A. To explain how the red pigment in leaves breaks down
B. To show that leaf color varies based on the tree species
C. To introduce a theory about why leaves turn a particular color
D. To explain how chlorophyll protects trees in autumn
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NARRATOR:Listen to part of a lecture in a botany class.
FEMALE PROFESSOR:It's autumn, and as you know, in most parts of the United States the leaves on the trees are changing color from green ... to yellow, orange, and lots of other colors.So this'll be a great time to talk about how and why some of these leaves turn one color in particular—and that's bright red.
Well, before we discuss why leaves turn red, first let's, um—look, I know this is very old material, but just to play it safe—let's first go over why leaves are usually green.It's chlorophyll, right? Leaves get their green color from chlorophyll—the chemical that's responsible for photosynthesis.
The chlorophyll in the leaves collects energy from the Sun, in the form of sunlight, and it converts this energy into sugar, which is food for the plant.It's chlorophyll that makes leaves green most of the time.Now, the classic explanation for why leaves change color is this: in autumn, the leaves start preparing for the winter and stop synthesizing new chlorophyll.
Since chlorophyll is sensitive to sunlight and to cold temperatures—both of which you get in autumn—the existing chlorophyll in the leaves breaks down.And since it's not being replaced by the new chlorophyll, the green color of the leaves gradually fades away.As this happens, the other pigments present in the leaf become visible.According to the classic theory, this is true for the red pigment as well. It was there in the leaf all along but it was hidden by the green chlorophyll.OK, so that's the classic explanation, and it's partially right.Why do I say "partially"? Well, it's probably true for pigments like yellow or orange, but it doesn't seem to hold for the red pigment.Let's back up a bit. Just what produces this red color in leaves? It's a red pigment called anthocyanin.
Here's where the classic explanation doesn't seem to apply to red.What's interesting is that during the summer, there was very little if any anthocyanin in the leaves, but in the weeks before a tree is about to drop its leaves, the production of anthocyanins increases significantly.In other words, unlike those other pigments, anthocyanins are not just unmasked by the breakdown of chlorophyll in autumn; they're actually created at this time.
So that raises a question. Why would a tree produce more anthocyanin just before dropping its leaves?Why does the tree spend so much of its resources doing this just before the leaves fall off?On the surface, this doesn't make sense. It'd be like spending money to, I don't know, to have your old car repainted when you know the car's not going to last more than a couple of months.All this extra anthocyanin in the autumn seems like a waste.But remember, nature is very economical with its resources, so that means anthocyanin must be serving some function that's important for the tree.
Today there are some theories about what that function might be. One of them involves predatory insects; another involves fungi.You know, the more I read about these theories and the related research, it always created more questions for me than answers.So I was really glad to learn about a totally different theory... a new one. It seemed to come with research and data that give a full explanation.So here it is. Remember I said the chlorophyll breaks down? Well, in autumn, a whole lot of other chemical constituents of the leaf break down as well.I don't mean they're totally destroyed, 'cause actually they break down into other, different chemicals that the tree can reabsorb from the leaves and reuse later.Now, this reabsorption process is very important for the tree, and—here's the key—it's sensitive to light, meaning that too much exposure to sunlight can interfere with this process.
So where does anthocyanin fit in here?Well, anthocyanin's more stable than chlorophyll. It's not harmed as easily by the Sun or the cold. So it's still working long after the chlorophyll breaks down.But what does it do? The theory is that anthocyanin protects the reabsorption process from the sunlight.For example, if you look closely at a red leaf on a tree, you'll notice that most of the red pigment is on the upper side of the leaf, the side facing the Sun.This new theory suggests that what the anthocyanin is doing there on top is shielding the rest of the leaf from the sunlight ... and more importantly, allowing those important chemicals to be reabsorbed by the tree.
旁白：请听植物学课上的部分内容。
教授：现在是秋天，正如你们所知，在美国的大部分地区，树上的树叶正从绿色变成黄色、橘色和很多其他颜色。所以现在是个绝佳的时机来谈论一些树叶是如何以及为什么会变成一种特别的颜色，那就是亮红色。
在我们讨论树叶为什么会变红之前，我们首先......我知道这是非常老的材料，但是为了保险起见，我们先来过一遍树叶为什么通常是绿色的。是叶绿素对吧？树叶从叶绿素中获得它们的绿色，这种化学物质是产生光合作用的原因。
树叶中的叶绿素从太阳的阳光中收集能量，然后将这种能量转化成植物的食物——糖。让树叶在大部分时候保持绿色的正是叶绿素。对树叶为什么会变颜色的传统解释是：在秋天，树叶开始为冬天做准备，停止合成新的叶绿素。
因为叶绿素对阳光和低温很敏感，而在秋天这两个因素都有，所以树叶中存在的叶绿素开始分解。由于没有新的叶绿素来代替它，树叶的绿色渐渐消退。这种现象发生时，树叶中的其他色彩开始显现。根据传统理论，这对红色也适用。它一直存在于树叶中，但是被绿色的叶绿素藏起来了。好了，这是传统的解释，部分是对的。为什么我说部分？因为这对像黄色和橘色这样的色素可能是正确的，但是似乎不适用于红色色素。我们往回倒一点。什么产生了树叶中的红色呢？是一种叫做花青素的红色色素。
这就是传统解释似乎不适用于红色的地方。有趣的是，在夏天，树叶中几乎没有花青素，但是一棵树开始落叶前几个周，花青素开始大量增加。换句话说，和其他的色素不同，花青素不是因为叶绿素在秋天分解而开始显现，它们实际上是那个时候才产生的。
那么这就提出了一个问题：一棵树在落叶之前为什么要产生出更多的花青素？为什么树要在所有的树叶掉落之前花那么多资源这么做？从表面来看这一点都说不通。就像在你花钱，我也不太清楚，给一辆你知道无法继续使用超过几个月的老车重新刷漆一样。所有这些秋天的额外的花青素似乎就是浪费。但是别忘了，大自然在它的资源方面是非常节约的。这就是说花青素一定有着对树木来说很重要的某种功能。
今天有一些关于这种功能可能是什么的理论。其中之一涉及了捕食性昆虫，另一种涉及了真菌。这些理论和相关的研究我读得越多，它们给我带来的疑问就越多过答案。所以我很高兴能看到一种完全不同的理论，一个新的理论。似乎有研究和数据能充分解释这一理论。这个理论是这样的：还记得我说过叶绿素会分解吗？在秋天，树叶的很多其他的化学成分也会分解。我不是说它们被完全破坏了，因为实际上它们分解成了其他不同的化学物质，树木能够从树叶中重新吸收这些化学物质并且以后重新使用。这个重新吸收过程对树很重要，关键来了。它对光很敏感，也就是说过多暴露在阳光下会干扰这个过程。
那么花青素在这里扮演着什么角色呢？花青素比叶绿素更稳定，不会轻易被太阳或寒冷损害。所以在叶绿素分解后很长时间它仍在工作。但是它做了什么呢？理论是花青素保护这个重新吸收过程不被阳光损害。比如说，如果你近看一棵树上的一片红叶的话，你会发现大部分红色色素都在叶子向上的那面，也就是对着太阳的那面。这种新理论认为，花青素在树叶朝上一面所做的就是挡住树叶其他部分的阳光，更重要的是，允许那些重要的化学物质被树木重新吸收。
如果对题目有疑问，欢迎来提出你的问题，热心的小伙伴会帮你解答。