Ethylene Modulates Needle Abscission In Root-Detached Balsam Fir

Post-harvest needle loss is a major problem for the Atlantic Canada’s Balsam fir Christmas tree and greenery industry. While the exact physiological trigger is not known, evidence from other herbaceous plants indicate that ethylene, a plant hormone, has been strongly suggested to induce abscission.  It is not known, however, whether ethylene triggers post-harvest needle loss in Balsam fir. It was hypothesized that ethylene induces abscission in Balsam fir and the intensity of needle loss relates to endogenous levels of ethylene. Two experiments were conducted to test the effects of ethylene on needle abscission, referred to as short-term and long-term. In the short-term experiment branches were exposed to ethylene at concentrations ranging from 0 ppm to 2000 ppm for 24 h. In the long-term experiment branches were exposed to 1000 ppm ethylene daily and compared to an untreated control. The response variables were needle retention duration (NRD), water use (WU), and xylem pressure potential (XPP) for each experiment and were submitted to an analysis of variance (ANOVA) to detect significant differences.  Contrary to our expectations, short-term exposure to ethylene delayed needle abscission. A concentration of 1000 ppm ethylene significantly (p < 0.05) increased NRD by 80%, but had no effect on WU or XPP. Long-term exposure to ethylene, however, accelerated needle abscission. Branches continuously exposed to 1000 ppm ethylene began losing needles on day 5 and completed needle shed was observed on day 14, compared to day 35 for the control. There was a 60 % decrease in NRD and 160 % decrease in XPP as a result of long-term exposure to ethylene. Also, daily water use increased after day 5 in those branches exposed to ethylene. Overall, the results suggest that ethylene may be the signaling molecule to induce needle abscission in Balsam fir. It is also suggested that short-term exposure to ethylene might help to precondition Balsam fir and delay needle abscission.