SUNBURN PHYSIOLOGY PHENOTYPIZATION BY REFLECTANCE

Sunburn could affect the grape quality both for chemical modifications and visual impact of the browning. Different varietal behaviors could support the selection of the best cultivars in relation to the environmental and cultural conditions. In this study, optical properties of 17 traditional white grapevine varieties of Lombardy were investigated in the visible and near infrared regions using a non-invasive instrument. Experiment was set up in the germoplasm collection of the Regional Research Station of Riccagioia (Lombardy, Northern Italy). Browning was induced by exposing clusters to sunlight from 11.30 am until 4.30 pm and mechanically removing the epicuticular waxes. All the varieties were treated the same day in the last phase of fruit ripening. Spectra were taken from 20 exposed and 20 shaded berries, in the same bunch. Polyphenols contents were evaluated after extractions, while chlorophyll and carotenoids were estimated using several reflectance indexes. Differences between varietal and burned spectra underlined mainly the lost in chlorophyll, the formation of brown compounds (λmax 540 nm), no changes in carotenoids content and a probable tissue structural variations (modifications in the NIR region). No direct relations were found between phenolic concentration and browning intensity, while chlorophyll content was correlated to the sun browning incidence. The higher the chlorophyll concentration, the stronger the browning symptoms appeared. Two different groups were evidenced showing that a higher carotenoids content could significantly decrease the browning symptoms at similar chlorophyll levels. Also if phenolics are expected to be the substrate for the “melanin-like pigments”, their content does not directly correlate with the browning intensity. Thus, it is presumable that the light harvesting plays the most important role in the sunburn physiology, secondarily related to the ability of the carotenoids to recycle the excess of energy. Phenolics seem to be only the last step of the energetic chain providing irreversible oxidative reactions. The development of reflectance techniques could support grape varieties selection and cultivar-specific site managements.