The effects on plant growth and stomatal physiology of alterations in light quantity and quality during development were investigated in the C3 monocot, Commelina communis. Reduction in light intensity resulted in decreased branching and stem elongation, with effects more severe under "neutral shade" (R:FR≥1.0) than under "leaf shade" (R:FR≤0.4) conditions. Shade treatments had no effect on the leaf area or stomatal density of newly expanded leaves. Gas exchange measurements on leaves that had expanded under the different treatments indicated that a reduction in light intensity decreased the magnitude and slowed the kinetics of stomatal responses to pulses of blue light, particularly in plants from the neutral shade treatment. These results indicate that the specific stomatal response to blue light is plastic, and is modulated by the light environment prevailing during leaf development.