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