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

Abstract

Postharvest needle abscission in balsam fir varies considerably depending on the season and may be dependent on endogenous hormones of roots or shoots. Samples were obtained from three trees each month from April 2013 to November 2013 and evaluated for retention. In addition, root and needle tissues were analyzed for abscisic acid, cytokinins, auxins, and gibberellins and their conjugates. There was a 3.5-fold increase in time until needle abscission commencement and 1.75-fold increase in time until complete abscission in autumn compared to spring. 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.