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Optimization of Light Interception, Leaf Area and Yield in “WA38”: Comparisons among Training Systems, Rootstocks and Pruning Techniques – A Summary

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Written by Brendon Anthony, Sara Serra, and Stefano Musacchi, WSU TFREC, Wenatchee, WA. June 2020.

Proper modern management of orchards should include the simplification of canopy architecture, with the efficient use of land area to maximize light interception, while balancing an even distribution of light across the entirety of the canopy. The ultimate goal of a new orchard’s design is to ensure consistent, high yields of premium quality fruit.

Leaf area index (leaf area : ground area) (LAI) and light interception have been demonstrated to relate to photosynthesis, yield and fruit quality. LAI is considered optimal in apple systems around 1.2 – 2.0, while light interception should be maintained around 65 – 75% (Jackson, 1978; Hampson et al., 2002; Robinson et al., 2013). Excessive LAI and light interception levels can negatively impact yield and quality, due to the increase of shaded areas in the canopy.

Training systems, rootstock selection and pruning techniques can all influence these parameters as they alter the way the canopy is arranged, controlled for size and/or manipulated. These horticultural selections are critically important for “WA38,” as it is a vigorous, Type IV (like ‘Granny Smith’), tip-bearing variety that has a tendency to produce “blind wood” (Lespinasse, 1977). A pruning technique, called “click” pruning (Figure 1A), has been demonstrated to ameliorate “blind wood” and shift the tip-bearing tendency of Type IV varieties towards more basal bud production in the branches (Musacchi and Greene, 2017). This helps increase production across the entirety of the canopy and keeps fruits closer to the central leader, thus eliminating “weeping” and exhausted branches that may bend excessively due to the weight of fruit set on terminal buds.

A trial assessing different training systems, rootstocks and pruning techniques was conducted from 2016 to 2017 (fourth and fifth leaf) in a “WA38” orchard planted near Wenatchee, WA in 2013. The training systems assessed were Spindle (3’ x 10’ ≈ 1,500 trees/acre) and V (1.5’ x 10’ ≈ 3,000 trees/acre), the rootstocks were M9-Nic29 (Nic29) and Geneva41® (G41), and the pruning techniques were “click” and bending. The “click” pruning entailed simplifying branches and heading back on one-year-old shoots to 5–10 cm stubs (Figure 1B). Bending consisted of the removal of upright shoots and manipulating branches below 90 degrees to induce flowering by reducing vegetative vigor imposed by acute crotch angles (Figure 1C). The trial evaluated how these horticultural selections impacted LAI, light interception, yield and bienniality.

Three images showing the difference between click and bending methods of pruning, depicting where the new shoots emerge.
Figure 1. (A & B) Pictures depict the “click” pruning method of shortening one-year-old wood and heading shoots with crotch angles around 45-degrees. This helps inhibit terminal fruiting, which bends the branch below 90-degrees and exhausts it. The “click” pruning in (A) and (B) also demonstrate the reduction of “blind wood” on the branches with a high incidence of flower buds across the totality of the branches. Whereas, the “bending” method (C) demonstrates the onset of “blind wood” (orange arrows) toward the basal portion of the shoots. Note that without the “click” pruning in (C), only the terminal tip flowers are predominantly present, as little lateral bud swelling has been stimulated.


In total, there were eight treatment combinations in this three-way experimental design (training system x rootstock x pruning). Leaf area index was estimated with a non-destructive canopy imager and light interception was measured with a 1-m long ceptometer throughout the summer (June – August) each year.

There were no differences in LAI in 2016, while Spindle-Nic29 combinations (regardless of pruning technique) demonstrated the lowest and most optimal LAI values in 2017 (Figure 2). Overall, the V training system (given its higher density) and trees grafted to G41 (given its increased vigor in our soils) demonstrated LAI values that exceeded optimal thresholds (> 2.0) (Figure 2).

Horizontal bar graph showing average leaf area index for each of the treatments.
Figure 2. Average leaf area index (LAI) non-destructive estimations across all treatment combinations in the summers (June–August) 2016 and 2017 for “WA38” trees grown in Wenatchee, WA. ANOVA indicated non-significant differences in 2016 (no lower-cased letters displayed) and a significance of: ** < 0.01 in 2017 (upper-cased letters). Student–Newman–Keuls (SNK) test for mean comparisons across all 8 combinations within a given year. Means to the right of each bar in the chart with the same letter indicate non-significance at p-value < 0.05. No differences among combinations in 2016, so no letters appear. Dashed vertical black lines indicate minimum and maximum optimal LAI thresholds based on literature. Errors bars ± SE, N = 3 trees.


Similarly, in respect to light interception, it appears the Spindle-Nic29 combinations (regardless of pruning technique) also generated values that are considered optimal, based on reported ranges in literature (65 – 75%), across both years (Figure 3).

Bar graph showing average light interception for each treatment.
Figure 3. Average light interception (LI %) at 20 cm (2016) and 40 cm (2017) from each trial tree across all treatment combinations in the summer (June–August) for “WA38” trees grown in Wenatchee, WA in 2016 and 2017. ANOVA indicated significance: *** (p < 0.001) for both 2016 (lower-cased letters) and 2017 (upper-cased letters). Student–Newman–Keuls (SNK) test for mean comparisons. Means to the right of each bar in the chart with the same letter indicate non-significance at p-value < 0.05. Dashed vertical black lines indicate minimum and maximum LI % optimal thresholds based on literature. Errors bars ± SE, N = 11 trees for Spindle plots, N = 10 trees for V plots.


In both years, Spindle trees produced more on a kg/tree basis, while V produced more on a per acre basis given the increased planting density. Rootstocks did not present differences in yield in either year. In 2016, “click” pruned trees produced more than bending, but there were no differences in 2017 between pruning techniques. This is due to the biennial nature of trees that received the bending treatment, with the bending trees experiencing an “on” year in 2017 matching the production in “click” trees. Vast differences in year to year production were seen in the bending combinations, with all of them receiving a biennial classification based on their 2016 – 2017 production (Figure 4). However, “click” pruned trees all received consistent bearing classifications (I < 0.50; Figure 4).

bar graph showing production and biennial index for each treatment.
Figure 4. Production (ton/acre) and biennial index (I) across all treatment combinations at harvest for “WA38” trees grown in Washington State in 2016 and 2017. ANOVA indicated no significance in 2016 and 2017 for calculated production. Biennial index (black triangles) from Hoblyn et al., 1936 comparing fruit count in 2016 and 2017. The classifications are quantified as: I = 0.90–1.00 = Strongly Biennial, 0.50–0.89 = Biennial, 0.10–0.49 = Consistent, 0.00–0.09 = Strongly Consistent. Black dashed line indicates the “biennial” classification threshold at 0.50. Errors bars ± SE, N = 3.


Bending branches in “WA38” does not regulate its’ tip-bearing tendency, which sets fruit terminally, and brings branches excessively below horizontal (i.e. “weeping). This exhausts the branch, leaving large amounts of “blind wood,” without flower buds and the ability to crop in the subsequent season, hence the onset of the alternate bearing cycle.

Inversely, “click” pruning heads one-year-old wood, effectively removing terminal flower buds and stimulates lateral bud break and flower formation. This allows for an increase in production, as “blind wood” is mitigated, but also fosters consistent production as new shoots and flower buds are stimulated each year. In combination to the “click” pruning strategy, a phloem-disruption technique, such as girdling and notching above dormant buds will help reduce “blind wood” and alternate bearing tendencies in “WA38.”

In sum, the treatment combination that maintained optimal LAI and light interception thresholds in both years, while maintaining a consistent bearing pattern was the Spindle-Nic29-“click” sequence. This combination of training-rootstock-pruning is categorized by a less vigorous rootstock (in our soil conditions) and a pruning style that maintains a narrow and flower bud rich canopy to ensure consistent yields of high-quality fruit.


  1. Hampson, C.; Quamme, H.; Brownlee, R. Tree density or training system-What is important in apple orchard design? Compact. Fruit Tree 2002, 35, 48–50.
  2. Hoblyn, T. Studies in biennial bearing. J. Pomol. Hortic. Sci. 1936, 14, 39–76.
  3. Jackson, J.E. Utilization of light resources by HDP systems, In Symposium on High Density Planting 65. Acta Hortic. 1978, 65, 61–70.
  4. Lespinasse, J.M.; Chol, P.; Dupin, J.; Terenne, E. La conduite du Pommier: Types de Fructification, Incidence sur la conduite de l’arbre; Brochure INVUFLEC: Paris, France, 1977; p. 80.
  5. Musacchi, S.; Greene, D. Innovations in Apple Tree Cultivation to Manage Crop Load and Ripening. In Achieving Sustainable Cultivation of Apples; Evans, K., Ed.; Burleigh Dodds Science Publishing: Cambridge, UK, 2017; pp. 195–237.
  6. Robinson, T.; Hoying, S.; Sazo, M.M.; DeMarree, A.; Dominguez, L. A vision for apple orchard systems of the future. N. Y. Fruit Q. 2013, 21, 11–16.

Full article citation

Anthony, B., Serra, S., and Musacchi, S., 2020. Optimization of Light Interception, Leaf Area and Yield in “WA38”: Comparisons among Training Systems, Rootstocks and Pruning Techniques. Agronomy, 10(5), p.689.

Link to full article:


Stefano Musacchi
Professor & Endowed Chair,
Tree Fruit Physiology & Management
WSU Tree Fruit & Research Extension Center
Wenatchee, WA 98801
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