The adoption of cover crops as soil management systems in Mediterranean rain-fed vineyards has been limited due to the concern of excessive water and nutrient competition between these crops and the vines (Lopes et al., 2008). However, a number of environmental and agronomical benefits could be obtained from cover crops in these agrosystems: soil protection, improvements in physical and biological properties of soils, increases in plant species diversity, enhancement of berry quality, reductions in vine vigour, etc. (Morlat & Jacquet, 2003; Peregrina et al., 2010; Ruiz-Colmenero et al., 2011; Virto et al., 2012). Despite these advantages, vineyards in Spain are usually managed through tillage in the inter-row and herbicides in the vine row (Ibáñez Pascual, 2013).

In Galicia (NW Spain), favourable temperatures and soil water availability during springtime enable a fast canopy establishment in the vineyards. This may induce a dense canopy, creating unbalanced vines and unfavourable microclimate at the cluster zone, which can be deleterious for berry health and ripening (Smart & Robinson, 1991), which is the case of Galicia due to high spring rainfall amounts and mild temperatures. Those highly vigorous vines need more intensive canopy management (shoot trimming and defoliation), increasing management costs. In these situations, the use of cover crops may provide a useful tool for reducing grapevine vegetative growth since soil water and nutrients can be extracted by those cover crops and thus induce a mild stress in the vines (Celette et al., 2008, 2009).

Nevertheless, research on competition for water resources between grapevines and cover crops has led to contradictory results, likely due to differing pedoclimatic conditions. Some studies showed a high water stress affecting grapevines when they were grown with a cover crop (Maigre, 1996; Monteiro & Lopes, 2007), whereas others reported that intercropped vineyards did not always exhibit higher water stress than those under bare soil (Celette et al., 2005; Lopes et al., 2008). Moreover, vineyards need a moderate water stress to produce the grape quality necessary for wine production (Dry & Loveys, 1998), and the characteristics usually considered for grape quality (total soluble solids content, titratable acidity, pH, malic and tartaric acid contents) have been reported to improve or not to be altered by the use of cover crops (Monteiro & Lopes, 2007; Lopes et al., 2008; Marques et al., 2010), although grapevine vigour and yield can be affected (Ruiz-Colmenero et al., 2011).

Therefore, more research is needed in order to characterize the effects that permanent cover crops may have on grapevine yields and quality, as well as in soil water and soil quality dynamics under different pedoclimatic conditions. In the case of Galicia and its traditional grapevine cultivars such as ‘Mencía’, the main problems that the use of cover crops may help to solve are the excess of vegetative growth and unfavourable microclimate at the cluster zone which, favoured by autumn rainfall, causes rotten harvests. Moreover, ‘Mencía’ is a cultivar with a low colour potential that may be increased by the use of cover crops. Although erosion is not a major problem in Galicia as compared to Mediterranean regions, under certain circumstances such as bare soil and heavy autumn rainfalls, soil losses may reach values similar to those observed in Mediterranean conditions (Mirás-Avalos et al., 2009); this problem would be reduced by the use of cover crops as soil management. Finally, tractor fuel and time consumption would decrease by the use of cover crops, reducing air pollution.

In this regard, the present study aimed to assess the effects of four different soil management systems in the vineyard, including soil tillage and three different permanent cover crops (native vegetation, sown ryegrass, and subterranean clover), on water relations, grapevine physiology, vegetative growth, yield, berry composition and wine attributes of ‘Mencía’ cultivar, the main red grapevine cultivar from Galicia (a sub-humid region in NW Spain). For this purpose, experiments were carried out over a period of 3 years with contrasting rainfall regimes.

Results of this 3-year study of soil management systems indicate that cover crops can be used as a tool for reducing vegetative vigour in vineyards located in humid and sub-humid regions. The three crops used in this study presented different plant coverages throughout the growing seasons; resident vegetation and sown ryegrass covered almost 70% of inter-row surface, whereas subterranean clover only reached 45% of soil coverage. This coverage may protect soil against water erosion, which in this region can be considerable (Mirás-Avalos et al., 2009); hence, cover crops should be favoured against soil tillage since they would not only protect soil against erosion but also reduce the high vegetative vigour of the vines in a humid climate (Caspari et al., 1997; Wheeler et al., 2005) such as the one of this region.

The decreasing pattern of the midday stem water potential along the season reflects the fall in soil water availability (Monteiro & Lopes, 2007). However, differences of vine water status among treatments (Fig. 1) were not very clear in 2012, which was the driest year but the wettest growing season, and reflected slightly higher values of midday stem water potential in vines under ST at the end of the season, due to the lack of competition from cover crops. However, in 2013 and 2014, with similar rainfall amounts, significant differences in water status were detected on several dates. Surprisingly, the less negative stem water potential values were observed in vines under the SC and ER treatments (Fig. 2), which may be due to the reduction of total leaf area that may have caused a lower plant transpiration in these treatments when compared with ST, as observed in some irrigation studies (Intrigliolo & Castel, 2010).

Despite the observed differences in water status, vine stomatal conductance was only slightly affected by the use of cover crops, indicating an adequate physiological status of the vines in each treatment (Fig. 1). Anyway, vines in the NV treatment showed lower stomatal conductance values, suggesting a greater competition between cover crop and vines in this treatment, probably due to the presence of broad-leaved species (Monteiro et al., 2008) and the fact that NV and ER covered around 70% of the interrow, whereas SC only covered 45% of the interrow, approximately. Stomatal conductances measured in our study were much greater than those reported by other authors (Williams & Araujo, 2002; Intrigliolo & Castel, 2009). These results might have been caused by sufficient soil water availability and a high relative humidity in the atmosphere (higher than 45% over the study period) during our study. Apparently, no water restrictions were experienced by the vines and they had normal vegetative and transpirative behaviour, which was close to the optimal value for stomatal conductance reported by Buckley et al. (2014). In addition, most of the published data come from studies carried out in semi-arid or Mediterranean regions (e.g. de Souza et al., 2003; Intrigliolo & Castel, 2009), where vines are subjected to greater water restrictions than those observed in our study; thus, we expected to obtain higher stomatal conductances. Moreover, stomatal density in grapevine leaves depends on the cultivar (Rogiers et al., 2009), which might have also exerted an effect when considering the high conductance values observed. Furthermore, Williams & Trout (2005) and Teszlák et al. (2013) found peak values of stomatal conductance greater than 0.8 mol H²O/m²/s in well-irrigated ‘Thompson Seedless’ and ‘Riesling’ vines (similar to those observed for Mencía in the current study). A recent study in Italy reported values greater than 1 mol H²O/m²/s in four Sicilian grapevine cultivars (Inzerillo et al., 2014). In fact, stem water potential and stomatal conductances measured in the current study are over the optima (-1.25 to -1.4 MPa and 0.12 to 0.15 mol H²O/m²/s, respectively) suggested by Romero et al. (2010) for Monastrell cultivar in semi-arid conditions, which might have been the reason for the lack of differences among treatments in the present study.

At the photochemical level, it was found that F_v/F_m was unaffected by soil management strategies in this cultivar, suggesting that this parameter is not an ideal indicator to evaluate the influence of cover crops on photosynthesis under field conditions. In addition, its values were close to 0.8, the threshold suggested for healthy terrestrial plants (Cavender-Bares & Bazzaz, 2004). Nevertheless, the measurements done in leaves previously exposed to sunlight demonstrated that Φ_PSII, ETR and PQ were significantly altered by soil management in our study. In agreement with these Chl a results, CCI measurements indicated a significant reduction of chlorophyll content in leaves from grapevines of the cover crop treatments. This suggests that leaf photosynthetic composition is sensitive to the competition exerted by cover crops, as it is for other stresses (Moutinho-Pereira et al., 2012), confirming that this index may be useful for ecophysiological studies (van den Berg & Perkins, 2004).

Vegetative growth attributes were similar among treatments in the first two seasons of the experiment. However, in the third season (2014), total leaf area and pruning weight were reduced by the cover crops. This effect of the sward treatments on grapevine vegetative growth reduction is a consequence of the competition for water and nutrients exerted by the sown and resident plant species that constitute the cover crops, as reported by other authors (Caspari et al., 1997; Wheeler et al., 2005; Lopes et al., 2008). In high vigor vineyards, such as the Galician ones, this effect can be beneficial to grape health and berry composition since it induces a more favourable balance between vegetative and reproductive growth, and allows a better microclimate in the cluster zone (Dokoozlian & Kliewer, 1996), which may reduce rotten harvest when there are rainfalls by end of summer and beginning of autumn.

The absence of significant effects of the cover crops on grapevine yield may be attributed to high water supply by rainfall, which was enough for fulfilling the water requirements of both the vineyard and the cover crops. However, some yield components, such as berry weight, were significantly reduced by the cover crops, in accordance with reports by other authors (Maigre & Aerny, 2001; Marques et al., 2010).

Berry composition was not significantly affected by the cover crop treatments, and the values observed for total soluble solids and titratable acidity under these treatments were very similar to those of the tillage system. These observations may indicate an excess of source compared to sink demand. However, they are different from those observed in vineyards from other regions under semi-arid conditions, where sink demands were not covered by source availability (Lopes et al., 2008; Marques et al., 2010), proving that Galician climate conditions might favour the use of cover crops for soil management in vineyards without compromising berry quality.

Nevertheless, wines from the cover crop treatments tended to show slightly higher contents in anthocyanins and colour intensity than those from the ST treatment. This trend could be related to the lower berry sizes observed under the cover crop treatments, which increased the relative skin exposure and color extraction during maceration. However, differences in alcoholic grade and titratable acidity among wines were very low. In spite of these slight differences, consumers tended to prefer wines from the cover crop treatments (especially those from SC) over those from ST, perhaps due to a better balance in its components (alcohol/acidity ratio). This suggests that cover crops had a positive effect on wine quality, and is in agreement with previous studies (Wheeler et al., 2005; Marques et al., 2010). These authors reported that wines from vineyards managed through cover crop treatments were mainly preferred due to a greater varietal intensity than those from bare soil.

In summary, soil management systems using cover crops can provide a useful tool for reducing vegetative vigour in vineyards located in humid and sub-humid regions such as Galicia (NW Spain) without compromising grapevine yield, as reflected by this study. Significant reductions on vine vegetative growth on the cover crop treatments were observed, whereas no significant reductions in yield were detected. Vine physiology was slightly affected by the cover crops even though stem water potential was more positive under ST. Must composition was not affected by cover crops; however, wines from the SC treatment showed a lower concentration in malic acid and higher values of colour intensity and tannins. Moreover, wines from the cover crop treatments were preferred by consumers compared with those from ST.

These results enable us to conclude that native vegetation may be the best choice for a cover crop under the studied conditions, since it does not need to be sown and, therefore, its implementation is cheaper. However, attention must be given to the species that constitute this vegetation (which might include many broad-leaved species) in order not to obtain an excessive competition for soil water and nutrients.