Seasonal variation of abscisic acid in needles of Pinus cembra L. at the alpine timberline and possible relations to frost resistance and water status

The seasonal variation of abscisic acid (ABA) and frost resistance was studied in needles of five Pinus cembra trees at the alpine timberline at Mt. Patscherkofel near Innsbruck. The mean ABA level was distinctly higher in the winter than during the growing season. While frost resistance remained constantly high in winter, concentration of ABA varied considerably for trees and sampling dates. It is suggested that the high ABA level during the cold season is responsible for a continuous closure of stomata. Two hypotheses are put forward to explain the high ABA concentration in needles during winter: (a) Stress during diurnal freeze-thaw cycles triggers ABA synthesis or (b) photodestruction of xanthophylls leads to an increased level of ABA in Pious cembra needles, possibly in combination with diminished ABA catabolism during winter. In both cases, variation in needle exposure would explain the large variation of ABA over time and among trees.