更多的花青素,保护叶子不会过早坠落,从而让树木得以从叶子中吸收更多的营养,以备冬天来临时使用。
专家们认为,Habinck对树木的研究与Hoch的假设相一致。Habinck说,花青素的出现实际上是一种功能,因为树需要更多的能量。北卡罗莱纳大学地球科学系的助理教授、土壤学家Martha Eppes是Habinck的导师,他说:“我们在秋天看到的彩虹般美丽的颜色不是大自然对人类的取悦,而是树木为生存付出的努力。”
Eppes打算继续这项研究,他计划通过卫星数据,进一步了解在更大范围内树叶颜色与土壤类型的广泛相关性。(科学时报)
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.