Timing of cluster light environment manipulation during grape development affects C₁₃ norisoprenoid and carotenoid concentrations in riesling

Sunlight exposure of winegrape clusters is frequently reported to increase C₁₃-norisoprenoids in resulting wines, but the timing and mechanism of this influence is not well understood. Fruit zone leaf removal was applied to Vitis vinifera cv. Riesling at three timings: 2, 33 and 68 days past berry set (PBS), and compared to an untreated control. Free and total 1,1,6-trimethyl-1,2- dihydronaphthalene (TDN), vitispirane and β-damascenone were measured in juice and wines, and carotenoid profiles were determined in grapes at midseason and maturity. Significantly higher total TDN was observed in grapes from the 33-day PBS treatment compared to the control and other treatments (195 μg/L vs 54-87 μg/L). Total vitispirane in juice was also significantly increased in the 33-day PBS treatment, while total β-damascenone was reduced in the 68-day PBS treatment compared to the control. Existing HPLC protocols were modified to allow for quantification of zeaxanthin in V. vinifera berries, and zeaxanthin was determined to be significantly higher in the 33-day PBS treatment than the control or other treatments (p < 0.05). Total TDN in juice correlated with free TDN in wine, with 11.0% ' 2.5% of total juice TDN converted to free TDN in wine. In contrast, total vitispirane increased significantly during fermentation, and was not correlated with vitispirane in juice. In summary, leaf removal at 33 days PBS significantly increased zeaxanthin in Riesling grapes midseason, total TDN and vitispirane in the juice of mature Riesling grapes, and free and total TDN in finished wine, while earlier or later leaf removal had no effect.