Crop yield response to water - Cra

of crop responses to water deficits grouped into three classes. Pathway (1) involves rootto-shootand shoot-to-root hydraulic and chemical signals, which have attracted profuseattention. Pathway (2) contains a very strong reinforcing loop, whereby initial reduction ingrowth of shoot, root or both, forms a loop that may eventually override other processes.Pathway (3) involves changes in radiation- and transpiration-efficiency; these efficienciesare stable except for conditions of severe stress. This model illustrates the interactions andinterdependence of the multitude of physiological processes involved in water relationsand plant growth and, as such, is an integral part of understanding crop responses to waterdeficits. This understanding is fundamental and forms the basis for the development ofimproved irrigation practices.Effects on yieldKnowledge of yield and fruit quality responses to water deficits is required to predict theorchard response to reductions in water supply in water-short years or in areas of waterscarcity, where it is not possible to supply the amounts needed to meet maximum ET c .Normally, the prospect of water deficits increases the risk of yield reductions and these fearsare often realized if the water deficits are severe enough and occur at critical stages whensome of the components of yield are determined. However, it is sometimes possible to avoidthe negative impacts of reduced irrigation supplies by confining the plant water deficits tostress-tolerant periods of the season. Moreover, there are some cases where it is possible toactually exploit the positive responses to water deficits to improve fruit quality and thus,enhance crop value with reduced consumptive use. Thus, growers would profit from bothhigher gross revenue and reduced water costs.To assess the response of fruit trees and vines to water deficits, multiyear trials are necessarybecause perennial plants require time to acclimatize to a new water regime and becausethere may be carryover affects of water deficits on subsequent season(s)' productivity.In other words, stress history is an important aspect of permanent crop deficit irrigation.Normally, stored soil water, shoot, leaf, root, and fruit development are all affected by thefirst season of water deficits and all that influences the results of that year. After the firstyear, a minimum of two additional years are needed to evaluate the response, primarily is theresult of both carryover impacts of stress and the alternate bearing tendency of many treespecies that would affect the yield, regardless of the water deficits. To characterize the yieldresponse to water for perennial crops it is necessary to conduct experiments for 3-4 yearsminimum, and preferably more. The tree-to-tree variability increases the number of replicateplots needed to detect statistically significant differences in both physiological processes andyield components among treatments. All these make tree and vine irrigation experiments verytime consuming and expensive and thus scarce, particularly those that are long term.Fruit and product qualityMany fruit quality features may be affected by water deficits and it is ultimately the cumulativeimpact on yield and quality of a product that will inform deficit irrigation strategies. Thus,only those quality factors that influence product prices should be considered when evaluatingthe effects of water deficits on quality. The size of fresh fruit is one of the most importantquality aspects affecting orchard revenue. Fruit size is generally reduced by water deficits284crop yield response to water