Phytohormones in needles of healthy and declining sliver fir (Abies alba Mill) .2. Abscisic acid

Abscisic acid (ABA) and trans-ABA (t-ABA) were quantified in needle extracts from silver fir (Abies alba Mill.) and spruce trees (Picea abies [L.] Karst.) using gas chromatography with electron capture detection. In current-year needles of fir trees in the Northern Black Forest, high concentrations of ABA (up to 126 nmol g dw(-1)) were found while needles elongated. Apart from this observation, levels of ABA exhibited no general seasonal trend. However, in autumn a <<short-term>> elevation in levels of ABA was detected. Therefore, it cannot be ruled out that other <<short-term>> alterations of levels of ABA, which have not been detected, do exist. Without water stress, ABA concentrations did not exceed 2 nmol g dw(-1) in mature needles and showed no clear variation with needle age. During water stress, levels of ABA increased up to about 19 nmol gdw(-1) with levels being highest in the youngest needles. t-ABA concentrations in fir needles were independent of needle ABA levels while levels of t-ABA in spruce needles rose with increasing levels of ABA. No differences were seen between healthy and declining fir trees, while trees were well supplied with water, but during water stress, levels of ABA in declining trees were always higher than in healthy trees. It is concluded that the factors that cause silver fir decline do not directly influence levels of ABA in the foliage but make trees more sensitive to water stress by affecting either the stomata or the root system.