气象专家介绍，在自然界的大多数绿色植物体内都含有叶绿素、胡萝卜素和叶黄素，但由于各种色素的含量和比例不同，其叶片就表现出不同的颜色。在气温较高的季节，特别是３０℃左右时，植物生长旺盛，叶绿素的比例较多，因此叶片多呈现出绿色。秋天到来之后，气温降低、空气湿度减少、光照减弱，叶绿素的合成受到抑制。当气温降低到１０℃以下，尤其是最低气温降至４℃以下时，会出现霜冻，植物体内叶绿素的比例随之明显降低，叶片就会显现出叶黄素、胡萝卜素所特有的黄色。

According to meteorologists, most green plants in nature contain chlorophyll, carotene and lutein, but their leaves show different colors due to different contents and proportions of various pigments. In the season with high temperature, especially around 30 ℃, the plants grow vigorously, and the proportion of chlorophyll is more, so the leaves are mostly green. After autumn, the synthesis of chlorophyll was inhibited by the decrease of temperature, humidity and light. When the temperature is below 10 ℃, especially the lowest temperature is below 4 ℃, there will be frost, and the proportion of chlorophyll in plants will be significantly reduced, and the leaves will show the Yellow characteristic of lutein and carotene.

在同样的自然条件下，枫树、黄栌、火炬树、山楂树、五叶爬山虎等少数树种“红脸”的秘密在于它们的叶子中除了含有叶绿素、叶黄素、胡萝卜素外，还含有一种其他植物少有的特殊物质——花青素。说它特殊，一是因为花青素具有“反潮流精神”。随着气温降低、昼夜温差增大，花青素的含量不但不会减少，还会迅速增多；二是花青素具有遇酸变红，遇碱“面不改色”的特点。据测定，只有枫树等少数几种植物叶子中的细胞液是酸性的，故而随着秋天的降临、花青素的增多，叶片会逐渐由绿变红。而对于大多数植物而言，由于所含的花青素很少，或者花青素含量虽多，但细胞液是碱性的，所以无论气候怎样变化，叶片都不会变红。

Under the same natural conditions, the secret of "red face" of a few tree species, such as maple, yellow croaker, torch tree, hawthorn tree and five leaf creeper, lies in that their leaves contain not only chlorophyll, lutein and carotene, but also anthocyanin, a special substance rarely found in other plants. The first reason is that anthocyanins have "anti trend spirit". With the decrease of temperature and the increase of temperature difference between day and night, the content of anthocyanin will not decrease, but will increase rapidly. Secondly, anthocyanin has the characteristics of turning red when encountering acid and "not changing color when encountering alkali". According to the test, only a few plant leaves, such as maple, have acidic cell fluid. Therefore, with the fall of autumn and the increase of anthocyanins, the leaves will gradually turn from green to red. But for most plants, because of the low content of anthocyanin or the high content of anthocyanin, but the cell fluid is alkaline, no matter how the climate changes, the leaves will not turn red.

由此看来，枫叶等少数植物的叶子秋天变红的内因是花青素与酸性细胞液发生化学反应，外因则是入秋后天气转寒，促进了花青素大量增多，即气象条件发生变化造成的。

It seems that the reason for the leaves of a few plants such as maple leaves turning red in autumn is the chemical reaction between anthocyanin and acid cell fluid, and the external reason is that the weather turns cold after autumn, which promotes a large number of anthocyanins, that is, the change of meteorological conditions.