EFFECTS OF SOIL FREEZING AND DROUGHT STRESS ON ABSCISIC-ACID CONTENT OF SUGAR MAPLE SAP AND LEAVES

In 1991 and 1992, mature maple trees (Acer saccharum Marsh.) were free ze-stressed or drought-stressed by preventing precipitation (snow or r ain) from reaching the forest floor under selected trees. Lack of snow cover caused a decrease in soil temperature to well below 0-degrees-C from December to April and a lowering of the soil water content to 10 %. The abscisic acid (ABA) concentration in the spring sap of deep-soi l frost-stressed trees was significantly higher than in control or dro ught-stressed trees. The increase in ABA concentration in the xylem sa p in the spring of 1991 and 1992 preceded symptoms of canopy decline a nd a decrease in leaf area that were observed during the summers of 19 91 and 1992. These results suggest a role for ABA in root-to-shoot com munication in response to environmental stress. The largest difference s in ABA concentration induced by the treatments was found in sap coll ected at the end of sap flow. The increase in ABA concentration in spr ing sap at the end of the sap flow could be used as an early indicator of stress suffered by trees during the winter. Not only did the incre ase in ABA concentration occur before any visible symptoms of tree dec line appeared, but the trees that showed the most evident decline had the highest ABA concentrations in the spring sap. Leaf ABA concentrati on was not a good indicator of induced stress.