PHYSIOLOGICAL RESPONSES OF WHITE GRAPE BERRIES TO SUNLIGHT EXPOSURE

Reflectance spectroscopy was used in 2013, to investigate about varietal behaviors to different agronomics condition: (i) composition and quantification of pigments by using non-invasive method; (ii) photosynthetic pigments assessment by new reflectance indices (iii) how different microclimatic bunch conditions could affect the appearance of sunburn. The experiment was performed in the Regional Research Station of Riccagioia (Lombardy region, Northern Italy), at the University of Milan, on 16 white grape accessions, during 3 phenological stages: pre-veraison (77 BBCH), veraison (81 BBCH) and harvest (89 BBCH). New specific indices for the evaluation and estimation of photosynthetic pigments were proposed on the basis of grape berry reflectance spectra. Validations with classical extraction analysis were done. About 200 berries were analyzed (over 1000 reflectance spectra were collected). First the chlorophyll a and b absorption maxima in the reflectance spectra were established: 675 and 650 nm respectively. These new equations are also able to discriminate between chlorophyll a and b. Indeed, the wavelengths of major interest for their absorption detection were identified. If chlorophyll quantifications were achieved directly from reflectance spectra, for carotenoids the absorption bands did not allow good reflectance correlations. Nonetheless, thanks to the physiological relation of photosynthetic pigments, the chlorophyll/carotenoid ratio was used to estimate carotenoid content. Because their proportion changes during berry development, the index coefficients can be adapted in relation to the BBCH phenological stage. These indices demonstrated good correlations with the destructive quantifications. Also, the degradation intensity of the chlorophyll was different from that of the carotenoid during maturation, leading to a change in their absorption proportion throughout ripening. This finding allows suggesting that in white berries, the colour change during berry development is not related to the activation of a specific biosynthetic pathway, but is mostly the result of catabolic processes. Chardonnay and Riesling showed different susceptibility to sunburn. The results suggest that for each variety, the timing of leaf removal during the day is fundamental to reducing the appearance of brown color in the berry skin. In Chardonnay it would be better to avoid any leaf removal especially in pre-veraison, in the morning. Because Riesling was more susceptible during the afternoon, it would be recommendable to remove leaves, if necessary, during the early morning. The aims of the last two years (2014-2015) data collection were: (i) to evaluate phenolics in Riesling and Chardonnay berries in response to sunlight exposure under different irrigation regimes; (ii) to study the relation of water stress to sunburn appearance. The experiment was carried out in the Columbia Crest vineyards (Columbia Valley, Washington State, USA). Chardonnay and Riesling berries were collected from two different vineyards. Two different irrigation regimes were applied in both vineyards. Full irrigation (FI): vines were irrigated to replace 100% crop evapotranspiration, from fruit set to harvest, with no water stress imposed. Deficit irrigation (DI): vines were irrigated to maintain a moderate water stress (stem water potential (stem) at midday between -0.7 and -1 MPa) from fruit set to harvest. For compositional analysis two cluster exposures were considered: one exposed to direct sunlight (sun), and the other totally shaded from sunlight (shade). Skin flavonol, flavan3ol and proanthocyanidin content were analysed by HPLC methods. Total tannins were measured by spectrophotometer. Six temperature/light sensors per irrigation treatment were randomly installed on 3 sun and 3 shade bunches. In terms of absolute concentration in both varieties, several statistical tests indicate greater amount of variance accounted for by the effect exposure and phenological stages, and their interaction. No effect was due to the irrigation, in both varieties. Chardonnay had much higher flavanol concentration than Riesling. Chardonnay had much higher flavanols in the pre- veraison than during ripening. In Chardonnay the amount of monomers, dimers, trimers and polymers was greater in sun exposed berries than in shaded berries. Unlike in Chardonnay, in Riesling no flavan3ols monomers were detected by HPLC during the pre-veraison and veraison. In Riesling the amount of monomers, dimers and trimers was greater in sun exposed berries than in shaded berries. In both varieties flavonol concentrations were much higher in sun exposed berries than in shaded berries, with absolute much higher concentration in Chardonnay. Flavonols increased during ripening, especially in the sun in both Chardonnay and Riesling. Under similar light conditions the difference in temperature within the two irrigation treatments in the sun, could be due to the less vigorous canopy of DI plants compared with FI plants, which overall lead to greater cluster exposure, in both varieties. Nonetheless, the temperatures were always lower in FI and higher in DI, supporting the hypothesis of a possible no (or little) effect on flavonol biosynthesis. Shaded-bunch temperatures were always lower as compared to sun exposed ones, as direct solar heating did not occur in the shaded-conditions.