The productivity of 'Vitis vinifera' L. Chardonnay and Cabernet Franc grapevines growing on trellising systems in the Niagara Peninsula, Canada, and the performance of interior shaded leaves in those canopies was investigated. The hypothesis tested was that grapevines on trellising systems containing a large proportion of leaves under low light stress were at a disadvantage, and the carbon export cost of supporting those leaves would result in reduced fruit yield, composition, and long-term survival of the vine. The low cordon system generally had the highest leaf layer number while vertiko and Scott Henry had the lowest. Pendelbogen, 2-tier flatbow, and 4-cane kniffin produced reasonably consistent growth and fruit composition across years and cultivars. The low cordon, vertiko, and Scott Henry systems produced variable growth and fruit of variable composition. Canopy density and light penetration to the canopy interior differed between trellising systems; however, leaf anatomy and morphology of interior and exterior leaves generally did not differ in a field setting. Leaf size, volume, volume of internal air spaces, volume of liquids and solids, and chlorophyll content all decreased when greenhouse-grown vines were subjected to severe shading (90 or 99%), but these characters were not affected by moderate shading (54%). Only leaf fresh weight, density, and thickness were reduced by moderate shading. Severe shading also reduced total plant dry weight and root to shoot ratio (i.e. reducing root weight by 84%). Light compensation point was reduced by 49% by moderate shading and 61% by severe shading. Vegetative shoots with a light-adapted leaf which had been fed 14CO2 translocated 26.1% more radioactivity to the roots and 12.7% more radioactivity to the trunk than shoots where a shade-adapted leaf was fed 14CO2. Light-adapted leaves exported 1.3% of labelled photosynthate to shaded shoots, while only 0.3% of labelled photosynthate was translocated from dark-adapted leaves to light-adapted shoots. When a vine contained multiple dark-adapted shoots, dark-adapted leaves fixed more 14C into the ethanol-insoluble fraction than light-adapted leaves. These experiments indicate the likely existence of a carbon export cost of having a large proportion of leaves in a canopy under low light stress.