THE IMPACT OF CONSTITUENT IONS OF ACID MIST ON ASSIMILATION AND STOMATAL CONDUCTANCE OF NORWAY SPRUCE PRIOR AND POST MIDWINTER FREEZING

Norway spruce seedlings were sprayed twice weekly with one of a range of artificial mists at either pH 2.5, 3.0 or 5.6, for three months. The mists consisted of either (NH4)2SO4 (pH 5-6), NH4NO3 (pH 5-6), water (pH 5-6), HNO3 (Ph 2.5), HSO4 (pH 2-5 or pH 3.0), or NH4NO3 Plus (NH4)2SO4 (pH 2-5). In late December 1988 and early January 1989 the light response of assimilation and stomatal conductance were assessed in the laboratory following a 4-day equilibration period at 12-degrees-C The intact trees were then subjected to a mild (-10-degrees-C), brief (3 h) frost in the dark and the recovery of light saturated assimilation (A(max)) was followed during the subsequent light period The same trees were then subjected to a second 3 h (-18-degrees-C) frost. The recovery of A(max) during the next day was followed All ion-containing mists stimulated A(max) and apparent quantum yield relative to control trees, irrespective of pH. The mists containing SO4 made stomatal conductance unresponsive to light flux density and caused the stomata to lock open. Frosts of -10-degrees-C and -18-degrees-C did not inhibit the A(max) of control trees for longer than 200 min into the light period In contrast, the ion-containing mists exerted a significant inhibitory effect upon the recovery of A(max). Nitric acid inhibited A(max) to 35% of the pre-frost value, whilst the remaining treatments inhibited A(max) between 15% and 40% of the pre-frost value. It is concluded that SO4 causes increased mid-winter frost sensitivity and NO3 ameliorates this effect. The results are discussed in relation to forest decline.