Influence of Mount St. Helens volcanic ash on growth and abscisic acid in Douglas-fir seedlings

Seedlings of Douglas-fir ( Pseudotsuga menziesii (Mirb) Franco) received an accumulation of ash, which was collected from an eruption of Mt. St. Helens, WA. Root and shoot dry weights were lower in ashcovered plants. Ash-treated seedlings also had smaller leaf dimensions, smaller stem diameters, and greater chlorophyll contents. No significant difference was seen in height. Morphological features and chlorophyll content suggest that shading is the primary effect of a thin ash cover. ABA concentration was significantly greater in ash-covered shoots (2.40) compared with control shoots (1.49), on a mg/kg, dry weight basis. Water potential was greater in ash-covered shoots than in controls, but a regression of ABA on the water percentages in root, shoot, and vermiculite showed only a significant negative correlation between vermiculite moisture and ABA of ash-covered and control roots, collectively. Forty percent of the variance in ABA content of roots was explained by the level of moisture in the environment of the root.