Spatial variability in vineyards is an important issue that affects both farmers and winemakers, especially when trying to obtain the best potential out of their vines. Land (topography and soil), climate and diseases affect crop development and, therefore, grape yield and quality throughout a single vineyard. Some of these sources of variation remain stable along time, as observed by Bramley & Hamilton (2004). The study of these patterns can lead wineries to a better understanding of the different qualities of batches, in order to plan different winemaking procedures and get a better profit from the grapes (Proffit et al., 2006; Bramley et al., 2011). Furthermore, this information allows targeted crop management in terms of inputs (irrigation, fertilizers, spraying) and cultural practices (pruning, shoot and grape thinning, canopy management); with the aim to achieve more uniform parcels (Bramley, 2005; Proffit & Malcolm, 2005; Arnó et al., 2009).

The concern about grape composition is increasing among winemakers, since most vineyards all over the world produce high sugar content grapes. This is mostly because of viticultural practices, which have usually been designed to this aim. Climate change has also contributed to increase the berry ripeness process naturally. Therefore, wines present higher alcohol content and also there is a trend towards a pH increase (Schultz & Jones, 2010). Generally, for the most part of winemaking regions, the warming trend from the last decades to date has led to higher quality wines, with higher concentrations in anthocyanins and polyphenols, especially in northern regions. However, predictions reveal an average trend of +2 °C by 2049 (Schultz & Jones, 2010). This considerable increase will affect very likely grapevine phenology, leading to high-temperature ripening processes and to technological, phenolic and aromatic ripeness mismatch (Schultz & Jones, 2010). In addition, anthocyanin synthesis is highly related to temperature. Whilst sugar activity ranges from 18 to 33 °C, pigment producing enzymes is limited to 17-26 °C (Iland & Gago, 2002; Sadras et al., 2007). Temperatures above 30 °C after véraison can inhibit anthocyanin formation (Kliewer & Torres, 1972; Mori et al., 2007).

Microclimate in the leaf and bunch area depends on leaf density and bunch and leaf arrangement, and affects light reaching the vine, as well as leaf and bunch temperature and humidity (Smart & Robinson, 1991). It is assumed that, the higher bunch exposure to light, the better the grape quality (Kliewer, 1970; Smart et al., 1985). In addition, grapes show higher concentrations in sugar, phenolics or anthocyanins, and less acidity and malic acid. Generally, it is agreed that an increase in light exposure favours polyphenolic accumulation, mainly flavonoids, like anthocyainins and flavonols (Haselgrove et al., 2000; Spayd et al., 2002; Tarara et al., 2008). Also, when there is enough and moderate light getting to the bunches, the limiting factor in anthocyanin synthesis is temperature. However, some authors have reported that over-exposure and therefore high temperatures in grape bunches may lead to reducing anthocyanin accumulations (Kliewer et al., 1967; Haselgrove et al., 2000; Spayd et al., 2002). In this sense, vine vigor can affect vine microclimate, light reception and/or temperature in the bunches.

To assess vineyard variability, one of the most powerful tools is the airborne remote sensing through the translation of reflectance into a single number or index. This index can be related to shape, size and photosynthesic activity of the vines. Usually, spectral vegetation indexes are ratios including red and near-infrared (NIR) reflectance: red wavelengths are absorbed by leaf chlorophyll and NIR wavelengths by the mesophyll tissues. The most basic vegetation index is RVI (Ratio Vegetation Index), which is simply the coefficient between NIR and red reflected light. In order to avoid the influence of the light intensity reflected by objects (whether they are shadowed or fully exposed), other normalised indexes have been developed, such as the Normalized Difference Vegetation Index (NDVI). Since this ratio diminishes atmospheric and luminous variations, it is very convenient in multi-temporal studies (Fischer, 1994) Several studies demonstrate that NDVI can provide information not only about vine vegetative status, but about yield and grape composition, pH, acidity, sugar content or phenolic compounds (Lamb et al., 2004; Arnó et al., 2011; Fiorillo et al., 2012; Martínez-Casasnovas et al., 2012). Nevertheless, latest technology developments have improved the spectral resolution of remote sensors, allowing for calculating new indexes based in other narrow wavelengths capable to detect chlorosis, water stress or nutrition deficiencies in grapevine. Some of them are TCARI (Transformed Chlorophyll Absorption in Reflectance Index) based on 550, 670 and 800 nm wavelenghts, OSAVI (Optimised Soil Adjusted Vegetation Index) at 670 and 800 nm, or PRI (Photochemical Reflectance Index) at 570 and 539 nm (Zarco-Tejada et al., 2005).

In this respect, the purpose of this paper was to study the feasibility of using airborne remote sensing to assess spatial variability of vine vigor and its relation to grape composition traits (e.g. anthocyanins, polyphenols, etc.), and consequently delineate differential management zones for precision viticulture purposes. Unexpected relationships between vine vigor and some grape composition traits are analysed and are particularly discussed.

In view of the summary statistics, the high year within-field variability shows the great importance of differential management and selective harvesting, as described by Bramley & Hamilton (2004). Relevant grape composition variability was observed, as reported previously by other authors (Bramley, 2005; Fiorillo et al., 2012; Martínez-Casasnovas et al., 2012). Lower values of variables expressing vigor, lower acidity and higher sugars and phenolic compounds, reflected that, generally, 2010 could be considered as an average year while 2011, and especially 2012, were extremely dry.

Both Pearson correlations and ANOVA suggested that NDVI is a good indicator of vine vegetative development, as found by Acevedo-Opazo et al. (2008). The three zones presented also a stable pattern through time.

Yield variables exhibited a trend towards less yielding in the low vigor vines. Berry weight, which is a very important factor for phenolic concentration on the basis of the skin/pulp ratio (Coombe & Iland, 2004), was significantly lower in the three years of study. Smaller berries lead to lighter bunches and therefore less yield. However, for the vine yield variable, the Pearson correlations were poor and ANOVA test showed that high vigor vines yielded lower or similar than medium vigor vines. This could be explained by an excess of vigor on some of those vines, which originated a poor fruit setting and/or reduced bud fruitfulness, as observed by Cortell et al. (2005).

Regarding grape composition traits, sugars were found higher in the low vigor vines. These results agree with reported reductions in sugar accumulation due to fruit shading and vegetative growth (Kliewer et al., 1967).

No great differences or relationships with vigor were found in terms of total acidity, although pH presented a positive relation with NDVI, showing higher values in the low vigor zones. For equivalent total acidity, this trend can be explained by a reduction in malic acid concentration, and higher proportion of tartaric acid, which has more acidic power. Low vigor vines usually have less malic acid contents (Smart et al., 1985), due to a decreasing synthesis and also because high exposed bunches experience high temperatures, that lead to malic combustion.

Concerning anthocyanins, the results showed a positive relationship with vigor, and the trend towards less concentration in the low vigor zone, disagree with most part of studies on the subject, where low vigor zones tended to exhibit the highest levels of anthocyanins (Lamb et al., 2004; Stamatiadis et al., 2006; Cortell et al., 2007; Martínez-Casanovas et al., 2012; Filippetti et al., 2013). Most part of these studies have been conducted under cool climates and/or other grape varieties. Fiorillo et al. (2012), however, found that, in some years, the highest values of anthocyanins were found in the medium vigor block. They reported that medium vigor vines may experience moderate water deficit and moderate-high light exposure in the fruiting zone compared with the other zones. Conversely, low vigor vines may suffer from strong water deficit, resulting in a severe photosynthesis restriction and ripening downshift (Hardie & Considine, 1976). Zarrouk et al. (2012) have reported lower anthocyanin concentrations in Tempranillo vines in stress conditions, claiming a bunch over-exposure to sunlight and excess of temperature. In this sense, Tempranillo seemed to present a different behaviour as other grape cultivars (Teixeira et al., 2013), due to a different genotypic expression of the biosynthetic pathways of anthocyanins.

Vegetation/yield ratios, commonly used to evaluate grape quality, were calculated and compared (data not shown), finding no differences within the zones, and being always above the minimum requirements for a proper ripening. For this reason, differences in anthocyanin contents may be explained by exposure effects. As explained before, several studies have reported that high sunlight exposures may lead to high berry temperatures and reduction of anthocyanin accumulation (Kliewer et al., 1967; Haselgrove et al., 2000; Spayd et al., 2002). In warm climates, grapes reach enough sugar levels for high quality winemaking, but it is not the same regarding colour (Iland & Gago, 2002). This decoupling between sugar and anthocyanins during grape ripening in high temperature environments has already been stated by Sadras & Moran (2012), proving that anthocyanin synthesis trigger occurs later than sugar’s. Then, vigorous vines create more shading effects in the bunch area, protecting grapes from high temperatures in August and September (data not shown), and leading to higher anthocyanin and colour contents. The positive effect of bunch shading on anthocyanins has been explained by De la Fuente et al. (2007). This suggests that vigor influence on anthocyanin accumulation depends, basically, on environmental factors and especially, on maximal temperatures. This means that both macroclimate (occurrence of high maximal temperatures) and microclimate (vigor level and its influence in leaf shading in the bunch area) are involved in the process, in addition of Tempranillo particular gene expression. In a hot year, vigorous vines may protect bunches from over-exposure producing more coloured grapes, and oppositely, in a cool year, the more exposed grapes may present the highest anthocyanin contents.

Total phenolics presented negative correlations with vigor, as stated by Cortell et al. (2005), due to reduced phenolic synthesis. The lower vigor zone presented always the lower values, according to most part of studies on this subject (Lamb et al., 2004).

The present research confirms the great spatial variability in vineyard response in all the studied variables, suggesting an opportunity for differential management. NDVI was shown to be good indicator of vine vigor. NDVI-based zonification in three areas showed significant differences of yield variables, specially bunch and berry weight, without any ambiguity. Grape composition traits, as pH, anthocyanins and sugar content, seemed to be more variable but, largely related to NDVI variation. In this respect, particularly relevant was the unexpected relationship between NDVI and anthocyanin content, which showed a positive correlation. This was due to important environmental influences and complex relationships between vigor and grape composition, involving vine microclimate and the specific Tempranillo cultivar genotypic expression. Although some authors have found a negative correspondence between anthocyanin content and vigor by NDVI-based zones, in this study the relationship was found to be positive, opening a new issue in the field of vegetation indexes applications in viticulture. Moreover, under a climate change scenario, it may be interesting to consider the NDVI zones for a selective harvesting on the basis of grape juice pH. Finally, most part of the analysed traits was not significant for the three defined areas, suggesting that only two zones are recommended for a practical differential management. In any case, NDVI mapping remains at the moment, the most affordable way to obtain vineyard spatial information easily and objectively.