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Summary• Whole-plant morphology, leaf structure and composition were studied together with the effects of light orientation on the dorso-ventral regulation of photosynthesis and stomatal conductance in Paspalum dilatatum cv. Raki plants grown for 6 wk at either 350 or 700 µl l -1 CO 2 .• Plant biomass was doubled as a result of growth at high CO 2 and the shoot:root ratio was decreased. Stomatal density was increased in the leaves of the high CO 2 -grown plants, which had greater numbers of smaller stomata and more epidermal cells on the abaxial surface.• An asymmetric surface-specific regulation of photosynthesis and stomatal conductance was observed with respect to light orientation. This was not caused by dorso-ventral variations in leaf structure, the distribution of phospho enol pyruvate carboxylase (PEPC) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) proteins or light absorptance, transmittance or reflectance.• Adaxial/abaxial specification in the regulation of photosynthesis results from differential sensitivity of stomatal opening to light orientation and fixed gradients of enzyme activation across the leaf.Key words: abaxial/adaxial leaf specification, CO 2 enrichment, phospho enol pyruvate carboxylase (PEPC), photosynthesis, ribulose 1,5-bisphosphate carboxylase/ oxygenase (Rubisco), stomatal patterning.

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