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Orchard Management


By Peter Waterman


Turning up the heat


Explanations of the impact and some coping strategies for a summer of high temperatures and low precipitation.


P


lants, including tree fruits, have optimum temperatures under which they grow well with a good balance of vegetative and reproductive growth. Growers are all very familiar with the effects of extreme winter cold, impacts of freezing and cold temperatures on bloom and fruit development.


Excessive heat and or prolonged dry periods can be very challenging. As we move into summer in the Okanagan we already have had early


to restrictions in water use due to low snow packs and subsequent low volumes in storage reservoirs. The recent spring rains aside, how hot the summer gets and how long we have periods of low precipitation will determine the impacts on growth and development of crop plants and fruits. Many physical processes are influenced by temperature, such as the diffusion gases and liquids in plants and the solubilities of nutrient ions, the viscosity of the water which affects the rates of transport and transpiration. Far more and varied effects of temperature are found in the rates of chemical reactions in plants. These rates generally increase with temperature but they vary with the type of reaction. There is a dynamic balance between incoming and outgoing radiation and energy, the plant must adapt by rapid energy exchange to avoid lethal temperatures. Optimum temperatures vary for each process and tissue. Root growth, for example, has a lower optimum temperature than shoot growth. Plant tissues, and in our cases tree fruits and in particular apples, are in a


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constant struggle to maintain the ranges for optimum development.


Root absorption and movement of moisture in the vascular system is in response to a pull from leaves and fruit. Low humidities at the leaf surface involve more rapid demand of moisture from roots, if this cannot be met then openings in the leaves called stomates start to respond by closing to restrict plant moisture losses to avoid loss of moisture in leaf cells and potential losses in moisture in the balance of the tree.


This is a real factor in shoot and leaf growth and also results in shrinking and expanding of fruits – these are daily changes in fruit growth, but excessive heat and moisture loss results in reduced fruit size, which has a direct impact on the grower’s pocket. Leaf thickness, shoot diameter and trunk diameter all respond to changes in tree water status and are parameters that have been used to determine plant moisture needs. The


use of atmometers which are tool that mimic tree leaf moisture status and whose use has been extensive at the Summerland research centre, can be directly wired to irrigation controllers to apply the correct amount of water based on tree use and demand. Despite the ability of technology, heat and direct sunlight can wreak havoc on trees and fruit quality. Fruit on the southwest side of trees can sunburn, resulting in yellow to brown patches and ultimately hard brown leathery areas on the fruit. The temperature of the surface of the fruit can be as much as 18 degrees C above air temperatures when the fruit is exposed to solar radiation and 8 to 9 degrees warmer than the shaded side of the fruit. The shifting of branches as the fruit load gains weight exposes


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