Effects of air pollution on citrus trees

Regardless of the canopy leaf area, daily normalized transpiration (per unit leaf area) of irrigated Tahiti acid lime trees were equivalent to the normalized reference evapotranspiration (per unit of grass leaf area). Normalized transpiration represented about 1/3 of the reference evapotranspiration (ETo) during the growing season. Such coefficient represents the inverse of the leaf area of a hypothetical reference surface. Tree phenology disturbed substantially the relationship during the physiological rest of autumn/winter when photoperiod was short and the minimum temperature was below 15°C. Normalized transpiration was nearly constant (» 0.4 L m-2 leaf d-1) and independent of ETo during the rest period. However, once the tree was physiologically prepared to bloom (late winter/early spring) minimum temperature was not a limiting factor to the transpiration. A large canopy leaf area (99 m2) played a major role in drastically reducing the normalized transpiration during summer days with high atmospheric demand (minimum relative humidity below 50%; maximum vapor pressure deficit above 2 kPa). During such days transpiration was reduced by 30 to 40% indicating that the shallow root system was not able to uptake soil water fast enough to supply the large transpiration surface under high demand. Conversely, a small tree (3 m2 of leaf area) did not show any transpiration restriction. This is an indication that a pruning or topping program would reduce the growth of the transpiring area to keep a potential transpiration rate. An individualized soil water balance approach for precision irrigation schemes is presented.