Foldable Lyre and Vertical Shoot Positioning Training Systems on Physiology and Yield of ‘Merlot’ Grapevines Grown in a Humid Temperate Region

The strategic choice of training system is essential for adapting viticulture to current climate change, ensuring a balance of physiological efficiency and the sustainability of productivity and oenological quality. This study evaluated the effects of vertical shoot positioning and foldable lyre systems (set at angles of 20°, 30° and 40°) on the physiological performance and yield of ‘Merlot’ grapevines. The experiment was conducted in a humid temperate region in Brazil over two consecutive seasons. The experiment followed a randomized block design. The variables evaluated included: the number of clusters per shoot, cluster weight, pruning weight, Ravaz Index, leaf area and yield; gas exchange parameters such as net CO2 assimilation rate, stomatal conductance, transpiration rate, rubisco carboxylation efficiency, intercellular CO2 concentration and photosynthetic photon flux density; and chemical composition of berries such as pH, Total Soluble Solids and Titratable Acidity. The data were subjected to an analysis of variance, and the means were compared using Tukey’s test at a 5% probability level. The results indicated that canopy architecture significantly influenced solar radiation interception, with the 30° and 40° foldable lyre systems achieving the highest mean daily radiation levels, exceeding the vertical positioning system by 73.7% and 76.6%, respectively. Although gas exchange at the leaf level remained comparable across all systems, agronomic performance varied considerably. The 40° foldable lyre system achieved the highest yield (22.99 t ha−1), representing a 63.1% increase over the vertical positioning system (14.10 t ha−1). The number of buds in the foldable lyre systems increased by around 70%, which is closely in line with the observed increase in yield. In addition, the foldable lyre systems provided about 40% more leaf area than the vertical positioning system. These findings suggest that divided canopy systems, such as foldable lyre systems, particularly at 30° and 40°, optimize bud load, fruitfulness per shoot, light interception and significantly increase yield without compromising individual physiological efficiency and berry chemical composition, with a balance between vegetation and fruit load preserved and with positive effects on the ripeness and quality of the grapes.