Changes in Endogenous Hormone Levels Explains Seasonal Variation in Balsam Fir Postharvest Needle Abscission

There are significant variations in postharvest characteristics of balsam fir trees, include the dynamics of abscission, water uptake, moisture content, and fluorescence. Changes in endogenous hormone levels in roots and needles are postulated to be related to these postharvest characteristics. Samples were obtained from 6 trees each month from April 2013 to November 2013 and evaluated for retention, chlorophyll fluorescence, water content, and water uptake. In addition, root and needle tissue were analyzed for abscisic acid, cytokinins, auxins, and gibberellins and their conjugates. Postharvest characteristics all improved in autumn as compared to spring, with a 3.5-fold increase in time to needle abscission commencement and 1.75-fold increase in time until complete abscission. There were several changes to root and needle hormones with respect to harvest time, but fewer hormones were linked to postharvest needle retention. Among all hormones analyzed, both isopentenyladenine and isopentenyladenosine in roots were strongly linked to needle retention. When isopentenyladenine was present in roots, needle abscission took 114% longer to begin and 58% longer to finish. Isopentenyladenosine in roots was the strongest hormonal predictor of needle retention (R² = 79%). Dihydrozeatin riboside (R² = 37%), trans-abscisic acid (R² = 50%), and isopentenyladenosine (R² = 59%) in the needles were also strong predictors of needle retention. The relative effectiveness of certain hormones in predicting needle retention and speculation pertaining to the roles these hormones could play are discussed.

Keywords: Abscisic acid, Abies balsamea, auxins, cytokinins, fluorescence, needle retention, postharvest, water content, water uptake