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 weight and berries per bunch was consistent across scions. Significant scion × rootstock interactions were also found for pH and YAN. For each variety, significant effects of rootstock on fruit composition were linked to growth characteristics. However, these relationships, based on correlation analyses, varied for each scion.Conclusions: The study has shown that growth characteristics and fruit composition of the major varieties was not consistent across seven rootstock genotypes, as significant scion × rootstock interactions were determined. Hence, different rootstocks may be required for each variety to optimise scion performance and fruit composition. The study has also shown that the new CSIRO rootstock selections, covering a range of vigour classifications, may be useful alternatives to those currently in use by industry.Significance and impact of the study: The study has shown that the performance of scion varieties, and to a lesser degree the fruit composition, is dependent on rootstock choice. The inherent vigour of the scion variety must be considered in rootstock selection. Furthermore, individual scion/rootstock combinations may require specific irrigation, pruning or canopy management to achieve vine balance and optimise fruit and wine composition.