To remove bunches, or not? How we choose to manipulate grapevine canopies can have a significant impact on vine performance, according to two projects completed in 2018.
A team at the University of Adelaide, led by Associate Professor Cassandra Collins and Dr Roberta De Bei, evaluated a range of canopy management treatments over three seasons on Shiraz and Semillon vines across five South Australian regions, with a focus on how these change vine vigour and yield. Vine performance was assessed using measures of yield and yield components, canopy architecture, and berry and wine quality.
The canopy can be manipulated directly by physically removing leaves, bunches or shoots. However, indirect approaches (such as a cover crop or leaving more nodes at pruning) that induce some form of competition can regulate canopy growth and reduce vigour at a fraction of the cost of direct methods.
‘In some of the experimental trials the greatest impact on fruit and wine composition was found when indirect methods were applied, suggesting that we think carefully about how we manipulate our canopies’, said Roberta.
The team tracked changes in the canopy architecture in response to shoot and bunch thinning treatments and found that if the treatments weren’t dramatic then they rarely made a significant difference.
‘For example, with shoot thinning, which is a very common practice in many vineyards, if you don’t remove a significant proportion of the shoots (at least 40–50 per cent) then often you don’t have any significant impact on the canopy and final fruit and wine composition’, Cassandra said.
The team used the VitiCanopy app to track changes in canopy architecture in response to the various treatments.
The performance and analysis of different canopy management practices can be found in the final project report here.
A significant overall finding was that canopy architecture measures, such as leaf area index and porosity, correlated better than yield with grape and wine quality. It was also observed that the early season canopy architecture measures are better indicators of quality than the veraison measures.
In addition, a greater growth rate early in the season correlated positively with grape and wine chemistry measures. Conversely, a greater late-season growth rate, from veraison to harvest, was detrimental to quality.
A concurrent project on Shiraz by the CSIRO and the National Wine and Grape Industry Centre (NWGIC) used three different techniques in three very different regions and found that ‘vine balance’[1] in itself did not affect fruit composition, suggesting that growers are better off concentrating on manipulating the bunch environment.
The three techniques used to change ‘vine balance’ 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’, said project leader Dr Everard Edwards, a Research Team Leader with the CSIRO in Adelaide.
‘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 Everard’s CSIRO colleague Dr Mandy Walker.
And the results?
‘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 canopy architecture.’
Everard 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 field studies were complemented by research using a new experimental system, designed by Jason Smith at the NWGIC, to study how the availability of carbohydrates during ripening influences berry composition and wine quality attributes. The intent of this study was to try to separate out quality responses to carbohydrates from any other responses that may be due to differences in exposure caused by canopy adjustments.
A whole canopy gas exchange system was designed to manipulate the carbohydrate/sugar accumulation in the berries, with experiments taking place on mature potted Shiraz vines within a series of transparent gas exclusion chambers, that allowed the amount of carbon dioxide in their air to be manipulated by using carbon-dioxide scrubbers, impacting photosynthesis by artificially reducing available carbon dioxide.
‘When we used this system to vary the amount of sugar available to the fruit by 50 per cent, without changing the canopy or bunch environment, the rate of berry colour formation varied exactly in sync with the berry sugar accumulation – there was no effect on the anthocyanin to sugar ratio’, Everard said.
More information is available in the December issue of BMC Plant Biology here.
The final project report can be found here.
[1] For the purposes of the work, vine balance was defined as the ratio of the carbon demand by the fruit and the carbon fixed by the canopy, approximated to canopy area.