Crop yield response to water - Cra

Figure 2Occurrence and duration of main phenological stages of olive trees during the growing season(n). Flower bud induction occurs during the summer of the previous year (n-1). Shoot and leafdevelopment are often inhibited by high temperatures and water deficit during the summer(vertical shading). Modified from Sans-Cortes et al., (2002).Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.LeafdevelopmentShootdevelopmentInflorescencedevelopmentFlower budinductionFloweringFruitdevelopmentPit hardeningRipeningOilaccumulationlowered the LWP down to -8.0 MPa, the trees rehydrated in less than a week following the onsetof rains (Connor and Fereres, 2005). Nevertheless, as for all terrestrial plants, expansive growthof olive leaves, branches, fruit, and trunk, is sensitive to water deficits. Water stress also affectsstomatal opening and photosynthesis. It is well known that olive stomata close partially duringthe day (Fereres, 1984) in response to increases in vapour pressure deficit, even if the treesare well watered, with corresponding decreases in CO 2 assimilation. Leaf photosynthetic rate isrelatively high, of the order of 12-20 µmol CO 2 m -2 s -1 , but is reduced under and following waterstress because of stomatal closure. When water deficits are severe (LWP below -4 to -5 MPa),photosynthetic rate does not exceed one-third of normal values, reaching a maximum early inthe morning and then declining as the day advances (Angelopulos et al., 1996).The olive root system is extensive and vigorous and, therefore, can explore the soil profilethoroughly after the first years of tree establishment. In general, it can be assumed that mostof the absorbing roots are in the first 1 m depth. However, in deep alluvial soils, roots canreach 2-3 m depth or more, whereas in marginal soils the rooting depth may be less than 0.5m. Given the capacity of olive trees to lower their LWP to -7 MPa or less, the soil water contentin parts of the root zone can easily reach values below the standard permanent wilting point.Given the shape of the moisture release curve of most soils, the amount of additional waterextracted would be small. Nevertheless, such small amounts may be critical for survivingextreme droughts.Olive 303