The influence of fruits on export and distribution of photosynthates was studied in Vitis vinifera L. cv. Tempranillo using 14C labelling. Also, the influence of water stress was analysed in fruiting and fruitless plants of Tempranillo and fruiting plants of cv. Alfonso Lavallée. In fruitless plants of Tempranillo, reserve organs (trunk, roots and lower shoot) represented 80% of total plant dry matter (DM), and imported up to 90% of the total 14C exported from the fed leaf. Therefore, the distribution pattern of photosynthates in these plants reflected mainly the sink size. However, the presence of fruits in Tempranillo strongly stimulated 14C export and changed the distribution pattern of assimilates. Fruits imported up to 70–80% of the total 14C exported, while representing only 25% of the total plant DM. Therefore, the strength of fruits as carbon sinks was independent of sink size, and it is discussed on the basis of a water potential gradient theory. Water stress caused a significant reduction of leaf water potential, photosynthesis and stomatal conductance, but caused only a slight, non-significant, decrease of carbohydrate export from the fed leaves, and did not affect the distribution pattern of 14C except in some minor fractions in Tempranillo. The 14C distribution into different fruit components was also unaffected. In contrast, in Alfonso Lavallée water stress resulted in a highly significant reduction of export, and an altered photosynthate distribution pattern. These differences could be due to the lower water potential attained in stressed Alfonso Lavallée plants.

Climate change threatens worldwide grapevine production, especially in Mediterranean areas. To assume this challenge, the replacement of plant material to choose one more adapted to the new environmental conditions has been proposed as one of the possible solutions. Thus, the study of genetic variability in water-use efficiency (WUE) is needed to guarantee the survival of viticulture in those critical areas. In this study, the variability of WUE in 23 Tempranillo clones growth in pots and submitted to well-watered conditions and moderate water stress was studied along two consecutive years. Leaf net photosynthesis (AN), stomatal conductance (g_s), and plant growth parameters were measured, in addition the instrisic WUE (AN/g_s), biomass production, and water consumed were calculated. Results show a clear genotype effect for most of the studied parameters, but, with an important year by year variability. We identified different clonal behavior in response to soil water availability, that permits to classify them as water-savers vs water-spenders. In general, there was no found relationship between leaf and plant WUE, even some genotypes as 232 or 1048 were coincident in higher leaf WUE showed highly productive in terms of biomass accumulation per unit of water applied.