Free monoterpenoid accumulation in 'Riesling' (Vitis vinifera L.) is light-sensitive and uncoupled from grape hexose accumulation.

Monoterpenoid biosynthesis mirrors hexoses accumulation in grape berries during ripening and is affected by environmental factors such as solar radiation. However, no research has confirmed the dependency of monoterpenoid accumulation on grape maturity. Using potted 'Riesling' (Vitis vinifera L.) vines, we girdled (GD) shoots at veraison to halt leaf photoassimilate translocation to berries and also utilized light-impenetrable bags to exclude light exposure (LE) to grape clusters to <2 μmol ms during ripening. GD and LE were compared to an untreated control (CT), and a combined (GDxLE) treatment. GD caused leaf photosynthetic assimilation to decline from one-week post-treatment until harvest maturity due to feedback inhibition of photosynthesis. This was coupled to a decrease in hexoses and ABA accumulation in GD berries. Meanwhile, LE did not affect photosynthetic assimilation, or hexoses and ABA accumulation. We hypothesized that the accumulation of free and bound monoterpenoids in 'Riesling' berries would also be reduced by GD, if monoterpenoid accumulation was dependent on grape maturity. LE significantly decreased free monoterpenoids compared to CT. However, GD did not alter total free and bound monoterpenoid concentrations. GD decreased free linalool oxide I and II and bound linalool oxide I compared to CT, and increased citral I and II and α-terpineol. Although exogenous jasmonate applications stimulate monoterpenoid biosynthesis in grapes, we showed that endogenous jasmonate concentrations were not altered by GD or LE; therefore, any monoterpenoid modulation by GD and LE treatments was not mediated by endogenous jasmonate levels. This study indicates that free monoterpenoid biosynthesis during ripening is uncoupled from hexose accumulation and is strongly regulated by solar radiation.