Luca Brillante and Kaan Kurtural
In grapevine a moderate water deficit reduces berry size and increases technological quality (higher sugar levels and lower acidity, for example). The reason is that the vegetative and reproductive organs are competing sinks for carbohydrates produced by photosynthesis. Apexes are the most important sinks when fruits are not present. When fruits develop, they become progressively more important sinks for carbohydrates. During water stress, apexes reduce and then stop their growth, but the reduction in the vegetative growth varies across vegetative organs and physiological processes (Pellegrino et al., 2005). If shoot growth stops before veraison, there is no competition for carbohydrates between fruits and apexes during ripening. Red wines benefit from a moderate water deficit, while sparkling or white wines do not, nor table grapes (Sadras and Schultz, 2012). Soils favourable to the installation of a moderate water deficit during the summer, which are generally well suited to the production of high-quality red wines, have been described in France (Seguin, 1975; Choné et al., 2001a; van Leeuwen et al., 2009), Italy (Storchi et al., 2005; Tomasi et al., 2013), Hungary (Zsófi et al., 2009), the USA (Chapman et al., 2005, Brillante et al., 2017a) and in many other regions in the world. Research into the effect of water deficit on the quality of white wines is rare, but references can still be found in the scientific literature (des Gachons et al., 2005; Brillante et al., 2017b). The effect of water deficit on grape quality potential can be negative, because it causes an increase in phenolic compounds, which is not considered favourable for the quality of white wine (Sadras and Schultz, 2012). White wine also needs a certain level of acidity, which is rapidly degraded during water deficit (Ollat et al., 2002).
The amount of plant-available water in soils varies according to soil characteristics, such as soil texture, amount of organic matter and gravel content. Soil characteristics also affect the absorption process and have a direct physiological effect on plants. When the texture of the soil is fine, the soil matrix potential is low because of greater forces retaining water in capillary pores and at the surface of clay minerals. Therefore, the plant water potential must be more negative to allow absorption, even if soil volumetric water content is higher in fine-textured soils compared to sandy soils. Indeed, at the wilting point, the soil volumetric water content of fine-textured soil is always higher than that of coarse textured soils (Kramer and Boyer, 1995). Water in macro and meso-pores is generally more easily available to plants, but it is also more mobile, as it is not retained by capillary forces. Sandy soils have higher macro- and meso-porosity than clayey soils, and the available water tends to be highly variable in time. Contact between roots and soil, which is necessary for absorption, is favoured in fine-textured soils and more difficult in coarse-textured soils, as well as in soils rich in gravels. These parameters influencing soil water potential and water absorption by vines have an important effect on vineyard variability. In Bordeaux vineyards, wines produced on clayey soils, where the soil matrix potential is lower, are higher in anthocyanin content than those produced on sandy soils (van Leeuwen et al., 2004). Grapes also ripen faster on clayey soils. In Tuscany, moderately saline soils have been shown to produce the best wines (as evaluated by a sensory panel) even if water is not limited, probably because the lower osmotic potential induces a moderate water deficit (Costantini et al., 2009, 2010). Soil texture modifies the plant’s response to drought, as shown by Tramontini et al. (2012), who studied the effect of texture on grapevine physiology in neighbouring soils during the same vintage. They observed that gravel soils limited stomatal conductance and predawn water potential more than clayey and sandy soils. In sandy soils, stomatal conductance was highly variable, while it was less in clayey soil. On gravel soils, stomatal conductance was constantly low, independent of the level of water stress. Some authors have attributed the reported physiological differences observed in various soils to differences in root–shoot signalling mediated by the abscissic acid (Lovisolo et al., 2010; Ferrandino and Lovisolo, 2014). The water-holding capacity of a soil varies with soil depth. In deeper soils, vine vigour is higher and phenology is delayed (Bodin and Morlat, 2006). Soil depth can also have a direct effect on plant physiology, independent of the water amount, which is known as the bonsai effect (Passioura, 2002). However, the influence of such physiological modifications in field conditions should be further investigated.
Soil is not a homogeneous medium, therefore its effect on grapevine water status is spatially variable and has huge consequences on fruit variability at the vineyard scale. During the first year of the Efficient Vineyard Project (2016) we investigated the space-time variability of plant water status (measured as stem water potentials, SWP figure 1) in a vineyard in Sonoma county, California. We related it to soil and topography variability using electrical conductivity as a proxy for soil texture (figure 2), and digital elevation models for topographical variables. We summarized the space-time variability in plant water status across the entire season and delineated two management zones using a clustering algorithm (figure 2). The spatial pattern of this clustering well corresponded to the variability in soil texture as measured using electrical conductivity. We differentially harvested and separately analyzed the fruit from these two zones. A huge differences in the final composition of fruit at harvest, in both primary (sugars and acids, figure 3) and secondary metabolites (anthocyanins, for example figure 4) was found. In the case of anthocyanins, the difference was not only quantitative, but also qualitative. The total amount of anthocyanins in mg/g berry was different, but the di-hydroxylated forms (less colored and stable) were much more reduced than the tri-hydroxylated forms (more colored and stable), figure 4.
Different strategies can be proposed to deal with such a large spatial variability in water status at the vineyard scale. Selective harvest is an off-the-shelf state-of the-art method that can also be performed mechanically with commercial harvesters. This approach was shown to be commercially viable and can be implemented to separate the harvest in one pass, or in two passes, at different time points (Bramley et al., 2011). Other strategies with the aim of reducing the variability in the water status could also be implemented such as differential irrigation or selective cover crop removal. Their effect on grape composition deserves to be investigated in future works during the course of the efficient vineyard project.
For further information check out (contact us or download from publisher site) this article were the result of this study are reported: Brillante et al., 2017a. Assessing Spatial Variability of Grape Skin Flavonoids at the Vineyard Scale Based on Plant Water Status Mapping. J. Agric. Food Chem., 65, 5255−5265.
This post was partially re-adapted from the previously cited paper, Brillante et al., 2017a and also from Brillante et al., 2015 The use of soil electrical resistivity to monitor plant and soil water relationships in vineyards. SOIL, 1, 273–286; and Brillante et al., 2017a.
Brillante, L., Mathieu, O., Bois, B., van Leeuwen, C., Lévêque, J. The use of soil electrical resistivity to monitor plant and soil water relationships in vineyards. SOIL, 1, 273–286, 2015.
Brillante, L., Martínez-Luscher, J., Yu, R., Plank, C. M., Sanchez, L., Bates, T. L., Brenneman, C., Oberholster, A., Kurtural, S. K. Assessing spatial variability of grape skin flavonoids at the vineyard scale based on plant water status mapping. Journal of Agricultural and Food Chemistry, 65 (26), 5255-5265, 2017a
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