Balsam fir is an important horticultural crop for provinces in Atlantic Canada, including Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland. Each year the region supplies over 3 million trees for local and international use as Christmas trees (MacDonald, 2010). However, a major challenge for the Christmas tree industry is early postharvest needle abscission, attributable possibly to earlier harvest dates, reduced cold acclimation periods, and longer shipping distances (Chastagner and Riley, 2003; MacDonald and Lada, 2008). As a result, there has been more interest in understanding the physiology of needle abscission in postharvest balsam fir.
Although several factors have been considered, the role of ethylene has contributed most to our understanding of postharvest needle abscission in balsam fir. Endogenous ethylene evolution reaches detectable limits (0.1 ppm with ethylene analyzer) 1 week after harvest and then peaks at ≈15 μL·g−1·h−1 before abscission occurs (MacDonald et al., 2010, 2011b). If a branch is instead exposed to exogenous ethylene, then abscission occurs 30% to 70% earlier than a control and is strongly dependent on the concentration of ethylene (MacDonald et al., 2010, 2011a). The use of an ethylene synthesis or action inhibitor such as aminoethoxyvinylglycine or 1-methylcyclopropene negates the effect of ethylene and doubles the time required for abscission (MacDonald et al., 2010). Endogenous ethylene accumulation or exposure to exogenous ethylene subsequently increases cellulase activity in needle tissue by ≈10-fold, weakening the cell walls and promoting needle abscission (MacDonald, 2010, MacDonald et al., 2011a).
Although a portion of the abscission pathway is understood, the trigger for ethylene evolution is still not known. Initially, it was thought that mechanical wounding from harvest and shipping would trigger ethylene, as observed in other species (O’Donnell et al., 1996). However, the considerable 1-week lag time between balsam fir harvest and ethylene evolution observed by MacDonald et al. (2010, 2011b) has caused doubt on the role of mechanical stress-induced abscission in postharvest Christmas trees. A second theory is that postharvest water deficit induces ethylene evolution and abscission, which has also been demonstrated in several other species (Morgan et al., 1990; Taylor and Whitelaw, 2001). There is some evidence to suggest that postharvest balsam fir branches are experiencing water deficit, because xylem pressure potential (XPP) in balsam fir decreases after harvest and is negatively correlated with needle retention (MacDonald, 2010), but these studies did not directly explore the link between water status and ethylene evolution. If decreased water status is the cause, then minimizing transpiration (thus maintaining plant water status) by manipulating relative humidity or temperature should reduce or delay ethylene evolution and needle abscission in balsam fir. Thus, the objectives of this study were to 1) determine the effects of temperature and humidity, independently, on ethylene evolution and needle abscission in balsam fir; 2) to determine if temperature or humidity could offset the effect of ethylene on needle abscission; and 3) to link VPD to needle abscission.
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