Branch Configuration Impacts on Production, Fruit Quality, and Leaf Minerals of ‘Aztec Fuji’ Apple Trees in an Upright Single Row High-Density Orchard System Over Five Years

Tree architectures play a critical role in the productivity of high-density orchards, but limited information is available in this subject. We studied effects of three branch configurations on tree growth, yield components, fruit quality and leaf mineral nutrients in ‘Aztec Fuji’ apple (Malus domestica Bork.) in a single row upright high-density system under southwest Idaho, USA conditions over 2012-2016. This study revealed that trees trained into a Tall Spindle (TS) had larger trunk cross sectional area (TCSA) than those with an Overlapped Arm (OA) system. Trees trained into a TS had higher number of fruit and yield per tree, three years after planting in 2012, than those with a Tipping Arm (TA) or OA system. However, in 2013, trees with a TA system had higher yield than those with a TS or OA configuration due to trees’ biennial bearing habit and higher spur formation in trees with a TA system. Trees receiving a TA training had lower biennial bearing index between all consecutive years. Trees with an OA training had smaller fruit than those with either a TA or TS training in all years between 2012-2016. Training systems did not have any effect on fruit color, soluble solids concentration, or starch degradation pattern at harvest. However, fruit from trees with an OA training had higher firmness and lower water core than those from trees with a TS or TA training. Leaves from trees receiving a TA training had greater leaf area, fresh weight, and potassium (K) and magnesium (Mn) concentrations than those with other trainings. Leaves from trees receiving an OA training had higher leaf iron (Fe), zinc (Zn), and copper (Cu) than those with a TS training. In this study, we concluded that training trees into a TA configuration rather than an OA system is recommended if the management and operation of apple production mandate the use of an “upright wall” structure to facilitate mechanical harvesting.

. Clements (2011) studied the performance of 'Honeycrisp' and 'McIntosh' apples on Bud 9, M.26, and MM.106 rootstocks with a central leader (CL), vertical axis (VA), or TS tree architecture systems. In both cultivars, trees on Bud 9 had the highest year cumulative yield over 2008-2010 (during 3 rd through 5 th leaf), followed by those on M.26 and MM.106 rootstocks. In that study, trees with TS had the highest production per hectare, followed by those with VA and CL. In Idaho, training 'Fuji' apples on Bud 9 rootstock with a TS system resulted in more regular cropping than those on Nic 9 with a CL system .
Limited information exists on the impact of branch manipulation on yields and fruit quality attributes of apples in high-density orchards. Therefore, the objective of this experiment was to study the effect of three branch configurations on tree growth, yield, fruit quality attributes and leaf mineral concentrations of 'Aztec Fuji' apple in a single row upright high-density system over five consecutive years between 2012 and 2016.

Orchard Establishment and General Cultural Practices
The experimental orchard was established at the Parma Research and Extension Center, University of Idaho, Parma, Idaho, USA in the spring and early-summer 2010. The experimental site was located at 43.7853° N, 116.9422° W, and had a semi-arid climate with an annual precipitation of approx. 297 mm on a sandy loam soil of approx. pH 7.3. In general, pest and diseases control practices were like those recommended for commercial orchards in the Pacific Northwest (Washington State University, 2020). Crested wheatgrass (Agropyron cristatum (L.) Gaertn.), a drought-tolerant grass, was planted as the orchard floor cover in all treatments. The materials and methods for irrigation system, mineral nutrients, bloom and post-bloom chemical applications and hand-thinning were similar to those previously described by  and Fallahi, Mahdavi, Kaiser, & Fallahi (2019). Trees were irrigated, using a drip system, twice a week, using 100% apple crop evapotranspiration (ETc) (Proebsting, 1994), but adjusted for the ground shading area (GS), as described by Allen, Pereira, Raes & Smith (1998) and Fallahi, Fallahi, & Shafii (2013).

Tree Architecture or Training Treatments
'Aztec Fuji' trees on Budagovsky 9 (Bud 9) rootstock (C & O Nursery, Wenatchee, WA) were planted at 0.91 × 3.66 m spacing in upright single rows with a north-south orientation. 'Snow Drift' crab apple (Malus × 'Snowdrift') on M.26 EMLA rootstock (C & O Nursery, Wenatchee, WA) was planted between every 10 'Aztec Fuji' trees in each row as a pollinizer. One pressure-treated pole, with 15 cm diameter and 4.9 m length, was installed at every 7.31-m spacing (between every 8 trees), with about 90 cm of the pole buried in the ground and 4 m above the ground. Seven rows of 4.96-mm 2 gauge galvanized wires were installed on the poles. The first wire was installed at 61 cm above the ground and the other 6 wires were installed 45 cm apart from each other in such a way that the last wire was installed at 3.31 m above the ground level. Branches were trained (configured) into one of the following three systems: 1) Tall Spindle (TS): In trees with a Tall Spindle training, only 14-18 "feathered small branches", equally spaced around the central leader of each tree, were remained and tied to the trunk at 110-degree angles from vertical, using cotton ties. In this technique, tree leaders were not removed, or were very minimally tipped (about 3 cm from the top) to eliminated meristems that were damaged during shipment. Tree leaders were maintained at approx. 3.75 m in height. No permanent scaffolds or branches were left on the main trunk and side branches were bevel cut back to 15-cm stubs when their diameters were thicker than 1/2 to 2/3 of the diameter of the main leader, to generate new fruiting shoots. 2) Overlapped Arm (OA): The basic structure of this training was similar to the TS system. However, in the OA system, only seven pairs of bilateral cordon arms were chosen or created in the north-south orientation (along the row) at 90 o in relation to and at about 45 cm apart along the main trunk and these arms were tied to the wires in a horizontal orientation during late dormant throughout the entire growing season. Other branches or feathers were eliminated. Thus, all arms of trees with an OA system were on the same plane. In this system, the tip of each arm remained uncut until it reached the main trunk of the next tree. Thus, arms of the two adjacent trees would "overlap" each other in an OA system. If an arm did not exist in the exact desired place on the trunk, a 3-4-cm cut (scoring) was made through the bark cambium layer at about 6-7 mm above an outward going bud in that place, using a sharp scoring knife. In this system, risers (shoot suckers) that grew from each arm were cut short to about 15 cm, preferably down to an outward growing shoot, at any time during the growing season to create spur structure, similar to the system described by Goodwin (2016); 3) Tipped Arm system (TA): Trees with a TA training system were identical to those with an OA system except that the tip of each arm in the TA was cut and maintained at half-way between the two adjacent trees, at 45 cm distance from the main trunk of each tree.

Tree Growth, Yield and Quality Attributes
To determine tree growth, tree trunk diameter was measured from 30 cm above the bud union (about 35 cm from the soil level), using a digital caliper, in early November of each year in 2012-2016, and trunk cross sectional area (TCSA) was calculated. Yield per tree was recorded at harvest time, and twenty fruits were randomly sampled from each tree for measuring quality attributes on October 17-20 each year. For quality attribute evaluation at harvest, fruits were weighed, and fruit color was visually ranked on a scale of 1 to 5, with 1 = 20% red, progressively to 5 = 100% red. Soluble solids concentration (SSC) was measured by a temperature-compensated refractometer (Atago N1, Tokyo, Japan). Fruit firmness was measured on three peeled sides of each fruit with a Fruit Texture Analyzer (Guss, Strand, Western Cape, South Africa). This texture tester measured the force needed to puncture a 7.9 mm-deep hole on each of the three peeled sides of the fruit, using an 11-mm tip. Starch degradation pattern (SDP) of equatorial slices of each fruit was recorded by comparison with the SDP standard chart developed for 'Fuji' apples (Bartram et al., 1993). In that chart, SDP is scaled from 1 to 6 (1 = least fruit SDP, progressively to 6 = most SDP or starch hydrolysis). Percentage of fruit water core at harvest was calculated by counting the total number of fruits with each of these incidences, divided by the total number of fruits in the sub-sample and multiplied by 100.

Leaf Weight and Mineral Nutrient Measurements
Thirty leaves per tree were sampled at random from the middle of the current-season shoots in mid-August each year. These leaves were put in a cooler and taken to the University of Idaho Pomology and Viticulture Laboratories where their fresh weights were measured. Leaf preparation and digestion for mineral analyses were similar to those described in earlier reports (Chaplin & Dixon, 1974;.

Experimental Designs and Statistics
The experimental design in each year was a randomized complete block with three branch configurations (trainings), each with four 2-tree blocks (total of eight trees per branch configuration treatment). Interaction between years for each parameter was also calculated. The assumption of normal data distribution was checked by computing univariate analyses for all tree responses in this study. Analyses of variance was conducted using SAS (SAS Institute, Cary, NC, USA), with GLM and means were compared by least significant difference (LSD) at P ≤ 0.05.

Interaction
Other than the cases in branch configuration systems for yield components (yield per tree, yield efficiency, crop efficiency, and number of fruits per tree) between years 2012 and 2013, no significant interaction was observed between branch configuration-year for any measurements in this study. Thus, only effects of direct tree architectures (branch configurations) are reported in Tables 1-5.  Note. z TCSA = trunk cross sectional area. y Mean and Significance denotations: Mean values within each column followed by different letters are significant at 5% (*), 1% (**) and those followed by the same letters are not different at 5% (ns), using least significant difference test. Note. z TCSA = trunk cross sectional area in cm 2 . y Mean and Significance denotations: Mean values within each column followed by different letters are significant at 5% (*), 1% (**) and those followed by the same letters are not different at 5% (ns), using least significant difference test. Note. z Mean and Significance denotations: Mean values within each column followed by different letters are significant at 5% (*), 1 (**) and those followed by the same letters are not different at 5% (ns), using least significant difference test. y Fruit color rank: 1 = least red color, 5 highest red color. SSC = soluble solids concentration. SDP = Starch degradation pattern: 1 = least starch degradation; 6 = highest starch hydrolysis.

Fruit N
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Fruit from branch tra TCSA (Ta ratio and t because of significant (Table 4).
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Leaf Growth and Mineral Concentrations
Leaves from trees receiving a TA branch configuration system had significantly greater leaf area, fresh weight, K and Mn concentrations than those with other branch training systems (Table 5), perhaps because leaves from trees with a TA branch configuration had higher rate of photosynthesis and this area deserves further study. Maintaining higher annual leaf K levels in a high-density system is extremely critical. For example, in a side-test, soil K levels in our experimental site at the time of planting was well in the sufficiency range. However, scion leaf K declined with every year of fruit production when supplementary K was not applied (data not shown). The lower BBI in trees receiving a TA configuration could be in part due to their higher leaf K concentrations and this area deserves further study.
Averaging over five years revealed that leaf K concentration in trees receiving a TA branch training system was in the sufficiency range while those in trees with OA and TS systems were bordering deficiency ranges based on Westwood (1978) and Mills and Jones (1996). Thus, it is extremely important to consider several genetic, environmental and cultural practices factors when interpreting results of leaf mineral analyses and before recommending any nutrients remedies. As an example, one may interpret and recommend the same rate of K fertilizer application in all apple trees with different training systems, without regard to their branch configuration, and resulting in under application of K in some and over application of K in other trees. In an earlier report in 'Gala apples', leaf K had a positive correlation with the rootstock vigor (Fallahi, Arzani, & Fallahi, 2013). In our present study, however, TCSA of trees with different branch configurations were not strongly correlated to their scion leaf K because tree vigor differences were created by manipulating branch architectures system rather than rootstock.
Leaves from trees receiving an OA branch configuration had significantly higher leaf Fe, Zn, and Cu than those with a TS system (Table 5), perhaps because leaves of trees with an OA branch configuration have thinner cuticle and thus absorb more of certain micronutrient sprays. This area also deserves further study.

Conclusions
This study revealed that trees trained into a TS system had larger TCSA than those with an OA branching system. Trees receiving a TA training had lower biennial bearing index between all consecutive years. Fruit from trees with an OA training had smaller fruit than those with either a TA or TS training in all years. Training systems did not have any effect on fruit color, SSC, and SDP at harvest. Fruit from trees with an OA branch configuration system had higher firmness and lower watercore than those from trees with a TS or TA training. Leaves from trees receiving a TA branch configuration had greater leaf area, fresh weight, and K and Mn concentrations than those with other trainings. Based on this long-term study, training trees into a TA configuration rather than an OA system is recommended if the management and operation of apple production mandate the use of a single row "upright wall" structure to facilitate mechanical harvesting.