EVAPORATIVE COOLING EFFECTS ON TREE TRANSPIRATION

Abstract

Overhead sprinklers or micro-sprays are used in apple orchards in Australia to reduce the incidence of sunburn. Evaporative cooling occurs while the fruit surface remains wet and this is an effective way to lower fruit temperatures on hot days. The aim of the evaporative cooling is to apply the water at the right time, and at the right amount to achieve effective cooling of fruit and at the same time avoid excess runoff to the understory. The impact of evaporative cooling on irrigation requirements is unknown. The objective of this study was to undertake some preliminary observations of the effects on fruit temperature and tree transpiration (T). The heat-pulse technique was used to determine T in a commercial 'Royal Gala' apple (Malus domestica) orchard where overhead micro-sprays were already being used for evaporative cooling. The application rate of the micro-sprays was fixed at 2.4 mm/h. Evaporative cooling commenced when air temperature exceeded approximately 34°C and water was predominantly applied in cycles of 15 min on and 30 min off. Six trees were instrumented with sap flow sensors. Three trees (treatment) received evaporative cooling while the other three trees (control) did not. A leaf wetness sensor was placed under the micro-sprays to deduce the surface wetness of the apples. T declined by approximately 50% when the overhead micro-sprays were applying water for evaporative cooling. In the control trees, T decreased by approximately 5% in response to changes in air temperature and relative humidity. T recovered rapidly soon after the water was cut off. These results suggest that EC reduces root-zone irrigation requirements.