Understanding Optimal Anthocyanin Accumulation of ‘Merlot' Grapes – Influence of Light Exclusion

Anthocyanins are important components to all red colored fruit, but are absolutely crucial for red wine grapes. Despite a century of research we still have no concrete understanding of how light and temperature impart their effects on anthocyanin accumulation and composition. Our aim was to elucidate the mechanistic response of anthocyanin metabolism in ‘Merlot’ grapes to microclimate, in this case sunlight. Unlike previous reports, this study was unique in that temperature, light intensity, and humidity were continually measured for the entire duration of the treatments. There were two controls and two treatments: control-shaded (CS; ambient), control-exposed (CE), light-excluded (LE), and aspirated light-excluded (ALE). Aspiration was initiated or terminated in reference to CS cluster temperature. Polypropylene boxes with white exterior and black interior were used to shade randomly selected healthy clusters at the onset of veraison (one to two berries turning color) for treatments (LE and ALE). All experimental clusters were on the west canopy aspect of a research vineyard in Prosser, WA. Treatments were imposed until a composite berry sample equaled commercial ripeness (~23 ºBrix). Harvested grapes were chemically extracted and extracts were analyzed for individual anthocyanins by HPLC/DAD. Detailed microclimate data will be summarized. All of the 15 anthocyanins previously reported in ‘Merlot’ grapes accrued in all clusters, indicating no accumulations were terminated from light exclusion during ripening. Overall, both LE and ALE clusters accumulated less anthocyanins (86.8 mg of malvidin-3-glucoside/100g berry) than controls (160.2 mg of malvidin-3-glucoside/100g berry). The proportions of the individual anthocyanins were altered by light exclusion. The ALE clusters had the least total anthocyanins (75.4 mg of malvidin-3-glucoside/100g berry), but more % acylated anthocyanins (52% of the total) compared to the other treatments (CS-36%, CE-37%, and LE-42%). This work contributes towards our growing body of knowledge on environmental stresses and anthocyanin development in fruit.