Humidity and Temperature Influence Ethylene Evolution and Needle Abscission In Balsam Fir

There has been increased attention placed on understanding postharvest needle abscission physiology in conifers. Recently, it has been shown that ethylene accumulation increases postharvest and culminates in needle abscission. However, the trigger for ethylene synthesis is still unknown. The purpose of this research was to investigate the link between water status and needle abscission in balsam fir. Two experiments were conducted to reach this objective. In the first experiment branches were exposed to 30, 60, or 90% humidity (vapour pressure deficit of 1.59, 0.91, or 0.23 kPa respectively); in the second experiment branches were exposed to 5, 15, or 25°C (vapour pressure deficit of 0.35, 0.68, and 1.26 kPa respectively) with growth chamber maintained at 60% relative humidity. Needle retention duration, average water use, xylem pressure potential, and ethylene evolution were the key response variables measured. Overall, it was found that reducing water loss by either lowering temperature or increasing humidity effectively delayed needle abscission, though the 90% humidity treatment had the most profound effects. In the absence of exogenous ethylene, branches placed in 90% humidity had a 5-fold increase in needle retention, 67% decrease in average water use, and had a final xylem pressure potential of -0.09 MPa. These findings demonstrate an effective means of controlling postharvest needle abscission by modifying temperature and/or relative humidity. In addition, these findings suggest that declining water potential may trigger ethylene synthesis and promote needle abscission, though discrepancies between observed and expected damage thresholds may suggest an unidentified underlying factor.