THE EFFECT OF TIME OF EXPOSURE TO SULPHUR DIOXIDE ON POTASSIUM LOSS FROM AND PHOTOSYNTHESIS IN THE LICHEN, CLADINA RANGIFERINA (L.) HARM.
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abstract
SummarySamples of the lichen, Cladina rangiferina, were exposed to various concentrations of aqueous SO4 for 10 min, and 0.5, 1, 3, 6 and 12 h. K+‐efflux curves (K+ release versus ppm SO2(wt/wt, aqueous)) were constructed for each exposure period. In separate experiments, the photosynthetic 14C fixation rates were measured after the SO2 exposures. Plots of % total 14C fixation v. ppm SO2 (wt/wt, aqueous) were found to be linear for the 3 h and 6 h exposure periods, while plots of log (% total 14C fixation) v. ppm SO2 (wt/wt, aqueous) were linear for shorter exposures (10 min, 0.5 and 1 h). Threshold SO2 concentrations for each exposure time were evaluated by extrapolating the K+‐efflux curves, or their linear transformations, to zero K+ release. Similarly, the threshold SO2 concentrations that did not cause a reduction in 14C fixation (relative to control samples not exposed to SO2) were determined by extrapolation of the linear relationships mentioned. The threshold values obtained from the K+‐efflux data were in good agreement with those evaluated from fixation rates. Aqueous threshold SO2 concentrations were converted to gaseous threshold values by an experimentally verified relationship. The resulting gaseous SO2 threshold concentrations, (SO2)g** in units of ppm(v/v) in air, were related to the exposure time in hours, t, through the following rectangular hyperbolic function: (SO2)g**=1.5 t−0.62. Notwithstanding the uncertainties involved, extrapolation of this expression to exposure times considerably longer than the experimental range yielded threshold values for 6 months and 1 year in good agreement with mean SO2 levels known from mapping studies to affect detrimentally sensitive lichen species.
