Crop yield response to water - Cra

Because olives flower late, the risk of low temperature damage is significantly reduced;however, the risk is increased of being affected by water and/or high temperature stress inthe Mediterranean climate, and also fruit growth is delayed into the summer, normally anextended period of water shortage. It has been observed that for rainfed trees in years whenwinter rainfall has been very limited, that flowering and fruit set are processes that are quitesensitive to water deficits. Thus, water stress should be avoided from floral development tofruit set in spring. However, in relatively humid climates (e.g. central and northern Italy) stressseldom develops in spring and irrigation is usually unnecessary. Once the fruit is set, it growsmore or less linearly by cell division and subsequent expansion, unless water deficits and hightemperatures slow its growth rate. Initial rapid fruit growth (Stage I) and the period when oilis actively accumulated in the fruit (Stage III), are also sensitive to water deficit.Given that water deficits reduce fruit growth, irrigation that avoids water deficits increasesfruit size, although the effect is mediated by the amount fruit borne on the tree. Once thefruit is set, its growth rate may be slowed down by water deficits, but growth quickly resumesupon relief from stress by rainfall or irrigation. Complete recovery of endocarp growth occurseven after several weeks of deficit, whereas recovery of mesocarp growth is less certain. Thepulp-to-pit ratio, an important quality feature for both table and olive oil fruit, is increasedunder irrigated conditions, but it has been observed that mild water deficits during fruitdevelopment have a positive effect on the pulp-to-pit ratio (Gucci et al., 2007).Soil water availability affects colour change and ripening of fruit. The effect of irrigation onoil content may be apparently contradictory as it affects water content and the process of oilaccumulation in the fruit in different ways. It is well known that severe stress during the oilaccumulation period (from mid-August through the end of October for most cultivars in theNorthern Hemisphere), reduces the oil percentage on a dry weight basis at harvest. As stressbecomes less intense the oil percentage increases, although at a relatively low rate. Undernon-stress conditions the variability of data on the relationship between predawn LWP andoil content increases and oil concentration may be even less than for mildly stressed fruit(Figure 3).While irrigation does not usually alter the fatty acid composition or basic parameters usedfor the classification of VOO (e.g. free acidity, peroxide value, spectrophotometric indexes),increasing volumes of water applied decrease the concentration of phenolic compounds inthe oil, namely the concentration of secoiridoid derivatives of oils, such as 3,4-DHPEA-EDA,3,4-DHPEA-EA and p-HPEA-EDA (Gomez-Rico et al., 2007 and Servili et al., 2007), which actas natural antioxidants during oil storage and play important functions in human diet andprevention of cardiovascular diseases. As a result, oils from irrigated orchards are usually lessbitter and pungent that those from rainfed ones (Servili et al., 2007).Assessment of tree water statusThe established method of measuring tree-water status is the measurement of stem-waterpotential or leaf-water potential. Exposed leaves are used for the latter while shaded or coveredleaves are used for SWP; in some cases, small terminal branches have been used to characterizetree water status. Predawn LWP is a reliable indicator of tree water status in mature trees, but it isinconvenient in practice because of the need for early morning measurement.304crop yield response to water