INFLUENCE OF ACIDIC MIST ON FROST HARDINESS AND NUTRIENT CONCENTRATIONS IN RED SPRUCE SEEDLINGS .1. EXPOSURE OF THE FOLIAGE AND THE ROOTINGENVIRONMENT

Two-year-old red spruce (Picea rubens Sarg.) was grown in replicated o pen-top chambers supplied with charcoal--filtered air near Edinburgh, Scotland. Between May and November 1989, plants were exposed to four m ist treatments, three containing sulphuric acid and ammonium nitrate i n equimolar concentrations at 0.005 mol m-3 (pH 5) or 1.0 mol m-3 (pH 2.7), and a fourth treatment with sulphuric acid alone at 1-0 mol m3 ( equivalent to 2 mm precipitation). Two dose rates were used for the pH 2.7 treatment equivalent to 2 and 8 mm of rain per week. Three subtre atments (soil surface exposed to mist, addition of extra sulphuric aci d to the soil surface, exclusion of mist from the soil) were included in each chamber. Frost hardiness was assessed by measuring rates of el ectrolyte leakage after controlled freezing of detached shoots. At the end of October, frost hardiness, expressed as the lethal temperature for 50 % of shoots (LT50), was decreased by 8-degrees-C in the 8 mm wk -1 treatment at pH 2.7, compared to pH 5. The 2 mm wk-1 treatment at p H 2.7 had no effect on frost hardiness either when ammonium nitrate wa s present or absent (i.e. sulphuric acid only). Excluding mist from th e soil, and adding extra sulphuric acid, both increased frost hardines s by about 3-degrees-C when compared with uncovered soil. Excluding mi st from the soil increased the amount of foliage initiated and produce d inside the chambers but neither subtreatment, excluding the mist nor providing additional sulphuric acid to the soil affected foliar nutri ent concentrations. Mist of pH 2.7 as sulphuric acid alone and in comb ination with ammonium nitrate both enhanced N uptake. Several observat ions concerning the effect of acidic mist on frost hardiness were conf irmed by this study: (i) preventing mist from reaching the soil/roots, improving conditions for root growth can ameliorate the effects of ac idic mist on shoot growth and frost hardiness; (ii) the effect was det ermined by the ion dose but not by the ion concentration in the mist; (iii) the effect was primarily mediated through foliar absorption; (iv ) the presence of high foliar nitrogen concentrations did not increase frost hardiness when foliar sulphur concentrations were also high; (v ) low N concentrations were more important for frost hardiness than hi gh foliar N concentrations.