Water is required for all life processes and often limits plant development. For example, when the grass does not receive sufficient water, its growth slows and ceases long before it starts to look sick and turn brown. Water is required to maintain cell turgidity and to provide a substrate and medium for chemical reactions and for the transport of mineral ions in the plant; also when transpired from the leaves, water is of some value in cooling and maintaining a plant temperature suitable for metabolic reactions.

Plants growing in natural environments are often prevented from expressing their full genetic potential for reproduction and are considered stressed. The best way of assessing this potential is by determining plant productivity under conditions that are nonlimiting. One method is to identify the highest yields attained by crops. Corn, for example, yields 4600 kilograms per hectare on average but also has had a record yield of 19,300 kilograms per hectare (Erdei 1998).

The effects of water stress on plant growth puts a major limitation on grain yields throughout the world. Silk growth and leaf growth is inhibited under water stress. The major economic consequence of insufficient water on maize and corn is yield production. A reduction in herbage biomass production also results from water stress. The levels of water stress are a major factor limiting the quantity and quality of plant growth (Lu 1998).

Temperature alone is not a good indicator of stress in corn plants. The term "discomfort index" often is used to describe how uncomfortable a person becomes on days when temperatures climb and relative humidity varies. With regard to humidity, the discomfort index of plants is opposite to that of people. People need to lose water to keep cool; plants need to retain water to avoid wilting. The discomfort level for plants is, therefore, highest on clear, bright days with low relative humidity. This is because on bright, sunny days with low humidity, transpiration may exceed the rate of water uptake by roots causing the plant to wilt. Loss of turgidity in plant cells also results from moisture stress. This loss of turgidity causes stomata to close, which lowers carbon dioxide uptake by the leaf and reduces dry matter accumulation. When humidity is high, crops can tolerate high temperatures because there is little moisture lost through evaporation (Erdei 1998).

A major limitation to maize grain yields throughout the world is water stress. Clearly determination of kernel number is a dynamic process. Silk growth is inhibited under water stress and a synchrony often limits yield potential. Surface cuticular wax protects silk and leaf tissue from desiccation (Undersander 1987).

Corn has a relatively high water requirement. In the central Corn Belt, the amount of water used by the crop, plus that lost by evaporation from the soil surface, generally exceeds normal season rainfall by 3 to 5 inches. This precipitation deficit is offset by water stored

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