Physiology of root to shoot interactions

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

Summary

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.