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Rootstock conferred effects on scion vigour and, to a lesser extent perhaps, physiology, have long been observed and reported in the literature. However, the mechanisms behind these effects have proved elusive to pin down. Previous Wine Australia supported work on rootstock:scion interactions suggested, indirectly, that root derived hormones may play a role. In addition, whole plant hydraulics are impacted by rootstock conferred effects across a number of commonly grafted woody horticultural species.
The project described in this report exhaustively tested these hypotheses, utilising highly controlled environment studies, controlled greenhouse studies, a study with field grown young vines to match the greenhouse work, and a mature field planting. Overall, there was little evidence to support a physiologically significant difference in root-derived hormone production in response to environmental changes, nor a rootstock-derived difference in scion hydraulics once differences in scion growth were accounted for. Microscopic examination of new wood structure also didn’t indicate any rootstock-derived changes in the scions.
Despite these results, consistent rootstock conferred effects on the scion were observed in all of our experiments. Notably, although there was some evidence for scion effects on rootstock growth, wherever root growth and production were examined, specific rootstocks had consistent root growth habits. Consequently, it is likely that rootstock conferred effects on the scion are largely driven by inherent differences in root system growth form, which provides specific limitations on water and nutrient uptake, dependent on soil type and conditions.
The implication of this is that there is unlikely to be simple genetic markers or easy to measure rootstock phenotypes, such as xylem vessel density, that can be used for breeding, planting or management decisions. This does not mean that it is impossible to improve on using current experience with rootstocks for those decisions, but that improving our knowledge of rootstock conferred effects on the scion are likely to depend on an extensive network of long-term rootstock trials, where rootstock genotypes are not in competition belowground with other rootstocks that differ in their root system growth.
Abstract
Rootstocks influence scion vigour and physiology, but the mechanisms remain unclear. This project tested the roles of root-derived hormones and hydraulics using controlled and field experiments. Results showed no significant differences in hormone production or scion hydraulics after accounting for growth variations. Microscopic analysis also found no structural changes in scion xylem due to rootstock genotype. However, rootstocks consistently affected scion growth and some links with root traits were observed. It seems unlikely that simple genetic markers exist, emphasising the need for long-term trials focussed on the root system itself to better understand and optimise rootstock effects on scions
Objectives
This project will further develop the understanding of the role of rootstock choice and grafting in the regulation of water and nutrient uptake and abiotic stress response, based on a mechanistic understanding of root to shoot communication.
Background
In addition to providing protection against nematodes and phylloxera, rootstocks are being selected for a range of other traits, including vigour and tolerance to abiotic stresses such as drought and salinity. Rootstock effects on scion vigour and fruit composition are driven by a variety of root-to-shoot signals, which vary further depending on the rootstock:scion combination. The scion response to soil water deficit is governed by root hormone signals, while rootstock conferred vigour is governed by a mechanism yet to be fully elucidated, but likely to involve hydraulics.
As the parent material of commonly utilised rootstocks originates in varied natural environments from wetlands to arid environments it is also likely that root signals of different genotypes vary in response to a given soil environment or stress. Better utilisation of these differences, based on an improved understanding of the processes and their regulation, provides an opportunity to improve rootstock choice for specific environments and develop management strategies for specific viticulture purposes.
Approach
Rootstock selector tools will be improved with information for specific viticultural situations based on a mechanistic understanding of root to shoot communication (e.g. vigour control, hormone signalling). Existing rootstock:scion combinations will be evaluated and key traits such as vigour and specific root to shoot signalling will be identified for future breeding targets. The use of existing deficit irrigation strategies for vigour control and water use efficiency will be examined as well as the potential to develop 'precision planting' utilising multiple rootstocks within a single vineyard block based on soil type.
Sector benefits
Improved rootstock choice for planting or replanting, reducing post-establishment difficulties in crop management through greater surety of vine growth, production and response to the environment.
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