Absorption of SO2by Pecan (Carya illinoensis(Wang) K. Koch) and Alfalfa (Medicago sativaL.) and its Effect on Net Photosynthesis

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Abstract
Absorption rates of SO2 by pecan (Carya illinoensis (Wang) K. Koch) leaflets exposed to 2.6, 5.2, and 7.8 mg SO2 m−3 were measured over a 2 h period. SO2 was rapidly absorbed by the leaflets in all treatments during the initial 30–50 min; the rate of uptake decreased to a rather constant level thereafter. Total SO2 absorbed during the 2 h period was 15.6, 25.6, and 38.9 nmol cm−2 for the low, medium, and high SO2 concentrations, respectively. Reductions in net photosynthetic rates were proportional to ambient SO2 concentrations and total SO2 absorbed. Partial photosynthetic recovery occurred in all treatments during a 2 h post-treatment period and full recovery occurred during a 12 h dark period. Exposure to SO2 resulted in slight increases in stomatal and boundary layer resistances to CO2 and substantial increases in residual resistances. Absorption rates of SO2 by alfalfa (Medicago saliva L.) exposed to 5.2 mg SO2m−3 for 1 h were approximately double those of pecan exposed to the same ambient SO2 concentration. Alfalfa net photosynthetic rates were reduced 74% after 1 h exposure to 5.2 mg SO2 m−3 while a depression of 42% occurred in pecan.