Getting good vine balance has a number of benefits, but it appears that getting better fruit quality isn’t one of them.
Research by the CSIRO and the National Wine and Grape Industry Centre (NWGIC) using three different techniques in three very different regions suggests vine balance in itself does not affect fruit composition and that growers are better off concentrating on manipulating the bunch environment.
It’s a relatively straightforward answer to what has always been a complex question, in part because there are different definitions of the term vine balance.
‘Vine balance is sometimes discussed in a way that merges in with terroir and wine quality and it becomes a very fuzzy thing that’s difficult to deal with, but in essence vine balance really comes down to vine growth and fruit load’, said project leader Dr Everard Edwards, a Research Team Leader with the CSIRO in Adelaide.
‘We used a clear definition. We took it as canopy size – the area available for photosynthesising, which is bringing the carbon, as sugars, into the vine – and the fruit load itself. It’s a source-sink relationship basically; the leaves are the source and the fruit is a sink.
‘So we defined it as that, then we wanted to find ways of changing vine balance in the field – manipulating that ratio between canopy size and fruit – and we wanted to replicate that in vineyards in different regions.’
Those chosen regions were the Murray Valley (reasonably high production, warm climate, yields around 23 tonnes/ha), Langhorne Creek (still warm but with cooler evenings, and with yields, by choice, at around 10t/ha) and Hilltops (cooler and with even lower, but often variable, yields).
The three techniques were early defoliation (taking off most of the fully expanded leaves just before flowering) to reduce fruit set, crop removal (simply taking off half the bunches) and late canopy removal (as is being trialled in various parts of Europe to try to delay ripening). The latter two were applied immediately pre-veraison.
‘To some extent they’re the obvious ones to pick – perhaps the early defoliation approach less so – but we also wanted to do things that are interesting to the sector in their own right’, Dr Edwards said.
‘The other thing was that we were sampling all the way through from flowering to harvest and were looking at gene expression around the key elements such as anthocyanin and tannin production; anything we see in the field we can try to tie it in to gene expression in the actual fruit.’ This area of the work was undertaken by Dr Everard’s CSIRO colleague Dr Mandy Walker.
And the results from four years of detailed work?
‘All the field work suggested there’s not much evidence there that we are getting an impact on the fruit by changing vine balance.’
‘What was interesting was that the early defoliation, which originally didn’t work quite as planned, ended up with better fruit. The wines were preferred when tasted, yet there was no change in vine balance – the change was in vine structure.’
Another part of the project was carried out in the NWGIC greenhouses in Wagga Wagga. As it’s almost impossible to change vine balance in the field without changing what’s happening around the fruit, Dr Bruno Holzapfel and Dr Jason Smith reduced CO2 rather than removing leaves to lower photosynthesis, thus having the same affect without affecting the grape environment.
Once again, there was virtually no impact on grape and resulting wine quality. Dr Celia Barril led the chemical analysis and wine sensory side of the project.
Dr Edwards says that the team went into the project with a completely open mind but he admits that from a vine physiology perspective the results don’t really surprise.
‘We didn’t really see why changing vine balance would alter composition. In extremes yes; if you take it to point where you can no longer get enough sugar, clearly that’s going to have an impact. But otherwise no.’
‘We knocked off 50 per cent of our bunches, in all three sites, cool climate to warm climate, and we had no measurable improvement in colour or other characteristics in the wines. No one could tell the difference with the controls. So that’s half your yield lost for no clear benefit.’
The project’s final report is now available online.
The final report for a partner project that also investigated vine balance has recently been published and we’ll present the findings from that project in a future issue of RD&E News.