The Effect of Leaf Trimming to Delay Grape Ripening: A Beneficial Technique for a Changing Climate?

Boiling it down to extremely simplified terms, the goal of the wine grape grower is to raise healthy grapes to optimal ripeness, in order to achieve a sense of balance or ‚Äúcompleteness‚ÄĚ in the wine made from them.¬† Traditionally, sugar levels are monitored throughout the growing season, with higher sugar levels indicating closer to ripeness and higher potential alcohol levels in the finished wine.¬† With rising temperatures in some wine region as a result of the changing climate, grapes are maturing at a much faster rate, leading to increased sugar levels and increased alcohol in the finished wine (to name just a couple things).

Winemakers are better able to achieve a more harmonious balance in their wines when the alcohol levels are relatively low, so altering vineyard management techniques to delay ripening and keep potential alcohol levels down will be and are already very important for vineyards in these regions seeing increased temperature due to climate change.

One common viticultural technique for attaining balance in the chemical composition of grapes by delaying grape ripening is to optimize the leaf area to fruit  ratio.  Studies have determined that the ideal leaf area to fruit  ratio should be between 0.8 and 1.2 m2/kg for optimal ripeness and overall balance to occur.  One downside to this practice that some studies have found is that reducing leaf area reduces carbohydrate

Photo by I, Jukiwi [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

Photo by I, Jukiwi [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

stores for the plant during the dormant period (you need leaves to make life-maintaining carbohydrates….fewer leaves = lower carbohydrate stores).¬† These studies concluded that decreasing leaf area, while beneficial for the chemical composition and balance of the grapes, not only reduces carbohydrate stores in the plant, but also reduces berry yield, shoot growth, and vine weight during the following season.¬† A happy medium of leaf area to fruit ¬†ratio is thus necessary in achieving better ripeness as well as maintaining appropriate carbohydrate stores during the dormant period.

The goal of the study presented today was to examine the effect of leaf area reduction on ripening rates in wine grapes, as well as effects on vine vigor and productivity in following seasons.

Methods

This study was performed over three seasons:  2010, 2011, and 2012 and was located in a commercial vineyard of Grenache variety grapes in Badarán, La Rioja, Spain.  The vineyard was managed without irrigation, with bush vines planted in rows going north-south.  Prior to experimentation, vines were routinely pruned to 12 buds per vine on spurs with two buds each.

Three different leaf trimming treatments during the growing season were applied to this vineyard (in a completely randomized design):  1) a control with untrimmed vines; 2) vines with one trimming after fruit set; and 3) vines with two trimmings, one after fruit set and another at veraison (when the grapes start to change color).

Photo By Mark Smith [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Photo By Mark Smith [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

The following were measured during the experiment:  veraison date, leaf area, grape yield, grape chemical composition, as well as vegetative and productivity in the following seasons.  For grape chemical composition specifically, the following were measured: soluble solids, pH, total acidity, total anthocyanins, and total phenols.

To determine the impact of leaf trimming on the vegetative and productivity characteristics the following season, ‚Äúvine capacity‚ÄĚ was measured and analyzed.¬† ‚ÄúVine capacity‚ÄĚ refers to the total annual dry matter of the grape vines minus the roots and trunk growth.¬† In other words, it refers to the dry weight of the grape clusters, dry weight of the pruned material, and dry weight of the leaves.

During the dormant period between experimental seasons, vines were pruned as they had been prior to experimentation and dry weight of the pruned material was determined at this time.

Results

  • Leaf area per vine for the single trim treatment vines decreased 58% in 2011, and decreased 26% in 2012.
  • Leaf area per vine for the double trim treatment vines decreased 92% in 2011, and decreased 77% in 2012.
  • For control vines (no trim), the leaf area/yield ratio was 0.63-1.83m2/kg.
    • For single trim vines, the leaf area/yield ratio was 0.50-0.80m2/kg (48% reduction from control).
    • For double trim vines, the leaf area/yield ratio was 0.50-0.80m2/kg (87% reduction from control).
    • For both trim treatments, berry bunch weight and individual berry weight decreased from between 8 and 10% compared with the control.
    • Veraison date was delayed between 18 and 20 days in both trim treatments compared with the control.
    • Grape Composition:
      • Soluble solids decreased by 3oBrix (12%) in grapes from the single trim treatment and 3.5oBrix (14%) in grapes from the double trim treatment compared with the controls.
      • pH decreased by 0.1 in grapes from the single trim treatment, and between 0.1-0.14 in grapes from the double trim treatment.
      • No significant differences were noted for total acidity between treatments and the control.
      • Total anthocyanins decreased by 10% in grapes from the single trim treatment, and 27% in grapes from the double trim treatment.
      • No significant differences were noted for total phenols between treatments and the control.
    • Impact of leaf trimming over multiple seasons:
      • Double trim treatment:
        • Bunch numbers per vine decreased by 35% in the two following seasons compared with control vines.
        • Berry weight decreased between 10-20% in the two following seasons compared with control vines.
        • Bunch weight decreased between 25-45% in the two following seasons compared with control vines.
        • Total vine yield decreased between 50-70% in the two following seasons compared with control vines.
        • Total leaf area per vine decreased between 20-50% in the two following seasons compared with control vines.
        • Vine capacity decreased by 50% in the two following seasons compared with control vines.
    • Single trim treatment:
      • No significant differences were found in any of the yield or leaf area parameters mentioned above compared with control vines.

Conclusions

In terms of leaf area and yield, this study found that the leaf area to fruit ratio changed primarily due to leaf area loss.  This result seems obvious to me, as when you trim away leaves, you get less leaf area and therefore an altered leaf area to fruit ratio.  Based on other studies, this decrease in leaf area to fruit ratio means that there are more resources available to the grapes since they aren’t being allocated to as many leaves, thus sugar concentrations and other compounds in the grapes can be enhanced over the growing season.  In other words, fewer bits (extra leaves) eating the yummy resources means more yummy resources for the bits that are still there (grapes and remaining leaves).

In terms of the veraison dates, the control vines that did not undergo any sort of leaf trimming treatment started veraison (color change) during the beginning of September when average temperatures in that area were 20oC (max temp 33oC).  For both trim treatments, this study found that the veraison date was delayed until the latter part of September when average temperatures in that area were 14oC (max temp 25oC).  Therefore, as a result of the leaf trimming process, veraison and subsequent ripening took place at a time when temperatures were cooler than if no trimming had occurred.  Since studies have shown that phenol development and synthesis of other important aromatic compounds in grapes are improved in grapes ripening during cooler periods, it can be surmised that delaying ripening by administering trimming treatments at certain times during the growing season will have this same beneficial effect.

In terms of the chemical composition of the grapes, both leaf trimming treatments showed reductions in soluble solids as well as pH.  Total anthocyanins also decreased in both treatments, however, the double trim treatment showed significantly greater reductions in total anthocyanins compared with the single trim treatment, indicating that the double trim treatment may result in too great of a loss of this important

Photo by Andrew Teubes [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

Photo by Andrew Teubes [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

chemical compound required for color stability and other related characteristics in the finished wine.

Finally, in terms of productivity over subsequent seasons, this study found that the double trim treatment significantly reduced all measured productivity end points (leaf area to fruit ratio, reduced carbohydrate reserves, etc.).¬† The leaf area to fruit ¬†ratio for the double trim treatment was found to be 0.15m2/kg, which is significantly lower than the ideal leaf area to fruit ¬†ratio that was discussed earlier in this post (between 0.8 and 1.2 m2/kg for optimal ripeness and overall balance to occur).¬† The single trim treatment did not see any significant reductions in productivity end point measurements, and had leaf area to fruit ¬†ratios between 0.5 to 0.8m2/kg.¬† While on the lower end of the ‚Äúideal‚ÄĚ spectrum, the authors of this study did not believe that these levels were low enough to have a negative impact on vine productivity in subsequent years, based on the fact that no differences in productivity measurements were seen.

In conclusion, the results of this study found that veraison was delayed on average 20 days due to leaf trimming treatments, allowing the grapes to mature and ripen more slowly and completely at lower temperatures.  The single trim treatment, with trimming occurring just once during fruit set, was found to be superior to the double trim treatment, occurring both at fruit set and at veraison, which saw marked decreases in vine productivity and grape quality (re: total anthocyanins) over time.

Overall, I thought this was a neat little study that clearly showed physiological grape ripening and vine productivity changes when leaf trimming treatments were applied.  If these results are repeatable under different vineyard management regimes, different grape varieties, and different locations, it may be a useful generalized protocol for delaying grape ripening, a quality highly desired in warmer climates and in those regions that are currently experiencing warming temperatures due to climate change.  Of course, this is just one study in one location using one type of grape variety, but I think the results are promising and worthy of experimentation in one’s own vineyard (if you have vines to spare, of course!).

What do you all think of this study?  Any questions?  Need more details?  Do any of you have similar experiences?  Please feel free to leave any and all comments!  Thank you for reading!

Source: Martínez de Toda, F., and Balda, P. 2013. Delaying berry ripening through manipulating leaf area to fruit ratio. Vitis 52(4): 171-176.

1 comment for “The Effect of Leaf Trimming to Delay Grape Ripening: A Beneficial Technique for a Changing Climate?

  1. March 25, 2014 at 6:19 pm

    I am interested in your comment in the opening paragraph that, “With rising temperatures in some wine region as a result of the changing climate, grapes are maturing at a much faster rate, leading to increased sugar levels and increased alcohol in the finished wine (to name just a couple things).”

    In my view, temperature is much more directly related to phenols and acidity in the grape than it is to sugars. Sunshine (light) is the important factor in production of sugars and sunshine and temperature are not always as directly related as we might expect.

    For example, Marlborough in New Zealand experiences relatively high sunshine hours but cool temperatures; result relatively high alcohols and high acidities. Hunter Valley in Australia is the opposite with relatively low sunshine hours yet warm temperatures; result moderate alcohols with low acidities.

    Leaf trimming may decrease carbohydrate production by reducing the area of leaf to perform photosynthesis, but this doesn’t necessarily form a correlation to temperature.

What do you think about this topic?