Acid rain can be the result of human activity such as industrial pollution and internal combustion engines or natural causes like volcanic activity that spews sulfur compounds into the atmosphere. Researchers from universities to grade-school science fairs have studied the effect of acid rain on the environment and have discovered some disquieting facts about its effects on seed germination and plant life.
Acid rain results when sulfur dioxide, nitrogen dioxide and other molecules in fossil fuel or natural emissions combine with water vapor. The vapor forms clouds and precipitation--acid rain--enters into the water cycle. Neutral rain has a pH (a measure of acidity versus alkalinity) near 7.0. Healthy garden soil is slightly acidic or alkaline with a pH of 6.8 to 7.2. Acid-loving rhododendrons and evergreens prefer soil with a pH no lower than 5.0. The Environmental Protection Agency has records of acidic lakes in the Northeast U.S. as low as 4.2.
Rice, wheat and rapeseed (canola) provided the test subjects for a study of seed germination and acid rain at China's Southern Yangtze University. Researchers found that, at a pH level of 2.0, no seeds germinated. Rice and wheat seeds germinated at values as low as 2.5 but with abnormalities. As the "rain" pH was gradually increased from 3.0 to 5.0, the germination rate, vitality, water absorption rate, respiratory rate (the process that the plant uses to make food) and other key indicators of successful germination rose. The results proved that acidic water can seriously inhibit germination and stress grain crops like those tested.
Before seeds form to drop on the ground, pollination must occur during flowering. Scientists at Turkey's Firat University discovered that acid rain limits the ability of plants to successfully fertilize themselves. "Rain" with a pH of 6.5 watered an apple tree control group. Two functions dependent on hydration were measured as the pH was lowered; pollen germination decreased by over 40 percent when the pH of the "rain" was lowered to 3.3 and pollen tube elongation decreased by nearly 25 percent at pH 3.4. At pH 3.1, the apple tree pollination process stopped altogether.
Researchers at Cornell University's Arnot Teaching and Research Forest discovered that the nitric and sulfuric acid in the rain dissolves calcium, an essential element for plant growth and increased relative levels of manganese which can be toxic to maples in greater than trace amounts. When researchers added slow-release calcium to an area where sugar maples were failing, they observed stronger foliage, more successful seedlings and healthier mature trees.
In addition to the stress put on the plant itself, acid rain appears to interfere with the reproductive process and alter soil chemistry. New, resistant varieties must evolve or acid rain would have to be eliminated for crops to survive.