Crop yield response to water - Cra

(Elkins et al., 2007). The appearance of flower structures takes place during Stage II. However,this process is influenced and modified by climatic conditions and a number of factors thatare not yet well understood. Although shoot extension growth is minimal during that time,branch thickening may occur if fruit load is low and water status is optimal.PostharvestBud will continue to develop during postharvest and at a slower rate throughout winter.During this period, buds will increase in size by 25 percent, which mainly corresponds withthe elongation of the carpels. During the postharvest period, there is a second peak of rootgrowth activity, and this period is also important for reserve accumulation in roots and stemsbefore the start of defoliation. This tends to occur somewhat sooner than in apple and otherdeciduous species and it is accelerated by low temperatures. Anomalous postharvest floweringcan occur in autumn after a period of severe postharvest water stress, if this stress is relievedby irrigation or rainfall a month before leaf die back (Naor et al., 2006).Responses to Water StressAlthough pear is not considered drought resistant, its organs and tissues can withstand acertain degree of dehydration, which surpasses the capacity of other deciduous fruit trees suchas peach, plum or apple. During summer, leaf turgor loss occurs in the cv. ‘Barlett’ at values ofmidday stem-water potential (SWP) close to -3.1 MPa (Marsal and Girona, 1997), which is quitelow relative to the other deciduous fruit trees. It has also been reported that recovery fromwater deficits is delayed if SWP reaches values below -3.5 MPa, suggesting this threshold as alimit for the occurrence of vascular embolism (Marsal et al., 2002b). Stomatal conductance andleaf photosynthesis decrease linearly with midday SWP in response to irrigation reductions.For European pear, nearly zero values in both stomatal conductance and leaf photosynthesishave been reported at SWP values of -2.5 MPa (Naor et al., 2000; and Marsal et al., 2002b).Trunk growth ceases at SWP below -2.2 MPa, but shoot extension growth stops sooner, atabout -1.7 MPa in the case of moderately low vigour conditions. For more vigorous conditions(i.e. young, defruited trees) extension growth precedes up to -2.0 MPa of SWP. Fruit growth(fresh weight) is somewhat less sensitive to water deficits, stopping at about -2.5 MPa, eitherduring Stage I or Stage II (Marsal et al., 2002b). The SWP values discussed above are indicativeof full impairment, but the processes, whether fruit or vegetative growth, are affected bymuch milder water deficits. For instance, for cv. ‘Conference’ it was found that to achieve freshmarket standards of fruit size for at least 50 percent of harvested fruit, SWP values below -1.1MPa should be avoided during the Stage II of fruit growing period.In terms of flowering, moderate water stress during the fruit-growing season (Marsal et al.,2002a) or postharvest (Naor et al., 2006) increases bloom the following season as comparedto fully irrigated trees. This behaviour is attributed to the fact that moderate water stresslevels hasten development of flower organs (Forshey and Elfving, 1989). However, severewater stress (SWP values below -2.8 MPa) can induce cropping deficiencies next season (Naoret al., 2006). The data in Figure 3 shows that moderate water stress was the best postharveststrategy in terms of subsequent season productivity (Naor et al., 2006). Contrary to Europeanpear, Asian pear seems to have a differential flowering response to water deficit the previousyear. In general, water stress reduced return bloom in Asian pears (Caspari et al., 1994).pear 379