Recovery of net photosynthesis and dark respiration on rehydration of the lichen, Cladina mitis, and the influence of prior exposure to sulphur dioxide while desiccated

SUMMARY The pattern of resaturation respiration and of rehydration recovery of net photosynthesis has been examined after pretreatment of desiccated thalli of the mat‐forming lichen species Cladina mitis (Sandst.) Hale & Culb, with sulphur dioxide. The recovery of photosynthetic activity lagged behind that of dark respiration in all treatments, pointing to the necessity of monitoring both processes when investigating rehydration phenomena. The exposure of desiccated thalli to sulphur dioxide concentrations of 0·2 and 1·0 μ11 −1 (v/v in air) for periods of 2 and 3 weeks prior to rehydration resulted in the enhancement of subsequent photosynthetic activity in all treatments. These effects were greatest in the 0·2 μ11 −1 treatment where, after 2 weeks fumigation, rates of net photosynthesis on recovery were nearly 40% higher than that of control replicates receiving no SO 2 . These results point to the importance of considering the effects of dry deposition on physiological processes and suggest that in some cases limited SO 2 exposure may result in short‐term enhancement of photosynthetic uptake.