Effect of severe summer pruning, leaf and bunch removal on nitrogenous compounds in Shiraz must in Australia

Viticultural practices are important in optimising berry ripening and enhancing grape composition; varying the leaf:fruit area ratio is a fundamental principle of grape production. Fruiting capacity and berry composition of grapevines are dependent on canopy area, with grapevine reserves also affected by crop load. During grape maturation, nitrogen (N) uptake is limited, therefore grapes obtain N mobilised from annual and perennial vegetative organs. Removing leaves or bunches could influence N accumulation in the fruit and consequently nitrogenous compounds that are important for the fermentation process. Early leaf removal was implemented prior to flowering, while severe summer pruning and bunch removal were applied shortly before veraison to Shiraz vines during three growing seasons in three vineyards located in Langhorne Creek (South Australia), Murray Valley (Victoria) and Hilltops (New South Wales). The implemented difference in leaf:fruit area ratio increased or reduced grape maturation rates, and harvest was conducted at similar berry ripeness. Berry samples were taken to assess basic juice parameters, including nitrogenous compounds prior to fermentation. In Hilltops, free amino N (FAN), ammonia (NH3) and consequently the yeast assimilable N (YAN) levels were lowered by leaf removal and this was more pronounced after severe summer pruning. In Langhorne Creek, only the NH3 fraction was lowered by early leaf removal, while Murray Valley had the lowest levels of nitrogenous compounds assessed with no treatment influence observed. The lower YAN levels in grapes coming from vines that had leaves or a combination of shoot/leaf sections removed could be due to the reduced availability of N. During maturation, the berry demand for N is commonly mobilised from the other annual or perennial tissues as N uptake from the soil is often insufficient. A diversion of N to other nitrogenous compounds such as proline in the berry could be another reason for the YAN reduction, often being elevated under abiotic stress. The variation of treatment responses between sites is likely due to the differences in irrigation and nutrition management.