COMBINED EFFECTS OF ACID MIST AND FROST DROUGHT ON THE WATER STATUS OF YOUNG SPRUCE TREES (PICEA-ABIES)

This investigation was focused on the effect of frost drought followin g acid mist exposure on the water status and CO2 assimilation of young spruce trees (Picea abies). Trees cultivated in special containers we re exposed to acid mist with pH 3 and a control mist with pH 5 for 10 weeks with three mist events per week during autumn. Frost drought: in dicating frozen soil with atmospheric conditions allowing transpiratio n, was simulated in spring for 6 weeks. The controls for frost drought treatments were treatments with humid non-frozen soil. Acid mist appl ication resulted in degradation of the epicuticular needle waxes with melted wax threads and larger lesions in the wax layer. Control needle s showed a well-developed epicuticular wax layer. Frost drought inhibi ted transpiration and CO2 uptake in both mist treatments almost comple tely due to stomatal closure. In contrast, trees showed considerable t ranspiration and CO2 uptake rates in the treatments with non-frozen so il. Needle water potential of trees (control soil) was normal (-0.60 M Pa), whereas water potential in trees of frozen soil was significantly lower, with -1.49 MPa in the control and -2.26 MPa in the acid mist t reatment indicating water stress. We suppose from this finding that th e deteriorated wax layer impaired the water retention capacity of need les and allowed higher cuticular transpiration compared with the contr ol treatment. Frost drought was associated with a large decrease in st arch concentration of needles paralleled by an increase of glucose and fructose concentration, which is suggested to have increased the osmo tic potential. This rise in osmotic potential obviously enabled plants of the frost drought treatment + control mist to maintain their turgo r which was not different from the turgor of needles from the non-froz en soil treatments. Needles of the frost drought + acid mist treatment , however, had a weak turgor, indicating that in this treatment a stro ng water stress prevailed. It is suggested that also under natural for est conditions frost drought preceded by acid mist exposure may cause severe forest damage and decline. (C) 1998 Elsevier Science B.V. All r ights reserved.