Crop yield response to water - Cra

initiation, vigorous canopy, profuse flowering, and mature later, but also have a reducedtuber yield potential. Nonetheless, the breeding of long-day adapted andigena resulted inlarge increases in yield (Bradshaw, 2009).Water Use & ProductivityPotato requires from 0.35 to 0.8 m 3 of water to produce 1 kg of tuber dry matter. Underfield conditions, this translates into a water requirement during the growing period of 350to 650 mm, which is dependent on climate and cultivar (Sood and Singh, 2003). The waterproductivity for yield of fresh tuber (WP fresh y/ET ), which contains about 75 percent moisture,is 4 to 11 kg/m 3 . Expressed as dry tuber mass, the yield water productivity (WP y/ET ) rangesfrom 1.3 to 2.8 kg/m 3 . Under conditions of limited water supply the available supply shouldpreferably be directed towards maximizing yield per hectare rather than spreading the limitedwater over a larger area. Savings in water can be made mainly through improved timing anddepth of irrigation application.Responses to stressesPotato is sensitive to water deficits. Water shortages may result in a reduced tuber yield,number and size, and loss of tuber quality. To optimize yield, generally the total available soilwater (TAW) should not be depleted by more than 30 to 50 percent. Water deficit in the earlystages, during stolon formation, tuber initiation, and after tuber initiation have the greatestadverse effect on final yield. Although leaf growth is very sensitive to water deficit, if thedeficit is moderate and short, leaf growth after the defict is released by rain or irrigation cancompensate, and the effect on yield would be minor. The senescence stage is less sensitive,provided the deficit is not severe enough to shorten the duration of green canopy markedly.In general, water deficits in the middle to late part of the growing period tend to reduceyield less than in the early part, but this can vary with cultivar. Some cultivars respond betterto irrigation in the earlier tuber formation stage while others show a better response in thelatter part of that stage.Soil and air temperatures affect growth and tuber yield. The temperature for frost or coldstress for potato is considered 1 o C and less, whereas that of heat stress is 35 o C or higher.However, potato cultivars vary in their tolerance to frost, cold and heat stress, and theextent of damage depends on stress intensity and duration. A base temperature of 2 o Chas been used in some potato crop models for growing degree days (GDD) computation(Stol et al., 1991). A base temperature of 0 o C can be used for the subspecies S. tuberosumandigena. Optimum soil temperature for tuber growth is 15 to 18 o C whereas air temperaturerequirements for growing potatoes are a diurnal temperature of 25 to 32 o C and nighttemperature of 12 to 18 o C. As temperature increases from the low end of the optimum,vegetative growth of potato is enhanced, whereas tuber initiation and growth begin to besuppressed as temperature rises further.Fertilizer requirements of the potato crop are relatively high. The amount of NPK removedby potato plants is estimated at 4 to 6 kg N, 0.6 to 1.1 kg P and 7 to 11 kg K per tonneof fresh tubers produced. Depending on nutrient status of the soil, to obtain high yieldPOTATO 187