How ozone harms plants
　　　　　　　　　　　　　　　　　　　　　－－By Randy Ringen

　　The most harmful pollutant for plants is ozone. Ozone is a molecule of pure oxygen, but in a form that differs from that usually found in nature. Most oxygen atoms combine in pairs to form "O2" molecules, but ozone is a molecule of three oxygen atoms. That additional atom is what transforms ozone into a toxin. Ozone destroys plants, as free radicals of oxygen atoms attack plant tissues.
　　Ozone can be beneficial-the ozone layer in the stratosphere helps protect us from harmful ultraviolet rays. It is the ozone near ground, produced by the mixture of sunlight and emissions from cars and industry, that is the problem. According to the U.S. Environmental Protection Agency, ozone causes at least $500 million a year in crop damage in the United States each year. This accounts for[1] approximately 90 percent of all crop damage due to air pollution.
　　Most of the observable ozone damage to plants occurs on their leaves, where they produce their food through photosynthesis[2]. If enough leaves are injured by ozone, a plant begins to starve. Just like hungry humans, underfed plants grow more slowly, are more susceptible[3] to disease and pests, and reproduce less often than healthy plants.
　　Not all plants are equally harmed by ozone. Grains are less affected than tobacco, spinach, tomatoes, and beans. But all plants seem to be affected by ozone; and the more[4] ozone in the air, the more damage it causes.

臭氧怎样影响植物生长？
　　　　　　　　　　　　　　　　　　　　　　－－作者：Randy Ringen

　　对植物危害最大的污染源就是臭氧。臭氧是纯氧分子，然而它与存在于自然界的氧气构成不同。大多数情况下，两个氧原子结合形成O2，但是臭氧却是由三个氧原子构成。就是那一个多出来的氧原子使臭氧变成了有害物质。因为游离的氧原子团侵袭植物组织，所以臭氧会危害到植物的生长。

　　臭氧也有它有益的一面——大气同温层中的臭氧会保护我们免受有害紫外线的伤害。而阳光、汽车与工业废气混合作用产生的近地面臭氧却是一个令人头疼的问题。美国环保局的数据显示：在美国，臭氧每年对农作物造成至少50亿美元的损失，这相当于空气污染造成农业损失的90%。

　　臭氧对植物的影响表现在它发生光合作用制造养料的叶面上。如果有太多的叶面受到臭氧的侵袭，植物就会饿死。与人类营养不良类似，营养不良的植物生长变得迟缓，越来越容易受到疾病和有害物的侵扰，再生的速度远不如健康的植物。

　　但是，并不是所有植物受臭氧影响程度都一样。谷类受害程度就没有烟草、菠菜、蕃茄和豆类严重。但是所有植物的生长都会因臭氧而受影响，而且空气中臭氧含量越高，其危害越大。