ASSIMILATION AND STOMATAL CONDUCTANCE RESPONSES OF RED SPRUCE TO MIDWINTER FROSTS AND THE CONSTITUENT IONS OF ACID MIST

Red spruce (Picea rubens Sarg.) seedlings growing outside in open-top chambers were sprayed twice weekly with artificial mists at either pH 2.5 or 5.6, for five months during the 1988 growing season. The mists contained one of the following: (water, pH 5.6 (control); (NH4)2SO4, p H 5.6; NH4NO3, pH 5.6; HNO3, pH 2.5; H2SO4, pH 2.5; or (NH4)2SO4 + NH4 NO3, pH 2.5. During January 1989, the light responses of assimilation and stomatal conductance were assessed in the laboratory following a 4 -day equilibration at 12-degrees-C. The aerial portions of the intact trees were then subjected to a mild (-10-degrees-C) frost for three ho urs during the night and the rate of recovery of light-saturated assim ilation (A(max)) was determined die following day using the same branc hes as were used for the assimilation studies before the frost treatme nt. The same trees were then subjected to a second frost of -18-degree s-C for three hours during the following night and the recovery of A(m ax) of the same branches was measured die next day. All of the acid mi st treatments increased A(max) and apparent quantum yield relative to the control treatment when measured before die frost treatments. Frost s of -10 and -18-degrees-C resulted in a significant decline in A(max) of seedlings in all treatments except the control. Stomatal conductan ce increased with increasing irradiance in seedlings in the acid mist treatments that did not contain SO42- ion. Stomatal conductance of see dlings in acid mist treatments containing SO42- ion was insensitive to changes in irradiance over the range 50-1500 mumol m-2 s-1. It is con cluded that acid precipitation increased the sensitivity of the assimi lation response to midwinter frosts that follow a brief warm period. T he SO42- ion appears to be significant in causing increased sensitivit y to frost and in causing stomatal insensitivity to light flux density .