Inflorescence Formation Part 3

On June 21, I blogged about Grapevine Inflorescence Formation---Parts 1 and 2. The timing of inflorescence production is what I'd like to tackle in this blog.
In the previous blog, 2 was circled, today's blog is a discussion about 3.¹

The steps between 2 and 3 in the illustration above is occurring from early spring to midsummer in the current season, that is, right now.
This constitutes formation of inflorescence that can be categorized into three separate processes:²
          1) Formation of anlagen or uncommitted primordia
              also-Inflorescence initiation (induction)
          2) Formation of inflorescence primordia
          3) Formation of flowers
The first two stages are completed during the current growing season. The formation of the inflorescence primordia or anlage (German for rudiment) itself has been divided into 7 Inflorescence Primordia (IP) Stages:²

IP   Stage	Description2,3	Illustration3
Stage   0	Depending on the cultivar, the shoot apical meristem (SAM) of a young latent bud forms a specific number of leaf primordia before the formation of the first anlage. The illustration to the right shows that the shoot apical meristem (SAM) is surrounded by leaf primordia at its ends and scale primordia on the flanks.
Stage   1	Bisection of the shoot apical meristem (SAM) to form the anlage. The anlage is opposite the youngest leaf primordium. The illustration to the right shows that the apex of the shoot apical meristem (SAM) is bilobed into two equal parts with the SAM at the end next to the youngest leaf primordium and the anlage (at stage 1) at the other end.
Stage   2	The anlage has separated from the apex and is developing into blunt broad obovate structure. The illustration to the right shows that the anlage (at stage 2) has separated from the shoot apical meristem (SAM) and is becoming more broad and blunt with no scales on its flanks. Leaf primordia are at the ends of the growing point.
Stage   3	Formation of a bract primordium from the abaxial flank of the anlage. Stage 3 is identified by the formation of the bract. The illustration to the right shows the bract surrounding the outer edge of the anlage.
Stage   4	Division of the anlage to form an inner arm and an outer arm. The inner arm becomes the main axis of the inflorescence and the outer arm becomes the proximal branch of the inflorescence. The illustration to the right shows that the anlage has separated into two arms. The outer arm is surrounded by a bract on its outer edge and is smaller than the inner arm. The inner arm is broad with a flat superior surface.
Stage   5a	Growth of the main axis (inner arm) to give rise to the first branch primordium (BP). The illustration to the right shows that the outer arm has little growth and no branching, and the inner arm has two branches. The outer arm is substantially covered by a bract that is clearly visible.
Stage   5b	The illustration to the right show that the outer arm has not branched and is almost fully covered by a large bract and the inner arm is much larger and has more than four branches.
Stage   6	Growth of the main axis of the inflorescence primordium to form several branch primordia and bract primordia. The illustration to the right shows that the outer arm has just started branching and has at least two branches. The inner arm is larger with many branches.
Stage   7	Differentiation of the branch primordium at bud burst and formation of the flower initials. The illustration to the right shows that the outer arm has more than two branches; the inner arm is larger with many branches. A large bract covers the outer arm and there is a distinct gap between the inner and outer arms.

One reason that I decided to look into the formation of inflorescences is that when we pruned our vines in early March, we left the cane a little longer than necessary in the event that there was a frost, the apical bud will break first and delay budbreak for the rest of the cane. As it turned out, we didn't have any frost event in early spring and we became busy with our planting of 4200 vines and left the canes longer than necessary. When we went back to prune the long canes, they were already at the 10-16 inch shoot stage and showing many inflorescences, which we unfortunately had to prune back. The remaining shoots, didn't show any inflorescences and I wondered why. This is my attempt to answer the question for myself. I'll stop here for today and maybe tackle the conditions necessary for bud fruitfulness in a future blog. Stay tuned.
Reference:
1. For the illustration the grape inflorescence growth cycle was taken from:
Wine Science - Third Edition, Ronald Jackson
The grape bud was taken from:
YouTube: The Grapevine Bud
The was taken from:
Jennifer Hashim-Maguire, Managing the vegetative canopy and its effects on bud fruitfulness and fruit quality.
2. Chinnathambi Srinivasan and Michael G. Mullins, Physiology of Flowering in the Grapevine a Review, Am. J. Enol. Vitic., Vol. 32, No. 1, 1981.
3. Peter W. Noyce, John D.I. Harper, Christopher C. Steel, Robyn M. Wood, "A Practical Method for Staging Grapevine Inflorescence Primordia in Season 1, with Improved Description of Stages" American Journal of Enology and Viticulture, Nov 2015, 66 (4) 492-501; DOI: 10.5344/ajev.2015.14135.
4. Markus Keller, The Science of Grapevines Anatomy and Physiology, 2010, Academic Press, pg. 43.