Physiology of Postharvest Needle Abscission as Influenced by Mechanical Stress

While postharvest processes of Christmas trees including shaking, baling, and transportation, are critical and integral components of tree processing, these factors may affect postharvest needle abscission as they cause mechanical stress. It is hypothesized that, baling of balsam fir trees causes mechanical stress, triggering changes in ethylene and VTCs, promoting postharvest needle abscission. Thus, the purpose of this study was to uncover the detrimental physiological effect of baling balsam fir trees on ethylene, VTCs and postharvest needle abscission. Thirty, six-year-old trees, 90 cm in height and similar in girth were randomly harvested and immediately subjected to various baling treatments such as bale of one, two, three, four, and five trees with 5 replicates for each treatment. Treated trees were then transported to the laboratory and set up in 3 L of water in 4-L glass jars untill a tree lost 60% of its weight and considered non-marketable. Needle loss (NL), needle retention duration (NRD), average water usage (AWU), ethylene and VTC evolution were measured. The study showed that non-baled trees lost the lowest, 11.26% of total needles compared to 24.45%, 19.91%, 16.65%, 14.28% and 13.44% for bale of one, two three, four, and five trees, respectively. The non-baled trees also lasted 1 and 6 days longer than the bale of two and four trees, respectively. The results on VTCs also supported the argument of VTCs role in postharvest needle abscission, showing that non-baled trees evolved significantly lower concentration (0.67 mM/g) of VTCs compared to 0.94, 0.79, 0.87, 0.83 and 0.69 mM/g for bale of one, two, three, four, and five trees, respectively. However, similar concentrations of ethylene were detected in all trees ranging between 0.48 and 0.5 µL·g^-1·h^-1, except in the bale of five trees that was as high as 0.59 µL·g^-1·h^-1. This study suggests that postharvest needle abscission is induced by baling, perhaps triggered by VTCs pathway not through ethylene pathway.