Thermal regime of the grape ( Vitis vinifera L. Syrah grapes) during maturation. Influence of the berry coloration, the compactness degree of the grape and the local wind regime

The authors present the results of an experimentation realized on Chiraz grapes with the aims to study the thermal regime supported by berries and clusters during their maturation and to determine the main factors responsible for the observed modifications. The results show that two important factors explain these modifications, firstly, the albedo of the berries which is strongly decreasing as a consequence of the light absorption and subsequent coloration of the berries during the veraison phase and secondly, the increase of the cluster compactness. Two thermic parameters allowing the characterization of the thermal regime of the Chiraz grapes have been defined: - the thermal sensibility of the clusters towards the wind velocity directly depending on the compactness or aeration degrees of the grapes, and - the maximum capacity to accumulate heat which is also linked to cluster compactness but essentially depending on the berries coloration. The effect of the compactness or aeration degree of the grape on the thermal regime in relation with the local wind regime has also been described. In practice, it would have been better to take attention to any cultural intervention modifying the wind regime around the vine stocks or surrounding the parcel. A photographic method for estimating the compactness degree of the grape clusters on the vine stock has been used.