Crop yield response to water - Cra

water stress should not be imposed on any of the fruit growth stages. However, irrigation can bereduced under certain conditions i.e. deep soils and low evaporimetric demand, without inducingplant water stress. The postharvest phase is the period for accumulation of reserves. Fruit setin the following spring will be affected by the level of carbohydrate reserves, which have beenaccumulated during the previous growing season, and more so if flower buds developed earlierthan vegetative buds as for cherries. Sweet cherry trees do not grow well without irrigation in areashaving dry and warm seasons (Proebsting et al., 1981). Providing accurate information on how toirrigate cherry trees requires a good assessment of plant water status. The standard method ofassessing plant water status in cherry orchards is to measure midday stem-water potential (SWP).Air vapour pressure deficit (VPD) has a definite impact on the measures of SWP in cherry treesand VPD reference lines need to be developed to account for this effect. In cherry, Ψstem valuesbelow -1.0 MPa during midseason are likely to be indicative of water stress conditions (Marsal,2009 and Marsal, 2010). Therefore, the level of water stress in a tree having a SWP lower than-1.0 MPa would depend on the prevailing VPD conditions. For instance, early season (from lateApril until the end of May, Northern Hemisphere), with typically low VPD (< 1.5 kPa) and a canopystill under development, a non-water stressed cherry tree would have a Ψstem less than -0.7 MPa(Marsal, 2009 and Marsal, 2010). The level of SWP is related to tree transpiration because, as waterstress increases leaf conductance to water vapour decreases. The response of leaf conductance toSWP for cherry trees follows a standard exponential function (Figure 3); however the response oftree transpiration to midday SWP has not yet been described. Incipient leaf wilting in cherry treescan be observed in the field at a SWP of -1.8 MPa. At this value, stomata are mostly closed andvegetative growth hastened (Figure 3 – midseason conditions). However, leaf wilting can be moreclearly observed at SWP values below -2.2 MPa.Figure 3Relationship between midday stem-water potential and midday leaf conductance in two differenttimes of its seasonal development (at harvest and at postharvest early September) in ‘Summit’sweet cherry, obtained in different irrigation treatments.HarvestSeptember350-2s-1)300Leaf conductance (mmol m250200150100y = 690.72e 1.5484xR² = 0.759350y = 3E+07e 13.379xR² = 0.68440-0.5-1-1.5-2Midday stem water potential (-MPa)452crop yield response to water