Rootstock vigor shifts aboveground response to groundcover competition in young grapevines

Aims: Vegetative groundcover and rootstock selection are popular growth control practices for fruit and nut crops. Theoretically, plant potential growth rate should influence competitive effectiveness; however, it is unclear if rootstock vigor alters crop productivity when groundcover is present. Methods: In a humid-climate vineyard we grew young grapevines on low- and medium-vigor rootstocks with and without groundcover. Vegetative growth was determined on dormant stems; yield was determined at harvest. Roots were extracted with soil cores and distribution, morphology, and mycorrhizal colonization were determined. Resource competition was assessed by water and nutrient depletion in vines and soil. Results: Compared to vines on low-vigor rootstocks, vines on medium-vigor rootstocks exhibited greater reductions in vegetative (40% vs 19%) and reproductive (22% vs 0%) growth by presence of groundcover. Irrespective of rootstock vigor, grapevine root systems coped with competition by increasing specific root length, decreasing absorptive root diameter, and redistributing roots deeper. Competition for nutrients was the apparent main cause of growth reductions with groundcover; however, mechanisms for differential rootstock responses remain unclear. Conclusions: This study suggests that in a wet year, young grapevines grafted on low-vigor rootstocks may be more tolerant of groundcover competition than those on medium-vigor rootstocks; however, both rootstocks cope with a similar root response.