D. Michael Glenn
Management of fruit tree canopy temperature requires an accurate measurement of temperature, knowledge of whether temperature is limiting, and an understanding of temperature-modifying strategies and their economic and physiological consequences. The maximum photosynthesis of deciduous tree fruit crops occurs between 20 and 30°C when light is not limiting. Leaf temperatures above 30°C reduce photosynthesis; therefore, managing canopy temperature can increase productivity. The temperature of a tree's canopy is generally cooler than the air because transpired water evaporates and cools the canopy below air temperature. If water becomes limiting to the leaves, transpiration is reduced and leaf temperature rises because the leaf tissue is still absorbing radiation but evaporation of water is restricted.
Historically, the question of whether leaves are cooler than the air has been debated since the late 1800s. Technology limited a clear answer in the debate because leaf temperature could be measured only with mercury thermometers or thermocouples and only on single leaves. It was impossible to measure the temperature of an entire canopy until infrared (IR) sensors were developed in the 1960s. With the advent of IR thermometry, new theories developed and experimental evidence was collected to demonstrate that the surface temperature of a plant canopy could be predicted based on the micrometeorological conditions around the leaf and the effectiveness of the stomates to regulate transpiration rate. This body of work predicted that in humid environments, leaf temperature will equal or exceed air temperature, but in arid environments, leaf temperature can be as much as 7°C below air temperature. Understanding the mechanisms regulating leaf temperature led to useful techniques of quantifying plant stress, scheduling irrigation to meet the environmental demand on plants, and identifying new production practices that optimize canopy temperature and improve yield.
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