Crop yield response to water - Cra

Stem-water potential (SWP) values at midday of well-watered trees on a typical summer dayoscillate between 0.5 MPa and -0.6 MPa. These values should be the reference threshold levelsof SWP. Mild water deficits induce midday SWP values between -0.6 and -1.0 MPa. More severewater deficits are indicated by SWP levels below -1.0 MPa reaching down to -2.0 to -3.0 MPa.Stomatal conductance values are somewhat above those of citrus leaves but below the valuesobserved in deciduous orchards. Average leaf conductance values around 0.3 cm/s have beenmeasured for well-watered trees (Ferreyra et al., 2007). Excess water in poorly drained soilsdoes affect avocado tree water relations and, therefore, its overall performance. Low oxygenlevels in the soil reduce leaf expansion rates as well as root growth and, if prolonged, maycause root necrosis and leaf abscission. Several studies measuring the oxygen diffusion ratehave shown that avocado roots are extremely sensitive to anaerobic conditions. While mostspecies vegetate well in soils, where just 10 percent of the pore volume is air filled, avocadoroots seem to require a minimum of about 30 percent pore volume air filled for optimumperformance (Ferreyra and Selles, 2007). In addition to the aeration problem, there is apathogen, Phytophthora cinamomi, which thrives in waterlogged soils and can kill avocadotrees upon infection (Stolzy et al., 1967).Avocado trees are also quite sensitive to salinity, the Mexican race being the most sensitive andthe West Indian, the least. While sodium is excluded by the roots up to some level, chloridemoves freely along with the transpirational stream and causes tip burn and leaf abscission.Water RequirementsAvocado is an evergreen tree that follows the evaporative demand of the environment.Studies in Chile and in California indicate that an average crop coefficient (K c ) between 0.7and 0.72 throughout the year adequately represents the ET c of avocado. One additionalstudy in California has suggested a somewhat lower K c of 0.64. However, there are tradeoffsbetween irrigation amounts, canopy size and production efficiency (Faber et al., 1995)Figure 4 shows that the canopy size increases and yield per unit canopy volume drops as thefraction of ET c increases from 60 to 115 percent of the requirements. Canopy size is thereforethe determining factor of actual ET c , but yields were maximized in the study when a K c of 0.64was used throughout the year (Faber et al., 1995).Part of the avocado water requirements are met by rainfall; in central Chile, annual grossirrigation needs vary between 800-900 mm, while in the drier areas of Southern California itmay reach values of over 1 000 mm. To determine the actual irrigation needs in an area, therewould be a need to carry out a water budget that takes into account the effective rainfall andthe actual ET c .Irrigation scheduling and Deficit IrrigationThe extreme sensitivity to water deficits and water excess indicate that irrigation schedulingin avocado must focus on maintaining adequate aeration at the same time that tree waterdeficits are avoided (Lahav and Kalmar, 1983). In this situation, irrigation frequency is animportant issue; in coarse-textured, well-drained soils, daily applications using drip or microsprinklersare appropriate. However, in heavier-textured soils that could suffer anaerobicavocado 445