ED effects on grape composition
To the best of our knowledge, this study is the first to report on the effect of ED in a relatively warm and dry climate. Since the original study by Poni et al. (2006), ED has been the subject of several investigations in relatively cool and humid viticultural sites (Diago et al. 2010, Poni and Bernizzoni 2010, Sabbatini and Howell 2010). Overall, the results agree with previous studies (Poni et al. 2006, Diago et al. 2010, Poni and Bernizzoni 2010), which showed that ED when severe, i.e. by removing all main leaves and lateral leaves of the first six nodes, increased TSS (Figure 3) and phenolic concentration (Figure 4). Other studies conducted in high light and high temperature areas have reported that berry colour could be reduced by defoliation (Bergqvist et al. 2001, Spayd et al. 2002). Those studies recommended caution as to the timing for applying leaf defoliation under warm and arid environments. Our results, similar to those of Palliotti et al. (2011), suggest that leaf removal can be effective early in the season, when air temperature is still mild and when berries are at an early stage of growth and development. Under these circumstances, it is likely that berries and vines can withstand sudden high exposure to sunlight by either promoting the growth of skin tissues (Poni et al. 2008) or a lateral regrowth to compensate for the defoliation (Poni et al. 2006).
There are three possible explanations for the positive responses reported here. First, berry mass was lower in the defoliation treatments. Smaller berries have a higher surface area to volume ratio, increasing the concentration of phenolic substances mainly present in skin tissues (Roby et al. 2004). In fact, the largest effect of leaf removal on berry TSS and phenolic concentration was observed in the LD treatment (Figures 3, 4), the treatment with a greatest effect on berry mass (Table 4). Second, crop level was also lower in the defoliated vines, and previous studies (Intrigliolo and Castel 2011) suggest that yield per se can affect TSS and berry colour. In addition, at least in the final two experimental seasons, vine crop load was lower (higher leaf area to yield ratio) in the defoliation treatments (Table 5). Third, it is possible that fruit microclimate was improved by defoliation. The fact that defoliation was applied early in the season and lateral apices were not removed might have led to more favourable light conditions, which avoided excessive sun exposure while only slightly increasing berry temperature. In support of this contention, berry temperature in defoliated vines did not exceed 35°C (Figure 2), a temperature above which has a deleterious effect on colour (Kliewer 1977, Mori et al. 2007). In addition, berries from the defoliation treatments had higher daily thermal variation compared with that of control vines, with night-time berry temperature being higher in the control vines (Figure 2). This was most likely due to the higher amount of foliage that control vines had in the fruit zone, which presumably increased the resistance to heat exchange between berries and the surrounding environment. Lower night-time berry temperature can favour berry coloration (Kliewer and Torres 1972, Tomana et al. 1979).
Although defoliation increased TSS and phenolics in berries, it should be noted that this was not the case on a whole vine basis (Table 5) as the reduction in grapevine yield in all treatments was larger than the increase in TSS and phenolic concentration in berries (Figures 3, 4). Thus, overall vine performance was not stimulated by the defoliation treatments. The positive response that severe defoliation had on leaf assimilation rates diminished during the last part of the season (Figure 1) coinciding with berry ripening, including sugar accumulation. In addition, contrary to a previous study on Sangiovese grapevines (Palliotti et al. 2011), Tempranillo grapevines grown in an arid climate did not exhibit regrowth potential (Table 3) possibly because of soil water limitation as only half of the potential evapotranspiration was replaced with irrigation. Indeed, the deficit irrigation applied limited plant functioning towards the end of the season as reflected in the leaf photosynthetic responses (Figure 1).
The only important negative effect that defoliation had on grape composition was the decrease in must TA and increase in must pH (Figure 3). This is particularly problematic for Tempranillo grapes grown in southern Spain, where must pH can often be too high for normal vinification practices. Despite the fact that in many other ED trials, must acidity was not modified by defoliation (Intrieri et al. 2008, Tardáguila et al. 2008, Poni and Bernizzoni 2010), the results reported here are not surprising. This is mainly due to the fact that leaf defoliation decreased berry malic acid concentration (Figure 3) probably because of higher berry temperature (Ford 2012), with berry GDD accumulation in the defoliated vines being higher than in control vines. In addition, berries from the defoliation treatment had higher TSS. Similarly, Diago et al. (2012) found defoliation-improved fruit ripeness. Interestingly, even though fruit was sampled earlier, berries from the LD vines had higher TSS and phenolic concentration than berries from the control vines, which were sampled 7–20 days later (Figures 3, 4). In addition, must pH from the early sample of the LD treatment was lower than that from control grapes picked in the later sampling (Figure 3). This suggests that when defoliation is applied in commercial Tempranillo vineyards under temperate warm and dry conditions harvest could be advanced to counteract the possible negative effect that defoliation might have on berry acidity. Further studies need to be conducted to understand how wine sensory properties are affected by defoliation as wine quality is not purely a function of TSS and berry acidity. For instance, several skin maturity components that were not analysed in the present research are known to be affected by defoliation (Diago et al. 2010, 2012) and by the degree of grape ripening (Keller 2010).
Timing and intensity of leaf removal
We initially hypothesised that removing leaves only from the east side could be a better practice. Unfortunately, these treatment vines were defoliated only at phenological stage H, which turned out to be the less effective of the two timings tested here. Indeed, when an average 54% of the total leaf area was removed at stage H, as in treatment EED, vine performance and fruit composition was not affected. It appears that defoliation needs to be severe to significantly affect vine performance. Tardáguila et al. (2008) found that removing the leaves in Grenache vines only from the first five nodes without eliminating lateral leaves did not affect yield. Vines can compensate in response to leaf removal by increasing the area of remaining leaves (Poni et al. 2006) and by increasing the rate of leaf CO2 assimilation (Poni et al. 2008). Both of these short-term responses were detected in this study (Table 3 and Figure 1). However, in the mid-term (second season of leaf removal application), only leaf assimilation was increased by severe defoliation treatments. Only in the first season did vines respond to defoliation through increasing leaf area. In subsequent seasons, shoot growth was not increased by defoliation, perhaps suggesting a reduction of vine regrowth capacity over time.
The more severe defoliation treatments did not affect fruitset even over three seasons of consecutive leaf removal (Table 4). Although the vine source reduction because of leaf removal affected berry growth, it was probably not severe enough to cause berries to be shed. Similarly, Diago et al. (2010) showed that severe defoliation of Tempranillo grapes in La Rioja (northern Spain) carried out by removing the first eight basal leaves at fruitset did not reduce the number of berries per bunch. More recently, Gatti et al. (2012) showed that fruitset was affected by defoliation after only two consecutive seasons of leaf removal. Research conducted on other grape cultivars, however, has shown that fruitset is often clearly reduced by defoliation while berry growth is unaffected (Poni et al. 2008, Sabbatini and Howell 2010). Intrigliolo and Lakso (2009) showed that berry abscission can be related to the berry growth rate in two species of the Vitis genus, highlighting important differences among plant materials. It is possible that different cultivars or different environmental conditions might determine different berry growth and berry drop patterns in response to a carbohydrate source limitation. It may be that Tempranillo grapes need a drastic decrease in berry growth in order to promote berry abscission. In any case, more research is needed to better understand and predict responses to leaf removal around flowering.
The fact that defoliation affected berry mass but not fruitset has implications not only for yield control but also for fruit composition because a reduction in individual berry mass might have a greater effect on final grape composition rather than a reduction in the number of berries per bunch. Although berry size per se is not the only determinant of must composition (Matthews and Nuzzo 2007), smaller berries could facilitate greater extraction during fermentation of phenolic substances localised in the berry skins, leading to more concentrated wines. More research evaluating the effects of defoliation on the skin tissues growth is needed. Recent studies by Poni et al. (2009) and Palliotti et al. (2011) have shown that relative skin mass in defoliated berries was higher than that in the control berries, even when comparing berries of similar size (Poni et al. 2009).
In contrast with previous findings obtained in La Rioja with Tempranillo, Grenache and Carignan vines (Diago et al. 2010, Tardáguila et al. 2010), showing preflowering defoliation to be more effective than post-flowering leaf removal for regulating yield, our results show that defoliation at fruitset (phenological stage J) had a greater effect on yield via berry mass. In addition, berry TSS was clearly increased by defoliation, particularly when applied at phenological stage J. This may be due to higher exposure to light in treatment LD compared with that in ED.
Mid-term effects of defoliation on vine performance
Particularly in the treatment ED, defoliation decreased both inflorescences per shoot and flowers per inflorescence. It is well known that a carbohydrate source limitation occurring around flowering can reduce bud fertility in the next season (May 2000). It is not clear, however, why the ED treatment decreased bud fertility more than the LD treatment (Table 4). Candolfi-Vasconcelos and Koblet (1990) in cultivar Pinot Noir showed that the period from flowering to 2 weeks after was the most critical for bud fruitfulness responses to defoliation applied the previous season. This period of time covers both the ED and LD treatments. Thus, for Tempranillo, bud fertility may be more sensitive to the period just before flowering than the period after flowering (phenological stage J). The greater effect that ED had on bud fertility might also be due to a greater limitation in carbohydrate supply. More leaves were removed with the treatment also lasting longer (earlier defoliation). Similar to the present results, Candolfi-Vasconcelos and Koblet (1990) showed, after two seasons of leaf removal, that even in a third season in the absence of leaf removal, vines still exhibited reduced bud fertility and berry mass. Vines needed two seasons without defoliation to recover. Other previous studies have not shown such marked reductions in bud fertility over time because of the ED (Poni et al. 2008, Poni and Bernizzoni 2010, Palliotti et al. 2011). Mainly, this has been attributed to any negative effects because of the source being counteracted by higher bud exposure to light.
Bud fertility varied substantially between seasons (Table 4). A recent review by Clingeleffer (2010) on crop management concluded that seasonal differences in bud fertility were a major determinant of vineyard productivity. The marked season-to-season differences in bud fruitfulness observed here had as much of an effect on yield as defoliation.
Overall, the long-term results obtained suggest that preflowering defoliation should not be carried out in deficit-irrigated Tempranillo vines under relatively warm conditions in contrast with what has been previously suggested for this cultivar in a cooler climate (Diago et al. 2010). These contrasting results clearly point out the importance of conducting local research before extrapolating results across environmental conditions.