179 1. Introduction Slender spindle bush types are commonly used for compact-sized fruit trees such as apple cultivars grafted on dwarfing rootstocks. Peach trees can be dwarfed and trained as slender spindle types when they are grafted on Prunus tomentosa and P. japonica ( Mizutani et al., 1985; Yaegaki et al., 2008 ). However, these rootstocks often show graft-incompatibility for some peach scion cultivars several years after grafting (Nakano and Shi- mamura, 1983; Yaegaki et al., 2008). It is difficult to maintain trees grafted on vigorous rootstocks as slender spindle types by winter pruning. Their shoot growth is so great that the inside parts of the tree are shaded, re- sulting in poor growth and even death of shoots near the trunk. In relation to shading, Neri et al. (2003) reported that shading caused leaf wilting, necrosis and abscission earlier under water-stressed conditions. It is important to maintain shoots and buds alive near the trunk to maintain the slender spindle types. Otherwise, shoots extend out- ward resulting in the crown type of tree. However, when the tree vigor is so great, severe annual winter pruning only repeats imbalanced vegetative-oriented growth cy- cles each year. Commercial fruit production is difficult under such conditions. Many reasons have been given to support the practice of summer pruning in peach trees. It has been reported that summer pruning reduces vegeta- tive growth, improves light penetration, enhances fruit quality, concentrates fruit maturation and increases the number of flower buds. In general, it is considered that the time of flower bud formation in deciduous fruit trees is around late July and August in the temperate zones of the Northern Hemi- sphere. Thus, the time of summer pruning is very im- portant in relation to flower bud formation, especially when heading back pruning is conducted. After summer heading back pruning, new shoots are regenerated from remaining twig parts. Even thinning out pruning some- times enhances branching and burst of buds which oth- erwise remain quiescent. In relation to shoot regrowth after summer pruning, Neri et al. (1992) reported that it was induced only when the whole root system was well irrigated. After summer pruning the regenerated shoots are considered physiologically young compared with the spring flushes. In apple trees, the earlier the prun- ing time, the greater the number of flower buds (Mizu- tani et al., 2000). Apple flower buds tend to bear in the apical buds of shoots. When summer pruning is carried out earlier, new plural shoots come out and bear flower Summer pruning to maintain slender spindle bush type of peach trees grafted on vigorous rootstocks F. Mizutani Faculty of Agriculture, Ehime University, Matsuyama, Ehime Prefecture, 790-8566 Japan. Key words: peach, pruning, shading, shoot heading back, summer shoot thinning. Abstract: The slender spindle bush type system is commonly used for compact-sized trees, especially grafted on dwarfing rootstocks. It is difficult to apply this system to trees grafted on vigorous rootstocks by winter pruning. Such practices only cause the trees to repeat imbalanced vegetative-oriented growth every year. Therefore, in the current work summer prun- ing was applied to slender spindle bush type of early maturing peach trees grafted on vigorous rootstocks. Three trials were conducted: summer shoot thinning, shoot heading back trials in the field and a shoot heading back and shading trial in the pot experiment. Summer shoot thinning reduced tree growth and recovered proper balance between vegetative and repro- ductive growth. The following season such shoot thinning enhanced bud burst and initial growth of new shoots but the final tree growth was less in the summer-pruned trees compared with winter-pruned trees. The fruit matured earlier and soluble solids content was greater and titratable acidity was lower in the summer-pruned trees. The summer shoot heading back trial revealed that it regenerates shoots, although they bear fewer flower buds compared with winter-pruned trees. Such heading back is effective to keep alive shoots and buds near the trunk in slender spindle bush type systems. Summer shoot heading back and shading experiments in the pot showed that shading reduced the number of regenerated shoots and flower bud formation and delayed flower blooming in the following year. Thus summer shoot thinning and heading back are ap- plicable to early maturing peach cultivars grafted on vigorous rootstocks to maintain the slender spindle bush type because thinning favors reducing tree vigor and light penetration near the trunk, and heading back keeps alive shoots and buds near the trunk which otherwise weaken or die back due to apical dominance and/or shading. Adv. Hort. Sci., 2011 25(3): 179-186 Received for publication 31 March 2011 Accepted for publication 31 August 2011 180 buds in each shoot apex resulting in greater flower bud numbers (Mizutani et al., 2000). However, later summer pruning diminishes such effects. It seems likely that a certain period is required for regenerated shoots to bear flower buds. Erez (1982) also reported that in the mead- ow orchard system of peach trees, four to five months are required for sufficient shoot regeneration and flower bud formation. Therefore he recommends that such systems are only applicable to early maturing cultivars, with a long enough growing season after fruit harvest. Three trials (summer shoot thinning, heading back in the field, and heading back and shading in the pot) were conducted in the present work to develop new methods to maintain slender spindle bush type peach trees grafted on vigorous rootstocks. 2. Maintaining tree shapes as slender spindle bush type in peach trees grafted on vigorous rootstocks by summer shoot thinning A. Objectives At first we tried to maintain tree shapes as slender spin- dle bush type with ‘AB-1’ (‘Akatsuki’x unknown peen- tao) peach trees grafted on vigorous rootstock (Prunus persica Batsch, wild form) by winter pruning. The trees grew well in the orchard (previously vineyard) in spite of the fact that chemical fertilizers were not applied. It was difficult to maintain tree shape as slender spindle type while producing quality fruit annually. To keep shoots and buds near the leader trunk, excessive severe winter prun- ing was practiced. Such practices resulted in an improper balance between vegetative and reproductive growth. The vegetative-oriented growth is represented by vigorous shoot growth, poor fruit set, much June drop and delayed fruit maturation. When the peach trees are vigorous, se- vere winter pruning only repeats such a tree cycle every year. Therefore, the objective of the first trial in the field was to determine whether summer shoot thinning can al- ter such imbalanced tree growth to the proper balance in the slender spindle type of peach trees grafted on vigorous rootstocks. B. Materials and Methods The orchard used in the experiment was formerly a vineyard to which chemical fertilizers had been applied according to a standard instruction in the Experimental Farm, Faculty of Agriculture, Ehime University. For the purpose of dwarfing peach trees grafted on vigorous root- stocks, we planned no application of chemical fertilizers during the course of experiments. By using nine-year-old ‘AB-1’ peach trees which had so far received winter prun- ing, we tried summer pruning to maintain the tree as slen- der spindle bush type. The harvest time of ‘AB-1’ peach is mid July. Summer pruning was conducted just after fruit harvest; most of it was conducted as thinning out methods not heading back. The weight of pruned shoots and leaves was measured. The following year new shoot growth, flowering, leaf mineral content, fruit growth and quality of harvested fruit were determined. C. Results Figure 1 shows summer- and winter-pruned trees just after summer pruning on 24 July. The weight of shoots removed by summer pruning was less than that removed by winter pruning (Fig. 2). However, in the case of winter-pruned trees, there were no leaves at pruning time so that the actual bio- mass removed from the winter-pruned trees was much Fig. 1 - Peach trees just after summer pruning on 24 July (left: tree after summer pruning; right: tree without summer pruning) Fig. 2 - Weight of shoots removed by summer and winter pruning from peach trees. Data are presented as mean ± standard error (se). S h o o t w e ig h t (K g F .W . / tr e e ) Summer pruning Winter pruning 181 greater than the shoot weight pruned. Three or four days earlier bud break and flowering were observed in sum- mer-pruned trees compared with winter-pruned trees in the following year (Fig. 3). The ovary size was greater in summer-pruned trees (Fig. 4). The number of flowers however was less in summer- pruned trees (Fig. 3), while mineral and carbohydrate con- tent in the new leaves and shoots was greater in the sum- mer-pruned trees (Figs. 5 and 6). This indicates that the shoots remaining after summer pruning received enough Fig. 3 - Effect of summer and winter pruning on the formation of new shoots, leaves and flowers of peach trees in the following sea- son on April 4. Fig. 4 - Effect of summer and winter pruning on ovary size of peach flowers on 4 April. Data are presented as mean ± standard er- ror (se). O va ry le n g th w id th ( m m ) 8 6 4 2 0 Length Width Fig. 5 - Effect of summer and winter pruning on mineral content of stems and leaves in peach trees on 4 April. Data are presented as mean ± standard error (se). N u tr ie n t c o n te n t (% ) 5 4 3 2 1 0 Summer pruning N u tr ie n t c o n te n t (% ) Winter pruning 5 4 3 2 1 0 Fig. 6 - Effect of summer and winter pruning on sugar and starch con- tent in stems and leaves in peach trees on 4 April. Data are presented as mean ± standard error (se). Summer pruning C o n te n t (m g /g d w ) Winter pruning 120 100 80 60 40 20 0 C o n te n t (m g /g d w ) 120 100 80 60 40 20 0 182 solar radiation to accumulate carbohydrate as reserves and mineral nutrient from the roots. On the other hand, in the winter-pruned trees, the shoots remaining near the trunk are supposed to have been shaded in the previous sum- mer and fall. Although the initial tree growth was slightly enhanced in the summer-pruned trees, the shoot growth was accelerated in the winter-pruned trees in the middle growing season and final tree size became greater in the latter group (Fig. 7). Fruit number and yield per tree were greater in the summer-pruned trees (Tables 1 and 2). Fruit weight was similar in both treatments but fruit seemed to mature earli- er in summer-pruned trees. In this regard, the total soluble solids content in the juice was greater and titratable acidity was lower in summer-pruned trees. Thus, summer shoot thinning seems to be applicable to vigorous peach trees grafted on vigorous rootstocks in slender spindle bush type system to recover balanced vegetative and reproduc- tive growth. 3. Shoot regeneration and flower bud formation after summer shoot heading back A. Objectives Because of apical dominant nature of shoots, the termi- nal shoot grows well, which retards the growth of lateral shoots. Whereas the apical part of buds on shoots burst and extend, the lower part of buds mostly remain quiescent. In the slender spindle bush type of tree it is very important to keep alive buds or shoots near the trunk. Without shoot heading back, terminal shoots extend outward, while the inside of the crown becomes shaded and shoots and buds near the trunk die back. In the second trial in the field, we examined the effects of summer shoot heading back on shoot regeneration, leaf chlorophyll content (SPAD value), leaf drop and flower bud formation by using peach tree grafted on vigorous rootstocks. B. Materials and Methods Five-year-old ‘Hikawahakuho’ peach trees grafted on vigorous rootstock (Prunus persica Batsch, wild form) which were trained as slender spindle type were used. The harvest time of this cultivar is early July. Trees had been trained as slender spindle bush type by winter prun- ing before the experiment started. No chemical fertilizers were applied, as mentioned above, because the orchard was formerly a vineyard where the recommended amount of fertilizers had been applied according to the standard instruction. Tree vigor of ‘Hikawahakuho’ peach grafted on vigorous rootstocks used in this experiment was less compared with ‘AB-1’ peach described above in the previ- ous section. Summer pruning consisted of heading back of current shoots to about 10 cm and removal of vigorous shoots, which was conducted after fruit harvest on 15 July. The number of regenerated shoots, shoot growth, flower bud formation, SPAD values and leaf drop were deter- Table 1 - Effect of summer pruning on peach fruit yield and quality in the following season (2001) Treatment Fruit/tree (No.) Yield (Kg/tree) Fruit weight (g) Fruit length (mm) Fruit diameter (mm) SSC (%) Titratable acidity (%) Summer pruning 136.0±36.7 13.0±3.1 102.2±5.3 59.4±0.7 62.2±0.7 9.27±0.26 0.27±0.01 Winter pruning 96.2±5.3 8.7±0.8 102.2±4.0 58.4±0.5 60.7±0.4 8.45±0.24 0.52±0.04 Data are presented as mean ± standard error (se). Table 2 - Effect of two-successive-year summer pruning on peach fruit yield and quality in the following season (2002) Treatment Fruit/tree (No.) Yield (Kg/tree) Fruit weight (g) Maturity degree (Z) SSC (%) Titratable acidity (%) Summer pruning 76.8±15.0 8.3±2.1 108.9±4.4 3.7±0.4 12.10±0.32 0.20±0.01 Winter pruning 66.4±8.0 6.6±1.0 103.5±3.5 2.4±0.3 9.31±0.29 0.45±0.08 (Z) For maturity degree, the score was given to green fruit=1 and ripen fruit=5. Data are presented as mean ± standard error (se). Fig. 7 - The photos show tree sizes on 24 July in the following year. 183 mined. The following year, tree growth and fruit yield and quality were determined. C. Results Figures 8 and 9 show the shoot regeneration after sum- mer pruning. There was no regrowth of shoots in winter- pruned trees. SPAD values of leaves were greater in sum- mer-pruned trees than winter-pruned trees (Fig. 10). Leaf retention was prolonged by summer pruning (Fig. 11). Mi- erowska et al., (2002) also reported that in apple spur leaf total chlorophyll content was higher in summer-pruned than non-pruned trees. These facts indicate that regenerat- ed shoots are physiologically young compared with spring flushes. The percent of flower buds were lower in summer- pruned trees than winter-pruned trees (Table 3); total shoot length in the following year was less in summer-pruned trees (Table 3). Fig. 8 - Effect of summer pruning on regenerated shoot numbers of peach trees. Data are presented as mean ± standard error (se). Fig. 9 - Effect of summer pruning on the regenerated shoot length of peach trees. Data are presented as mean ± standard error (se). Table 3 - Effect of summer pruning on flower bud formation and total shoot length of peach trees in the following year Treatment Flower buds (%) Total shoot length in the following year (m) Summer pruning 17.2±4.5 21.8±4.9 Winter pruning 65.7±8.7 45.0±9.0 Data are presented as mean ± standard error (se). Fig. 10 - Effect of summer pruning on SPAD (chlorophyll content) in the leaves of peach trees in September and November. Data are presented as mean ± standard error (se). Fig. 11 - Effect of summer pruning on leaf drop in peach trees from September to late January. Data are presented as mean ± stan- dard error (se). 184 Therefore it can be said that the tree size of summer- pruned trees was reduced compared with winter-pruned trees. Figure 12 shows single year and two-successive- year summer pruning on the weight of shoots pruned. Two-successive-year summer pruning reduced the weight of pruned shoots. Fruit quality of harvested fruit as affect- ed by single year and two-successive year summer prun- ing is presented in Table 4. Summer pruning enhanced maturation and increased soluble solids contents and re- duced titratable acidity. 4. Shoot regeneration and flower bud formation after summer shoot heading back under shaded conditions A. Objectives It is important to keep alive shoots and buds near the trunk to maintain peach trees as a slender spindle type. Since shoots have apical dominance, terminal shoots ex- tend outward so that inner parts of the crown become shaded without pruning. We further examined the effects of shading and summer shoot heading back on shoot re- generation and flower bud formation by using peach trees grafted on vigorous rootstocks in the pot trial. B. Materials and Methods Potted (30 cm diameter) one-year-old ‘Hikawahakuho’ peach grafted on vigorous rootstock (wild form) were used in the trial. Fertilizers (N, P2O5, K2O=15%, 15%,15%) were applied at the rate of 30 g and 15 g per pot in mid March and early September, respectively. Treatments con- sisted of shading with white and black cheesecloth, shoot heading back (to 5 cm length from shoot base) and their combinations. Summer shoot heading back was conducted on 1 September. The number and length of regenerated shoots, leaf drop, SPAD values, flower bud formation and flowering time were determined. C. Results No shoot regeneration was found without summer shoot heading back under either non-shaded or shaded conditions (Fig. 13). Summer shoot heading back re- generated shoots but shading reduced their number (Fig. 13). Heavy shading (black cheesecloth) accelerated shoot growth as compared to light shading (white cheesecloth) and non-shading (Fig. 14). The number of flower buds was reduced by summer pruning and the tendency was acceler- ated by shading (Fig. 15). Shading tended to delay bloom in the following spring (Fig. 16). This indicates that the inner side shoots of the tree crown delayed bud burst and initial shoot growth as described in winter-pruned ‘AB-1’ peach in the previous section (Fig. 3). Fig. 13 - Effect of late summer pruning and shading on the number of regenerated shoots of peach trees. SP = summer pruning; WP = winter pruning, WC = white cheesecloth; BC = black cheesecloth. Data are presented as mean ± standard error (se). Fig. 12 - Effect of summer and winter pruning on pruned shoot weight of peach trees. Summer-pruned shoots include leaves. SPⅠ, WPⅠ and SPⅡ, WPⅡ indicate single-year and two-successive- year pruning, respectively. Data are presented as mean ± stan- dard error (se). Table 4 - Effect of summer and winter pruning on fruit yield and quality in peach trees Treatment Fruit No./tree Fruit weight (g) Yield (Kg/tree) Maturity degree (Z) SSC (%) Titratable acidity (%) Summer pruning I (y) 28.1±4.1 131.0±0.5 3.7±0.7 3.5±0.4 12.8±0.2 0.27±0.03 Winter pruning I(y) 41.1±5.8 128.1±0.4 5.3±1.0 2.7±0.2 11.8±0.2 0.39±0.03 Summer pruning II 22.2±3.6 136.2±6.0 3.0±0.6 3.9±0.3 13.7±0.3 0.24±0.02 Winter pruning II 30.4±4.3 128.6±0.4 3.9±0.8 2.9±0.3 12.7±0.2 0.38±0.03 (Z) For maturity degree, the score was given to green fruit=1 and ripen fruit=5. (Y) Indicate single-year and two-successive-year pruning, respectively. Data are presented as mean ± standard error (se). 185 5. Discussion and Conclusions Slender spindle bush type training systems have been easily adopted in compact-sized trees grafted on dwarfing rootstocks (Mizutani et al., 1985). However, when this system is applied to trees grafted on vigorous rootstocks, imbalanced vegetative-oriented cycle between vegetative and reproductive growth occurs. For commercial qual- ity fruit production, a good balance between vegetative and reproductive growth is necessary. As suggested in the present work, application of summer shoot thinning can reduce vegetative growth in such vegetative-oriented trees of spender spindle type trees grafted on vigorous rootstocks. Neri et al. (2010) also reported that summer pruning reduced vigor in apricot trees. To maintain the slender spindle type, it is crucial to keep alive shoots and buds near the trunk. However, shoots exhibit apical dominance by nature so that they extend outward, thus the space near the trunk becomes shaded. Shading accel- erates dieback of shoots and buds inside the crown. With regard to shading, Neri et al. (2003) reported that shading caused leaf wilting, necrosis and abscission earlier under water-stressed conditions. Thinning of shoots is effec- tive for the penetration of solar radiation near the trunk in the crown. Furthermore, heading back of shoots near the trunk induces shoot regeneration from the buds at the base, which are otherwise quiescent or died back. Un- less heading back is conducted, the base parts of shoots become bare without alive shoots or buds. Summer shoot heading back is efficient to keep such shoots and buds near the trunk, even under shaded conditions. Readers in- terested in further detailed information for current work are also referred to Hossain et al. (2004, 2005, 2006). 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