In cultivated grapevines (Vitis vinifera L.), sub-optimal photoassimilatory conditions during flowering can lead to inflorescence necrosis and shedding of flowers and young ovaries and consequently, poor fruit set. However before this study it was not known if carbohydrate reserves augment fruit set when concurrent photoassimilation is limited. Carbohydrate reserves are most abundant in grapevine roots and soil temperature moderates their mobilisation. Accordingly, we grew potted Chardonnay grapevines in soil at 15°C (cool) or 26°C (warm) from bud-break to the onset of flowering to manipulate root carbohydrate reserve status. Then to induce photoassimilatory responses we subjected the plants to low (94 Âµmol mol-1) CO2 or ambient (336 Âµmol mol-1) CO2 atmospheres during fruit setting. Analyses of photoassimilation, and biomass and carbohydrate reserve distribution confirmed that fruit set was limited by concurrent photoassimilation. Furthermore, the availability of current photoassimilates for inflorescence development and fruit set was conditioned by the simultaneous demands for shoot and root growth, as well as the restoration of root carbohydrate reserves. The study indicates that great seasonal variability in grapevine fruit set is a likely response of cultivated grapevines to photoassimilatory stresses, such as shading, defoliation and air temperature, and to variations in carbohydrate reserve status prior to flowering.