The effect of leaf removal on canopy microclimate, vine performance and grape phenolic composition of Merlot (Vitis vinifera L.) grapes in the continental part of Croatia
Introduction
Microclimatic conditions within the grapevine canopy- temperature and solar radiation, as well as the balance between sugar source and sink are important parameters that directly regulate grape ripening parameters: sugars, organic acids and secondary metabolites (Spayd et al. 2002, Pastore et al. 2013). UVB radiation is important for the chemical composition and quality of grapes, since some phenolic compounds have the function of absorbing and screening UVB radiation and protecting the plant from its harmful effects (Berli 2010). Solar radiation has been reported to increase sugar and pH in grape juice (Song et al. 2015) and increase phenol concentration in grapes and wine (Gregan et al. 2012, Song et al. 2015). On the other hand, high temperatures (>35°C) and excessive sun exposure can inhibit ripening, lower titratable acidity and reduce the synthesis of metabolites, especially phenolic compounds (Spayd et al. 2002). It is therefore important to evaluate different canopy manipulation treatments and their effects on grape microclimatic conditions, as they may contribute to the accumulation of numerous chemical components, especially the phenolic composition of grapes.
Leaf removal in the cluster zone is one of the common canopy management practices used to manipulate microclimate of grapevines by increasing light penetration and temperature in the fruiting zone (Feng et al. 2015, Young et al. 2016) and reducing canopy density (Zhang et al. 2017), which affects relative humidity and reduces the incidence of pests and diseases (Mosetti et al. 2016, Van der Weide et al. 2020). By improving microclimatic conditions and balancing the ratio of older, less photosynthetically active leaves to younger, more photosynthetically active leaves (Intrieri et al. 2008), basal leaf removal can improve grape and wine composition by increasing the concentration of polyphenolic and aromatic compounds (Hickey et al. 2018, Bubola et al. 2019, Ferlito et al. 2020) and reducing titratable acidity, especially malic acid (Riesterer-Loper et al. 2019, Verdenal et al. 2019).
Although measurements from traditional weather stations are consistent for comparing measurements from different sites and are useful for characterising the climatic conditions of a growing season, they do not represent the specific microclimatic conditions under which grapes ripen and are affected by the leaf removal treatment. In our research sensors were positioned at the cluster level, within the leaf zone of the vine, and continuously monitored UV radiation, air temperature and relative humidity in the fruiting zone, which can provide more detailed information to better understand the conditions to which grapes are exposed. While many previous studies have focused on te PAR measurements in the fruiting zone, only a limited number of studies have up to now been conducted with canopy microclimate measurements which include UV light measurements.
The effects of different timing of leaf removal are of interest to many producers in the wine industry. While leaf removal is a standard management practice in the continental Croatia, it is usually done at veraison. The aim of this study is to investigate the influence of leaf removal treatment carried out at berry set on the vine microclimate, as well as on the grape composition and wine production of the Merlot variety grown under the environmental conditions of the Croatian hillside. Merlot is a widely planted variety in Croatian vineyards and is know to produce high quality wines.
Section snippets
Plant material
The study was conducted in 2018 and 2019 on the Jazbina experimental field of the Department of Viticulture and Enology, Faculty of Agriculture, University of Zagreb (long. 45°51′N, lat. 16°0′E). The vineyard is located on the slope with slightly southwestern exposure. The soil type was anthropogenic pseudogley with clay texture. Merlot (Vitis vinifera L.) was planted in 2005 on the SO4 rootstock. The vines were trained on the bilateral Guyot system, 80 cm above the ground. The rows were
Weather conditions and phenological phases
Weather conditions for the 2018 and 2019 seasons in terms of precipitation and air temperatures are shown in Table 1. The average temperatures recorded in the two seasons differed especially at the beginning of the growing season, during berry formation and during berry ripening. As a result of lower temperatures in April and May 2019, flowering started 16 days later than in 2018. After flowering, phenological development followed a similar pattern in both seasons, and in two seasons the grapes
Discussion
The main objective of this study was to investigate the effect of leaf removal performed at berry set on vine microclimate, vine performance and grape phenolic concentration of the Merlot variety grown in the continental climate of Croatian hillside. Leaf removal vines received more UV radiation than the control vines, which was also reported in Gregan et al. (2012) and Joubert et al. (2016). Due to a NE-SW row orientation, UV radiation in the cluster zone showed one peak at 09:00, possibly
Conclusion
Modifications of the canopy microclimate significantly improved the phenolic concentration of Merlot berry skin, with a minor effect on primary grape composition. Seasonal climatic conditions during grape ripening influenced leaf removal efficiency, with leaf removal having more influence on berry skin anthocyanin and flavonol concentration under cooler berry ripening conditions. Determining the vintage effect by identifying the differences in microclimatic conditions during different stages of
CRediT author statement
Marina Anić: Data curation, Writing- Original draft preparation,
Mirela Osrečak: Visualization, Investigation,
Željko Andabaka: Formal analysis,
Ivana Tomaz: Resources,
Željko Večenaj: Software, Conceptualization,
Damjan Jelić: Software,
Bernard Kozina: Supervision,
Jasminka Karoglan Kontić: Validation,
Marko Karoglan: Writing- Reviewing and Editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study is supported by the HrZZ project (VITICLIC) (PKP-2016-06-2975) which is funded by the Environmental protection and energy efficiency fund under the Government program (Ministry of Environment and Energy & Ministry of Science and Education) for the Promotion of research and development activities in the field of climate changes for the period 2015-2016.
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