Leaf Area/Crop Weight Ratios of Grapevines

Recently, I was refreshing myself on leaf area/crop weight ratios in grapevines by re-reading a 2005 research article written by W. Mark Kliewer and Nick K. Dokoozlian.¹ Somewhere in the recess of my mind was a vague recollection that it took 15 leaves to ripen a bunch of grapes. Is that really true?
In this study, Kliewer and Dokoozlian reviewed the various definitions of vine balance:

• Winkler and Williams defined a grapevine as being well balanced and not overcropped when the vine brings its fruit from flowering to a given degree Brix, depending on the use to which the fruit will be used, with a given summation of degree days of heat, which is constant for a given variety
• Shaulis defined vine balance as dormant vine pruning weights, expressed as kg per meter of canopy length with neither too little nor too much growth with values of 0.3 to 0.6 kg pruning weight per meter of canopy length generally considered to be in the optimal range
• Another vine balance measure is crop load or the ratio between crop yield and dormant vine pruning weight vines with crop load values between 5 to 10 are considered in the optimal range.

Smart and Robinson devised an 80-point scoring system to evaluate vineyard balance and potential fruit quality assurance through the use of point quadrant analyses of canopy density and microclimate.
The Smart and Robinson 80-point scoring system took into account:

• leaf layer number
• percent canopy gaps
• percentage interior and exterior leaves and clusters

And five other measures related to the physiological status of grapevines:

• leaf size
• leaf color
• shoot length
• lateral growth
• presence of active shoot tips during the ripening period

These latter five characteristics are assessed by visual observation and require some advanced experience of what is desirable. Each of the eight characteristics is assigned zero to 10 points (maximum). Open-type canopies that have moderate shoot vigor and minimal amounts of lateral shoot growth are rated highest.
The Kliewer and Dokoozlian study defined the optimum level of cropping in terms of leaf area required per unit weight of fruit, expressed as m²/kg, to produce berries of maximum total soluble solids, skin color, and total sugar accumulation in fruits.
The optimal leaf area/crop weight ratio of single- and divided-canopy training systems were compared from field experiments of several cultivars including Thompson Seedless, Tokay, Chenin blanc, and Cabernet Sauvignon located at Davis or Oakville, California.
The caveat was that since temperature and light for maximum photosynthesis are frequently more limiting in cool-climate regions, it is likely that the leaf area/fruit weight ratios for optimal fruit composition and wine quality will be somewhat higher in cool-climate regions.
Since we are growing Cabernet Sauvignon and Chenin Blanc, my attention was focussed only on those two varieties.
For Cabernet Sauvignon, the published values of crop weight/pruning weight ratios within the range of 5 to 10 as indicative of vines well balanced (not over or undercropped), then the corresponding leaf area/crop weight ratios fall between 0.8 to 1.2 m² leaf area per kg fruit and agree very closely with the fruit total soluble solids leaf area/crop weight relationship.
For Chenin Blanc, the authors found that the number of primary leaves required per cluster to ripen fruit to 22 Brix for V-, lyre-, and GDC-trellised vines was 12 to 13, 10 to 11, and 10 to 11, respectively.
So my guess of 15 leaves to ripen a cluster was pretty close.
References:
1. W. Mark Kliewer and Nick K. Dokoozlian, Leaf Area/Crop Weight Ratios of Grapevines: Influence on Fruit Composition and Wine Quality, Am. J. Enol. Vitic., 56:2 (2005). Proceedings of the ASEV 50th Anniversary Annual Meeting.