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Effects of boundary layer conductance on substomatal pressures of carbon dioxide

Abstract. Gas exchange measurements were made on single leaves of three C3 and one C4 species at air speeds of 0.4 and 4.0 m s−1 to determine if boundary layer conductance substantially affected the substomatal pressure of carbon dioxide. Boundary layer conductances to water vapour were 0.4 to 0.5 mol m−2 s−1 at the lower air speed, and 1.2 to 1.5 mol m−2 s−1 at the higher air speed. Substomatal carbon dioxide pressures were about 5 Pa lower at low boundary layer conductance in the C3 species, and about 3 Pa lower in the C4 species when measurements were made at high and moderate photosynthetic photon flux densities. No evidence of stomatal adjustment to altered boundary layer conductance was found. Photosynthetic rates at high photon flux densities were reduced by about 20% at the low air speed in the C3 species. The commonly reported values of substomatal carbon dioxide pressure for C3 and C4 species were found to occur only when measurements were made at the higher air speed.