Effects of late winter pruning at different phenological stages on vine yield components and berry composition in La Rioja, North-central Spain

Aims: Under global warming, the desynchrony between technology maturity and phenolic maturity of wine grapes is a worthy concern. Late winter pruning (LWP) has been proved to be an effective way to delay the grape phenological stages. The aim of this study was to evaluate the effects of LWP at different phenological stages (based on Baillod & Baggiolini system) on the delay of the grape ripening, on vine yield components as well as on berry composition, among others, the anthocyanin to sugar ratio. Methods and results: The two-year (2015 and 2016) trial was conducted in Rioja wine region (North of Spain) on Maturana vines and in each year, four pruning treatments were carried out taking apical buds/shoots as reference: (1) winter pruning at stage A (WPA; dormant bud); (2) LWP at stage C (LWPC; green shoot tip) in 2015 and at stage F (LWPF; inflorescence clearly visible) in 2016; (3) LWP at stage G (LWPG; inflorescences separated); (4) LWP at stage H (LWPH; flowers separated). LWPC failed to delay the late phenological stages and did not exert important influence on vine yield and berry composition. LWPG and LWPH succeeded to delay all the phenological stages of grapes to a great extent and created a considerably cooler and longer ripening period compared to WPA. Vine yield was not affected by LWPF and was reduced significantly (averagely by 41%) by LWPG. LWPH lead to great losses in yield (averagely by 67%), especially in 2015. LWPG did not change the fruit composition while LWPH increased the ratio of anthocyanin to sugar and helped to maintain a relatively high level of acidity in berries. Conclusions: The primary cause of the decline in production seems to be the losses of flowers and/or the reduction in fruit set percentage in the current season, instead of the losses in inflorescences within buds in the previous season. For Maturana grapes, LWP after the stage F would reduce the vine yield and could be applied as an alternative to the time consuming cluster thinning to meet the needs of yield control. Delaying the winter pruning to stage H could improve the fruit quality in spite of the greater risk of botrytis and a serious decline in production. Significance and impact of the study: The outcomes of this research open a door for the winegrowers to realize the yield control in a simple way. Also, for those who only pursue wines of top quality (regardless of production), a very late winter pruning might provide them with high-quality grapes. Moreover, as can be seen obviously from our results, viticulturists could postpone the budburst date to whatever extent they wish thus reducing the risk of spring frost injury to zero, though this point is not our focus in the study.