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ABSTRACT
Several of the conifer species are increasingly adopted as Christmas trees worldwide. These species have become integral parts of the horticultural economies of North American and European countries. Postharvest characteristics such as needle abscission/retention, color, fragrance and rehydration abilities vary with species and these complex physiological traits are strongly modulated by hormonal and environmental factors. A large body of research indicates that prevalence of low temperature before harvest evokes cold acclimation responses that involve an increase in complex sugar concentrations, alterations in membrane structures and enhancements in scavenging abilities promoting postharvest needle retention. Adverse postharvest environmental factors, for example, high temperature and vapor pressure deficit are found to increase water stress, cause dehydration and accelerate needle abscission and/or discoloration. Postharvest water stress/cellular dehydration is one of the fundamental biophysical signals that triggers a cascade of hormonal changes, leading to needle abscission. Abscissic acid levels increase during cold acclimation as well as prior to abscission indicating a complicated and paradoxical role in abscission. Ethylene levels increase before abscission and are well proven to instigate the needle fall. Concentrations of cytokinins, auxins and polyamines decline postharvest. However, their interactive roles with other phytohormones orchestrating the abscission process still remain elusive. This review presents and discusses our current knowledge of the physiological aspects of pre-and postharvest environmental factors on needle abscission.