Crop yield response to water - Cra

Under certain growing conditions, stomata remain slightly closed at the end of the postharvestseason, even when the trees are not water stressed (Figure 3 – September conditions). This hasbeen found in cherry orchards growing under warm Mediterranean conditions. Since the cherrymarket in these regions sets pressure for the use of early ripening cultivars, the postharvestperiod may last more than four months. During this period, little growth is being accomplishedbecause tree growth is checked by the application of growth regulators. After harvest, leaf netphotosynthetic rates tend to decrease with time (Figure 4). Reductions in photosynthesis usuallycome with reductions in stomata aperture and consequently, the transpiration also declines withtime. This is referred in the physiology literature as photosynthetic down-regulation.Water UseAlthough some preliminary work on modelling cherry tree transpiration has been done recently(Antunez, 2006), specific reports on measurements of cherry ET c or K c values are lacking in theliterature. Cherry irrigation requirements could be approximated by using the informationdeveloped for peach trees (see Peach Section) where the K c is related to tree intercepted radiationat midday. An example of the seasonal evolution of K c is provided in Figure 5 where cherry treeintercepted radiation was measured every two weeks until mid-August. A steady decrease in K cis assumed after mid-August. This K c decline is due to a leaf die-back process, but also to thepreviously referred effect of the down-regulation of photosynthesis, which may already apparentby late July (Figure 5). Two different tree vigour conditions are considered in Figure 5, and theeffects on crop intercepted radiation and estimated K c is evident. Low vigour conditions hadnoticeable lower K c , resulting in a 18 percent reduction in annual water requirements for theconditions in the Ebro basin, Spain (Marsal, 2010). This emphasizes the need to adjust the K c valuesto the specific orchard conditions.Suggested RDI RegimesThe application of RDI is currently widespread in some regions along with the use of growthregulators. However RDI before harvest is rarely used because besides reducing fruit growthand fruit final size (Werenfels, 1967), it can also increase cracking if stress is relieved duringripening (Sekse, 1995). RDI is more commonly applied after harvest. The reason for the groweracceptance of using postharvest RDI is because it decreases tree internal shading and controlsexcessive vigour. However, deficit irrigation is often applied after harvest in the absence ofresearch-based recommendations. For instance, in certain areas irrigation is commonly reduceduntil visual leaf wilting without being aware of possible carryover effects during the followingseason. From the few research reports published it can be inferred that, under certain conditions,postharvest water deficits can negatively affect cherry quality the following season, withexcessive water stress exacerbating this problem. A study on postharvest RDI in New Star sweetcherry grown in the semi-arid climate of Catalonia, Spain found a significant linear relationshipbetween reduction in cherry firmness and soluble solids with the average midday stem-waterpotential experienced the previous postharvest season (Marsal, 2009). Another issue related tothe use of postharvest RDI is the possibility of applying excessive water stress and negativelyinfluencing fruit set and crop load in the next season, as it has been reported for peach andalmond. In a recent study on Summit cherry, a postharvest RDI treatment, receiving 50 percentof the water given to a Control treatment, reduced fruit set and crop load in the following454crop yield response to water