Sulfur Dioxide Flux into Leaves of Geranium carolinianum L. : Evidence for a Nonstomatal or Residual Resistance

Abstract

The concurrent exchange of SO(2) and H(2)O vapor between the atmosphere and foliage of Geranium carolinianum was investigated using a whole-plant gas exchange chamber. Total leaf flux of SO(2) was partitioned into leaf surface and internal fractions. The emission rate of SO(2)-induced H(2)S was measured to develop a net leaf budget for atmospherically derived sulfur. Stomatal resistance to SO(2) flux was estimated by two techniques: (a) R(s) (SO(2)') from SO(2) data using analog modeling techniques and (b) R(s) (SO(2) ) from analogy to H(2)O (i.e. 1.89 R(s) (H(2)o)).The emission of H(2)S was positively correlated with the rate of SO(2) flux into the leaf interior. An accounting of the simultaneous, bidirectional flux of gaseous sulfur compounds during pollutant exposure showed that sulfur accumulation in the leaf interior of G. carolinianum was 7 to 15% lower than that estimated solely from mass-balance calculations of SO(2) flux data (i.e. ignoring H(2)S emissions).The esimate of stomatal resistance to pollutant flux from the SO(2) data (R(s) (SO(2)')) was consistently less than the simultaneous estimate derived from analogy to H(2)O vapor (R(s) (SO(2) )). The resultant of R(s) (SO(2)') - R(s) (SO(2) ), which was always negative, is indicative of a residual resistance to SO(2) flux into the leaf interior. On a comparative basis, SO(2) molecules experienced less pathway resistance to diffusion than effluxing H(2)O molecules. It is proposed that the SO(2):H(2)O path length ratio is less than unity, as a consequence of the pollutant's high water solubility and unique chemical reactivity in solution. Thus, the diffusive paths for H(2)O and SO(2) in G. carolinianum are not completely synonymous.