131-174_Botany 11/8/04 12:06 PM Page 165
Uptake and Use of Water, Minerals, and Light
Most often, soils are made more acid by the addition of sulfur or organic materials such as peat moss or sawdust; limestone (calcium carbonate) is widely used to increase the alkalinity. The mineral content of irrigation water may alter the soil pH, particularly when the water evaporates and residual chemicals accumulate in the soil—a frequent, acute problem in arid regions. Normally, rain leaches excess minerals deep into the soil and helps to restore the pH to more favorable conditions. But in some areas, industrial emissions are contaminating rain and affecting soil chemistry. When the air pollutant sulfur dioxide combines with atmospheric moisture, it falls to earth as a mild sulfuric acid solution called acid rain, which has a devastating effect on species ranging from simple nitrogen-fixing bacteria to forest trees. Increased soil acidity, whether from acid rain or natural sources, causes aluminum, manganese, and iron to be liberated from harmless, insoluble forms in the soil to concentrations that slowly poison cells. Furthermore, free aluminum and iron cause phosphates to precipitate and interfere with the uptake of calcium by roots, thereby adding the deficiency of those macronutrients to other problems of plants in acidic soils. At the other end of the pH scale, toxic amounts of molybdenum are released in alkaline conditions. Alkalinity also makes phosphates and calcium unavailable to roots when they combine to form insoluble calcium phosphate; and manganese and iron bind into tight chemical complexes. Because the resultant iron deficiency is fatal to many species, gardeners who work with alkaline soils must supply the metal in chelated form. Chelates (soluble organic compounds to which iron is bound) make the element available to plants without toxic effects. The chelate is eventually broken down by microorganisms. Two commonly used chelating agents are known by the acronyms EDTA and EDDHA. THE PHOTOSYNTHETIC APPARATUS
The electron microscope was one of the great inventions of the twentieth century. The instrument reveals tantalizing images of the intricacies of cell structure, at magnifications thousands of times their actual size. Chloroplasts, the site of photosynthesis, and other cellular organelles possess such refined details that it stretches the imagination to fathom how each is made. The black-and-white photograph shows two chloroplasts taken with an electron microscope. They are separated by the walls of the two cells in
165