When canopy temperatures exceed 30°C, additional production practices are required to maintain high photosynthetic rates. Two general strategies are utilized to reduce the heat load on the tree: (1) evaporative cooling with water and (2) application of reflective materials. Evaporative cooling utilizes the latent heat of vaporization of water evaporation to cool the wetted leaf surface up to 14°C. Scheduling of crop cooling is based on air temperature and relative humidity and requires an automatically programmed irrigation system. Application rates vary from 3 to 10 millimeters of water per hour. Evaporative cooling maintains a thin film of water on the exposed canopy that evaporates and cools the leaves. Crop cooling and the subsequent reduction of heat stress increase yield, color development, and internal fruit quality. Evaporative cooling requires additional capital investment for the water distribution system, utilizes tremendous volumes of water, and necessitates that additional disease control be anticipated. When canopy temperatures do not significantly exceed 30°C, materials can be applied that reflect incoming solar radiation, thus lowering the heat load on the canopy. Kaolin is a common reflectant used in orchards. Some photosynthetically active radiation (PAR) is also reflected by the kaolin, but the value of reducing leaf temperature appears to overcome the loss of PAR on a whole-canopy basis, with similar fruit benefits as evaporative cooling. Reflectants have wide appeal because they can be applied with orchard sprayers at or prior to excessive heat periods and so do not require the capital and pumping costs of irrigation. Also, water resources are conserved. A benefit of both evaporative cooling and reflectants is the reduction of fruit sunburn damage.
Evaporative cooling can also be used in the dormant season to delay fruit bud development. Less water is required for evaporative cooling in the dormant season than in the growing season due to the reduced energy load of the environment. Overhead sprinkler irrigation, wind machines, and heaters are used in the spring to protect fruit flowers from freezing temperatures and are discussed in the chapter on spring frost control.
Tree canopy temperature management strategies utilize the physical properties of water and mineral materials to modify the microcli mate of a fruit tree to reduce environmental damage. These strategies improve fruit quality and the stability of orchard productivity by reducing water stress, sunburn, and other temperature-related injuries.
Related Topics: FRUIT COLOR DEVELOPMENT; FRUIT GROWTH PATTERNS; IRRIGATION; LIGHT INTERCEPTION AND PHOTOSYNTHESIS; SPRING FROST CONTROL; TEMPERATURE RELATIONS; WATER RELATIONS
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