Stress Physiology of Postharvest Balsam Fir Trees As Influenced By Shaking and Baling

It has been established that environmental conditions such as temperature and humidity have significant bearing on postharvest quality of Christmas trees. However, much is not known on stress physiology of Balsam fir Christmas trees subjected to various postharvest handling processes such as shaking and baling. It was, therefore, hypothesized that shaking and baling of balsam fir trees induce mechanical stress, causing membrane injury, stomatal closure, and increase in tree temperature under storage and transportation conditions leading to the trigger of ethylene and volatile terpene compounds (VTCs), promoting postharvest needle abscission. In all, fifty, six-year old trees, 90cm in height with similar girth were used. Twenty five trees were immediately subjected to shaking treatments of 0, 15, 30 and 60 seconds. Remaining 25 were baled at 0, 1, 2, 3, and 4 trees per bale. Tree core temperature was monitored using Thermal Infrared Imaging Technology, membrane injury of trees were studied by estimating membrane injury index (MII). Ethylene and VTC evolution were studied using designed air-tight chamber for gas trapping, airtight syringe for ethylene and solid phase microextraction (SPME) kit for VTC extractions. Analysis of ethylene and VTC was accomplished using GC-FID. The study showed that shaking duration and tree baling have significant effect (p<0.001) on tree core temperature. Shaking and baling of trees increased temperature by 2.3 and 3.5%, respectively, compared to control. Baled trees exhibited 35.1% higher MII compared to shaken trees, with MII ranging between 14.54 and 28.08%. MII for shaken trees ranged between 14.5 and 20.7%, with 10 seconds shaken trees attaining the highest MII of 20.7%. The MII of shaken trees was 16.6% higher than the control. Shaking and baling of trees had a significant (p<0.001) effect on ethylene evolution.  5-fold increase in ethylene evolution (4.66 µL·g^-1·h^-1) in baled trees compared to control (0.99 µL·g^-1·h^-1) was observed. A significant increase (68.1%) in VTC when trees were baled compared to control, on the other hand we observed 18% VTC increase in controlled treatment compared to shaken trees. This study suggests that postharvest handling of trees plays significant physiological alterations possibly contributing to postharvest needle loss.

Key words: Christmas tree, shaking, baling, stress.