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 in the soil from early season rainfall. During rapid growth in the later vegetative stages of development, a corn plant will use about 0.2 to 0.25 inches of water per day. This may increase to almost 0.33 inches of water per day during pollination. Stress conditions will become evident in the corn plant when 50 percent of the available soil water has been depleted (Schoper 1986).
Corn is perhaps the most completely domesticated of all field crops. Its perpetuation for centuries has depended wholly upon the care of man. It cannot exist as a wild plant. Corn plants increase in weight slowly early in the growing season. But as more leaves are exposed to sunlight, the rate of dry matter accumulation gradually increases. Cell division in the leaves occurs at the growing tip of the stem. Leaves enlarge, become green, and increase in dry weight as they emerge from the whorl and are exposed to light. The leaves of the plant are produced first, followed by the leaf sheaths, stalk, husks, ear shank, silks, cob and finally grain (Kiesselbach 1949).
Enough leaves are exposed to sunlight so the rate of dry matter accumulation is rapid, under favorable conditions, this rapid rate of dry matter accumulation in aboveground plant parts will continue at a nearly constant daily rate until near maturity. High yields will be obtained only where environmental conditions are favorable at all stages of growth (Bassetti 1993). Unfavorable conditions in early growth stages may limit the size of the leaves. In later stages, unfavorable conditions may reduce the number of silks produced, result in poor pollination of the ovules and restrict the number of kernels that develop; or growth may stop prematurely and restrict the size of the kernels produced (Schoper 1987).
A synchronous floral development and abortion of fertilized ovaries are responsible for much of the kernel loss caused by drought during flowering in maize. Inhibition of silk elongation and ear growth at low water potential both contribute to the failure of silks to emerge during pollen shed. Rapid and sustained ovary growth also is essential to maintain kernel set during drought. A decrease in assimilate production in drought plants coupled with inhibition of carbohydrate metabolism within the ovaries leads to a dramatic decrease in carbohydrate partitioning to the ear, and ultimately, kernel abortion (Sadras 1985).
A primary response of developing plants to suboptimal water availability is the inhibition of leaf growth, which can lead to reductions in the final size and yields of crop plants. In the uninhibited state leaf growth is dependent on the massive and irreversible expansion of new daughter cells that are produced by meristematic divisions. Imposition of moderate water stress can result in rapid decreases in growth. Water stress may rapidly limit cell expansion by decreasing the water potential gradients driving water uptake and the turgor pressure that drives the expansion of the cell walls (Westgate 1985).
WATER STRESS SYMPTOMS
If maize shows curling of leaves and a darkening color it is showing symptoms of water stress. Maize needs adequate water from germination to dent stage for maximum production. This is a physiological defense mechanism of the crop that is evident on hot, windy afternoons when the crop cannot transpire fast enough, even if the water is readily available in the soil. If the crop does not recover from these symptoms overnight, the crop is
suffering from water stress. Any changes in crop appearance due to water stress may mean a reduction in yield (Erdei 1998).
Improvements in the adaptation of plants to adverse environments can make major contributions to agricultural production in the United States. The adaptation approach would conserve resources because enhanced nutrient acquisition, drought resistance, ion toxicity avoidance, temperature tolerance, and so forth would be achieved without large-scale modification of the environment (Westgate 1985). As an example, consider corn cultivation in Champaign County, Illinois. When drained, most of the soil in this area has no physicochemical limitations, and irrigation is usually unnecessary. Yields have increased over the years. To determine how much of this increase is associated with genetic adaptation, old and new hybrids were planted at modern population densities in a fertile environment in Iowa. Genetic adaptation has accounted for 50 percent (single cross) and 53 percent (double cross) of the increased yield on the farm (Undersander 1987).
Drought is considered to be a pre-dominant factor both for determining the global geographic distribution of vegetation and for restricting crop yields in agriculture. Water stress is a limiting factor for a wide range of physiological processes in plants. Water availability is one of the most important determinants of plant growth globally (Herrero 1981).
In the evolutionary struggle of native vegetation, certain traits provide an advantage over the competition. When these are understood, we will be in a position to markedly improve plant types and hence to bring about major increases in plant productivity (Lu 1998).
Environmental stresses restrict crop production worldwide. The most severe stresses are caused by salt, cold, and water stresses. By using exogeniously applied osmoregulatory compounds that are naturally accumulated by several halophytes under stress conditions some of the crop losses might be prevented.
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