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Canadian writer Jay Ingram investigates the mystery of why leaves turn red in the fall

加拿大作家Jay Ingram探索了树叶一到秋季就变红之迷

AOne of the most captivating natural events of the year in many areas throughout North America is the turning of the leaves in the fall. The colours are magnificent, but the question of exactly why some trees turn yellow or orange, and others red or purple, is something which has long puzzled scientists.

A 在遍及北美的许多地区，一年中最引人入胜的自然界事件之一便是秋季的树叶 变色。那些颜色绚烂夺目，但到底为什么有些树叶转为黄色或橙色，而另一些则变成红色或紫色，则是长久以来一直另科学家们迷惑不解的问题。

BSummer leaves are green because they are full of chlorophyll, the molecule that captures sunlight and converts that energy into new building materials for the tree. As fall approaches in the northern hemisphere, the amount of solar energy available declines considerably. For many trees – evergreen conifers being an exception – the best strategy is to abandon photosynthesis* until the spring. So rather than maintaining the now redundant leaves throughout the winter, the tree saves its precious resources and discards them. But before letting its leaves go, the tree dismantles their chlorophyll molecules and ships their valuable nitrogen back into the twigs. As chlorophyll is depleted, other colours that have been dominated by it throughout the summer begin to be revealed. This unmasking explains the autumn colours of yellow and orange, but not the brilliant reds and purples of trees such as the maple or sumac.

B 夏季的叶子之所以呈现绿色是因为它们充满了叶绿素，这种分子捕捉阳光，并将那种能量转化为树木用以自我建设的新材料。随着秋季在北半球的临近，可用太阳能的量大幅度减少。对于许多树种来说——四季常青的针叶树除外——最好的策略就是先放弃光合作用等到来年开春再拾。因此在整个冬季，树木不再供养当前已变的冗余无用的叶子，而是储存起自身宝贵的能源并抛弃叶子。但是在弃置这些叶子之前，树木会先分解它们的叶绿素分子并将其中有价值的氮元素运回枝干中去。随着叶绿素被耗尽，其它哪些在此之前的整个夏季里一直受其压制的颜色变开始显现了。这样一种显露过程解释了秋天里的黄色和橙色，但却无法解释例如枫树和漆树这些树种光彩艳丽的红色和紫色。

CThe source of the red is widely known: it is created by anthocyanins, water-soluble plant pigments reflecting the red to blue range of the visible spectrum. They belong to a class of sugar-based chemical compounds also known as flavonoids. What’s puzzling is that anthocyanins are actually newly minted, made in the leaves at the same time as the tree is preparing to drop them. But it is hard to make sense of the manufacture of anthocyanins – why should a tree bother making new chemicals in its leaves when it’s already scrambling to withdraw and preserve the ones already there?

C 红色的来源广为人知：它来自于花青素，一种可以反射从黄到蓝的这一段可见光谱的可溶于水的植物色素。它们属于别名为类黄酮的一类糖基化合物。令人迷惑之处在于，花青素其实是当季新生产出来的，它们在叶子中被制造出来的时候正好就是树木正在准备丢落这些叶子之时。但是很难理解为什么要制造花青素来——一棵树为什么要在已经争分夺秒地准备回收营养，维持现有叶片的时候还特意在其叶片中制造新的化学物质呢？

Dome theories about anthocyanins have argued that they might act as a chemical defence against attacks by insects or fungi, or that they might attract fruit-eating birds or increase a leaf’s tolerance to freezing. However there are problems with each of these theories, including the fact that leaves are red for such a relatively short period that the expense of energy needed to manufacture the anthocyanins would outweigh any anti-fungal or anti-herbivore activity achieved.

D 一些关于花青素的理论认为，它们的角色也许是作为一种抵制昆虫或真菌侵袭的化学防御，又或者他们也许是为了吸引果实为生的鸟类或增加叶片的抗冻耐受力。然而，这些理论中的每一种都存在漏洞，其中之一便是事实上树叶保持红色的时期相对较短，制造花青素所消耗的能量消耗代价比任何抗真菌或防食草动物行为所取得的成果都要更大。

EIt has also been proposed that trees may produce vivid red colours to convince herbivorous insects that they are healthy and robust and would be easily able to mount chemical defences against infestation. If insects paid attention to such advertisements, they might be prompted to lay their eggs on a duller, and presumably less resistant host. The flaw in this theory lies in the lack of proof to support it. No one has as yet ascertained whether more robust trees sport the brightest leaves, or whether insects make choices according to colour intensity.

E 还有一种理论认为，也许树木自造出生动的红色是为了使那些食草型的昆虫相信：它们十分健康和强壮，轻而易举就能发起抗害虫侵扰的化学防御。这个理论的缺陷在于缺乏理论支持。到目前为止，还没有人确认过是否更佳健康的树木会展示出色彩最明丽的叶片，或者昆虫是否会根据颜色的浓度来作出选择。

FPerhaps the most plausible suggestion as to why leaves would go to the trouble of making anthocyanins when they’re busy packing up for the winter is the theory known as the ‘light screen’ hypothesis. It sounds paradoxical, because the idea behind this hypothesis is that the red pigment is made in autumn leaves to protect chlorophyll, the light-absorbing chemical, from too much light. Why does chlorophyll need protection when it is the natural world’s supreme light absorber? Why protect chlorophyll at a time when the tree is breaking it down to salvage as much of it as possible?

F 在解释叶片为什么在已经忙着存储能量迎接冬季的时候还要煞费工夫的制造花青素时，也许最令人信服的建议就是“光屏障”假说。它乍听上去有些自相矛盾，因为这些假设背后的理念是：秋叶中所产生的红色素是为了保护叶绿素这种吸收阳光的化学物不受过多光照的伤害。叶绿素是自然界中的吸光高手，为何需要此种保护？树木此时已经在分解叶绿素以挽救尽可能多的有用物质了，又为何要在此时去保护叶绿素？

GChlorophyll, although exquisitely evolved to capture the energy of sunlight, can sometimes be overwhelmed by it, especially in situations of drought, low temperatures, or nutrient deficiency. Moreover, the problem of oversensitivity to light is even more acute in the fall, when the leaf is busy preparing for winter by dismantling its internal machinery. The energy absorbed by the chlorophyll molecules of the unstable autumn leaf is not immediately channelled into useful products and processes, as it would be in an intact summer leaf. The weakened fall leaf then becomes vulnerable to the highly destructive effects of the oxygen created by the excited chlorophyll molecules.

G 尽管叶绿素已经经过了精密的进化以捕捉阳光的能量，有时却也会被过多阳光所伤害，尤其是在干旱、低温或营养不足的情况下。而且，对光过度敏感这个问题到了秋季变得更为严重，因此，此时树叶正忙于分解其内部机制以准备过冬。这时，生存状态并不稳定的秋叶中的叶绿素分子所吸收的那些能量并不会像在一片完好无损的夏日叶片中那样，被立刻输送到有用的生成物和过程当中。功能减弱的秋叶因此十分容易遭受活跃叶绿素分子所制造的氧所带来的具有高度破坏性的影响。

HEven if you had never suspected that this is what was going on when leaves turn red, there are clues out there. One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree which gets most sun. Not only that, but the red is brighter on the upper side of the leaf. It has also been recognized for decades that the best conditions for intense red colours are dry, sunny days and cool nights, conditions that nicely match those that make leaves susceptible to excess light. And finally, trees such as maples usually get much redder the more north you travel in the northern hemisphere. It’s colder there, they’re more stressed, their chlorophyll is more sensitive and it needs more sunblock.

H 即使你从未怀疑过着就是树叶变红的真相，也有很多提示线索。有一条十分直白：在很多树上，最红的叶片都位于光照最多的那一侧。不但如此，红色在叶片向上的一面也更为鲜艳。几十年来这一点也一直为人所知：制造浓艳红色的最好条件是光照，阳光充足的白天和凉爽的夜晚，而这些条件恰好契合于那些造成叶片对过度光照尤为敏感的条件。最后还有一条：你在北半球向北走，像枫树这样的树叶就越是红的热烈。那里气候更为寒冷，它们感受到了更多压力，它们的叶绿素更为敏感，也需要更多的防晒保护。

IWhat is still not fully understood, however, is why some trees resort to producing red pigments while others don’t bother, and simply reveal their orange or yellow hues. Do these trees have other means at their disposal to prevent overexposure to light in autumn? Their story, though not as spectacular to the eye, will surely turn out to be as subtle and as complex.

I 然而，我们未能充分理解的还有一点：为什么有些树木会动用生产红色素的机制而另外一些则无动于衷，仅仅只是显露它们的橙色或黄色即可？这些树木是否有其它可以随意使用的方法来阻止自身过度暴露在秋日的阳光下？也许表面看来它们的故事没有那么辉煌多彩，但一旦公之于众必定也是同样的精妙而复杂。