1. Objectives To determine if observed genotypic variability in needle retention duration (NRD) of balsam fir can be attributed to differences in ethylene evolution or sensitivity of ethylene response Background Ethylene has recently been shown to function as a signal molecule in needle abscission of balsam fir. Endogenous ethylene increases approximately 2 days before abscission, exogenous ethylene induces abscission, and inhibiting ethylene action delays abscission (MacDonald et al. 2010). It has also been shown that there is a lot of genotypic variability in needle abscission. For example, one genotype may lose needles in 6 days while another may take 60 days (MacDonald and Lada 2008). It is hypothesized that genotypic differences in ethylene evolution or sensitivity may explain needle abscission patterns in different genotypes. Experimental Design The experiment followed a 2 x 7 factorial design with 4 replicates. Factor 1: Genotype (low or high needle retention) Factor 2: Continuous ethylene exposure (0, 10, 50, 100, 250, 500, or 1000 ppm) Response variables were ethylene evolution and needle retention duration (NRD), which was defined as the number of days for complete needle shed. Conclusions Genotypes characterized as having low NRD were found to release endogenous ethylene 47% faster than high NRD genotypes. Initial analysis suggested no difference in ethylene sensitivity, but further analysis with regression detected that poor NRD genotypes respond more dramatically to low concentrations of exogenous ethylene Results – Ethylene Evolution There was no difference in ethylene evolution rates due to exogenous ethylene concentrations. However, low needle retention genotypes evolved ethylene significantly faster. Is there a relationship between ethylene evolution, ethylene sensitivity, and needle abscission in root- detached balsam fir? MacDonald MT 1, Lada RR 1, Martynenko AI 1, Dorais M 2, Pépin S 3, Desjardins Y 3 Methods 1. All branches were from a clonal orchard in Debert, NS and then transferred to a growth chamber. 2. Each branch is wrapped in cotton gauze to increase stability and limit evaporation 3. Each branch is placed in a 250 mL flask and supplied with 200 mL of distilled water 4. Each flask is placed in a sealed chamber to measure ethylene concentrations Results – Ethylene Sensitivity Continuous exposure to ethylene at any concentration decreased NRD, though 1000 ppm was most effective. It was determined that the slope of the linear equation for low needle retention genotypes was significantly steeper (P = 0.038) than the slope of high needle retention genotypes. Day 15: 0 ppm Day 15: 1000 ppm Discussion Ethylene is able to induce abscission in balsam fir at concentrations as low as 10 ppm. In addition, there is a greater response to low concentrations of ethylene in genotypes known to have poor needle retention characteristics. Finally, low needle retaining genotypes release ethylene approximately 50% faster. The combination of increased ethylene evolution and sensitivity help us understand why needle retention is so consistently poor in certain genotypes. References MacDonald and Lada RR. 2008. Cold acclimation can benefit only the clones with poor needle retention duration (NRD) in balsam fir. HortScience. 43: 1273 MacDonald MT, Lada RR, Martynenko AI, Dorais M, Pepin S, Desjardins Y. 2010. Ethylene triggers abscission in root-detached balsam fir. Trees. IN PRESS 1 Department of Plant and Animal Sciences, Nova Scotia Agricultural College, Bible Hill, NS, B2N 5E3 2 Agriculture and Agri-Food Canada, Université Laval, Quebec, QC, G1V 0A6 3 Horticultural Research Center, Université Laval, Quebec, QC, G1V 0A6