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Sulfur Dioxide Inhibition of Translocation in Bean Plants ¹

Department of Botany, Ohio State University, Columbus, Ohio 43210

Exposure of the source leaf of bean (Phaseolus vulgaris L. cv. Black Valentine) for 2 hours to 2.9 microliters per liter SO₂ inhibited the net photosynthetic rate an average of 75% and, simultaneously, the translocation rate an average of 45%. Calculations indicated that the experimentally determined translocation rates from SO₂-stressed leaves were lower than were the rates expected on the basis of the observed reductions in photosynthesis. It is inferred that, under SO₂ stress, the phloem-loading system becomes a major limiting step in controlling the translocation rate.

Following removal of SO₂, photosynthesis recovered quite rapidly (to about 60% of its preexposure rate within 2 hours), but the translocation rate failed to increase during this time interval. This delayed response of translocation to removal of SO₂ does not appear to be due to an injury effect of SO₂, inasmuch as a similar effect was obtained by exposing the source leaf to a short (2-hour) interval of dark.

³ To whom reprint requests should be addressed.

¹ Supported by National Science Foundation Grant PCM 73-01313-A01 and by the Graduate School of Ohio State University.