The influence of vine row position in terraced Merlot vineyards on water deficit and polyphenols – case study in the Vipava Valley, Slovenia

Abstract

A study was conducted in the Vipava Valley (Slovenia) to understand the effects of positioning rows of Merlot (Vitis vinifera L.) vines on terraces on plant available water, yield, and grape composition. Seven vineyards planted on dry, (hiper) skeletal Eutric Cambisol terrace slopes in a relatively limited area with similar mesoclimate were studied. The vines are planted in two rows: the inner rows, which are close to the slope and where the soil is more compact, are expected to have less available water, while the vines in the outer rows are more likely not to experience drought. Vineyard parameters (e.g., crop size, leaf area to yield ratio) were standardised in all selected vineyards as described by Sivilotti et al. (2020), and stem water potential (SWP) was measured during the growing season (Deloire and Heyns, 2011). Berry samples were randomly collected in triplicate from the inner and outer rows of the terraces at harvest time in 2019 and 2020. The skins and seeds were separated and extracted by accelerated extraction in organic solvents. The content of total anthocyanins (TA), free anthocyanins, total phenols (TP), proanthocyanidins (PAs), catechins and PAs reactive with vanillin (vanillin index) in grape skins and seeds was determined spectrophotometrically (Rigo et al., 2000). The structural characteristics of PAs (mean degree of polymerization (mDP), percent galloylation (%G), and percent prodelphinidins (%P)) in seeds and skins were analysed by UHPLC-DAD-MS /MS (Calderan et al., 2021). The profile of free anthocyanins in the skins was determined using HPLC-DAD (Vanzo et al., 2008). In early July 2019 at pre-verasion, statistically significant higher levels of SWP were measured on vines growing in the inner rows, while in late July and August, higher water deficit was measured in the outer rows of the terraces. The water deficit in early July 2019 was most likely related to the lower leaf area and yield in the inner rows. Grape seeds from inner rows contained more TP, vanillin reactive PAs, and PAs in dry weight and PAs had higher % G. Grape skins from inner rows contained more PAs in dry weight. On the other hand, grape skins from outer rows contained more TP, TA and more anthocyanidin 3-glucosides in fresh weight. In 2020 there was significantly higher SWP in late August and yield in the outer rows, and no differences in grape composition. The results indicated that the position of the vines on the terraces could influence the grape phenolic content and PAs structural characteristics. In this study, we confirmed differences in phenolic composition of Merlot grapes at the microscale within the vineyard, where the water deficit of the vines could have an important effect. Differences between row position of the vines suggest that it would be possible to improve the mechanical work with an excavator when establish new terraces in order to reduce water deficit of vines.

References

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