Assessing thermal efficiency for berry anthocyanin accumulation in four different sites and field-growing conditions

In this work we evaluated the thermal efficiency for berry skin anthocyanin accumulation in two red grape cultivars, Bovale Grande (syn. Carignan) and Cannonau (syn. Grenache), cultivated in Sardinia, Italy. Two-season data of berry skin temperatures, collected from veraison until harvest, and total anthocyanin content, sampled at two-weeks intervals, were considered. The study was carried out in four different Sardinian vine growing areas (Gallura, Oristano, Nurra and Trexenta), considering four different experimental trials where vineyard or canopy management treatments (namely basal leaf removal, solar UV-screening, cover-cropping and shoot trimming) modified sunlight microclimate at the fruit zone as compared to untreated controls. The accumulated normal heat hours (NHH) and the duration of low and high temperatures (HT<15°C, lower than 15°C and HT>35°C, higher than 35°C) were calculated. The significance of these three variables to predict total anthocyanin content was tested by multiple linear regression analysis. Principal component analysis (PCA) was carried out to separate the effects of cultivar, vine growing conditions and permanence of high or low temperatures on anthocyanin accumulation. The results showed that both HT<15°C and HT>35°C improved model performances, even if the weight and significance of the predicting variables varied with cultivar and experimental site. The relationship between dependent and independent variables ranged from strong to very strong (correlation coefficients ranging from about 0.7 up to 0.9). The Adjusted Normal Heat Hours model allowed to account for varietal differences and to weight the contribution of high and low temperatures to anthocyanin accumulation under different growing conditions.