GROWTH AND ABA RESPONSES OF MAPLE SEEDLINGS TO ALUMINUM

We assessed the impacts of low pH and 2.0 mM aluminum (Al) on the grow th of sugar maple (Acer saccharum Marsh.) seedlings over a 13-week per iod. At Week 9, total leaf area of Al-treated seedlings was reduced by 27%; however, by Week 13, leaf area was similar for seedlings in all treatments. None of the other growth parameters examined were negative ly affected by the treatments at either Week 9 or Week 13. The ABA con centration in the xylem sap, which is an indicator of tree stress in t he field, was not affected by any of the treatments and was highest du ring periods of high evaporative demand in June and August. We conclud e that the duration of exposure to Al is critical when assessing a thr eshold concentration for Al toxicity because plants can acclimate to a n Al concentration previously considered toxic. Although Al stress did not appear to reduce the vigor of sugar maple seedlings directly, it could facilitate an inciting factor such as winter frost to induce tre e decline.