The Effect of Irrigation Methods and Levels on Several Crops

^^ under adequate irrigation 32 VI induced transpiration reduction and cites results obtained by Viets.Viets found that higher water-use efficiencies can usually be obtained with low irrigation rates applied frequently as contrasted with less frequent, high irrigation rates.Water-use efficiency may be largely influenced by an adequate supply of water to plants without oversupply.Plant water stress that is induced by under supply of water to plants is often more detrimental to water-use efficiency than oversupply of water to plants.Carolus (1969) has concluded that when light and nutrients are not limiting, internal plant water stress is usually the dominant factor that governs plant development.Usually indicated by wilting that is due to internal water deficits, water stress affec.tsnearly every plant process.Direct effects may include a decreased ability of the cell protoplasm to photosynthesize, decreased translocation rates of numerous organic compounds, and physical damage that is due to dehydration of plant tissue.Indirect effects may include decreased gaseous exchange rates due to stomatal closure, decreased growth rates, and possible lowering of yield and quality.Long continued, severe stress may result in a condition from which the plant will not readily recover on supplemental moisture, or its complete destruction.Although plant water stress is generally considered a synonym of internal water deficits, suboptimal production conditions may occur when internal water deficits approach zero.For instance, conditions that are conducive to minimal internal water stress may cause tomatoes to exhibit blotchy ripening.Kramer (1969) notes that wheat is known to have higher protein content when grown under conditions of moderate stress.Increased vegetative growth and delayed maturity that is due to low water stress may be critical under normal production practices.Optimal' production therefore appears to be usually dependent upon maintenance of an intermediate degree of water stress.Currently, regulation of internal water stress is accomplished primarily through water supply and its subsequent uptake by plants.Unfortunately, the factors that govern the rate of water uptake differ markedly from those factors that govern the rate of water loss.The inequity resulting from different rates of uptake and loss may be best exemplified by temporary wilt during periods of high climatic demand.Howell and Hiler (1971) concluded that, under high climatic demand, plants may undergo temporary wilt even when the soil water potential approaches that of free water.Certainly, soil water supply is an inefficient and often ineffective regulator of plant water stress unless some degree of control is imposed upon plant water loss.Jensen and others (1969) point out that optimum soil moisture levels may be viewed as dependent upon the highly variable climatic factors that influence transpirational water loss.Transpiration is the primary source of plant water loss and soil water depletion.Its regulation is essential to effective control of internal water stress.Chemical methods of transpiration control have been studied but have met with limited success on a practical basis.Genetic manipulation of the plant that would produce physical alterations sufficient to reduce transpiration rates has thus far been either inadequate or non-existant.A third possibility of obtaining control of transpiration rates may lie in the modification of the plant's atmospheric environment.According to Jensen (1969) the atmospheric factors that most influence the rate of transpiration include the temperature of the plant.the temperature of the air, incoming radiation, wind, and the relative humidity near the stomatal openings.Numerous methods of alteration of one or more of these factors have been employed but are impractical on other than very small areas.Carolus (1969) reported however, that forty years ago the solid-set, low-volume irrigation systems that were later abandoned because of their considered evaporational inefficiency often prevented temporary wilt.The low-volume water application, while providing little supplemental soil moisture during periods of high climatic demand, apparently reduced the vapor pressure gradient from the plant to the atmosphere to the point that water uptake by plant roots did not lag behind transpirational water loss.Also, reduced plant temperatures during these periods may have reduced the incidence of plant injury that is commonly associated with high temperatures.Recent investigators have found alteration of the microclimate over large areas practical with the use of solid-set, low-volume irrigation systems.Howell and others (1971) reported that the cost of such a system ranges from 300 to 400 dollars per acre.Although more expensive than movable systems, the cost difference appears to be equitable in view of the possible advantages.Applications of water to plants in the form of a fine mist during periods of high climatic demand has been found to induce favorable yield responses with respect to both quantity and quality.Several detrimental effects have been noted, but may have been a result of defective methodology.Misting at rates ranging from 0.1 to 0.9 inch per hour should effectively lower the soil, leaf, and air temperatures while increasing the relative humidity within the plant canopy.Subsequent increases in yield and quality may result.Howell and others (1971) found that misting at a rate of 1.25 mm per hour decreased the leaf temperature of southern peas an average of 4 C while increasing the water potential of the leaf by as much as 3.5 bars as compared to non-misted plots.Yield increases, as compared to check''plots, were found to be 10 percent, 20 percent, and 60 percent, depending upon the level of available soil moisture maintained.Further, the efficiency of soil water-use was greatly increased by the use of mist irrigation.Chesness and Brand (1970) found that misting at a rate of 0.2 cm per hour reduced the soil surface temperature 3.3 C, the ambient air temperature 9.0 C and the leaf temperature of strawberries 15.8 C. Gilbert and others (1970), working with mist irrigation in vineyards, were able to reduce the ambient air temperature 5.5 C and the leaf temperature 14.0 C while increasing the relative humidity within the plant canopy 10 to 20 percent.Utilizing mist irrigation, Carolus (1964) recorded significant increases in strawberry yields without a corresponding increase in total water use.Increased fruit size with better marketing quality was also reported.