FIG. 3. Stomatal responses to CO2 in leaves of X. strumarium at four
irradiance levels. Photosynthesis was allowed to proceed uninhibited. Each
curve represents the average behavior of the sum of the conductances of
the abaxial and adaxial leaf surfaces of four leaves.
grown in the fall of 1978 possessed stomata that were sensitive to
CO2 even without a supply of exogenous AbA (Fig. 2). The
reduction of stomatal sensitivity to CO2 caused by increased
irradiance was greatest at CO2 concentrations that normally occur
in leaves of X strumarium (150 to 250 IlI 1-'). Stomata of leaves
used in later experiments (Table II, Fig. 3) did not respond to
CO2 in strong light, in agreement with earlier observations (15)
but in darkness or low irradiance, they did respond to CO2 (Fig.
3). Decreasing stomatal sensitivity to CO2 with increasing irradiance
was also observed in P. frutescens and P. vulgaris but not in
Z. mays (Fig. 4).