Crop yield response to water - Cra

nil at EC e of 13 dS/m in some studies. For practical purposes, the irrigation water salinitythreshold for long-term use is about 3.5 dS/m on sandy soils, 2.0 dS/m on loamy soils and1.2 dS/m on clay soils (Ayers and Westcot, 1985). During fruit ripening, light saline stressescan improve the fruit quality without any detrimental effect on yield. In fact, fruit qualityis better with moderate salinity but at the cost of reduced yield, as shown by a number ofstudies in Israel.Irrigation practiceSurface irrigation by furrow is still commonly practised. Pressurised irrigation methods(sprinkler, mini-sprinkler and drip irrigation) are now common in many main croppingcountries. Large-scale tomato growers have long learned to limit irrigation as the cropapproaches maturity. Because of its vegetative vs. reproductive changes in response to waterstatus, irrigation management should focus on water saving while maintaining yield andenhancing fruit quality. In recent years, experiments with deficit irrigation have been directedat these objectives, with either the deficit maintained at a selected level over a long time(often referred to as DI), or with the irrigation being deficit only at selected stages of thecrop’s life cycle, referred to as regulated deficit irrigation (RDI) (Battilani et al., 2008). In morearid areas preplant irrigation is practised when past rainfall is insufficient to replenish the soilprofile. Preplant irrigations are also used to leach more saline soils. Frequently, if soil is wellcharged initially, one to two irrigations over a 2-4 week period are used for stand establishmentafter transplant or seeding. During canopy development and much of the flowering period,irrigation needs to be sufficient to ensure fast canopy growth and yet not so much as to causeexcessive leaf growth and the associated dropping of flowers and young fruit. Soon after fruitcolour change, irrigation should be reduced, but the start of irrigation cutback depends onthe water remaining in the root zone of the soil, and the ET rate for that period. These arereadily simulated by AquaCrop. The cutback over the ripening stage saves water but needsto be optimal to improve fruit quality while allowing last fruits to reach commercial size andmaintaining the canopy coverage to protect fruit from sunburn.YieldYield of processing tomatoes has increased by more than 50 percent over the past 30 years inCalifornia and Mediterranean countries. A good commercial fresh fruit yield ranges from 60to 120 tonne/ha for processing tomatoes and up to more than 150 tonne/ha for fresh marketcultivars. Yield can be much higher in greenhouse production for fresh markets. In drier areas,rainfed crop production ranges from 40 to 70 percent of irrigated. Soluble solids content ofthe juice of most widely used cultivars can vary between 4.2 and 5.5 percent. Factories requiresa minimal quality for processing tomatoes: juice acidity must range between 0.34 and 0.40g/100 ml, reducing sugars between 2.5 and 3.0 g/100 ml and Bostwick consistency between 8and 12 cm/30 s. Dry matter content of fresh fruit ranges from 4.0 to 7.0 percent (Leoni, 2002).Harvest Index (HI, the ratio of yield measured as dry matter to total above ground biomass)normally ranges from 0.5 to 0.65. HI decreases when plants are over watered or receiveexcessive nitrogen fertilization because of excessive vegetative growth, but yield may not beaffected or even slightly increased, as long as the increased biomass production compensatesfor the lower HI. On the other hand, yield can be negatively affected when vegetative growthTOMATO 197