Uptake Efficiency of Nitrogen Forms and Kinetic Parameters of Grafted ‘Chardonnay’ and ‘Cabernet Sauvignon’ Vines

Nitrogen (N) is a key nutrient for grapevine development, influencing from biomass formation to photosynthetic efficiency and grape quality. However, despite the widespread adoption of grafted plants in modern viticulture, understanding of how different scion–rootstock combinations modulate the uptake of different forms of N present in the soil remains limited. In this context, assessing the nutrient uptake efficiency of grapevines can be a strategy for selecting efficient cultivars, especially in nutritionally poor environments. This study aimed to assess the uptake efficiency of N forms by ‘Chardonnay’ and ‘Cabernet Sauvignon’ grafted onto rootstocks ‘IAC 572’, ‘Paulsen 1103’ and ‘SO4’. Vines were subjected to Hoagland’s nutrient solution at 50% total strength for 21 days, followed by nutrient depletion and a 72 h kinetic uptake assay. Morphological, physiological, biochemical and uptake-related parameters—Vmax, Km, Cmin and influx (I)—were assessed. ‘Chardonnay’ grafted onto the ‘IAC 572’ rootstock was the most efficient in the uptake of both NO3− and NH4+, as it showed the lowest Km and Cmin values and a high influx in relation to the other grapevines evaluated. In general, the ‘Cabernet Sauvignon’ grafted onto the ‘Paulsen 1103’ and ‘IAC 572’ exhibited the highest affinity (i.e., lower Km) for N forms, indicating that these combinations are more adaptable to environments with low N availability or require lower N inputs. These findings highlight the importance of using kinetic parameters in plant selection, because they can point out the efficient use of and ability to uptake different N forms, in addition to selecting plants that are efficient at uptaking nutrients in nutritionally depleted soils, or even physiologically efficient with low fertilization rates.