How can we combine leaf removal and irrigation techniques to optimize flavonoid concentration in grape berries? Focusing on the hot, arid winegrowing region of San Joaquin Valley, Runze Yu et al studied the impact of different leaf removal schemes and water deficit on yield and berry phenolics accumulation. The study was published on the Journal of Agricultural and Food Chemistry in October 2016.
The authors conducted the experiment in 2013-2014 on a vineyard that grew Merlot grapes in northern San Joaquin Valley. They tested two leaf removal schemes – pre-bloom leaf removal and post-fruit set leaf removal – against the control scenario. In addition, two deficit irrigation schemes were also tested. As noted by the authors, both years experienced extreme hot temperatures compared to the regional 5-year average.
Impacts on yield
Interestingly, leaf removal treatments did not negatively affect yield in 2013 but reduced berry skin mass especially with post-fruit set leaf removal. There are several possible reasons for these results. A paper by Percival et al. posits that grape vines tend to produce more leaves than necessary in warm climates; and therefore, leaf removal treatment may not trigger yield decrease. With regard to berry skin mass, the pre-bloom removal scenario resulted in more skin mass relative to the post-fruit set removal scenario. According to the authors, this could be attributed to the long-term adaptation of berry thickening in response to prolonged inflorescence and cluster exposure in the pre-bloom scenario.
In contrast, deficit irrigation did not affect either yield or berry skin mass.
Impacts on berry phenolics accumulation
In general, the authors found higher flavonol concentration under leaf removal treatments. Meanwhile, water deficit had no effect on flavanol concentration per berry. The two water deficit scenarios consist of: (a) sustained deficit irrigation, which aimed to keep mid-day leaf water potential at -1.2 MPa from fruit set to harvest, and (b) regulated deficit irrigation, which aimed to keep mid-day leaf water potential at -1.5 MPa. While vineyard managers typically deem these targets as negative, the results of the experiment demonstrate that vines can withstand these levels of water deficit. The results are consistent with what Fruition Sciences data suggests when comparing vine transpiration data with berry composition.
Impacts on tannin concentration and composition
The authors investigated several subunits of tannins. They found that post-fruit set removal resulted in higher skin tannin and longer tannin polymer relative to pre-bloom leaf removal. Interestingly, water deficit had no effect on skin tannin, possibly because the irrigation treatment was not severe enough.
It’s interesting to see how leaf removal and irrigation techniques can be applied to manipulate yield, canopy and berry composition. The article implies important opportunities for regions subject to drought to use alternative deficit irrigation methods which don’t require a sacrifice in yield or important quality variables.
Our product Sap Flow enables winemakers to enhance fruit and wine quality while saving significant amounts of water. We have also discussed water scarcity issues in California here and plan to provide an update and best practices surrounding this issue at the Napa Vintage Report conference on January 18, 2017. Tickets to the conference are available at https://www.vintagereport.com/en/napa-2016.