Characterization of phytohormonal and postharvest senescence responses of balsam fir (Abies balsamea (L.) Mill.) exposed to short-term low temperature

To fulfill the US Thanksgiving and Christmas tree markets, balsam fir (Abies balsamea (L.) Mill.) is generally harvested before the cold season, anecdotally leading to premature needle senescence. Accordingly, we tested the hypothesis that LT exposure before harvest induces specific hormonal changes and delays postharvest senescence and/or abscission in balsam fir. Two hundred and six seedlings exposed to two temperature treatments for 48 h, LT at 5 A degrees C and controls at 22 A degrees C were severed off roots and monitored for their postharvest needle senescence. Root and shoot (needles and buds) tissues were examined for major endogenous hormone metabolites. LT increased shoot ABA (2,007 ng g(-1) DW) by 2.5x and decreased GA(44) (9.84 ng g(-1) DW) by 3.5x over those in roots. LT did not alter cytokinins, auxins or any root hormonal concentration. With auxins, only IAA, IAA-Asp, IAA-Leu and IAA-Glu were detected and the concentrations of IAA and IAA-Asp in shoots were lower than those found in roots. Among cytokinins, shoot c-ZR (58.95 ng g(-1) DW) and t-ZR (4.17 ng g(-1) DW) were 3x higher than those in roots. Apart from GA(44), GA(9) (136.76 ng g(-1) DW) was abundant in shoots. The PBL and PNL were 46 and 1.2 %, irrespective of treatments. LT seedlings held needles 11 days longer than the controls (122 days). In balsam fir, short-term LT exposure augmented ABA and decreased GA(44) levels in shoots and delayed postharvest needle senescence.