Morpho-structural and physiological traits of Sangiovese and Montepulciano varieties (Vitis vinifera L. - two red grapes widely cultivated in Italy), grown outside under non-limiting water supply conditions were evaluated in 2007 and 2008 and results were correlated with yield components and grape composition. The 2-year analysis showed intraspecific differences in canopy characteristics, leaf and shoot properties, photosynthetic ability, water use efficiency, vine yield and grape composition. Compared with Sangiovese, Montepulciano was able to assure a higher whole-canopy seasonal net CO2 exchange rate during the season (+38% in mid morning and +49% in mid afternoon). It also had higher water use efficiency (especially early in the morning and in late afternoon) and a higher vine yield (+16%). Furthermore, total soluble solids (+1.7 degrees Brix), anthocyanins (+0.44 mg cm(-2) berry skin) and phenolic compounds (+0.88 mg cm(-2) berry skin) were higher in the grapes. To ensure this performance, Montepulciano vines have to support higher costs of growth and maintenance processes, made possible because of the increased respiration activity of the canopy during the night. We confirmed that vine yield and grape composition is strictly dependent on the seasonal photosynthetic capacity of the canopy. Therefore, Montepulciano should be put in a position to fully realise this substantial photosynthetic potential, by avoiding or reducing environmental stress. Sangiovese is structurally and morpho-physiologically better able to withstand any stress during the summer than Montepulciano. Sangiovese xylem tissue had larger mean vessel density and smaller mean vessel diameter and hydraulic conductance than Montepulciano, holding the hypothesis of less susceptibility to conduit damage.