1. In certain conditions, ethylene may promote or reverse needle abscission in root-detached balsam fir M.T. MacDonald, R.R. Lada, A.I. Martynenko, M. Dorais, S. Pepin, Y. Desjardins Background Objectives Results: Short-term exposure to exogenous ethylene The balsam fir is typically found in eastern and central Canada and is the principal Christmas tree species grown in Atlantic Canada, preferred by consumers for its unique fragrance and needle characteristics. In Nova Scotia alone there is approximately 25,000 acres grown, with about 1.5 – 2.0 million trees harvested annually valued at approximately $30 million. In all of Atlantic Canada, over 3 million trees are harvested, which is worth $72 million annually (CTCNS 2009). Accelerated needle loss is presenting a major challenge to Christmas tree growers due to zero tolerance from consumers towards needle loss and increased competition from artificial trees. The precise mechanism through which needle abscission occurs in root-detached balsam fir is unknown, but it is speculated that ethylene (known to induce abscission in a variety of other species) is the signal molecule. (1) To determine whether ethylene can induce needle abscission in balsam fir (2) To determine whether the duration of ethylene exposure can influence the effect of ethylene Methods All branches were collected from the Tree Breeding Center in Debert, Nova Scotia. Two experiments were conducted to test effects of 0, 10, 100, 500, and 1000 ppm ethylene Discussion Ethylene exposure duration is an important component modifying abscission responses. A 24 h exposure period increased NRD by approximately 50 to 100%, while daily ethylene exposure decreased NRD by approximately 30 to 70%. Ethylene is known to stimulate several hydrolytic enzymes that weaken cell walls in the abscission zone, so it was not surprising that long-term exposure induced abscission in balsam fir. However, there is limited information available on ethylene delaying abscission.. It is known that continuous presence of ethylene is required to maintain an accelerated rate of weakening in abscission zones. For example, ethylene induced transcripts disappeared in bean leaf abscission zones when the partial pressure of ethylene was reduced using hypobaric chambers (Sexton et al. 1985). The abscission process not only halts if ethylene declines before a certain point, but will reverse and break strength increases (Abeles et al. 1992; Biggs 1971). It is suggested that a balance exists between cell wall synthesis and degradation in the abscission zone. A limited exposure to ethylene favors cell wall synthesis; prolonged exposure favors cell wall degradation. Results: Long-term exposure to exogenous ethylene Short-term exposure to ethylene delayed needle abscission in balsam fir. Branches treated with 500 ppm and 1000 ppm ethylene for 24 hours had the lowest percentage needle loss after 70 days (13.6% and 39.3% respectively) than the control (70.3%). Needle retention duration (NRD), the time it takes to lose 50% of mass as needle abscission, produced similar results. All concentrations of ethylene performed better than the control. There were no significant differences in average daily water use (0.05 mLg -1 d -1 ), chlorophyll index (12.07), or XPP (-0.33 MPa) in response to ethylene Figure 3. Needle retention duration of balsam fir branches after 24 hour exposure to ethylene Long-term exposure to ethylene accelerated needle abscission in balsam fir. The lowest concentration of ethylene tested (10 ppm) was enough to significantly decrease NRD, however 1000 ppm ethylene had the most profound effect.. Exposure to 1000 ppm ethylene significantly decreased NRD and XPP. Exposure to 1000 ppm ethylene significantly increased average daily water use. There was no significant influence on chlorophyll index (19.94) Table1. Significant changes to NRD, XPP, and water use after daily exposure to ethylene Conclusions (1) Ethylene can induce abscission in balsam fir and, thus, may be the signal molecule for abscission in post-harvest conditions (2) Short-term exposure to ethylene delays abscission. Long-term exposure to ethylene accelerates abscission Acknowledgements References Abeles FB, Morgan PW, Saltveit ME. 1992. Ethylene in Plant Biology. 2nd ed. Academic Press, San Diego, CA Biggs R. 1971. Citrus abscission. Hort Science. 6: 388-392 CTCNS. 2009. Christmas Tree Council of Nova Scotia. www.ctcns.com, accessed on June 23, 2009 Sexton R, Lewis LN, Trewavas AJ, Kelly P. 1985. Ethylene and abscission. In: Roberts JA, Tucker GA, (eds) Ethylene and plant development. London, UK: Butterworths, 173- 196 1. Branches are wrapped in cotton wool and placed in flasks supplied with 200 mL distilled water 2. Each branch and flask placed in an 80 L ethylene incubation chamber Short-term exposure Long-term exposure 3. Placed in growth chamber after 24-hour exposure 3. Exposed to ethylene daily in incubation chambers OR 0 10 1005001000 Figure 1. Balsam fir branches on day 70 after 24-h exposure to 0, 10, 100, 500, and 1000 ppm ethylene Figure 2. Relationship between percentage needle loss and exogenous ethylene concentration Treatment NRD (days) XPP (bars) Water Use (mL·g -1 ·d -1 ) Control (0 ppm) 35.4 ± 1.51.60 ± 0.190.05 ± 0.01 Ethylene (1000 ppm) 14.2 ± 0.24.20 ± 0.660.09 ± 0.01 Day 0Day 8 Day 11 Day 13 Figure 4. Progression of needle loss in balsam fir after daily exposure to ethylene Figure 5. Needle retention duration of balsam fir branches after daily exposure to ethylene 1000 ppm0 ppm Figure 6. Comparison of branches treated with 0 ppm and 1000 ppm ethylene daily after 13 days 0 ppm1000 ppm