The significance of scion × rootstock interactions

Aims: Rootstocks provide protection against soil-borne pests and are a powerful tool to manipulate growth, fruit composition and wine-quality attributes. The present study aimed to assess the consistency of rootstock effects on the growth and fruit composition of scion varieties and identify scion × rootstock interactions.Methods and results: Vine performance and fruit composition of hot-climate, drip-irrigated, spur-pruned Chardonnay, Cabernet Sauvignon and Shiraz, grafted on seven rootstocks, was assessed over five seasons from 2013 to 2017. Rootstocks included Ramsey, 1103 Paulsen and 140 Ruggeri and four promising selections from the CSIRO rootstock development programme. Vines were trained as quadrilateral cordons on a two-wire vertical trellis of height 1.8 m and 3.0 m row × 1.8 m vine spacing, and irrigated with 5.5–6.0 ML/ha of water each season. The study was conducted with mature vines, established in 2006, as a randomised block design with five replicates. There were significant effects of both variety and rootstock on yield, number of bunches, bunch weight, berry weight (scion only), berries per bunch, pruning weight and the Ravaz Index (yield/pruning weight). Despite identical management practices, there were large differences between scion varieties in key growth characteristics across rootstocks. Chardonnay produced a high yield (mean 25.2 kg/vine) with low pruning weight (2.3 kg/vine) and a high mean Ravaz Index value of 12.1. Shiraz had the highest yield (27.4 kg/vine) with high pruning weight (5.1 kg/vine) and a Ravaz Index of 6.3. Cabernet Sauvignon had the lowest yield (15.9 kg/vine) and highest pruning weight (6.6 kg/vine) and a very low Ravaz Index value of 3.0. Effects of rootstock on growth characteristics were smaller than the effects of variety, with mean yields ranging from 19.5 to 25.9 kg/vine, pruning weights ranging from 3.24 to 6.13 kg/vine and mean Ravaz Index values ranging from 5.54 to 8.63. Each variety was harvested when the mean total soluble solids reached 25.0 °Brix. Significant effects of variety and rootstock on fruit composition, included pH, titratable acidity (scion only), malate, tartrate (scion only), yeast assimilable nitrogen (YAN); and for the red varieties, total anthocyanins (scion only) and phenolic substances (scion only). Significant interactions between scion variety and rootstocks were found for yield, number of bunches, berry weight, pruning weight and Ravaz Index. The effect of rootstock on bunch we