2019 Volume 88 Issue 3 Pages 315-319
Moderately vigorous shoots of mature rabbiteye blueberry ‘Tifblue’ bushes were pruned in summer to clarify the effects on vegetative and reproductive traits. Treatments included un-pruned, 25% pruned (removing 25% of the shoot length), 50% pruned (removing 50% of the shoot length) and 75% pruned (removing 75% of the shoot length). Vegetative and flower bud number per shoot decreased with increasing pruning severity. Consequently, the number of laterals per shoot dropped. However, lateral length increased with increasing pruning severity. Shoots pruned at 75% produced a small number of laterals, but they were the most vigorous. Pruning severity induced the transition of vegetative buds to reproductive buds in areas lower than the cut position. The flower bud number per shoot decreased with increasing pruning severity. However, yield per shoot was not affected due to a compensatory increase in berry weight. Total soluble solids content and titratable acidity of the berry juice, as well as, the number of berries per flower bud were not affected. Hence, to reduce the unproductive parts of the plant, and to produce bigger berries, September removal of either 50% or 75% of the length of mature rabbiteye blueberry shoots under conditions similar to the Kanto region of Japan is recommended.
Pruning of blueberries (Vaccinium spp.) is a crucial practice to balance fruit load and vegetative growth (Jorquera et al., 2014; Müller, 2011; Strik and Buller, 2005), and to ensure adequate bearing wood for the next season of fruit production (Yarborough, 2006). If done properly, pruning renews the canopy and increases productivity by stimulating vegetative and reproductive growth (Kovaleski et al., 2015b). According to Gough (1991), firstly, pruning must always be light enough to ensure a significant crop for the current season. Secondly, pruning must be severe enough to ensure large berry size, and thirdly, pruning must be severe enough to ensure enough good quality bearing wood for the next season.
Fruits of blueberries are produced in the apical area of one-year-old shoots (Darnell, 1991; Hall et al., 1970; Phatak and Austin, 1990). At the same time, vegetative buds are produced in the basal area (Kovaleski et al., 2015b). Blueberries are pruned annually during late winter and early spring in the northern United States (Gough, 1991). In Japan, either severe or slight winter pruning of blueberry plants is applied. Severe cutting of vigorous shoots removes the apical productive area, and consequently, no fruit can be harvested in the next season from the same shoot. However, slight cutting of shoots done to maintain the yield results in expanding the unproductive parts of the plant. The Rabbiteye blueberry (Vaccinium virgatum Ait.) is more vigorous than highbush or lowbush blueberries (Davies, 1983). To limit the height of the bushes that are cultivated in small orchards (e.g., U-Pick operations), the mature bushes may be pruned in the summer after harvest or during the dormant winter period (Yarborough, 2006). Summer pruning consists of cutting back shoots during the plant’s active growth period from early summer through autumn to reduce the height of the bush and to stimulate branching and flower bud formation. However, most of the summer pruning studies on blueberries have been done in Chile and the United States (Bañados et al., 2009; Jorquera et al., 2014; Kovaleski et al., 2015a, b), where agroclimatic and management conditions are far different from those in the Japanese blueberry production areas. Müller (2011) found that summer pruning increased berry weight and diameter by reducing the total yield of the southern highbush blueberry in Stellenbosch, South Africa. However, in research at the Tokyo University of Agriculture and Technology, Karimi et al. (2017) found that June pruning of shoots induced laterals, whereas September pruning suppressed laterals but led to differentiation of vegetative buds to flower buds in the basal area of the shoot (below the pruning cuts). They recommended summer pruning as a substitute for, or to complement, winter pruning.
Pruning severity may need to be adjusted to balance the production of high yield and quality fruits with adequate shoots for subsequent seasons (Rana et al., 2011). Increased pruning severity decreases yield and increases the berry size and weight of highbush blueberries (Siefker and Hancock, 1987). Jorquera et al. (2014) reported that berry number and yield per plant decreased with increasing pruning severity, but canopy leaf area increased. Heavy summer pruning reduced the yield of the Rabbiteye blueberry compared with moderate and no pruning but did not affect fruit size (Spiers et al., 2002). However, research-based recommendations on the appropriate severity of summer pruning in the Rabbiteye blueberry are still lacking. This study aimed to determine the effects of summer pruning severity on the vegetative and reproductive traits of the Rabbiteye blueberry.
The experiments were conducted on field-grown mature ‘Tifblue’ bushes established 50 years previously in the research orchard of Tokyo University of Agriculture and Technology, Tokyo, Japan. Average plant height was 2.71 m in July 2016, and the shoot lengths at the time of treatment are presented in Figure 1. Treatments correspondeding to un-pruned, 25% pruned (removing 25% of the shoot length), 50% pruned (removing 50% of the shoot length) and 75% pruned (removing 75% of the shoot length) were applied in September 2015. Four of the current season’s moderately vigorous primary shoots were selected per treatment and each treatment was replicated on four different bushes. In January 2016, the total number of leaf buds and flower buds was counted.
The actual length of shoots before application of summer pruning in September 2015. 25% pruned: removing 25% of the shoot length, 50% pruned: removing 50% of the shoot length, 75% pruned: removing 75% of the shoot length. Different letters indicate significant differences at P < 0.05 by Tukey-Kramer’s test (n = 16).
In summer 2016, quantitative parameters of harvested berries such as berry diameter, berry weight, the number of berries and total berry weight per shoot, and the number of berries per flower bud, were calculated. Internal qualitative parameters such as total soluble solids (TSS, °Brix), and titratable acidity (TA, %) were also measured. TA was expressed as cumulative titratable acids per 100 mL fruit juice. Harvest was started in July 2016 at four-day intervals and continued through mid-August 2016. Fully mature (100% blue) fruits were harvested. Fruits from the first harvest were compared among treatments for fruit quality, berry weight, and diameter. TSS and TA were measured using freshly prepared juice. The juice of 10 randomly selected healthy fruits per treatment from each replication was manually extracted, filtered through a muslin cloth, and the TSS measured using a digital refractometer (PR-101α; ATAGO CO. LTD., Tokyo, Japan). TA was determined by the colored indicator method, diluting a proportion of 0.5 mL juice to 2.5 mL using distilled water and adding a drop of phenolphthalein, then titrating with NaOH (0.05 M). The results were expressed as percent concentration of succinic acid in the juice. For berry diameter, 10 fruits of the first harvest were randomly taken from each replication, and their diameters (mm) were measured using a digital vernier caliper. Average berry weight was determined by randomly selecting 10 fruits per replication at first harvest. Berry weight was measured using a digital scale (TA3001JP; Ohaus Corporation, China). The total number of berries and yield per shoot were obtained from cumulative values of all harvest intervals. The average number of berries per flower bud was determined by dividing the total number of berries per shoot by the total number of flower buds per shoot. To understand the effects of pruning severity on vegetative traits, the total number of laterals and the length of the most vigorous laterals per shoots were recorded in October 2016.
Statistical analysis was done using MS Excel 2013 software, Japanese version (Microsoft Japan, Tokyo, Japan) with the Statcel add-in, version 3 (OMS Publishing Inc, Saitama, Japan). Significant differences were determined by the Steel-Dwass’s test (P < 0.01 or 0.05) or Tukey-Kramer’s test (P < 0.05).
Production of strong fruit-bearing wood for the next cropping season is an objective of pruning in blueberries. We previously reported that early summer pruning (June) broke apical dominance and encouraged auxiliary bud breaking (Karimi et al., 2017). June pruning also stimulated the growth of secondary shoots, which produced flower buds at the top area. No secondary shoots developed but vegetative buds that were present at the new top area of primary shoots differentiated into flower buds after September pruning. From these results, we concluded that summer pruning could maintain the balance between growth and reproductive development. In this study, late summer pruning was not observed to stimulate the growth of secondary shoots, and vegetative buds at the new top area of primary shoots differentiated to flower buds (data not shown). This result was in agreement with our previous research (Karimi et al., 2017). The number of vegetative and flower buds per shoot decreased with increasing pruning severity (Fig. 2). Consequently, the same trend was observed in the number of laterals per shoot. Pruning severity increased the length of the most vigorous laterals by reducing their number (Fig. 3). In general fruit production, the growth of laterals originating from severely pruned shoots is more vigorous than that of light pruned shoots. According to Maust et al. (1999), increasing inflorescence bud density decreases overall growth and dry weight of shoots in highbush blueberry cultivars. Summer pruning increased the vigor of vegetative growth in highbush blueberries compared to not pruning (Kovaleski et al., 2015b; Yarborough, 2006). Müller (2011) found that severe pruning reduced the number of shoots and led to development of the longest average shoot length. In southern highbush blueberries, 60% pruning stimulated stronger vegetative growth, and consequently a larger canopy leaf area, compared to 30% pruning (Jorquera et al., 2014). Horticultural trees accumulate photosynthetic carbohydrates, before entering dormancy in winter. The next spring, these reserved carbohydrates are remobilized for reproductive and vegetative growth (Darnell and Birkhold, 1996; Garcia, 2002). In this study, in un-pruned and 25% pruned shoots, the number of laterals per shoot was higher and the length of the most vigorous laterals was shorter than in 50% and 75% pruned shoots (Fig. 3). These results indicate that the initial cell division phase in the laterals decreased in response to limited resources due to the large total number of buds in un-pruned and 25% pruned shoots, possibly arising in response to apical dominance.
Effects of summer pruning severity on the number of vegetative and flower buds per shoot. Different letters indicate significant differences between treatments at P < 0.01 by Steel-Dwass’s test (n = 16). The bars indicate standard errors.
Effects of summer pruning severity on the number of laterals per shoot and the length of the most vigorous laterals. Different letters indicate significant differences between treatments at P < 0.01 by Steel-Dwass’s test (n = 16). The bars indicate standard errors.
Shoots pruned at 75% had significantly fewer flower buds than un-pruned and 25% pruned shoots (Fig. 2). The number of berries per 25% pruned shoot was larger than for 75% pruned shoots (Table 1). No differences were observed in the number of berries per shoot between the other treatments, and yield per shoot was not affected by treatment. However, a significant increase in berry weight, which was reflected by an increase in berry diameter, was observed between the 25% and 75% pruned treatments. There were no differences between treatments in the number of berries per cluster and number of berries per flower bud. In the subsequent cropping season, the initial vegetative and reproductive growth and development of woody fruiting plants depends on reserved carbohydrates (Darnell and Birkhold, 1996; Garcia, 2002; Maust et al., 1999). Final fruit size is directly related to the number of cells produced in the period immediately following pollination, and mechanisms that regulate cell production before bloom and during fruit development determine fruit size (Johnson et al., 2011). According to Gillaspy et al. (1993), cell division before bloom and immediately after pollination and fertilization is the primary factor driving early fruit growth in blueberries. In the the Rabbiteye blueberry, cell production occurs before bloom and during the initial stages of fruit growth (Cano-Medrano and Darnell, 1997; Darnell et al., 1992; Edwards et al., 1970). On the other hand, an increase in the number of floral buds decreases overall vegetative and reproductive growth by limiting the resources in the period between dormancy and bloom (Maust et al., 1999). Therefore, the strong effect of pruning severity on berry size and weight suggests that severe pruning induces more cell division in the fruit by reducing the competition for reserve nutrients with other fruits, while un-pruned and 25% pruned shoots probably experienced resource limitation due to their larger number of flower buds (Fig. 2) and growth suppression of laterals (Fig. 3). All shoots that were either pruned at 50% or 75% produced flower buds below the cut position (Fig. 4). This suggests that summer pruning treatments stimulate flowering even in the basal area of shoots.
Effect of pruning severity treatments on qualitative and quantitative parameters of harvested berries of rabbiteye blueberry ‘Tifblue’.
Severe summer pruning treatments stimulated flowering below the cut position area of shoots. The arrows indicate the pruning cut position. (A) No flower buds were induced in the area below the cut position of conventionally winter pruned shoots. The picture was taken in April 2016. (B) Severe summer pruning in September 2015 stimulated flower buds formation in the area below the cut position of the shoot. The picture was taken in April 2016. (C) Severely pruned shoots produced bigger berries. The picture was taken in July 2016.
In the present study, despite an increase in berry weight and diameter with increasing pruning severity, quality parameters such as TSS and TA were not affected (Table 1). Possibly, in the advanced phases of fruit growth, the required photosynthetic assimilates were provided by an adequate leaf area of the larger number of laterals in un-pruned and 25% pruned shoots, and by vigorous laterals in severely pruned shoots, since in those phases, photosynthetic factors have more effect on fruit quality than fruit size (Henton et al., 1999).
Summer pruning severity stimulated stronger laterals. Yield per shoot was not affected by increasing pruning severity due to a compensatory increase in berry weight. Hence, to reduce the unproductive parts of the plant, and to produce bigger berries, September removal of 50% or 75% length of the mature Rabbiteye blueberry shoots when grown under conditions similar to that of the Kanto region of Japan is recommended.
This study provides new evidence that severe pruning of rabbiteye blueberry shoots in September, induces not only flower buds but also an increase in berry size and weight. Therefore, further studies are required to elucidate the regulatory mechanisms, such as the molecular basis, for these changes.