Crop yield response to water - Cra

Figure 3Effects of postharvest irrigation levels on next season crop yield (cumulative of two years).Midday stem-water potential was -2.8 MPa, -2.4 MPa, and -1.5 MPa in the Low, Medium andHigh irrigation levels, respectively (source: Naor et al., 2006).Total crop yield (tonne/ha)403020bab100High Medium LowPost harvest irrigation levelIn general terms it can be stated that for favourable growing conditions reference middaySWP values for unstressed pear trees oscillate between -0.65 and -0.95 MPa depending onevaporative demand; values below -1.1 MPa are indicative of water stress conditions. On theother hand, it is difficult to diagnose early waterlogging effects from SWP values.Water stress responses during Stage IPear trees are highly responsive to seasonal water stress. Water stress during Stage I decreasesshoot growth, fruit growth, final fruit size at harvest, and can increase fruit drop (Marsalet al., 2000; and Naor et al., 2000). Water stress during Stage I can potentially affect fruit celldivision, cell enlargement or both processes (Marsal et al., 2000). Only in few cases and undermoderate water stress conditions (LWP above -2.5 MPa), final fruit size was not impairedor favoured by the application of early water stress (Behboudian et al., 1994; and Mitchellet al., 1989). These authors argued that such responses were achieved by the occurrence offruit osmotic adjustment that increased fruit growth after the early water stress. However,other interpretations of these positive effects, such as the different conditions in the timingand duration of the applied water stress, are also possible (Naor et al., 2006). Other factorssuch as vigour and tree-to tree shading conditions can also be added to this controversy. Theearly literature from Australia described the application of water deficits to cv. ‘Barlett’ peartrees in high density orchards (from 2 500 to 5 000 plant/ha) growing on largely vigorousrootstocks (Pyrus calleriana). In those experiments, fruit under RDI during Stage I sized largerthan Control fruit at harvest, and water stress during Stage I helped reduce vegetative growththat was considered excessive. The growing conditions in these studies were site-specific anddo not represent the typical pear-growing conditions around the world where canopy shadingis optimized by the introduction of new vigour controlling rootstocks. Experiments carried380crop yield response to water