Exploring the Impact of Vascular Alignment and Grafting on Grapevine Physiology and Growth

Grafting has been fundamental in viticulture since the phylloxera crisis of the late 19th century; nevertheless, the functional consequences of vascular connection on the graft union remain poorly understood. The effects of grafting on Vitis vinifera cv. Tempranillo (Te) were evaluated using two complementary approaches: (1) cambial alignment, comparing completely aligned (CA) versus partially aligned (PA) unions; and (2) grafting and scion-rootstock interaction, comparing heterografts (Te/110R and Te/RG8), homografts (Te/Te), and ungrafted Te cuttings. These approaches were tested through three experiments: a vineyard trial and two pot trials under well-watered (WW), moderate water stress (MWS), and recovery (R) regimes. In the vineyard, CA plants exhibited greater vegetative growth and gas exchange, particularly on 110R, whereas the vigorous RG8 rootstock mitigated the effects of misalignment. Under MWS conditions, CA adopted a drought-avoidant strategy with earlier stomatal closure and higher root allocation, whereas PA maintained higher stomatal conductance, recovered photosynthesis faster after rewatering, and prioritised shoot and rootstock growth, especially on RG8. Finally, grafted plants were more sensitive to water stress than ungrafted plants, while homografts accumulated the greatest biomass and root investment, suggesting more efficient vascular connectivity compared with heterografts. Our study highlights that cambial alignment, grafting, and partner interactions influence plant development and physiological performance; however, long-term studies are needed to clarify how vascular connectivity at the graft union affects transport processes, stress responses, and ultimately vine longevity under different scion-rootstock combinations.