Summer pruning effects in apple — a review

Scientia Horticulturae, 30 (1987) 253-282

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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Summer Pruning Effects in Apple m a Review MAX C. SAURE

Dorfstr. 17, D-2151 Moisburg ( F.R.G.) (Accepted for publication 16 October 1986)

ABSTRACT Saute, M.C., 1987. Summer pruning effects in apple - - a review. Scientia Hortic., 30: 253-282. Summer pruning effects in apple have generally been attributed to improved light penetration and to reduced carbohydrate supply. However, the common basis to most of these effects seems to be the retardation of senescence, a process under hormonal control. Summer pruning causes a temporary loss of apical dominance except when thinning cuts are used. It also causes a temporary increase in cytokinin supply, presumably mainly by increased export from the roots. Both effects presumably result from reduced auxin availability. Depending on its timing, the resulting rejuvenation may consist of mobilization and redistribution of nutrients and phytohormones, breaking of axillary buds, inhibition of flower induction, delayed fruit development and later induction of dormancy. The extent of pruning responses increases with the vigour of the tree, the earliness of pruning and its severity. Heading and stubbing are more effective in provoking these responses than thinning of shoots. In areas with a short season, summer pruning may not only delay, but also prevent, the onset of dormancy, with adverse effects on winter hardiness. The mostly positive influence of summer pruning on fruit colour by factors in addition to better light penetration is discussed. Summer pruning, where considered necessary, needs to be adjusted to local conditions and coordinated with other cultural practices.

CONTENTS

1. Introduction 2. Results of summer pruning 2.1. Modification of growth 2.2. Modification of yield 2.3. Modification of flowering 2.4. Modification of fruit development 3. Sources of variation in summer pruning effects 3.1. Selection of standards 3.2. Pruning procedures 3.3. Environment 3.4. Tree characteristics 0304-4238/87/$03.50

© 1987 Elsevier Science Publishers B.V.

254 4. The physiology of plant responses 4.1. Removal of leaves 4.2. Removal of buds and shoot tips 5. Summer pruning as an integrated cultural practice Acknowledgements References 1. INTRODUCTION Summer pruning has been known to European apple growers as a cultural practice since about the middle of the 17th century (Kemmer, 1948). However, it was widely neglected for several decades of the 20th century, when commercial apple production expanded rapidly and fruit science became established. Most probably, this is due to the partially inconsistent effects of summer pruning, as described by Gardner et al. (1952) in their excellent review of the earlier literature. Recently, summer pruning has regained the attention of apple growers, as a means of: limiting the size of trees, especially in modern high-density plantings; - - improving fruit development in these for better fruit finish and better storage quality. Recent experimental work has been summarized by Kluge (1983), Barden and Marini (1984) and Ferree et al. (1984). These investigations have not unequivocally confirmed the benefits of summer pruning in overcoming the negative side-effects of modern highly intensive production methods. The reasons for the diverse and partially contradictory results are still not fully understood. Without such an understanding, it remains very difficult to decide whether summer pruning should be done as an integrated part of standard cultivation methods, and if so, how to adapt it properly to different growing conditions with respect to timing, method and .degree of pruning. This review tries to reconcile the contradictions between results of different investigations and to identify a common basis of various pruning responses. It is hoped that suggestions resulting from it will encourage further research. The nuclear physicist W. Heisenberg (1973) pointed out that one of the benefits of new theoretical approaches is that they may permit the finding of new facts, and known facts may appear in a new light. A new approach to summer pruning is needed. This review is confined to recent research results. For earlier publications, it refers to the extensive work of Gardner et al. (1952), comprising 95 references. The term "summer pruning" will be used in its widest sense, i.e. the removal of leafy branches, shoots or parts thereof, irrespective of the timing, method or severity. The different types of pruning procedures will be dealt with in Section 3.2. -

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255 2. RESULTS OF SUMMERPRUNING

2.1. Modification o/growth Gardner et al. (1952) stated that summer pruning does not necessarily have either a dwarfing or an invigorating influence, and therefore does not generally retard growth more than dormant pruning. However, during the last decade, summer pruning has been recommended repeatedly as a proper practice to reduce tree vigour, e.g. in Italy (Innerhofer, 1974), The Netherlands (Lemmens, 1975, 1977), Germany (Utermark, 1976) and the U.S.A. (Stembridge, 1979). Indeed, in most experiments summer pruning reduced stem thickening, although there was great variability in the extent of reduction (Lord et al., 1979a; Marini and Barden, 1982a, e; Nelgen, 1982; S~ik5 and Laurinen, 1982; Greene and Lord, 1983; Mika et al., 1983; Myers and Ferree, 1983b; Polefska, 1983; Ferree et al., 1984; Domoto, 1985; Saure, 1985a; Wagenmakers, 1985). Subsequent shoot growth (regrowth) in the season of pruning was often limited, and the canopy was thus drastically reduced. Leaves of the regrowth were generally small (Maggs, 1965; Taylor and Ferree, 1981; Ferree et al., 1984a) and contained less chlorophyll a and b ( Singha and Baugher, 1985 ). The susceptibility of the wood to winter injury sometimes increased substantially compared with unpruned or dormant-pruned trees (Lord et al., 1979a; Domoto, 1985; Saure, 1985a). All these effects have been interpreted as devitalization. However, these results do not confirm a general dwarfing effect. As an immediate response to summer pruning, growth rate of the remaining shoots may be enhanced, and termination of growth is often delayed. Leaves left on the tree turn dark green, and their abscission is postponed ( Maggs, 1965; Lord et al., 1979a; Taylor and Ferree, 1981; Marini and Barden, 1982b; Nelgen, 1982; S~ik5 and Laurinen, 1982; Myers and Ferree, 1983a; Polefska, 1983). In the year after summer pruning, shoot growth is generally not restricted or is even more vigorous (Lord et al., 1979a; Prigge, 1981; Marini and Barden, 1982a, e; Nelgen, 1982; Greene and Lord, 1983; Mika et al., 1983; Myers and Ferree, 1983a; Taylor and Ferree, 1984a; Wagenmakers, 1985). Saure {1985a) noticed a reduced total growth increment of young apple trees in the season after severe pruning, but length of individual shoots was increased. The increased shoot length was partially due to a larger number of internodes, but even more to longer internodes. Most of these observations point to a redistribution of growth rather than to devitalization, and Myers and Ferree (1984) interpreted summer pruning as a localized, invigorating influence. Summer pruning often results in hastened bud break of axillary buds, depending on pruning method and plant vigour (cf. Section 3). Lord et al. (1979a) observed this proliferation of growing points mainly below the cuts on terminal and vigorous upright shoots. Marini and Barden (1982a), Saure

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(1985a) and Wagenmakers (1985) reported that bud break on apple trees pruned in August or September, respectively, started earlier the next season as compared to dormant-pruned trees. Flower bud opening in spring may be also hastened (Preston and Petting, 1974; Marini and Barden, 1982a; Taylor, 1982; Link, 1984; Taylor and Ferree, 1984a). This could explain why Engel and Lenz (1981) found flower buds in summer-pruned trees to be more susceptible to late spring frosts. However, Morgan et al. (1984) did not find this influence on flower bud opening in spring, and Link (1984) reported that summer pruning increased frost survival of flower buds. Occasionally, summer pruning may induce flowering in autumn (Lord et al., 1979b ). In regions with warm winters, bud break may be delayed by summer pruning, due to an increased chilling requirement of the buds (Chandler and Brown, 1957). In the season after treatment, the previous promotion of lateral bud development by early summer pruning may cause a more favourable branch orientation in certain cultivars (Aselage and Carlson, 1977). However, Myers and Ferree (1986) did not succeed in distributing vigour more evenly along the limb and away from the distal section. According to Gardner et al. (1952), light summer pruning tends to encourage fruit spur development.This has also been observed by Lemmens (1977), Stembridge (1979), Belter and Thomas (1980) and Domoto (1985). Myers and Ferree (1983c) noted that summer pruning, under certain conditions, may prevent the decline in the amount of leaf area in spurs which usually occurs in the interior of unpruned trees. Such effects are not limited to apple but have also occurred in sweet cherries (Prunus avium), with more fruiting wood in the lower part of the canopy (Widmer and Zbinden, 1984), and in peach (Prunus persica), with more but less vigorous growing points (Rom and Ferree, 1985). Stiles (1980) agreed that summer pruning stimulates lateral shoot growth, but reported flower formation was reduced (cf. Section 2.3).

2.2. Modification of yield One of the objectives of summer pruning is to get higher, earlier and more regular yields,as expressed in the proverb cited by Gardner et al. (1952) "Prune in winter for wood and in summer for fruit".Link (1984) noted that trees that were summer-pruned rather than dormant-pruned always produced highest yields. In the long run, summer-pruned trees yielded about 25% more than dormant-pruned trees. However, he concluded that the beneficial effects of pruning exclusively in the summer are not the immediate result of summer pruning, but of avoiding pruning in the previous winter. Lemmens (1979, 1982) consistently reported higher yields after summer pruning, especially in the triploid 'Karmijn de Sonnaville'. Quinlan and Preston (1971) noticed that regular pinching of bourse-shoots, starting shortly after full bloom, resulted in increased yields. Pinching may further cause earlier yields by reducing exces-

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sive shoot growth and favouring spur formation, as observed by Zbinden and Widmer (1980) in cherry and plum. Other authors could not confirm a consistent effect of summer pruning on cropping (Preston and Perring, 1974; Marini and Barden, 1982d; Jackson et al.,1983; Myers and Ferree, 1983b; Ferree, 1984; Bootsma, 1984; Domoto, 1985; Wagenmakers, 1985), and Belter and Thomas (1980) were unable to make apple trees more precocious. In many of the more recent experiments, summer pruning even reduced yield under certain conditions (Bos, 1974; Engel, 1974; Borsboom, 1976; Aselage and Carlson, 1977; Belter and Thomas, 1980; Brunner and Droba, 1980; Stiles,1980; Prigge, 1981; S~ik5 and Laurinen, 1982; Sus and Prskavec, 1983; Akkerman, 1984; Taylor and Ferree, 1984a; Williams, 1985) and depending on the standards selected. Taylor (1982) got a great reduction of yield on a per-tree basis,but not per unit of canopy volume. Since "yield" is a very complex subject, the obvious contradictions mentioned above need to be analyzed in order to get a more general understanding of these summer pruning effects.In principle, low yields may be the result of reduced flowering or of impaired fruitdevelopment.

2.3. Modification of flowering Occasionally, flowering in apple may occur from lateral buds of the last season's extension growth. More regularly, flowering occurs from terminal buds on spurs and medium-long shoots. In any case, formation of flower buds requires: induction of flower initials; proper development of these flower initials. However, very few reports dealing with the influence of summer pruning on subsequent flowering reveal the way in which flowering may have been affected. Gardner et al. (1952) concluded from the literature that summer pruning may promote flower initiation if done early enough and with the appropriate pruning method. Later investigations have confirmed a beneficial effect of pinching on flower bud formation (Mika et al., 1983 ). Lord et al. (1979a) also reported that the ability of secondary shoots to form flowers was increased the earlier pruning was done. Several authors reported an increased formation of fruit spurs after summer pruning (cf. Section 2.1). Taylor (1982) noticed a higher proportion of inflorescences on terminal spurs, and Link (1984) found a higher flowering density, probably resulting from the larger number of spurs. Conversely, Myers and Ferree (1983c) reported fewer fruit spurs but more flowers per cluster the year after summer pruning, indicating an invigoration of flower buds. Marini and Barden (1982a) noted a swelling of the terminal buds below the pruning cuts, but most of these buds, although resembling flower buds, did not bloom. A promotion of flowering following summer pruning has

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258 been claimed, especially in less vigorous trees (Dietz, 1984 ) and on more basal parts of shoots (Lemmens, 1977). Taylor and Ferree (1984a) could not generally confirm an influence of summer pruning on flower cluster density and quality, and Lord et al. (1979a) did not recommend summer pruning for this purpose because of unpredictable results. Greene and Lord (1983), Morgan et al. (1984) and Wagenmakers (1985) did not find consistent effects of summer pruning on flowering. Reduced flowering, under certain conditions, has been reported by Ferree and Stang (1980), Nelgen (1980), Stiles (1980), Engel and Lenz (1981), Lemmens (1982), Marini and Barden (1982a) and Mika et al. (1983). 2.4. M o d i f i c a t i o n o / f r u i t d e v e l o p m e n t

A modification of fruit development by summer pruning may influence both yield and quality. Belter and Thomas (1980) did not find a single fruit after harvest from flower buds differentiated adjacent to summer pruning cuts. They assumed that in the distal buds, the flower initials may have been incompletely developed. Also, Marini and Barden (1982a) noted in 'Delicious' that none of the few distal buds induced to flower as a result of summer pruning ever produced a fruit. Ogata et al. (1986) reported that after summer pruning, some flowers developed terminally on secondary shoots and produced abnormallyshaped fruits. F r u i t set. ~ Fruit set was not influenced, or in some instances it was reduced,

in experiments by Lord et al. (1979a). It was definitely diminished in a study by Link (1984), and there was a tendency towards reduced fruit set after early summer pruning done by Myers and Ferree (1983c). Pruning treatments did not affect fruit set in experiments by Greene and Lord (1983) and by Taylor and Ferree (1984a, b ). Both shoot removal and shoot tip removal starting 15 days after full bloom increased initial fruit set (Quinlan and Preston, 1971 ). Furthermore, Ferree and Stang (1980) stated that fruit set can be increased by summer pruning during the first half of the growing season, but that there was no effect the year after pruning. Reports on the influence of summer pruning on fruit abscission are as contradictory as they are on fruit set. However, it should be noted here that a clear distinction between the two is not always possible, as some authors have reported only the percentage of fruit set at harvest. This does not explain whether only the initial fruit set, or also the fruit drop at certain periods thereafter, was involved. Quinlan and Preston (1971) observed that early removal of complete bourse-shoots resulted in fewer fruits at harvest, although it promoted fruit set, whereas repeated pinching of the bourse-shoots increased the number of fruits retained to harvest. The reduction of fruit abscission by F r u i t abscission. ~

259 early pinching has been confirmed by Bos (1974), and also by Varga (1971) in pears. Promotion of June drop by severe pruning was reported by Lord et al. (1979b). Link (1984) found fewer fruits per cluster on summer-pruned trees, with a large percentage of clusters shedding all their flowers or fruitlets. Summer pruning consistently reduced pre-harvest drop in 'Delicious' and 'Stayman', probably as a result of delayed ripening (Marini and Barden, 1982d; Barden and Marini, 1984). Previous season's summer pruning reduced the effect of NAA thinning in years with a generally good fruit set, probably due to an increased proportion of terminal flower buds (Taylor and Ferree, 1984b). s i z e . ~ Fruit size is often seriously reduced after summer pruning (Perring and Preston, 1974; Terblanche et al., 1977; Lord et al., 1979b; Van der Boon, 1980; Brunner and Droba, 1980; Janse, 1980; Myers, 1981; Marini and Barden, 1982d; Nelgen, 1982; Ferree, 1984; Link, 1984; Domoto, 1985; Williams, 1985). However, this effect may vary depending on the time, method and severity of pruning, on the year, and on the cultivar (cf. Section 3 ). Some authors found no tendency towards smaller fruits (Schumacher et al., 1974; Lord and Greene, 1982; Miller, 1982; Wagenmakers, 1985). There are only few reports on increased fruit size (Engel, 1974; Silk5 and Laurinen, 1982; Taylor and Ferree, 1984b, 1986), and this was probably the result of a reduction in yield caused by summer pruning rather than a direct influence. Little information is available as to the causes of fruit size reduction. Nelgen (1982) stated that summer pruning did not visibly influence cell number, but late summer pruning slightly reduced cell size.

Fruit

~ Delayed development of the fruits could also affect maturation. Marini and Barden (1982d) noted a delayed conversion of starch, especially in fruit from the interior of the canopy. They considered the delay in fruit starch depletion ~ like suppressed pre-harvest drop - - to be an indicator of delayed maturation. This effect is not limited to apple. Protracted maturation has also been observed in peach after topping the trees 3 weeks prior to harvest (Johnson and Coston, 1985). Struklec (1981) noted less chlorophyll reduction in apples during the whole storage season, and Nelgen (1982) observed that the skin and flesh of apples from summer-pruned trees were greener. However, fruit background colour, considered to be a reliable maturity index for apples, was not significantly affected in pruning experiments by Morgan et al. (1984). On the contrary, higher rates of ethylene evolution of maturing fruits on summer-pruned trees have provided some evidence that summer pruning hastened maturity of the apple fruit, but other maturity factors were not affected in these apples (Taylor and Ferree, 1984b).

Maturation.

s o l i d s , m Marini and Barden (1982d) observed that increase of soluble solids was prevented within 2 weeks after summer pruning. A lower content of

Soluble

260 soluble solids at harvest time has been confirmed by many other authors, e.g. Perring and Preston (1974), Lord et al. (1979b), Lemmens (1980), Nelgen (1980, 1982), Myers (1981), Prigge (1981), Barden and Marini (1984) and Taylor and Ferree (1984a). Again, like other responses to summer pruning, the reduction in soluble solids may be inconsistent, for several reasons (cf. Section 3). ~ No consistent effect of summer pruning on acidity has been observed. While acidity was generally increased in experiments by Struklec (1981), Nelgen (1982) got a partial decrease, especially if pruning was done in late summer, and no significant difference was found by Perring and Preston (1974).

Acidity.

~ Fruit firmness, another factor showing some relation to maturation and ripening, also does not respond unequivocallyto summer pruning. No effect was observed by Miller (1982), Marini and Barden (1982d), Greene and Lord (1983) and Taylor and Ferree (1984a), whereas Myers and Ferree (1983b) and Nelgen (1980, 1982) sometimes found reduced firmness, but only after pruning late in summer. Increased firmness of fruit flesh in apple was noted by Lemmens (1975), Lord et al. (1979b) and Struklec (1981), and in peach by Johnson and Coston {1985). Firmness.

c o l o u r . ~ Gardner et al. (1952) reported that the positive influence of summer pruning on the development of red colour in the skin of apples and other fruits depended on the amount of sunlight reaching the fruit directly. Since most fruits develop their colour shortly before ripening, they concluded that pruning may be done relatively late in the season to enhance colour. A promotion of red colour by summer pruning has since been confirmed by many authors, such as Bos (1974), Engel (1974), Preston and Perring (1974), Lord et al. (1979b), Belter and Thomas (1980), Lemmens and Spruit (1980), Janse (1980), Bootsma (1984), Morgan et al. (1984) and Williams (1985). However, the effect may vary, depending on the year (Miller, 1982; Taylor and Ferree, 1984a) and on the treatment (Lord and Greene, 1982). Sometimes, summer pruning had no effect on colour formation (Greene and Lord, 1983; Myers and Ferree, 1983b; Wagenmakers, 1985 ), and sometimes its effect was even negative after early pruning of a difficult-to-colour cultivar like 'Jonagold' (Gebert et al., 1986). Since blush and maturity are not closely related, an increase in red colour does not necessarily indicate an acceleration of maturation (Morgan et al., 1984).

Red

f i n i s h . ~ Skin finish contributes to the attractiveness of apples. Lemmens (1979) pointed out that summer pruning may reduce russeting in the triploid 'Karmijn de Sonnaville', which often suffers from severe russeting around the calyx. However, he later reported that pruning too early may increase Skin

261 russeting (Lemmens, 1982). This also holds true for very severe summer pruning (Bos, 1974 ). Russeting was not influenced by summer pruning treatments in 'Golden Delicious' and 'Melrose' (Ferree, 1984). Storage quality. ~ Summer pruning is often considered to have a beneficial

effect on storage quality. This has been confirmed, to varying degrees, for: bitter pit (Bangerth and Link, 1972; Preston and Perring, 1974; Lemmens, 1975, 1982; Borsboom, 1976; Terblanche et al., 1977; Lord et al., 1979b; Van der Boon, 1980; Nelgen, 1980; Schumacher et al., 1980; Struklec, 1981; Marini and Barden, 1982b; Myers and Ferree, 1983b; Link, 1984; Kluge, 1985); - - internal breakdown ( L e m m e n s , 1975,1982; Lord et al., 1979b; Van der Boon, 1980; Struklec, 1981; Nelgen, 1982); m w a t e r core (Myers, 1981; Marini and Barden, 1982b; Myers and Ferree, 1983b); --rotting:lenticel rot, gloeosporium, and unspecified (Preston and Perring, 1974; Lemmens, 1975, 1982; Nelgen, 1982). However, bitter pit was not reduced in experiments by Utermark (1976), Greene and Lord (1983) and Sus and Prskavec (1983), or if summer pruning was only light (Terblanche et al., 1980) or rather late (Myers and Ferree, 1983b). Sometimes, summer pruning reduced bitter pit only in combination with calcium sprays (Schumacher and Fankhauser, 1967; Engel, 1974), and occasionally it even increased bitter pit, especially if done early or heavily (Schumacher and Fankhauser, 1972; Schumacher et al., 1974). Internal breakdown was not affected in experiments by Schumacher et ah (1974), Utermark (1976), Lord and Greene (1982) and Greene and Lord (1983). A tendency towards increased incidence of water core after summer pruning instead of a reduction was noted by Taylor and Ferree (1984a). A lower incidence of bitter pit can often be related to a reduction in fruit size, which may result from summer pruning (Schumacher et al., 1980; Terblanche et al., 1980). Moreover, the occurrence of this and other physiological disorders has also been attributed to reduced levels of Ca, or to increased levels of K and Mg in the fruits, i.e. to an increased (K+Mg)/Ca ratio. This view finds some support by observations that summer pruning raised the level of Ca in apple fruits (Perring and Preston, 1974; Terblanche et al., 1977; Nelgen, 1980; Struklec, 1981 (but not in the first year); Lemmens, 1982; Link, 1984), or --lowered the K or K + M g level in the fruits (Perring and Preston, 1974; Lemmens, 1982; Nelgen, 1982; Link, 1984). However, Borsboom (1976), Terblanche et al. (1980), Lord and Greene (1982), Marini and Barden (1982d), Nelgen (1982), Greene and Lord (1983), Kluge (1985) and Taylor and Ferree (1986) found no, or inconsistent, effects of summer pruning on fruit Ca. Perring (1985) observed that late summer pruning increased the redistribution of Ca from the core to outer fruit parts -

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262 during storage. Taylor and Ferree (1986) reported that summer pruning increased K concentration in harvested fruit, but Struklec {1981 ) concluded from her experiments that differences in fruit K may be attributed to differences in yield rather than to direct effects of summer pruning. In the leaves, increased Ca levels as a result of summer pruning have been recorded by Utermark (1976), Struklec (1981) and Jang and Ko (1985), in the lower leaves in particular by Van der Boon (1980), and in spur leaves in general by Taylor and Ferree (1986). However, in leaves of shoot regrowth after summer pruning, the level of Ca and Mg was lowered, but the level of K and N was increased (Taylor and Ferree, 1986 ). Reduced levels of leaf K have been noted by Utermark (1976), and Nelgen (1982) observed that leaf Ca may be reduced after early summer pruning but leaf K was not affected. 3.S O U R C E S OF VARIATION IN S U M M E R P R U N I N G EFFECTS

There is a great diversity of tree responses to summer pruning, with many contradictions still not resolved. For a better understanding, an identification of the underlying mechanisms is required. Lord et al. (1979a) emphasized that the results of many pruning experiments are difficult to interpret for lack of sufficient experimental description or statistical design. Marini and Barden (1982a) also complained that evaluation and comparison of the early studies are difficult because of different locations, poor design or lack of statistical analysis, different treatments, and incomplete descriptions.

3.1. Selection of standards One of the most important causes of differences in the results and conclusions of pruning experiments is the use of inappropriate standards. Most recent investigations used standards where either summer pruning differed in method and/or severity of dormant pruning, or the control trees were not pruned at all, or summer pruning was compared to dormant ÷ summer pruning, thus causing differences in the amount and type of wood removed by the respective treatments. This appears feasible where only physiological processes triggered by summer pruning are to be investigated, or where the only objective of pruning is to improve fruit quality immediately by better exposure to light. However, where the influence of pruning season on the whole plant system, i.e. on growth responses, flowering and fruiting, is to be investigated, Saute (1980) postulated that no other factor except timing should be altered, as one equation with two or more unknowns cannot be solved. Accordingly, Myers and Ferree (1984) pointed out that the interpretation of summer pruning effects on tree size control can be strongly influenced by the control that is used as basis for comparison, and by the point of time that is selected for evaluation. Actually, only few studies compare the effect of season without altering other

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variables (e.g. Prigge, 1981; Marini and Barden, 1982a,b,d,e; Mika et al., 1983; Saute, 1985a), but even if identical dormant pruning was selected as standard, the results may differ depending on the basis selected for comparison. By summer pruning, Taylor (1982) got a yield reduction per tree, or per ha, but not per m 3 canopy volume (cf. Section 2.2). Another important factor is whether dormant pruning in the previous winter or that in the next winter serves as control. The appropriate standard will depend mainly on the timing of summer pruning and on the general objectives of the experiment. The proper timing of evaluation is also essential to separate direct and indirect pruning effects. In many experiments, they were not distinguished properly. This problem is aggravated when experiments are continued for more than one season. Saure (1981, 1985a) has pointed out that pruning predominantly causes short-term effects which are compensated by feed-back reactions of the plant, and may additionally be disturbed by the cultural practices they provoke. For example, it has been shown that light interception may be improved in the year of summer pruning, but may be reduced in the next year because of vigorous regrowth (Porpiglia and Barden, 1981) - - which fruit growers tend to remove early. Similarly, fruit size, which is generally reduced by summer pruning, may be larger in the following season because of reduced yield (cf. Section 2.4). Because of the wave-like course of plant responses (contrary to the often expected linear responses), pruning experiments need permanent, concrete observations and records. Instead of being extended beyond the following season, they ought to be repeated with new, uniform material in order to assess the effects of uncontrollable factors such as light, temperature, rainfall and crop load. 3.2. Pruning procedures

Summer pruning may be done in many different ways with respect to timing, method and severity. Timing. m Gardner et al. (1952) concluded that summer pruning may have quite different effects depending on its timing. They suggested that very early summer pruning was most effective in "diverting the energy of the plant into other developing or already developed tissues", resulting in " . . . a more efficient functioning of the remaining tissues". It was also most effective in encouraging fruit-spur development. Pruning later during the period of most rapid vegetative growth resulted in one of the following effects: it may lead to a greater accumulation of nutrients, with beneficial effects on the formation of flower buds, especially on spurs and on the basal and median portions of the growing shoots;

264 - - i t may result in the opposite effect, through the consumption of nutrients, by forcing axillary buds into growth, leading to the formation of secondary shoots instead of flower buds. They also suggested that pruning late in the season nearly always promoted the accumulation of nutrients in the remaining shoots, since no new growth would take place to utilize the stored foods. However, this accumulation would occur too late to benefit flower bud initiation. Regarding fruit development, Gardner et al. (1952) mentioned that pruning could be done rather late not only for better fruit colour, but also for less sunburn of the fruit, which is likely to occur after early summer pruning. Some of the more recent publications further investigate the importance of timing. Myers and Ferree (1983c) observed that in late-pruned trees the point at which lateral shoot growth begins on vertical shoots of the previous season is lower than in early-pruned trees. There are some reports that early summer pruning may be beneficial for spur development and subsequent flower bud formation in the same year (Lord et al., 1979a; Domoto, 1985; Ogata et al., 1986), but other reports show that flowering is rather negatively affected when summer pruning is done early (Nelgen, 1980; Lemmens, 1982; Dietz, 1984; Domoto, 1985). However, early summer pruning may improve fruit set (Quinlan and Preston, 1971; Ferree and Stang, 1980). In certain cases, the incidence of bitter pit and internal breakdown was reduced when summer pruning was done early (Schumacher et al., 1974 ), but russeting was increased (Lemmens, 1982). Some authors found more adverse effects of early summer pruning on fruit size as compared to later pruning (Lemmens, 1982; Myers and Ferree, 1983b; Morgan et al., 1984), but fruit growth was more reduced by late than by early summer pruning according to observations by Domoto (1985) in apple, which agrees with findings by Akkerman (1984) in pear. Schumacher et al. (1974) found no specific effect of timing within growing season on fruit size. The reduction of soluble solids is generally greater after late than after early summer pruning (Nelgen, 1980; Morgan et al., 1984; Ogata et ah, 1986), although according to Gardner et ah (1952), the timing of pruning generally had little influence on maturation. ~ In principle, summer pruning may consist of the following methods: (1) removal of the apical parts of shoots, either green tips ("pinching"), or - - a portion of more lignified shoots ("heading"), or upper parts of branches, leaving stubs of older wood ("stubbing"); (2) removal of complete shoots or branches back to their point of origin, or back to an apically dominant side branch/side shoot ("thinning"). This can be done at the periphery or in the interior of the canopy, and the various methods may be used separately or in combination. It appears from the earlier literature (Gardner et al., 1952) that summer

Method.

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heading tends to stimulate vegetative growth, thus reducing flower bud formation, whereasthinning, under certain conditions, may encourage fruit spur formation and promote flower bud formation. Pinching has the reputation of promoting spur and flower formation, but frequently has not provided the expected response unless it is followedby successive pinching of secondary and tertiary shoots. More recently, Lord et al. (1979a) noted that the effects of pinching and heading on regrowth were comparable in some instances, but heading frequently caused more regrowth, fewer flowers and less stem thickening than did pinching. On the other hand, Miller (1982) considered stubbing to be more devitalizing than heading because stubbing resulted in significantly less regrowth of either current season's or last year's shoots. Comparing heading and thinning, Mika et al. (1983) recorded no difference in the number of flower buds or yield of fruit when the same amount of wood was removed. Prigge (1981) found heading of all shoots in summer to be more detrimental to fruit size than dormant-type summer pruning consisting mainly of branch thinning. June-drop was reduced by repeated pinching of bourse-shoots, but was increased by complete removal of bourse-shoots (Quinlan and Preston, 1971). S e v e r i t y . ~ Gardner et al. (1952) reported that increasing severity of summer pruning increasingly induced breaking of axillary buds, thus theoretically causing a dissipation of stored nutrients. This would cause a weakening of the remaining shoots (and perhaps the whole plant) as they entered the dormant season less richly supplied with stored nutrients. This would certainly not promote the formation of fruit spurs or flower buds, as was sometimes observed after light early shoot thinning. Rom and Ferree (1985) increasingly reduced leaf, shoot, root and total dry weight by increasing pruning severity in young peach trees which have some responses to summer pruning treatments in common with apples (Rom and Ferree, 1984). Saure (1985a) pointed out that young apple trees pruned very severely suffered most from early winter frosts. The physiologicallystimulating effect of severe summer pruning has been confirmed by Ferree et al. (1984), who observed higher transpiration and photosynthesis rates with increasing severity of summer pruning. With respect to fruit development, Bos (1974) warned against excessively heavy summer pruning as this may cause shock, resulting in an inhibition of fruit growth and promotion of russeting. Lemmens (1982) pointed out that severe summer pruning may lead to small and poorly coloured fruits. Nelgen (1980) reported a reduced relative fruit weight and more bitter pit in severely vs. moderately pruned trees. 3.3. E n v i r o n m e n t

Gardner et al. (1952) stated that environmental conditions, particularly nutrient supply, soil moisture and light, greatly influence the nature of response

266 to summer pruning, and many contradictions between different reports can be explained in this way. For instance, Marini and Barden (1982d) measured a significant suppression of fruit diameter 3 weeks after pruning 'Delicious' trees during the 1979 season, but there was no influence on fruit size in 1980. Taylor and Ferree (1984a) suggested that the effect of summer pruning on fruit size was mitigated by crop position, and they observed varying effects on fruit colour in different years. However, only a few reports explicitely relate variations in external conditions to variations in pruning effects. Gardner et al. (1952) reported that atmospheric conditions favouring a high transpiration rate promoted fruit spur and flower bud formation with certain summer-pruning treatments. They further warned that promoting shoot growth by summer pruning in order to hasten the development of young fruit trees is advisable only where the growing season is long enough to permit proper maturity of the secondary shoots. In support of this, Stik5 and Laurinen (1982) did not recommend summer pruning under Finnish conditions because of the short growing season there, which interferes with the delayed hardening of the trees after pruning. When soil conditions, particularly moisture and nutrient supply, encourage new vegetative growth, summer pruning is less likely to promote fruit spur and flower bud formation than when less moisture and less nitrogen are available (Gardner et al., 1952 ). Miller (1982) confirmed that the amount of regrowth, as well as flower bud initiation on regrowth from stubs, varied with moisture conditions during 2 growing seasons. 3.4. T r e e c h a r a c t e r i s t i c s C u l t i v a r . ~ There are several observations that cultivars respond differently to summer pruning with respect to growth, flowering or certain fruit characteristics. However, there is no general understanding about why they respond differently. Regarding vegetative growth, the final shoot length is influenced by the vigour of the cultivar (Mika et al., 1983). Aselage and Carlson (1977) noted that each variety responds differently to summer pruning treatments, and the proper timing of pruning, in particular, very much depends on the cultivar concerned. Marini and Barden (1982d) pointed out that summer pruning is advisable mainly in cultivars suffering from poor colouration, but it may contribute little to the colouration of high-colouring strains.

Vigour of apple trees does not depend only on environmental factors (cf. Section 3.3 ) or on cultivar, but can also be influenced by rootstock, cropload and age. Bos (1974) reported that the inhibition of growth by summer pruning occurred only on M 9, and not on more vigorous rootstocks. Mika et al. (1983) noted that the pruning response increased with the vigour of the scion and rootstock, although the scion/rootstock influence was less pronounced aftersummer pruning than afterdormant pruning. Myers and Ferree Vigour. w

267 (1983b) suggested that differences in scion/rootstock combinations may explain many of the differences and contradictory conclusions in summer pruning research. In trees with a full crop load, there was a distinctly greater reduction of stem thickening caused by summer pruning than in non-bearing trees, according to experiments by Taylor and Ferree (1984a). 4. THE PHYSIOLOGYOF PLANT RESPONSES Shoots which are removed by summer pruning while plants are non-dormant, or in the state of predormancy (Saure, 1985b), have leaves, buds and often growing shoot tips. However, most of the diverse reponses to summer pruning in growth, flowering and fruit development, and most of the theoretical conclusions drawn from these summer pruning effects, have been attributed to the loss of foliage only. 4.1. Removal of leaves

There are mainly two aspects of leaf removal that have been considered in relation to summer pruning effects: - - loss of carbohydrate supply due to the reduction of photosynthetically active leaf surface; better penetration of light to the interior of the canopy. Reduced carbohydrate supply. - - Gardner et al. (1952} suggested that the immediate effect of any summer pruning on the tree as a whole is to reduce the carbohydrate supply and the rate of carbohydrate production, and to increase the supply of water and nutrients available to the rest of the plant. They speculated that under certain conditions this may cause a depletion of the reserves for the following season, and that growth may be correspondingly restricted in that year. They further suggested that where photosynthetic processes are seriously reduced, fruit quality would certainly be impaired. In apple fruits, a reduction of soluble solids levels has been confirmed by most authors (cf. Section 2.4). H. Prigge (personal communication, 1981) observed that such fruits were damaged on the tree by early winter freezes, whereas those from unpruned trees were not. Similarly, a reduction of carbohydrates throughout the winter has been observed by Maggs (1965) in young, non-fruiting apple trees pruned in August. Their stems contained no starch and their late-formed wood was not lignified. Priestley (1964) found the amount of new growth of young apple trees did not depend on the amount of initial reserves, but Head (1969) and Kandiah (1979) maintained that the effect of reserves cannot be excluded completely. Engel and Lenz (1981) observed that summer pruning in September, removing 60% of the leaves after termination of growth, caused a slightly lower starch and sucrose content of the shoots and

268 buds, and the resistance of the flowers to late spring frostswas severely reduced in the next season compared to winter pruned trees. However, the lower frost resistance was not closely correlated to the lower carbohydrate content. Even complete defoliation, resulting in a great loss of starch and sucrose in shoots and buds, produced fewer frost-damaged flowers than the summer pruning, and partial defoliation (44%) of all current season's shoots did not alter the frost resistance of the flowers. Taylor and Ferree (1981) and Myers and Ferree (1983a) reported that summer pruning reduced the relative amount of dry matter accumulation in roots, as indicated by lower values for the dry-weight root/shoot ratio. Complete defoliation of young apple trees 4-6 weeks before natural leaf-fall greatly reduced root growth within a few weeks of treatment (Head, 1969). This is in agreement with earlier findings of Richardson (1957) with Acer saccharinum, in which defoliation of seedlings brought about a cessation of root elongation. However, Taylor and Ferree (1986) stated that 11 weeks after summer pruning the concentrations of gross water-soluble reducing sugars and insoluble hydrolyzable carbohydrates in the roots was not significantly affected by that pruning; those of sucrose and fructose even increased with increasing severity of summer pruning. Maggs (1964) observed that partial defoliation gave rise to a compensatory increase in photosynthetic activity in the remaining leaves. Taylor and Ferree (1981) also reported that after summer pruning net photosynthesis and the transpiration increased substantially, especially in older leaves, with the extent depending on the severity of pruning. The differences were noticeable even over 1 month after pruning. These findings have been confirmed by Marini and Barden ( 1982c ) and Myers and Ferree (1983a), who stressed that summer pruning effects on photosynthesis resulted from a delayed decline in photosynthesis as compared to unpruned controls. They agree with results in other crops such as mulberry (Morus alba) (Satoh et al., 1977) and peach (Rom and Ferree, 1985). Taylor (1982) concluded that summer pruning effects on shoot elongation, fruit growth and root growth do not appear to be controlled by carbohydrate levels or type of carbohydrate present. Gardner et al. (1952) reported that certain parts of the plants can be favoured by summer pruning, but other parts may suffer. De Haas and Hein (1973) noted an almost complete cessation of root growth at the same time as new secondary shoots were being formed after summer pruning. These findings may be taken as an indication that there could be a redistribution of assimilates to the pruned parts of the plant rather than an absolute deficit. This has been further investigated by Mika and Antoszewski (1973), Marini and Barden (1983) and Rom and Ferree (1985). This redistribution is probably controlled by the buds rather than the leaves (Nagarajah, 1975 ), as will be discussed in Section 4.2.

269 Improved light penetration, a Improved light penetration for better fruit colouration is one of the main objectives of summer pruning. It is usually done rather late in the season in order to limit adverse effects on fruit development (cf. Section 2.4). Gardner et al. (1952) pointed out that early pruning, especially when heading cuts are used, may cause the formation of many secondary shoots and result in heavier shading of the leaves and fruits in the lower parts of the tree. A highly positive correlation between percentage of red blush and photosynthetic photon flux density has been established by Morgan et al. (1984) after early ( 73 days after full bloom ) and late (108 days after full bloom or 4 weeks before first harvest) pruning, but more importantly, the increase in red blush was relatively greater in those heavily shaded zones of the trees where light transmission was not markedly increased by summer pruning and where, independently of the pruning treatment, the mean concentration of soluble solids was lowest. No explanation for this observation has been offered. Gardner et al. (1952) noted that moderate early summer pruning, preferably done by thinning, allows more light to reach leaves on the lower parts of the shoots, and therefore tends to encourage fruit spur and flower bud formation. This view has been supported by many authors since, but there is no experimental evidence that such an effect of summer pruning is caused specifically by better light penetration. Porpiglia and Barden {1981) could confirm that the diffuse photosynthetically active radiation increased immediately after summer pruning, with the percentage increase depending on whether it was measured in the periphery of the tree or within the canopy, but the rates of net photosynthetic potential and dark respiration of interior spur leaves remained consistently lower 3 weeks after summer pruning than those of leaves from the periphery. Actually, they did not differ significantly from leaves of a similar location on unpruned trees. Heading even reduced light penetration throughout the canopy the year after treatment, due to a proliferation of shoots developing immediately below the heading cuts. Marini and Barden (1982b) also measured an increase in light penetration throughout the tree canopy for the remainder of the season after summer pruning. The specific leaf weight of peripheral leaves did not decline following summer pruning, in contrast to the decline in dormant-pruned trees. The response was probably due to improved light conditions and/or delayed leaf senescence. In the next season, light penetration was reduced and the specific leaf weight of interior leaves was lower than that of the interior leaves of dormant-pruned trees. As well as allowing better light penetration, summer pruning could improve the penetration of pesticides and other sprays. Bangerth and Link (1972) assumed that summer pruning was partially effective in reducing the incidence of bitter pit by enabling better deposition of Ca sprays in the tree interior. 4.2. Removal of buds and shoot tips The effect of removing buds and growing shoot tips has rarely been discussed when trying to understand plant responses to summer pruning. However,

270 growing shoot tips are known to be important sources of auxins which play a central role in apical dominance (Phillips, 1975 ). Saute (1971) has postulated that auxins have a key role in the control of growth and developmentby acting as controllers in the cybernetic system based on the interaction of shoots and roots. Via their influence on root activity, they may cause a positive or negative feedback, depending on their concentration. Accordingly, pruning would interfere with this natural regulation by removing active or potential sites of auxin production. This hypothesis has been outlined comprehensively elsewhere (Saure, 1981 ). Adjacent leaves, and subsequently growing shoot tips and terminal buds, are known to prevent the breaking of lateral buds. This correlative inhibition is one aspect of apical dominance. Removing important sites of auxin production by summer pruning, except if done by thinning cuts, temporarily eliminates the inhibition imposed on the lateral buds, and consequently promotes their outgrowth. However, this promotion of axiUary bud growth can only be achieved when the buds are still under the external control of leaves or shoot tips. Later in the season, when they have reached the state of true dormancy which is under the control of factors inside the buds, pruning is ineffective in causing bud break (Saute, 1985b). Release [rom correlative inhibition. - -

The other component of apical dominance is the sink activity exerted by sites of high auxin activity. Apical dominance causes the preferential movement of nutrients, and probably of phytohormones like cytokinins (Lockard and Schneider, 1981), to the growing shoot tips, especially in non-fruiting trees. In fruiting trees the fruits, by their own auxin production, have even greater capacity to attract nutrients than shoot tips. Thus fruits are strong competitors m and are at least as strong as growing shoot tips in their ability for correlative inhibition of the shoots below them. Therefore removing the sites of auxin production not only removes correlative inhibition, but may also cause a temporary redistribution of nutrients. Quinlan and Preston (1971) showed that repeated pinching of a growing bourse-shoot modified the distribution pattern of assimilates. Apical photosynthate movement out of a primary leaf towards the growing shoot tip was shifted by the pinching to mainly basipetal transport towards the flower clusters and the fruitlets, which is the usual direction of transport in bourse-shoots after termination of growth. However, Mika and Antoszewski (1973) observed that after early (June) pinching in young, vegetative apple trees, the absence of strongly competing sinks meant that photosynthates accumulated mainly below the pinching point and were hardly translocated to more distal parts of the plant. Roots are considered to be poor competitors within a plant for photosynthetic assimilates (Radin et al., 1978 ). Only after termination of shoot growth, when the auxin activity of the shoot tips is lowered, will the nutrients be transRedistribution of nutrients, m

271 ported increasingly to the roots (Hansen, 1967). The stabilizing mechanism maintaining the balance of distribution between the various regions in young trees, which Maggs (1965) was looking for, probably works in this way. As indicated in Section 4.1, root development may be reduced by summer pruning, but this reduction apparently starts only after new shoot growth has started. The amount of reduction was related to the intensity of shoot growth (Head, 1967). Consequently, inhibition of shoot regrowth may promote root growth, as shown shortly after treating young apple trees with the growth retardant Alar by Schumacher et al. (1971). In experiments by Polefska (1983), tipping + partial disbudding ( no regrowth) resulted in the greatest dry matter accumulation in the root system of young apple trees, 2 and 4 weeks after treatment; pruning or defoliation, causing regrowth, reduced dry matter accumulation as compared to unpruned controls. Marini and Barden {1983) noticed that in young apple trees pruned in September after termination of growth, photosynthate from basal leaves was primarily transported to regrowth 3 weeks after pruning, but root-directed transport - - similar to unpruned trees and immediately after pruning - - was maintained if regrowth was prevented by NAA treatment. Experimental evidence that it is IAA that directs the transport of photosynthate has been provided by Altman and Wareing (1975), who applied IAA to the base of cuttings and caused basipetal transport of assimilates and accumulation of sugar in this region. The increased transport caused by auxin probably also accounts for the known induction of transport channels (sieve tubes) by auxins ( Sachs, 1975). However, the effects of summer pruning on root growth cannot be attributed only to an altered nutrient supply caused by a modification of the sink capacity of the shoots. From defoliation and disbudding experiments in young A c e r sacc h a r i n u m seedlings, Richardson (1957) concluded that hormones were involved because defoliation led only to a cessation of root elongation, but disbudding completely suppressed root formation. A promotion of root development by leaves and buds seems to contradict the inhibition of root growth by growing shoots, as mentioned above. However, these differences may be taken in support of the hypothesis that the roots act as some kind of correcting unit in a control loop in which growth can be controlled by positive or negative feedback depending on the concentration of auxins (Saure, 1971, 1981). This hypothesis is supported, for example, by the observation of Pilet et al. (1979) that the inhibitory effect of exogenously applied auxin on growth of root segments can be shifted to growth promotion by reducing the endogenous auxin concentration. Further information could be obtained by comparing the immediate effect of pinching in young, nondormant apple trees having few growing shoots with the effect in older trees, but so far such root investigations are known for young trees only. A c t i v a t i o n o f roots, w

The promotion of regrowth by summer pruning is often

272 considered to be a resultof an altered root/shoot ratio,which implies a greater concentration of root-produced growth promoters in remaining leaves (e.g. Polefska, 1983 ). However, there is strong evidence of an initialroot activation after eliminating the inhibition caused by the shoots. Skene (1968), who observed a substantially increased cytokinin level in the bleeding sap of grapes after growth was retarded by C C C treatment, was not sure whether this was directly related to cytokinin synthesis by the roots, or to the larger root meristem. Beever and Woolhouse (1974), who removed apical and lateral buds from vegetative plants of Perilla frutescens, noted that a decreased rate of root elongation was accompanied by an increased cytokinin export from the roots. Certainly, an increased level of cytokinins in the uppermost remaining leaves was recorded soon after pinching of mulberry shoots (Satoh et al., 1977), and in sprouting grape buds after summer topping (Matsui et al., 1979). However, it needs to be determined whether the cytokinins observed are always of root origin, or whether they may also be transformed from storage forms which are found in the bark and older leaves (Van Staden and Davey, 1979). Retardation of senescence. ~ Senescence is a very important process, not only in the life cycle of the trees but also in the annual cycle of development in apple trees, as this phenomenon is associated, e.g., with: E spur development (reduced apical dominance; Wareing, 1970); accumulation of nutrients; reproductive development (flower induction); ripening; abscission; induction of dormancy; frost hardiness. Cytokinins are known to inhibit senescence, i.e. to prevent or even reverse the senescence of leaves. The effects of cytokinin treatment, such as leaf enlargement, re-greening with increased levels of chlorophyll, protein and RNA, and consequently increased photosynthesis, are identical to those observed after summer pruning in apple (Taylor and Ferree, 1981; Marini and Barden, 1982c; Myers and Ferree, 1983a; Ferree et al., 1984), peach (Rom and Ferree, 1985) and mulberry (Satoh et al., 1977). Satoh et al. (1977) stated that the removal of axillary buds additionally to pinching aggravated the effects of shoot pruning, e.g. with respect to increased photosynthesis, delayed starch increase, maintained leaf chlorophyll, and continued mesophyll cell division. These results suggest that an increase in free cytokinins after summer pruning seems to be an important factor in pruning-responses of apple trees. Factors inhibiting the cytokinin export from the roots, e.g. root pruning in apple (Geisler and Ferree, 1984), peach (Richards and Rowe, 1977) and other crops (McDavid et al., 1973), or waterlogging (Childers and White, 1942) or drought, cause opposite effects in that they promote leaf and plant senescence.

273

Additionally, summer p r u n i n g - - if heading cuts or pinching are u s e d - - may prevent or delay senescence in a more direct way also. It has been shown that leaf senescence can be promoted by nearby growing regions, and the elimination of younger organs has consequently retarded or reverted senescence (Nood~n and Leopold, 1978; Thomas and Stoddart, 1980). This agrees with the proposed double function of growing shoot tips in controlling growth directly by correlative inhibition, or indirectly by controlling root activity. In apple trees, the rejuvenation caused by summer pruning was accompanied by a great reduction of ABA activity in the buds within 6 days after pruning (Myers, 1981 ), and in mulberry trees the ABA level of the uppermost leaves remaining after summer pruning was also substantially lowered within 6 days ( Satoh et al., 1977). Obviously, the rejuvenation caused by summer pruning does not result only from the removal of buds and growing shoot tips. Taylor et al. (1984) observed a reduction in the ABA activity in the apex tissues of closed terminal buds on short shoots, and a marked increase in gibberellin-like substances, also after defoliation, but then the increase in cytokinin-like substances was only slight. The effect on ABA and GA content, and the amount of bud break, were more marked when defoliation was done early, pointing to an advance in true dormancy with later defoliation. However, from the observation of Engel and Lenz (1981) on frost damage in apple blossom after various summer treatments (cf. Section 4.1), we may conclude that the rejuvenation by summer pruning is stronger than that caused by total defoliation, provided the assumption is right that the reduced frost hardiness was primarily caused by disturbed onset of dormancy. It is not known whether the same mechanisms acting in leaf senescence are also responsible for the delay in certain characteristics of fruit maturation (cf. Section 2.4). In tomato (Lycopersicon lycopersicum), Varga and Bruinsma (1974) observed that a reduction in foliage considerably increased the cytokinin level in the fruits, and that ripening was progressively retarded by increasing levels of endogenous cytokinins. This observation seems to explain the effect of summer pruning on fruit maturation better than an explanation based only on reduced carbohydrate supply following a reduction of supporting leaves. It could also explain the increased ethylene levels, as observed by Taylor and Ferree (1984b) in maturing apples of summer-pruned trees (cf. Section 2.4). There are numerous reports indicating that ethylene production is raised at higher cytokinin activity, but simultaneously its inhibiting effects are prevented by cytokinins (Saute, 1985b). In support of the observations in tomatoes, apple growers in Northern Germany have observed that early removal of water sprouts may halt fruit development for several weeks (verbal communication), although water sprouts consume rather than supply carbohydrates. Unfortunately, the effect of cultural practices on the content of

274 phytohormones in apple fruit, and the influence of the resulting hormonal changes on physiological processes, still await investigation (Ebert and Bangerth, 1985). Cytokinins could also play a role in the predominantly positive effects of summer pruning on anthocyanin synthesis which occurs even at low light levels, and despite protracted dry matter increment and delayed maturation (cf. Section 2.4). Besides increasing light penetration to the fruit, summer pruning could increase sensitivity to light, i.e. an increased colouring capability could exist, similar to that achieved by temporary covering of apple fruits during fruit development (Kikuchi, 1964; Proctor and Lougheed, 1976). Cytokinin treatments were found to cause an intensive red colouration under certain conditions in young shoots of conifers (Kossuth, 1978; Mulgrew and Williams, 1985). However, this aspect of colour promotion has been widely neglected so far and still requires a lot of research. The information available for discussion in this section is complex and not well elucidated, and the picture drafted of the physiology of summer pruning effects is far from being complete. For instance, it is well established that auxins exert their influence on senescence and ripening at least partially by their influence on, and their interaction with, ethylene. Further, it is known that the primary effects of cytokinins are accompanied by variations in the biosynthesis and/or activity of gibberellins (Grochowska et al., 1984 ). However, the role of gibberellins in rejuvenation, and their interaction with cytokinins, are still insufficiently understood. Yet, even the limited information presented here will be sufficient to indicate the necessity for re-evaluation of some established theories regarding the physiology of summer pruning effects for the benefit of commercial fruit production. 5. SUMMERPRUNING AS AN INTEGRATEDCULTURALPRACTICE As shown above, summer pruning, like pruning in general, is a cultural practice altering the capacity of the trees to control their growth. The endogenous regulation usually becomes active when growth exceeds a certain threshold. If this balance is disturbed, plants tend to re-establish it by increased growth/regrowth ("parachute effect"; Saure, 1981). Therefore, both summer pruning and dormant pruning must be considered primarily as growth-promoting practices, but the invigoration is rather short-term. Accordingly, Maggs (1965) pointed out that dormant and summer pruning are not such diametrically opposed treatments as issometimes suggested, and other authors, e.g. Prigge (1981), Marini and Barden (1982c) and Saute (1985a), confirmed that the effects of both treatments on growth vigour are similar. The fundamental difference between them is that summer pruning is done while the trees are non-dormant or in the state of pre-dormancy, which is at least partially reversible. Therefore, summer pruning delays all phenomena related to senescence (cf. Section 4.2 ). Dormant pruning deals with trees that

275 have completed this process, or have already overcome true dormancy rather recently. Therefore, while dormant pruning can only influence the source/sink ratio still to be established in the coming season, summer pruning a d d i t i o n a l l y interferes with an active shoot/shoot, shoot/root, or shoot/fruit interaction. G r o w t h r e d u c t i o n . ~ It may be concluded that growth of apple trees with a well-functioning root system generally cannot be reduced by any kind of pruning (this is probably different for newly planted trees). Therefore, although pruning is the only way to reduce tree size, and especially the unproductive interior of the canopy, it should be avoided where shoot vigour is to be reduced. The growth promotion and - - in the case of summer pruning - - the delaying effect on dormancy are more pronounced the more invigorating factors prevail such as fertile soils, warm and humid climate, vigorous rootstocks, juvenility of the plant, and poor fruit set. The growth responses will also increase with the severity of pruning, and will be stronger after heading or stubbing than after thinning. If it is necessary for other reasons, any kind of pruning should be accompanied by weakening cultural practices such as bending of shoots, reduced N supply, or reduced irrigation.

To influence flower bud formation, the timing of pruning is decisive. Flower bud formation may be taken in principle as a 3-step process, consisting of: (1) the development of vegetative meristems in the bud initials; (2) the conversion of these meristems into generative meristems; ( 3 ) the further development of these generative meristems. Consequently, early summer pruning may be expected to benefit flower initiation in the apical buds of spurs and short shoots only if tree vigour is poor, by promoting Step 1. Late summer pruning may then promote the further development of these flower initials ( Step 3) and, if thinning cuts are used, may encourage flower initiation in the lateral buds of current season's shoots. However, in vigorous trees, early summer pruning can be expected to disturb flower initiation in the apical buds by preventing Step 2, which requires a temporary inhibition of bud growth. This negative effect of early pruning in vigorous trees will be aggravated by increasing severity of pruning, by heading and stubbing instead of shoot thinning, and by accompanying invigorating factors. In lateral buds, repeated pinching may encourage flower bud formation by promoting Step 1.

Flower bud formation. --

In vigorous trees, the eating quality of fruits is likely to be impaired most by early and severe summer pruning, especially when the growing season is short. The risk of June-drop will increase in these trees correspondingly. Repeated early pinching may reduce June-drop, but would be very laborious. Light summer pruning, preferably by thinning, may help to

Fruit development. ~

276

improve fruit quality only where external conditions prevent vigorous growth. In the season after summer pruning, larger fruit can be expected provided that flower bud formation has been reduced in the previous season, but this would be at the expense of yield. The necessity of heavy late summer pruning for better fruit colour ("exposure pruning") may be taken as an indication of undesirable tree development due to natural growth factors and/or inappropriate cultural practices that need to be corrected, but better should have been avoided. ACKNOWLEDGEMENTS

Dagmar Geisler-Taylor and Dr. B.H. Taylor have been very helpful in critically reading the manuscript, and I am very grateful for their suggestions. I thank all those who have provided me with literature, and who have encouraged me to prepare this review.

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