Sulfur dioxide inhibition of translocation in bean plants

Exposure of the source leaf of bean (Phaseolus vulgaris L. cv. Black Valentine) for 2 hours to 2.9 microliters per liter SO(2) 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 SO(2)-stressed leaves were lower than were the rates expected on the basis of the observed reductions in photosynthesis. It is inferred that, under SO(2) stress, the phloem-loading system becomes a major limiting step in controlling the translocation rate.Following removal of SO(2), 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 SO(2) does not appear to be due to an injury effect of SO(2), inasmuch as a similar effect was obtained by exposing the source leaf to a short (2-hour) interval of dark.