托福official43听力lecture1 Leaves Turn Color原文解析+翻译音频

[00:00.00]NARRATOR: Listen to part of a lecture in a botany class.
[00:05.51]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. [00:15.12]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. [00:23.78]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. [00:36.09]It’s chlorophyll, right?
[00:38.05]Leaves get their green color from chlorophyll—the chemical that's responsible for photosynthesis. [00:45.09]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. [00:54.10]It's chlorophyll that makes leaves green most of the time. [00:57.96]Now, the classic explanation for why leaves change color is this.
[01:02.47]In autumn, the leaves start preparing for the winter and stop synthesizing new chlorophyll. [01:08.42]Since chlorophyll is sensitive to sunlight and to cold temperatures—both of which you get in autumn—[01:14.99]the existing chlorophyll in the leaves breaks down. And since it's not being replaced by the new chlorophyll, [01:21.40]the green color of the leaves gradually fades away. [01:25.01]As this happens, the other pigments present in the leaf become visible. [01:29.97]According to the classic theory, this is true for the red pigment as well. [01:34.59]It was there in the leaf all along but it was hidden by the green chlorophyll. [01:40.00]OK, so that's the classic explanation, and it's partially right. [01:44.88]Why do I say “partially”?
[01:47.09]Well, it's probably true for pigments like yellow or orange, but it doesn't seem to hold for the red pigment. [01:54.42]Let's back up a bit. [01:55.93]Just what produces this red color in leaves? [01:59.34]It's a red pigment called anthocyanin.
[02:04.58]Here's where the classic explanation doesn't seem to apply to red. [02:09.04]What's interesting is that during the summer, there was very little if any anthocyanin in the leaves, [02:15.81]but in the weeks before a tree is about to drop its leaves, the production of anthocyanins increases significantly. [02:23.14]In other words, unlike those other pigments, anthocyanins are not just unmasked by the breakdown of chlorophyll in autumn; [02:32.04]they're actually created at this time.
[02:35.45]So that raises a question. [02:42.73]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? [02:48.13]On the surface, this doesn't make sense. [02:50.92]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. [02:59.51]All this extra anthocyanin in the autumn seems like a waste.
[03:03.65]But remember, nature is very economical with its resources, [03:08.65]so that means anthocyanin must be serving some function that's important for the tree. [03:16.22]Today there are some theories about what that function might be.
[03:21.52]One of them involves predatory insects; another involves fungi. [03:26.36]You know, the more I read about these theories and the related research, it always created more questions for me than answers. [03:35.00]So I was really glad to learn about a totally different theory…a new one. [03:40.00]It seemed to come with research and data that give a full explanation. [03:44.40]So here it is.
[03:46.00]Remember I said the chlorophyll breaks down? [03:48.88]Well, in autumn, a whole lot of other chemical constituents of the leaf break down as well. [03:55.10]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.
[04:04.72]Now, this reabsorption process is very important for the tree, and—here's the key—[04:10.46]it's sensitive to light, meaning that too much exposure to sunlight can interfere with this process. [04:17.95]So where does anthocyanin fit in here?
[04:21.05]Well, anthocyanin's more stable than chlorophyll. [04:24.89]It's not harmed as easily by the Sun or the cold. [04:28.77]So it's still working long after the chlorophyll breaks down. [04:33.06]But what does it do?
[04:34.72]The theory is that anthocyanin protects the reabsorption process from the sunlight. [04:40.69]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. [04:51.74]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.