Sauvignon blanc: The effect of pre-budburst temperature on phenology, flowering, fruit-set and bunch composition

In this study, researchers focused on the effect that temperature during winter dormancy and budburst has on the timing of key phenological stages as well as inflorescence primordia and grape composition.

Project layout
Individual Sauvignon blanc grapevines in a commercial vineyard were heated for different periods of time from mid-winter (July) to budbreak (Oct).
Treatment 1: Heating 1 July to budbreak (mean daily temp about 2°C higher than control)
Treatment 2: Heating July only
Treatment 3: Heating 1 August to budbreak
Treatment 4: Control
Buds and inflorescence structures on shoots, at every position along the cane were monitored during the whole growing season.

Results
-          Heating during dormancy had no effect on the number, or position of inflorescences on a shoot;
-          Heating during dormancy decreased the presence of side bunches. The presence of a tendril in lieu of a side bunch increased, especially higher up on the shoot.
-          Heating during July to budbreak and August to budbreak advanced the date of budbreak by 12-14 days and advanced the start date of flowering by 14 days, compared to the control and Treatment 2;
-          Despite the above difference, the time taken from 50% flowering to reaching 14 °Brix, did not differ between treatments. It did however differ within each vine, where bunches higher up on the shoot took longer to reach 14 °Brix.
-          The time taken from 50% flowering to reaching 14 °Brix also differed between the main bunch and the side bunch, in all treatments. This difference also increased at the higher shoot node positions.
-          Heating during July to budbreak and August to budbreak decreased the number of flowers per inflorescence, but increased fruit set and bunch mass;
-          Mean temperatures 12 days before budbreak significantly affected number of flowers per shoot. Every 1°C increase in mean daily temperature decreased flower number per shoot by 54.
-          Flower number per inflorescence and temperature during flowering together influence fruitset.

Significance of the study
This study showed that much of the variation within a vine can be attributed to the position of a bunch on a shoot and the presence of side bunches. Side bunches ripen significantly later than the main bunch, especially at higher node positions on a shoot. Removing side bunches on apical bunches could help to reduce fruit variation on individual vines.



Image: Shutterstock

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