An intra-block study of bunch zone air temperature and its impact on berry and wine attributes

Abstract

Context and purpose of the study – Temperature is a key environmental factor affecting grape primary and secondary metabolites. Even if several mesoscale studies have already been conducted on temperature especially within a Protected Designation of Origin area, few data are available at an intra-block scale. The present study aimed at i) assessing the variability in bunch zone air temperature within a single vineyard block and the temporal stability of temperature spatial patterns, ii) understanding temperature drivers and iii) identifying the impact of temperature on grape berry attributes.

Material and methods – The experiment was carried out on a 0.51 ha Guyot trained Syrah vineyard from the South West of France. Loggers displayed in solar radiation shields were positioned at 19 points in the vineyard to monitor air temperature within the bunch zone every ten minutes between veraison and harvest. At each logger, a sampling area of 21.5 m2 was delimited to collect data on topography, soil stoniness, vine behavior and fruit characteristics at harvest. Rotundone, a sesquiterpene responsible for the black pepper typicality of Syrah wine which is known to be affected by berry temperature, was also determined in wine prepared by microvinification techniques (1-L Erlenmeyer). Data were spatialized using GIS tools and used to calculate several climatic indexes over the measuring period. Dh25, Dh30 and Dh35, the percentage of degree hours above 25°C, 30°C and 35°C respectively were also determined. The whole data set was treated through principal component analysis (PCA).

Results – Average temperature varied across points from 20.93°C to 21.62°C. The amplitude of variation was greater for cool night index and maximum air temperature which fluctuated from 12.49°C to 13.92°C and from 30.36°C to 33.28°C respectively. A relative stability in temperature spatial patterns was observed on the block over the maturation period. Surprisingly, the warmest area in the morning in the center of the block turned out to be the coolest part of the block during the afternoon and the night. Maximal air temperature and cool night index were best explained respectively by stem water potentials and the distance to the southern end of the vineyard which was characterized by a slightly higher elevation and a greater stoniness. Surprisingly rotundone was poorly correlated to Dh25 while Dh25 spatial pattern tends to visually overlay the anthocyanins map. Our results indicate that bunch zone air temperature can differ largely within a single vineyard block and suggest that variations in this environmental factor can affect berry and wine volatile compositions.