A literature review on the leaf-to-fruit ratio in viticulture and its adaptation in the context of global warming

Abstract

The most notable effects of global warming on grapevine response include an earlier phenological timeline, increased sugar concentration in grapes, and decreased acidity. To mitigate these effects, various adaptations are possible, among which adjusting the leaf-to-fruit ratio is a viable option. This ratio is measured by comparing the exposed leaf area (in m²) to the harvest weight (in kg). It is an indicator of the balance between the “source” function (the vine’s capacity to produce carbon compounds, particularly sugars) and the “sink” function of the grapes, provided that soil and climate conditions including temperature, water supply, nitrogen nutrition, and sunlight are not limiting factors.

The first scientific studies specifically examining the influence of the leaf-to-fruit ratio on grape composition were primarily conducted between the 1970s and the early 2000s, before or at the onset of the period of accelerated global warming. Increasing the leaf-to-fruit ratio was a way to optimize the interception of solar radiation to achieve grape ripeness. Today, the goal is rather to adjust this ratio to, conversely, limit sugar production while affecting the other berry parameters as little as possible.

Studies conducted on the source-sink balance have yielded optimal values ranging from 0.6 to 2 m²/kg, representing significant variability depending on the study context and grape variety. The lowest value was obtained in Chile (Maule Valley) for Syrah while the highest value was obtained in France (Midi-Pyrénées) for Malbec, Duras, and Négrette. Therefore, the leaf-to-fruit ratio can be adjusted according to the soil and climate conditions of the vineyard block, depending on the grape variety and production objectives.

Viticultural practices (particularly pruning and fertilization) must be managed to achieve an optimal yield and fruit quality per vine that is balanced with the leaf area (the latter being constrained by spacing and training system). At the same time, during each growing season, the main tools available for trellised vines are the thickness and height of canopy trimming, while suckering, leaf removal, and green harvesting remain complementary tools to be considered on a case-by-case basis. These last three practices alter the microclimate of the fruiting zone and influence the health and composition of the berries at maturity (though they increase production costs).

Under increasingly hot conditions, with potentially higher water stress, reducing canopy area can be used to some extent as a tool to limit or delay sugar accumulation in the berries, while also limiting the plant’s water loss.

References

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Assefa, M. K., Creasy, G. L., Hofmann, R. W., & Parker, A. K. (2024). Changes in total soluble solids concentration, fruit acidity, and yeast assimilable nitrogen in response to altered leaf area to fruit weight ratio in Pinot noir. OENO One, 58(4). https://doi.org/10.20870/oeno-one.2024.58.4.8225

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