Crop yield response to water - Cra

When nutrition is limiting, yield potential and canopy expansion are constrained, reducing thetotal water requirement of the crop. Excessively high nutrient levels, particularly N, result inluxuriant vegetative growth and high water consumption but usually without a commensurateincrease in grain yield. A possible cause is reduced HI since an excessive number of tillers areformed and many of them either do not have time to form heads or die off as a result of heavyshading by older tillers.TemperatureTemperature requirements of wheat, especially with respect to cold temperature, have alreadybeen discussed under Growth and Development. Temperatures above 34 o C are possible inmost wheat-growing regions during grain filling. Such temperatures accelerate senescenceand can cause significant reduction in grain yield through reduced grain size and increase theproportions of shrivelled and undersized grain. Cultivars are available that can tolerate hightemperatures to some degree, to minimize heat stress damage.SalinityWheat is considered moderately tolerant to soil salinity. The reduction in shoot growth withincreasing sodium concentration in a sand or solution culture is approximately linear witha concentration of 100 mM (about 10 dS/m) reducing shoot growth by around 45 percentin bread wheat and about 50 percent in durum. By comparison, the reduction in barley isaround 40 percent and in rice about 75 percent. Wheat cultivars with higher salt tolerance arebecoming available.Irrigation practiceWhile much wheat is grown solely under rainfall and stored soil water, the importance offully or partially irrigated production is very high in some countries. Irrigation practices forwheat production are diverse. In arid areas, or when grown in the dry season of monsoonalregions, wheat may be grown under full irrigation. In Mediterranean and semi-arid systems,supplemental irrigation may be used to alleviate intermittent drought or to reduce the impactof increasing water deficits as spring progresses (Oweis, et al., 1999).On a global scale the most common method of application is flood irrigation in bordered basins.Furrow application and overhead application by a variety of sprinkler methods are also used.Irrigation is frequently applied to wheat with little knowledge of its moisture requirementsor the available soil moisture at the time of application. Because wheat is so widely grown,a variety of scheduling systems and tools have been developed including methods based onwater budgets, in-field soil moisture measurement and canopy temperature. However theextent of their use in commercial production is very limited, particularly in less economicallydeveloped countries. Where rainfall is low and irrigation water supply is limited, the precedingcrop and the interval between the crops, in combination with the rainfall pattern and soil watercharacteristics, dictate whether there is a need for irrigation at or before seeding to establisha crop stand. Subsequently, generally speaking, irrigation should be managed to avoid or98crop yield response to water