Can berry composition be explained by climatic indices? Comparing classical with new indices in the Portuguese Dão region

Climatic data collected between 1963 and 2010 in the Portuguese Dão wine growing region were analysed to evaluate the relationship between climatic indices, seasonal weather and the berry composition of the red cultivar 'Touriga Nacional'. The trends over time for the classical temperature-based indices (growing season temperature, growing degree days, biologically effective degree days, Huglin index and cool night index) were significantly positive and can be mostly attributed to the effects of climate change. The dryness index showed a negative trend although not significant. These indices were able to explain 9 and 45% of the variability in total soluble solids and titratable acidity, respectively, using a multiple stepwise regression analysis. The proportion of explained variability was much improved, to 52% for total soluble solids and 65% for titratable acidity, when new climatic indices were used. The new indices resulted from the generalisation of the classical indices based upon chronological time specification as well as taking into consideration the phenological time instead. Our data shows that the classical climatic indices were not able to sufficiently explain the berry composition, and that new climatic indices should be used for a better understanding of the climate drivers of berry quality.