Grape Berry Development - Heading into Véraison

Right around this time of the year is a good time to review a very relevant article called Understanding berry development by James Kennedy.
August is typically the month that our grapes enter véraison or the onset of ripening. The grapes have been in the first growth stage and are now in the lag phase. This stage lasts approximately 60 days after bloom. In our case, since our bloom occurred during mid-June, we would expect véraison to begin around mid-August.¹

Another way to look at the same information is:²

A grape berry only starts to accumulate significant sugars from the time of véraison indicated by:

• berry softening
• skin colour changing
• recommencing of rapid growth

This dramatic increase in sugar accumulation at véraison is accompanied by profound physiological variations:³

• sugar phloem loading shifts from symplasmic to apoplasmic pathway
• transcription of some genes encoding sugar transporters is enhanced
• activity and expression of key sugar metabolism enzymes are increased
• water influx pathway changes from predominantly xylem to phloem

Here is yet another way to look at the grape berry development path:⁴

References:
1. Illustration of the double sigmoid curve from: Moschou, Panagiotis & Aziz, Aziz & Roubelakis-Angelakis, Kalliopi, Chapter 7, Polyamines and grape berry development, The Biochemistry of the Grape Berry, (2012), 137-159.
2. Kennedy, James, Understanding berry development, Practical Winery and Vineyard, 2002, 24.
3. Zhan Wu Dai, Philippe Vivin, Thierry Robert, Sylvie Milin, Shao Hua Li and Michel Génard, "Model-based analysis of sugar accumulation in response to source–sink ratio and water supply in grape (Vitis vinifera) berries", Functional Plant Biology, 2009, 36, 527–540.
4. Rogiers Suzy Y., Coetzee Zelmari A., Walker Rob R., Deloire Alain, Tyerman Stephen D., "Potassium in the Grape (Vitis vinifera L.) Berry: Transport and Function", Frontiers in Plant Science, Volume 8, 2017, pg. 1629.
5. Xiao-Yan Zhang, Xiu-Ling Wang, Xiao-Fang Wang, Guo-Hai Xia, Qiu-Hong Pan, Ren-Chun Fan, Fu-Qing Wu, Xiang-Chun Yu, and Da-Peng Zhang, "A Shift of Phloem Unloading from Symplasmic to Apoplasmic Pathway Is Involved in Developmental Onset of Ripening in Grape Berry", Plant Physiology, September 2006, Vol. 142, pp. 220–232.