Balsam fir is a tree typically found in eastern to central Canada and northeastern United States (Burns and Honkala, 1990). Balsam fir is also the principal Christmas tree species grown in Atlantic Canada (a region that includes the provinces of Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland). Over three million trees are harvested annually from Atlantic Canada, valued at $72 million. Of these trees, over 80% of all balsam fir are shipped to the United States (CTCNS, 2009). Balsam fir trees are preferred by consumers for their unique fragrance, color, and needle density (Burns and Honkala, 1990). To meet demands, balsam fir harvest begins in mid-October, although trees are expected to remain in good condition until the end of December. Unfortunately, balsam fir trees suffer from inferior needle retention compared with many other species used as Christmas trees. It has been estimated that as many as one in three balsam fir trees harvested in October will not last until the end of December (MacDonald and Lada, 2008). Postharvest needle loss between October and December is threatening the Christmas tree industry, largely as a result of decreasing tolerance from consumers toward needle loss and increased competition from artificial trees (Davis, 1996).
Identifying the physiological mechanism of needle abscission in balsam fir has been difficult as a result of complications from several other factors. For example, there is considerable genetic variation in needle retention characteristics. An experiment by MacDonald and Lada (2008) screened 197 different balsam fir genotypes and found abscission occurred between 6 and 60 d, although it remained relatively consistent within each genotype. In addition, it was proposed that earlier harvest dates and warmer fall temperatures result in accelerated needle abscission in some species (Chastagner and Riley, 2003; Mitcham-Butler et al., 1988). This was later confirmed in balsam fir by comparing needle retention of cold-acclimated trees with non-cold-acclimated trees. Cold acclimation generally improved needle retention, although the degree of improvement also varied between genotypes (MacDonald and Lada, 2008).
Ethylene is a gaseous plant typically associated with abscission, although it is involved in several physiological processes. Ethylene is a regulator of seed germination, seedling growth, leaf and petal abscission, organ senescence, and stress response (Abeles et al., 1992). In broad terms, ethylene triggers senescence and influences plant growth and morphology (Bleecker and Kende, 2000). Ethylene evolution typically increases after stress conditions such as water deficit, chilling, or mechanical stress (Morgan and Drew, 1997). For example, ethylene evolution increased by 120% to 300% in jack pine (Pinus banksiana) and by 110% in white pine (Pinus strobus) under water deficit (Islam et al., 2003; Rajasekaran and Blake, 1999). As a result of the mechanical stresses associated with harvest and shipping, it has been speculated that balsam fir trees may be exposed to physiologically significant concentrations of ethylene, which accelerate abscission. Accelerated abscission resulting from ethylene occurs in several herbaceous species, including tomato, tobacco, and ornamental flowers (Aharoni et al., 1979; Bleecker and Kende, 2000; Gepstein and Thimann, 1981). It is believed that ethylene induces abscission by increasing production of several hydrolytic enzymes such as cellulase or polygalacturonase, which weaken cell walls of the abscission zone (Sexton and Roberts, 1982; Tucker et al., 1988). Ethylene has also accelerated senescence and abscission by decreasing auxin transport (thereby increasing the sensitivity of a plant to ethylene) and by producing the forces necessary to facilitate abscission (Beyer and Morgan, 1971; Wright and Osborne, 1974).
Only a few studies have been published regarding the role of ethylene in needle abscission of conifers, although it is speculated to work in a similar manner as other plants. In Norway spruce (Picea abies), ethylene increased linearly with needle damage and abscission (Wilksch et al., 1998). In silver fir (Abies alba), increases in ethylene production in response to disease infection were correlated with chlorophyll degradation followed promptly by accelerated abscission of needles (Fuhrer, 1985). Previous work with balsam fir has implicated continuous ethylene exposure as a signal to induce abscission after root detachment (MacDonald et al., 2010). Exogenous ethylene induced abscission after several days of exposure and inhibition of ethylene doubled the time required for abscission Curiously, abscission often occurs in less than 24 h of ethylene exposure in other species (Brown, 1997) prompting investigations into possible benefits of acute ethylene exposure. A preliminary experiment with balsam fir suggested a 24-h exposure time in balsam fir does not induce abscission but may be beneficial in delaying abscission (MacDonald et al., 2009). It was hypothesized that acute exposure to ethylene may precondition balsam fir to future ethylene responses. Thus, the objective of this study was to investigate the effects of both short- and long-term ethylene exposures on needle abscission of 2-year-old balsam fir branches.
Azuma, T., Hatanaka, T., Uchida, N. & Yasuda, T. 2003 Enhancement of transpiration by ethylene is responsible for absence of internodal elongation in floating rice at low humidity J. Plant Physiol. 160 1125 1128
Burns, R.M. & Honkala, B.H. 1990 Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654 U.S. Department of Agriculture, Forest Service Washington, DC 2 877
Islam, M.A., Blake, T.J., Kocacina, F. & Rajasekaran, L. 2003 Ambiol, spermine, and aminoethoxyvinylglycine prevent water stress and protect membranes of Pinus strobus L. under drought Trees (Berl.) 17 278 284
Jacob-Wilk, D., Holland, D., Goldschmidt, E.E., Riov, J. & Eyal, Y. 1999 Chlorophyll breakdown by chlorophyllase: Isolation and functional expression of the Chlase1 gene from ethylene treated Citrus fruit and its regulation during development Plant J. 20 653 661
MacDonald, M.T. & Lada, R.R. 2008 Cold acclimation can benefit only the clones with poor needle retention duration (NRD) in balsam fir HortScience 43 1273 (abstr.).
MacDonald, M.T., Lada, R.R., Martynenko, A.I., Dorais, M., Pepin, S. & Desjardins, Y. 2009 Ethylene modulates needle abscission in root-detached balsam fir HortScience 44 1142 (abstr.).
MacDonald, M.T., Lada, R.R., Martynenko, A.I., Dorais, M., Pepin, S. & Desjardins, Y. 2010 Ethylene triggers needle abscission in root-detached balsam fir Trees 24 879 886
Merritt, F., Kemper, A. & Tallman, G. 2001 Inhibitors of ethylene synthesis inhibit auxin-induced stomatal opening in epidermis detached from leaves of Vicia faba L Plant Cell Physiol. 42 223 230
Mitcham-Butler, E.J., Hinesley, L.E. & Pharr, D.M. 1988 Effect of harvest date, storage temperature, and moisture status on postharvest needle retention of Fraser fir J. Environ. Hort. 6 1 4
Patterson, S.E. & Bleecker, A.B. 2004 Ethylene-dependent and -independent processes associated with floral organ abscission in Arabidopsis Plant Physiol. 134 194 203
Rajasekaran, L.R. & Blake, T.J. 1999 New plant growth regulators protect photosynthesis and enhance growth under Jack pine seedlings J. Plant Growth Regul. 18 175 181
Sexton, R., Lewis, L.N., Trewavas, A.J. & Kelly, P. 1985 Ethylene and abscission 173 196 Roberts J.A. & Tucker G.A. Ethylene and plant development Butterworths London, UK
Sisler, E.C., Serek, M., Dupille, E. & Goren, R. 1999 Inhibition of ethylene responses by 1-methylcyclopropene and 3-methylcyclopropene Plant Growth Regulat. 27 105 111
Tanaka, Y., Sano, T., Tamaoki, M., Nakajima, N., Kondo, N. & Hasezawa, S. 2005 Ethylene inhibits abscisic acid-induced stomatal closure in Arabidopsis Plant Physiol. 138 2337 2343
Wilksch, W., Schmitt, V. & Wild, A. 1998 Ethylene-biosynthesis in conifers: Investigations on the emission of ethylene and the content of ACC and MACC in Norway spruce (Picea abies) and silver fir (Abies alba) Chemosphere 36 883 888
Wright, M. & Osborne, D. 1974 Abscission in Phaseolus vulgaris. The positional differentiation and ethylene induced expansion growth of specialized cells Planta 120 163 170