Crop yield response to water - Cra

water stress and thus highly responsive to irrigation (Naor, 2006; Naor et al., 1995 and Gironaet al., 2010). It is also highly responsive to crop load (Naor et al., 2008). Assimilate availability isthus the limiting factor for fulfilling potential fruit size (Naschitz et al., 2010). Water stress notonly limits cell and fruit expansion but also reduces photosynthesis (the source for assimilates),while primarily crop load determines the demand for assimilates and to a certain extent thephotosynthetic rate.Unlike peach, apple does not have distinct stages of fruit growth and a large part of vegetativeand reproductive growth overlap during the growing season. For this reason, deficit irrigationresearchers normally use the terms ‘early-season’ or ‘late-season’ to describe the timing of theirtreatment application. Early season normally indicates the time period before flowering budsare formed for the next season fruit. For most varieties, early season will be before July in theNorthern Hemisphere and before January in the Southern Hemisphere.Early-season water stress reduces apple fruit size (Failla et al., 1992 and Rufat et al., 2003).It also reduces fruit set by dramatically increasing fruitlet drop in temperate zones; in oneexperiment (Powell, 1974), final fruit set decreased from 24 percent for irrigated trees to8.7 percent for non-irrigated trees. In a semi-arid area of Israel, final fruit set of fully-irrigatedtrees was 15 percent decreased to 8 percent in severely stressed trees. Water storage fromwinter precipitation avoids rapid development of severe water stress in most temperateclimatic conditions, but for containerized apples with a limited rooting zone, severe waterstress may cause up to 100 percent fruitlet drop. This suggests that growers should be awareof the risk of severe water stress development early in the season especially during droughtyears, and/or in very shallow soils having low water-holding capacity.Late-season water stress that occurs in the post-reproductive cell division stage affects appledepending on the degree of severity. Moderate water stress up to 102 days after full bloomreduced canopy growth (Behboudian et al., 1998), whereas water stress after this period hadno effect. This indicates that shoot growth ends within three months after bloom (Forsheyand Elfving, 1989). An early deficit created moderate water stress and resulted in a lowerreturn bloom, whereas no reduction in return bloom was apparent in a late-deficit treatment(Behboudian et al., 1998). However, if water stress is severe during these later stages it may affectnext year’s growth (Ebel, 1991 and Girona, 2010a). Reductions in return bloom and productivityunder severe water stress have been found in apple. Fruit numbers in the following years wereaffected by severe stress the previous year generated by terminating irrigation in early summer.This was particularly evident in early varieties (Ebel, 1991).The other, above-mentioned studies, where return bloom was not reduced, did not involve sucha severe level of water stress (Behboudian et al., 1998). The reduction in the proportion of returnbloom of apple trees that were moderately stressed early in the season could reduce the size of thebourse shoot that emerges from apple flower buds below the threshold required for its terminalbud to produce a viable flower bud (Lauri et al., 1996). It may well be that bourse shoots had alreadyreached the threshold length prior to the start of late-deficit treatments thus no effect on returnbloom during late water stress was apparent. It has been observed that excessive shading by a densecanopy can also negatively affect return bloom.In most studies early water deficits (for about 2 months post reproductive cell division) reducedapple fruit size (Naor, 2006 and Rufat et al., 2003). In general, total crop yield increases with336crop yield response to water