FUMIGATING MATURE SPRUCE BRANCHES WITH SO2 - EFFECTS ON NET PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE

Branches on mature red spruce trees (Picea rubens Sarg.) were fumigate d with SO2 in late summer of 1990 and 1991. There were four SO2 treatm ent levels (0, 0.1, 0.2, and 0.4 ppm) and 12 fumigation chambers. Each treatment was replicated on three neighboring trees. Net photosynthes is and stomatal conductance of current-year foliage were determined wi th a Li-Cor photosynthesis system. Rates of foliar SO2 absorption were estimated using regression models, with leaf temperature, relative hu midity, atmospheric CO2 concentrations, and photosynthetically active radiation as predictors. Fumigation effects became visible in terms of needle yellowing and browning after a few weeks of treatment. Effects on net photosynthesis and stomatal conductance were generally negativ e and in direct proportion with cumulative foliar SO2 absorption. It w as found that, in shade-adapted foliage, visible symptoms were delayed , and stomatal conductance was slightly stimulated at low SO2 levels. In other foliage, leaf yellowing and browning occurred earlier, and st omatal conductance was depressed. Net photosynthesis was depressed at about the same rate regardless of branch position. Foilage subjected t o high SO2 concentration treatments did not recover from SO2 damage 1 year after treatment: needles had fallen off the twigs, and needle and twig length of new foliage was reduced.