Crop yield response to water - Cra

that occur during the periods of fruit growth. However, the impact depends on stress timing;early season growth rate reductions may not be evident at harvest because of acceleratedfruit growth if and when full irrigation is restored. Regardless, since large fruit at harvest isgenerally desirable, water deficits should be avoided during most species' fruit growth. On thecontrary, the eating quality of many pome and stone fruit is enhanced by mild water deficits,normally by increasing their sugar content, or by increasing the sugar/acid ratios and otherchemical compounds responsible for flavour and aroma. There are other positive responsesof fruit quality to water deficits, such as improved colour, but the incidence of a number ofdisorders have been known to increase with water stress. The responses are certainly speciesand even cultivar-specific (see specific crop Sections). In the case of grapevines, the qualityissue is of paramount importance for wine production. The price that grapes fetch may differby an order of magnitude in some regions, depending on their fruit size, chemistry, andcolour, and water management is the primary factor determining this quality, as discussed inthe Section of Grapevine.WATER PRODUCTION FUNCTIONS IN TREE CROPS AND VINESFrom the practical standpoint, it is important to know how yield and quality responds tovariations in water used; i.e. the relation between water and yield, which is termed the waterproduction function. Such functions quantify the sensitivity of yield to a reduction in theconsumptive use or ET below the maximum potential, and therefore describe the expectedyield response to water. Water production functions were defined in the FAO I&D No. 33Publication for most field crops, but at that time, there were insufficient data to formulatesimilar functions for the tree crops and vines. An important goal of the following paragraphsis to develop the production functions for different fruit tree species based on researchconducted over the last 30 years. While each species has its own response, there is a generalpattern of response for most fruit tree species, as shown in Box 18.The generalized relationship between applied irrigation water (AIW), ET c and yield of treecrops shown in Box 18 has different response regions. Starting at Point 1, maximum yield isnormally achieved at maximum ET; at this point, the level of AIW is such that there are nodrainage losses and the level of allowable depletion has been reached. To ensure that ETneeds are met, some growers apply more irrigation (Region A) but the yield is maintained atmaximum, albeit with some drainage losses. For many tree crops, there is an irrigation levelbeyond which yield starts to decline, either as a result of the direct effects of waterlogging orthe indirect effects caused by diseases associated with very high soil water levels, which mayeven cause tree death in extreme cases (Region B).When AIW is reduced below the level of Point 1 (deficit irrigation), initially, yield may not bereduced (Region C, Box 18); the extent of this region depends on the species and the irrigationregime, but is generally limited to a range of actual ET c between 75 and 100 percent. As AIWis further reduced, yields finally decline due to the irrigation deficit (Region D), and will befurther reduced at a faster rate if AIW is decreased further (Region E). In this last region, thelikelihood of large yield losses is high because of the sharp decline in yield in response to thedecrease in AIW. Note that because of the steep slope in Region E, the water productivity(yield per unit AIW) of the initial irrigation amounts is highest.Yield Response to Water of Fruit Trees and Vines 285