Crop yield response to water - Cra

several others since that time have found that Stage II is not sensitive to water deficits interms of negatively impacting yield. It has been shown (Girona et al., 2003) that significantwater deficits applied during Stage II may induce some dehydration of the fruit, but thatsubsequent recovery of fruit growth is usually complete after the water stress is relieved atthe onset of Stage III, and that Stage II water deficits have no impact on final yield.Even though there has been an initial report showing (Chalmers et al., 1981) increased fruitsize, relative to fully irrigated controls, when applying RDI in Stage II, no other publishedpapers reported such results, with one exception (Girona et al., 2003) for a single season in athree-year study, where low temperatures at blooming time damaged many fruit and the finalfruit load was very low. A comprehensive analysis of the effect of fruit load on the response toRDI at Stage II (Girona et al., 2004), detected larger fruit in the RDI treatment compared withan unstressed control only with low fruit loads, and as the fruit load increased, no effects weredetected and in some cases, even a reduction in fruit size was observed. Presumably the RDI inStage II enhances fruit growth relative to unstressed controls by directing more carbohydratesto fruit growth, but this phenomenon apparently occurs only with low fruit loads (Girona etal., 2004). There have been reports of less fruit drop before harvest under RDI (Girona et al.,2003), and this could explain the few observations where water deficits during Stage II hadpositive effects on yield, relative to fully-irrigated treatments.Vigorous fruit expansion takes place during Stage III when the rate of fruit expansion ishighest and most sensitive to water deficits. Fruit water content is more sensitive to waterdeficits than fruit dry weight during this period. A reduction of 25 percent in fruit watercontent occurred with Stage III water deficits in a medium peach cultivar (Girona et al., 2004).Water deficits that affect fruit dry matter accumulation must be quite severe because, notonly must they decrease photosynthesis but they must also counterbalance the tendencyof many fruit trees, including peach, where assimilate allocation to fruit has higher priorityrelative to its distribution to other tree parts (DeJong et al., 1987). Leaf photosynthesis andtree transpiration in peach are not affected by water deficits until more than 50 percent ofthe available water in the root zone is depleted (Girona et al., 2002). When water deficitsoccur under these conditions, the peak of daily Tr moves from a plateau between noon and14:00 hours towards the morning hours, and by the time Tr was reduced by 70 percent, themaximum Tr rate occurred at 9:00 am hours (Girona et al., 2002).Indicators of peach tree water status are used to quantify the water stress levels. A comparativestudy among different indicators (Goldhamer et al., 1999) found that indices derived frommicrometric measurements of trunk diameter fluctuations were the most sensitive forwater stress detection, followed by stem-water potential. Other indicators such as stomatalconductance, leaf photosynthesis, and leaf temperature were less sensitive (Goldhamer et al.,1999).Water deficits may have a negative impact on fruit appearance in the next season. Anincreased frequency of fruit doubles and deep sutures have been observed in water-stressedpeach trees (Johnson and Phene, 2008). These problems have been overcome by relievingthe water stress shortly before and during carpel differentiation (Johnson et al., 1992). Withearly-season cultivars, this stress-sensitive period is in August and September and suggestsavoidance of water deficits during these months (Johnson and Phene, 2008). For a midseasoncultivar, the increase in occurrence of double and deep suture fruit is highly correlated with398crop yield response to water