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Aspects of stress physiology of citrus

Syvertsen, J. P.
Acta horticulturae 2017 no.1177 pp. 51-58
Citrus, Nematoda, biotic stress, carbon dioxide, carbon dioxide enrichment, cold stress, crops, drought, evaporative demand, flowering, fruit trees, fruit yield, gas exchange, leaves, light intensity, nutrient deficiencies, pests, physiological response, relative humidity, root rot, roots, rootstocks, salinity, salt stress, salt tolerance, seedlings, spring, stress tolerance, temperature, transpiration, tree growth, water use efficiency, water utilization, winter
Citrus, one of the most important fruit tree crops in the world, is sensitive to many environmental stresses including drought, salinity, nutrient deficiency, high irradiance, high temperature, and high atmospheric evaporative demand. The negative effects of stresses usually reduce tree growth, fruit yield and quality. Under natural conditions, long-lived evergreen citrus trees often experience multiple stresses at the same time so there are direct and indirect interactions between almost all abiotic and biotic stresses. The physiological responses of citrus trees exposed to two or more stress factors, can vary depending on stress intensity or duration. For example, since citrus leaf Cl accumulation has been linked to water use, decreased leaf transpiration from elevated CO2 concentration and high relative humidity can improve leaf water use efficiency and increase salt tolerance. In addition, even in relatively salt tolerant rootstocks, salinity stress has direct effects on roots predisposing trees to biotic environmental stressors including attack by root rot, nematodes and bacterial disease. Root pests can, in turn, affect the salt tolerance of citrus roots and may increase salt uptake. Sun-exposed leaf temperatures usually exceed optima so reducing leaf temperature with shade can increase leaf gas exchange and tree growth. Moderate drought or salinity stress in winter, however, can reduce physiological activity and growth allowing citrus seedlings to survive cold stress and can even enhance flowering in the spring. To help citrus growers cope with environmental stress problems, researchers should study the underlying mechanisms of stress tolerance and understand their modes of interaction with biotic stresses.