A部分

One 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部分
在许多北美地区，秋天的叶子变色是一年中最迷人的自然景观之一。这些色彩华丽，但为什么有些树会变成黄色或橙色，而其他树却会变成红色或紫色，这个问题长久以来一直困扰着科学家们。

B部分

Summer 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部分
夏天的叶子之所以是绿色，是因为它们富含叶绿素，这种分子能吸收阳光并将其转化为树木的新建材料。随着北半球的秋天的临近，可利用的太阳能明显减少。对于许多树木来说，除了常绿针叶树外，最好的策略就是放弃光合作用，直到春天再恢复。因此，在冬季来临之前，树木会将已不需要的叶子舍弃掉，以节省宝贵的资源。在舍弃叶子之前，树木会分解其中的叶绿素分子，并将其中宝贵的氮元素返还到枝干中。随着叶绿素的消耗，夏季一直由叶绿素主导的其他颜色开始显现出来。这种显露可以解释黄色和橙色的秋天颜色，但不能解释枫树或漆树等树木的鲜艳红色和紫色。

C部分

The 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部分
红色的来源是被广泛知晓的：它是由花青素产生的，花青素是一种水溶性植物色素，能反射可见光谱中的红色至蓝色范围。它们属于一类基于糖的化合物，也被称为黄酮类化合物。令人困惑的是，花青素实际上是新制造出来的，与树木准备脱落叶子的同时制造出来。但是，很难理解为什么树木在叶子中制造新的化学物质，当它已经忙于回收和保存已有的化学物质了呢？

D部分

Some 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部分
关于花青素，有一些理论认为它们可能起到了防御昆虫或真菌侵袭的化学防线的作用，或者可能吸引食果鸟或增加叶片对寒冷的耐受力。然而，这些理论都存在问题，包括叶子变红的时间相对较短，制造花青素所需的能量消耗会超过任何抗真菌或抗食草动物活动的效果。

E部分

It 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部分
还有一种观点认为，树木可能会产生鲜艳的红色，以说服食草昆虫它们是健康而强壮的，能够轻松地对抗虫害。如果昆虫关注这样的广告，它们可能会选择在颜色较暗淡、预计抵抗力较弱的寄主上产卵。这个理论的缺陷在于缺乏支持它的证据。迄今为止，还没有人确定更强壮的树木是否拥有最亮的叶子，或者昆虫是否根据颜色强度做出选择。

F部分

Perhaps 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部分
也许关于叶子在忙于准备冬天时为什么要制造花青素这一问题的最可行的解释是“光屏障”假说。这个假说听起来似乎矛盾，因为它的背后的想法是秋叶中产生红色色素是为了保护叶绿素，即吸收光线的化学物质，免受过多的光照。为什么叶绿素在需要尽可能多地回收的时候需要保护呢？

G部分

Chlorophyll, 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 channeled 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部分
叶绿素虽然精确地进化出了吸收阳光能量的能力，但有时候它会被阳光压倒，特别是在干旱、低温或营养不足的情况下。此外，在秋季时，叶子正在忙于拆除其内部机制，因此受到的叶绿素分子的激发所产生的能量并没有立即转化为有用的产物和过程，就像完整的夏季叶子那样。这使得变弱的秋叶容易受到叶绿素分子产生的氧气的极具破坏性的影响。

H部分

Even 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 recognised 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部分
即使你以前从未怀疑过叶子变红时发生的事情，实际上有一些线索可以说明问题。其中一个线索很简单：在许多树木上，最红的叶子是树木朝向太阳的一侧的叶子。不仅如此，叶子的上面红色更亮。几十年来，人们已经认识到，干燥、阳光充足的日子和凉爽的夜晚是产生强烈红色的最佳条件，这些条件与使叶子容易受到过多光照的条件相吻合。最后，像枫树这样的树木在北半球越往北走，变红的程度就越高。那里更冷，它们更受压力，叶绿素更敏感，需要更多的防晒霜。

I部分

What 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.