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Effects of leader pruning on vine architecture, productivity and fruit quality in kiwifruit (Actinidia deliciosa cv. Hayward)
Scientia Horticulturae 91 (2001) 189±199
Effects of leader pruning on vine architecture, productivity and fruit quality in kiwifruit (Actinidia deliciosa cv. Hayward) S.A. Miller*, F.D. Broom, T.G. Thorp, A.M. Barnett HortResearch, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand Accepted 6 March 2001
Abstract The effects of two summer pruning strategies on vine architecture, fruit yield and quality were compared over three growing seasons. With `leader pruning', all vigorous vegetative shoot growth close to the central leader zone was removed continually through the growing season. With `conventional pruning', this new shoot growth was left virtually un-pruned over summer with only summer pruning to shorten fruiting laterals in the fruiting zone to three to four leaves past the last fruit. Leader pruning resulted in signi®cant increases in the ratio of second year wood to ®rst year wood, and in the number of self-terminating canes per vine. The basal diameter of canes was signi®cantly reduced in leader pruned vines. Leader pruning was associated with an increase in total fruit yield. This increase was a result of increased fruit size on leader pruned vines, coupled with a small increase in the number of fruits per unit area. The number of winter buds per square metre did not differ from that of control vines, suggesting that the slight increases in fruit number could be attributed to an increased number of ¯owers per winter bud. Changes in vine composition as a result of leader pruning included a decrease in leaf number, particularly in the upper zone of the canopy, and these changes were associated with signi®cant changes in fruit quality, with fruits from the upper zones being larger, and having a higher soluble solids content after storage than fruits from lower parts of the vines in the same treatment. The effects of pruning strategy on vine resource balance allocation are discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Leader pruning; Fruit quality and yield; Kiwifruit; Resource allocation; Actinidia deliciosa
*
Corresponding author. Tel.: 64-7-858-4671; fax: 64-7-858-4700. E-mail address: [email protected] (S.A. Miller). 0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 0 1 ) 0 0 2 5 9 - X
190
S.A. Miller et al. / Scientia Horticulturae 91 (2001) 189±199
1. Introduction Commercially cultivated kiwifruit (Actinidia deliciosa (A. chev.) cf. Liang and A.R. Ferguson var. deliciosa) are trained onto trellis systems designed to support the weight of vines under full production. The currently preferred supporting structures in New Zealand are the T-bar trellis and the over-head pergola. Vines are trained with a straight trunk about 1.8 m high, and a single strong leader along a centre wire in each direction. The leaders are normally permanent, while a balanced system of replaceable fruiting arms is trained at right angles to them (Sale, 1990). Vines are pruned in various ways according to local and traditional practices in order to optimise yield and fruit quality. Most pruning strategies are based on a system of cane replacement to remove spent fruiting wood during winter. New fruiting wood is produced on 1-year-old canes developed during the previous growing season. Spurs, short fruiting laterals and 1-year-old replacement shoots comprise the fruiting structures on which the next season's fruit will be produced (Sale and Lyford, 1990). Generally, an excess of cane and shoot growth is produced during summer, and summer pruning techniques are applied to reduce the amount of vegetative growth. A recent trend has been the adoption of a vine management strategy known as `leader pruning', where vigorous shoots along the centre of the vine are removed at regular intervals during the growing season. Shoots are removed when they emerge so as to minimize carbohydrate losses from the plant. The aim of this process is to reduce vine vigour and promote the development of less vigorous replacement shoots whilst maintaining a framework of main and secondary branches. General observations show that changes in architecture are apparent when vines are converted to a leader pruned system. Because vigour of the vine is continually being suppressed by the selective removal of shoots, fruiting canes are generally thin, with short internodes, and have a tendency to self-terminate. Spurs form readily particularly along the main leader. A large proportion of the leaf area in these vines is located away from the leader zone, leaving this area open to light penetration, and access for bees during pollination. Air movement within the vine is increased and relative humidity reduced, thus minimizing environmental conditions conducive for fungal diseases such as caused by Botrytis. Most previous research on pruning effects have concentrated on single aspects such as cane characteristics (Henzell et al., 1990; Wilson, 1989; Volz et al., 1991), or leaf area density (Snelgar et al., 1998) and few trials have continued beyond one season. This paper compares canopy structure, productivity and fruit quality attributes of vines managed by a leader pruning system, with that of vines managed by conventional cane replacement. It shows that fruit weight, fruit
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quality and yield in kiwifruit can be improved over a number of seasons by the combined effects of a change in pruning strategy. 2. Materials and methods 2.1. Plant material Mature kiwifruit vines (cv. Hayward grafted on to seedling rootstocks) at HortResearch, Te Puke, New Zealand (378490 S 1768190 E), were used for this trial. The vines were trained onto a T-bar trellis system and the proportion of male:female vines in the block was 1:5. Vines were planted at a spacing of 5 m within rows and 5 m between rows to give a density of 320 female vines/ha. Row orientation was north±south. Two different pruning strategies, `leader' and `conventional cane replacement', were compared. Leader pruning involved removal of all vigorous, upright shoots (canes) growing from the centre of the vines (leader zone) adjacent to the main leaders. These shoots were easily identi®ed when ®rstformed by their dense covering of bright red hairs. Shoots were snapped from their parent structure rather than cut to prevent re-growth from basal buds. The pruning was started about two weeks before ¯owering and was repeated every 14±21 days throughout the summer. Only suf®cient canes required for the next season's fruiting canes (approximately 3.5 canes/m of leader) were retained. These canes commenced growth in October or November, grew comparatively slowly, and most had terminated extension growth by the end of January. In addition to shoot removal from the centre of the vine, non-fruiting lateral shoots were removed from the hanging curtain (`fruiting zone') of the vine. Transition to leader pruning was initiated in November 1996 and was continued for a further 3 years. In the cane-replacement system, practically no pruning was undertaken in the leader zone apart from shortening back any shoots that were starting to curl and tangle. Under this system at least 6 canes/m of central leader were available for potential use a fruiting canes in the following season. Fruiting laterals in the fruiting zone were cut back to three to four leaves past the last fruit and removal of non-fruiting laterals. In winter, standard winter pruning techniques were applied to all leader and conventional vines, with pruning targets of 3.3 canes/m of leader and 40 winter buds per square meter of canopy. Fruits were commercially thinned to remove undersize and misshapen fruits and to prevent over-cropping of weak fruiting laterals. Pruning treatments were applied to four blocks of 30 vines (three adjacent rows with 10 vines per row, two blocks per treatment. Six trees from each block were selected for measurement and data analysed as a randomised complete block
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design. Because the between-vine variation was greater than the between-plot variation, plots were dropped from the analyses. 2.2. Canopy characteristics In late winter, after pruning, the number of winter buds per square meter of cordon and the date of bud-break (10% of buds 1 cm long) were recorded. In November 1997, at the beginning of the growing season, and again in February 1998, once a full canopy had developed, leaf characteristics in 1 m2 quadrats were recorded. These quadrats were placed at eight different positions in the canopy with two quadrats in each quarter of the vine, one in the upper part of the canopy (leader zone), and one on the side curtain (fruiting zone), approximately 1.5 m above the ground. Quadrats were placed at an angle parallel to the planar surface of the vine. Average leaf size was calculated from length and breadth measurements of the youngest mature leaf (YML), and used as an estimate to represent the size of all the leaves present. Leaves on the inside of the canopy, in shaded positions, and on the outside in well-lit or exposed positions were measured. In each case, the leaf chosen for measurement was located on the shoot which had extended the greatest distance either into the centre of the vine (shaded), or away from the vine, into a well-lit position. The distance between these two leaves was measured to give an estimate of canopy depth. Leaf discs (three) were cut using a 1 cm diameter cork borer from the innermost and outermost leaf in each quadrate, placed in a sealed plastic bag and weighed immediately to determine fresh weight. Relative chlorophyll concentration of these same leaves was measured using a SPAD-502 chlorophyll meter (Minolta). Three readings per leaf were taken, and were assumed to be representative of leaves in that zone. 2.3. Fruit quality Fruits were harvested once they had attained commercial maturity of 6.2% soluble solids concentration (SSC) according to the standard sampling procedure of Harman (1981). A sample of 15 fruits from each experimental vine was then harvested using the sampling scheme developed by Miles et al. (1996). Each fruit was weighed and packed into single-layer trays with polyliners, as used commercially, which were then placed in a cool store at 08C for 12 weeks. A second set of 15 fruits was sampled according the same method and assessed immediately after harvest for dry matter content. Fruits were weighed, sliced and oven-dried at 608C, then weighed again to determine percent dry matter content. In 1998 and 1999, vines were segregated into leader and fruiting zones for fruit sampling. The leader zone extended 0.6±0.7 m out from each side of the leader,
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while the fruiting zone included all fruits beyond this point to the edge of the fruiting canopy. Sampled fruits were sorted and stored according to their position on the vine. Fifteen fruits were sampled from each zone. Remaining fruits were harvested and weight graded according to standard pack-house procedures. All fruits were harvested on the same day. For SSC measurement, fruits were removed from the cool store then ripened by exposure to 50 ml ethylene in a 468l container for 24 h. They were then left to equilibrate at room temperature to eating-ripeness (approximately 13 days). SSC was measured on a combined sample from stalk and blossom end of each fruit using an Atago digital PR-1 refractometer. 3. Results 3.1. Canopy composition After 3 years, leader pruned vines had a greater proportion of 2-year-old wood than vines pruned by conventional cane replacement, and a tendency to produce more spurs (short shoots, with internodes <15 mm long). One-year-old canes were signi®cantly thinner on leader pruned vines, and a greater percentage of the canes had terminated their extension-growth before the end of the growing season (Table 1). Winter bud densities were similar after pruning for conventional and leader pruned vines with 39.9 and 39.5 buds per square meter, respectively. Bud- break began on approximately the same date (early September) in each year on vines in both treatments. Eighteen months after the start of the experiment, estimated leaf area from quadrate measurements showed that leader pruned vines had approximately 20% less leaves (40.5 per square meter of cf. 51.9) but this had no signi®cant effect on canopy depth, or area of individual leaves. Table 1 Canopy composition after winter pruning of vines managed either by conventional or leader pruninga Canopy composition
Conventional
Leader pruned
LSD (P 0:05 )
Ratio of 2:1-year-old wood No of spurs per vine Basal diameter of 1-year-old canes Percentage of canes self terminated
1:80 11.4 15.0 22.8
1:17 15.1 13.7 35.1
± 3.9 0.6 9.7
a
After the transition year, data collected for the remaining 2 years of the trial were similar (results from 1998 are shown).
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Table 2 Differences in characteristics of leaves in shaded positions on the vine compared with those in exposed (well-lit) positionsa
Leaf length (cm) Leaf breadth (cm) Leaf area (cm2) Petiole length (cm) Relative chlorophyll content (SPAD) Fresh weight(g) of leaf disc (0.8 cm2) a
Shaded
Exposed
LSD (P 0:05 )
11.95 13.47 148.1 11.49 56.98 0.296
13.48 16.56 204.0 8.99 56.29 0.397
0.66 0.97 18.6 1.21 1.44 0.016
Combined data from conventional and leader pruned vines.
For both treatments, leaves in shaded positions, in the innermost parts of the canopy were smaller than those in more exposed positions and had longer petioles. Leaf discs from shaded leaves also weighed less. There were no signi®cant differences in the relative amount of chlorophyll present in the leaves (SPAD measurements), and this was consistent in both pruning methods (Table 2). 3.2. Fruit production Crop yields varied each year and were particularly low in 1999 because inadequate winter chilling resulted in poor bud-break and ¯owering. The numbers of fruits per vine were slightly greater on leader pruned vines for 2 of the 3 years of this experiment. Average fruit weights on leader pruned vines were consistently greater, and in the third year (1999) this translated into an improvement in yield (Table 3). That year, it was found that there were signi®cantly (P 0:05) more fruits on leader pruned vines that weighed 120 g or more than on the conventional vines (Fig. 1). Also, fruits from the upper part of the canopy (leader zone) weighed signi®cantly more than fruits from Table 3 Crop load, fruit weight and yield on vines maintained by either conventional or leader pruning management practice Yield (kg)
1996 1997 1998 1999
LSD (P 0:05 )
Conventional
Leader pruned
69.8 76.1 78.7 51.1
83.7 79.0 59.8
7.4 6.6 7.9
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Fig. 1. Distribution of fruit weights at harvest for leader pruned (*) and conventional (*) vines for the 1999 season.
lower down the canes, and this was consistent for both the pruning treatments (Table 4). 3.3. Fruit quality In the ®rst year of the experiment there were no signi®cant differences between treatments in dry matter and soluble-solids content of the fruit. Mean dry matter at harvest in 1997 was 14:6 0:3 for the conventionally pruned vines and 14:9 0:5 for leader pruned vines, while the mean SSC at harvest for both treatments was 7:0 0:3 Brix. In year 2 (1998), when fruits were sorted according to their position on the vine, and stored for 12 weeks at 08C, fruits from the leader (upper) zone appeared to have higher soluble-solids content than fruits from the fruiting (lower) zone and this was con®rmed the following year for both the pruning treatments (Table 5). Table 4 Positional differences fruit size from the upper and lower zones of vines managed by either cane replacement or leader pruning Fruit weight (g)
Conventional Leader pruned LSD P 0.05
LSD (P 0:05 )
Upper zone
Lower zone
121.3 127.1 3.7
116.6 118.3 3.7
3.8 3.8
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Table 5 Positional effects on soluble-solids content of stored fruit harvested from the leader zone (upper) and fruiting zone (lower) of kiwifruit vines managed by either conventional or leader pruning techniques Conventional Upper
Lower
Soluble-solids content (Brix8) 1998 12.8 12.6 1999 13.5 13.0
Leader pruned LSD (P 0:05 )
Upper
Lower
LSD (P 0:05 )
0.2 0.3
13.3 14.3
12.7 13.3
0.2 0.3
4. Discussion 4.1. Fruit yield and quality Leader pruning of kiwifruit vines led to more open canopies and resulted in a change in canopy architecture, so that fruit yield and soluble solids concentration were improved. The differences between treatments in fruit quality were most pronounced for fruit harvested from positions close to the leader. Fruit in the leader zone might have been exposed to higher light levels through the growing season due to the constant removal of vegetative shoots along the centre of the vine. Biasi et al. (1993) note that in kiwifruit, both leaves and fruits need exposure to light for high fruit quality. 4.2. Resource allocation Spatial difference in fruit attributes were reported by Smith et al. (1994), who also found that fruits with the highest quality occurred in positions close to the leader. They state that vines with a denser leaf canopy will have a greater proportion of fruits with superior characteristics. Our hypothesis is that the distribution of leaves over the whole canopy is a more important factor than leaf area index per se. Estimation of leaf area index from extrapolation of quadrat measurements suggest that leader pruned vines had approximately 20% less leaf area, and yet, these vines had higher yields. It is possible that continual removal of vegetative shoots from the centre of leader pruned vines changed the radiation environment in this area so that photosynthetic capacity of the inner leaves was improved. As noted by Buwalda (1994), variation in radiation interception in both time and space within a plant canopy, can affect fruit size distribution through variation in supply of assimilated carbon. Although photosynthesis was not measured in this trial, other research has shown that leaves growing in the shade have lower rates of assimilation (Smith and Buwalda, 1994). While leaves are able to acclimate to low light levels, their contribution to overall canopy
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photosynthesis is possibly reduced. Indirect evidence of lower light on the inner parts of the canopy were seen in this work with the increase in petiole length in shaded leaves. Alternatively, there may have been a change in the leader pruned vines in carbohydrate partitioning. Famiani et al. (1997) showed that the assimilate can be easily translocated within the plant to support fruit growth on shoots with a small leaf:fruit ratio, thus fruits located in areas of the vine that have few leaves can be supplied with assimilate from some distance away. Snelgar and Thorp (1988) and Lai et al. (1988) also found that the photo-assimilate was freely translocatable in kiwifruit vines and reported that laterals ®ve to six nodes away from a fruit can supply the carbohydrate required for growth. However, recent work by Piller et al. (2000), using girdled canes, indicated that the assimilate may move less freely down the cane, from the base to the apex, than in the reverse direction. Thus positioning more leaves at the ends of the canes may enhance growth of fruits near the leader. A direct effect of leader pruning is that although cane number is reduced the number of leaves per fruiting lateral further down the canes is increased. Work by Amano et al. (1998) showed that shoot elongation during fruit growth adversely affects the distribution of photosynthate into fruits, whilst the results of Greer (1999) imply that competition between fruit and vegetative sinks has a regulatory role on photo-assimilation of leaves. Leaves that emerged in spring with axillary ¯owers and fruits had both a higher initial rate of photosynthesis and an earlier time of maximal capacity than leaves not subtending fruits (Greer, 1999). Therefore cultural practices such as leader pruning which remove vegetative growth and retain self terminating shoots may be an effective means to enhance assimilate uptake by fruits. Our results are consistent with observations that removal of excessive vigour improves productivity and fruit quality in kiwifruit vines (Sale and Lyford, 1990). At the present time, returns to growers are based on fruit numbers and size, with maximum pro®t being gained through production of large fruits. Vines managed by leader pruning produced larger fruits. Cost bene®t analysis after 3 years showed that annual returns to growers using this system could be increased by up to NZ$ 5,400/ha. 4.3. Effects on vine health The long-term effects of leader pruning on vine health are unknown. Reductions in leaf potassium concentration were measured in our work during the transition year to a leader pruning system (Miller et al., 1997), but this did not recur in subsequent years. Work by Buwalda (1991) suggests that altering leaf/fruit ratios may effect root turn-over, and that insuf®cient resource allocation to the roots will reduce performance of the entire vine. There were no visual indications in our
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work over 3 years that the vines were under stress. This suggests that overall carbohydrate demand by the vines was being satis®ed. It is thought that assimilate can be transferred within the vine to areas of high demand (Lai et al., 1988), but the rate at which a vine can adjust to altered demands by individual components has not been determined, and deserves further investigation. Our work would suggest that the combined effects of a pruning strategy are cumulative over a number of seasons and can result in improved fruit yield and quality. Provided that leaf:fruit ratios are maintained on a whole vine basis at two to four leaves per fruit (Famiani et al., 1997), then we believe that leader pruning is a sustainable management strategy that growers can use to improve pro®tability of the orchard. Acknowledgements We thank Barbara Dow for the help in statistical analyses, and also several colleagues for providing constructive comments during preparation of this manuscript. The research was funded by the Foundation for Research, Science and Technology. References Amano, S., Yui, T., Yamada, H., Mizutani, F., Kadoya, K., 1998. Effect of growth habit of bearing shoot on the distribution of 13C-photosynthates in kiwifruit vines. J. Jpn. Soc. Hortic. Sci. 67, 875±879. Biasi, R., Costa, G., Manson, P.J., 1993. Light in¯uences on kiwifruit (Actinidia deliciosa) quality. Acta Hortic. 379, 245±251. Buwalda, J.G., 1991. Root growth of kiwifruit vines and the impact of canopy manipulations. Dev. Agric. Manage. For. Ecol. 24, 431±441. Buwalda, J.G., 1994. The impact of canopy growth and temporal changes in radiation on the dynamics of canopy carbon acquisition for kiwifruit (Actinidia deliciosa) vines during spring. Env. Exp. Bot. 34, 145±151. Famiani, F., Antognozzi, E., Boco, M., Tombesi, A., Battistelli, A., Moscatello, S., Spaccino, L., 1997. Effects of altered source±sink relationships on fruit development and quality in Actinidia deliciosa. Acta Hortic. 444, 355±360. Henzell, R.F., Briscoe, M., Gravett, I., 1990. Little things can mean a lot. In: New Zealand Kiwifruit Natural Resourses Conference. Special Publication No. 3, 15±16. Rotorua, June 24±25. Greer, D.H., 1999. Seasonal and daily changes in carbon acquisition of kiwifruit leaves with and without axillary fruit. N.Z. J. Crop Hortic. Sci. 27, 23±31. Harman, J.E., 1981. Kiwifruit maturity. The orchardist N.Z. 54 (126-127), 130. Lai, R., Woolley, D.J., Lawes, G.S., 1988. Patterns of assimilate transport from leaves to fruit within a kiwifruit (Actinidia deliciosa) lateral. J. Hortic. Sci. 63, 725±730. Miles, D.B., Smith, G.S., Miller, S.A., 1996. Within plant sampling procedures Ð fruit variation in kiwifruit vines. Ann. Bot. 78, 289±294. Miller, S.A., Broom, F.D., Thorp, T.G., Barnett, A.M., 1997. Kiwifruit canopy management Ð transition to leader pruning. The Orchardist N.Z. September, pp. 39±41.
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Piller, G.J., Greaves, A.J., Mowat, A.D., Meekings, J.S., 2000. Effects of fruit:leaf spatial organisation on fruit maturation in kiwifruit. Ann. Bot., (in press). Sale, P.R., 1990. Kiwifruit growing. GP Books, Wellington, New Zealand. Sale, P.R., Lyford, P.B., 1990. Cultural, management and harvesting practices for kiwifruit in New Zealand. In: Warrington, I.J., Weston, G.C. (Eds.), Kiwifruit Science and Management. Richards Publisher, Auckland, pp. 247±296. Smith, G.S., Buwalda, J.G., 1994. Kiwifruit. In: Schaffer, B., Andersen, P.C. (Eds.), Environmental Physiology of Fruit Crops. CRC Press, Boca Raton, FL. pp. 135±163. Smith, G.S., Gravett, I.M., Edwards, C.M., Curtis, J.P., Buwalda, J.G., 1994. Spatial analysis of the canopy of kiwifruit vines as it relates to the physical, chemical and postharvest attributes of the fruit. Ann. Bot. 73, 99±111. Snelgar, W.P., Thorp, T.G., 1988. Leaf area, fruit weight and productivity in kiwifruit. Sci. Hortic. 36, 241±249. Snelgar, W.P., Hopkirk, G., Seelye, R.J., Martin, P.J., Manson, P.J., 1998. Relationship between canopy density and fruit quality of kiwifruit. N.Z. J. Crop Hortic. Sci. 26, 223±232. Wilson, G., 1989. Self-terminating canes are inferior. N.Z. Kiwifruit, September 6. Volz, R.K., Gibbs, H.M., Lupton, G.B., 1991. Variation in fruitfulness among kiwifruit replacement canes. Acta Hortic. 297, 443±449.
Effects of leader pruning on vine architecture, productivity and fruit quality in kiwifruit (Actinidia deliciosa cv. Hayward) S.A. Miller*, F.D. Broom, T.G. Thorp, A.M. Barnett HortResearch, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand Accepted 6 March 2001
Abstract The effects of two summer pruning strategies on vine architecture, fruit yield and quality were compared over three growing seasons. With `leader pruning', all vigorous vegetative shoot growth close to the central leader zone was removed continually through the growing season. With `conventional pruning', this new shoot growth was left virtually un-pruned over summer with only summer pruning to shorten fruiting laterals in the fruiting zone to three to four leaves past the last fruit. Leader pruning resulted in signi®cant increases in the ratio of second year wood to ®rst year wood, and in the number of self-terminating canes per vine. The basal diameter of canes was signi®cantly reduced in leader pruned vines. Leader pruning was associated with an increase in total fruit yield. This increase was a result of increased fruit size on leader pruned vines, coupled with a small increase in the number of fruits per unit area. The number of winter buds per square metre did not differ from that of control vines, suggesting that the slight increases in fruit number could be attributed to an increased number of ¯owers per winter bud. Changes in vine composition as a result of leader pruning included a decrease in leaf number, particularly in the upper zone of the canopy, and these changes were associated with signi®cant changes in fruit quality, with fruits from the upper zones being larger, and having a higher soluble solids content after storage than fruits from lower parts of the vines in the same treatment. The effects of pruning strategy on vine resource balance allocation are discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Leader pruning; Fruit quality and yield; Kiwifruit; Resource allocation; Actinidia deliciosa
*
Corresponding author. Tel.: 64-7-858-4671; fax: 64-7-858-4700. E-mail address: [email protected] (S.A. Miller). 0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 0 1 ) 0 0 2 5 9 - X
190
S.A. Miller et al. / Scientia Horticulturae 91 (2001) 189±199
1. Introduction Commercially cultivated kiwifruit (Actinidia deliciosa (A. chev.) cf. Liang and A.R. Ferguson var. deliciosa) are trained onto trellis systems designed to support the weight of vines under full production. The currently preferred supporting structures in New Zealand are the T-bar trellis and the over-head pergola. Vines are trained with a straight trunk about 1.8 m high, and a single strong leader along a centre wire in each direction. The leaders are normally permanent, while a balanced system of replaceable fruiting arms is trained at right angles to them (Sale, 1990). Vines are pruned in various ways according to local and traditional practices in order to optimise yield and fruit quality. Most pruning strategies are based on a system of cane replacement to remove spent fruiting wood during winter. New fruiting wood is produced on 1-year-old canes developed during the previous growing season. Spurs, short fruiting laterals and 1-year-old replacement shoots comprise the fruiting structures on which the next season's fruit will be produced (Sale and Lyford, 1990). Generally, an excess of cane and shoot growth is produced during summer, and summer pruning techniques are applied to reduce the amount of vegetative growth. A recent trend has been the adoption of a vine management strategy known as `leader pruning', where vigorous shoots along the centre of the vine are removed at regular intervals during the growing season. Shoots are removed when they emerge so as to minimize carbohydrate losses from the plant. The aim of this process is to reduce vine vigour and promote the development of less vigorous replacement shoots whilst maintaining a framework of main and secondary branches. General observations show that changes in architecture are apparent when vines are converted to a leader pruned system. Because vigour of the vine is continually being suppressed by the selective removal of shoots, fruiting canes are generally thin, with short internodes, and have a tendency to self-terminate. Spurs form readily particularly along the main leader. A large proportion of the leaf area in these vines is located away from the leader zone, leaving this area open to light penetration, and access for bees during pollination. Air movement within the vine is increased and relative humidity reduced, thus minimizing environmental conditions conducive for fungal diseases such as caused by Botrytis. Most previous research on pruning effects have concentrated on single aspects such as cane characteristics (Henzell et al., 1990; Wilson, 1989; Volz et al., 1991), or leaf area density (Snelgar et al., 1998) and few trials have continued beyond one season. This paper compares canopy structure, productivity and fruit quality attributes of vines managed by a leader pruning system, with that of vines managed by conventional cane replacement. It shows that fruit weight, fruit
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quality and yield in kiwifruit can be improved over a number of seasons by the combined effects of a change in pruning strategy. 2. Materials and methods 2.1. Plant material Mature kiwifruit vines (cv. Hayward grafted on to seedling rootstocks) at HortResearch, Te Puke, New Zealand (378490 S 1768190 E), were used for this trial. The vines were trained onto a T-bar trellis system and the proportion of male:female vines in the block was 1:5. Vines were planted at a spacing of 5 m within rows and 5 m between rows to give a density of 320 female vines/ha. Row orientation was north±south. Two different pruning strategies, `leader' and `conventional cane replacement', were compared. Leader pruning involved removal of all vigorous, upright shoots (canes) growing from the centre of the vines (leader zone) adjacent to the main leaders. These shoots were easily identi®ed when ®rstformed by their dense covering of bright red hairs. Shoots were snapped from their parent structure rather than cut to prevent re-growth from basal buds. The pruning was started about two weeks before ¯owering and was repeated every 14±21 days throughout the summer. Only suf®cient canes required for the next season's fruiting canes (approximately 3.5 canes/m of leader) were retained. These canes commenced growth in October or November, grew comparatively slowly, and most had terminated extension growth by the end of January. In addition to shoot removal from the centre of the vine, non-fruiting lateral shoots were removed from the hanging curtain (`fruiting zone') of the vine. Transition to leader pruning was initiated in November 1996 and was continued for a further 3 years. In the cane-replacement system, practically no pruning was undertaken in the leader zone apart from shortening back any shoots that were starting to curl and tangle. Under this system at least 6 canes/m of central leader were available for potential use a fruiting canes in the following season. Fruiting laterals in the fruiting zone were cut back to three to four leaves past the last fruit and removal of non-fruiting laterals. In winter, standard winter pruning techniques were applied to all leader and conventional vines, with pruning targets of 3.3 canes/m of leader and 40 winter buds per square meter of canopy. Fruits were commercially thinned to remove undersize and misshapen fruits and to prevent over-cropping of weak fruiting laterals. Pruning treatments were applied to four blocks of 30 vines (three adjacent rows with 10 vines per row, two blocks per treatment. Six trees from each block were selected for measurement and data analysed as a randomised complete block
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design. Because the between-vine variation was greater than the between-plot variation, plots were dropped from the analyses. 2.2. Canopy characteristics In late winter, after pruning, the number of winter buds per square meter of cordon and the date of bud-break (10% of buds 1 cm long) were recorded. In November 1997, at the beginning of the growing season, and again in February 1998, once a full canopy had developed, leaf characteristics in 1 m2 quadrats were recorded. These quadrats were placed at eight different positions in the canopy with two quadrats in each quarter of the vine, one in the upper part of the canopy (leader zone), and one on the side curtain (fruiting zone), approximately 1.5 m above the ground. Quadrats were placed at an angle parallel to the planar surface of the vine. Average leaf size was calculated from length and breadth measurements of the youngest mature leaf (YML), and used as an estimate to represent the size of all the leaves present. Leaves on the inside of the canopy, in shaded positions, and on the outside in well-lit or exposed positions were measured. In each case, the leaf chosen for measurement was located on the shoot which had extended the greatest distance either into the centre of the vine (shaded), or away from the vine, into a well-lit position. The distance between these two leaves was measured to give an estimate of canopy depth. Leaf discs (three) were cut using a 1 cm diameter cork borer from the innermost and outermost leaf in each quadrate, placed in a sealed plastic bag and weighed immediately to determine fresh weight. Relative chlorophyll concentration of these same leaves was measured using a SPAD-502 chlorophyll meter (Minolta). Three readings per leaf were taken, and were assumed to be representative of leaves in that zone. 2.3. Fruit quality Fruits were harvested once they had attained commercial maturity of 6.2% soluble solids concentration (SSC) according to the standard sampling procedure of Harman (1981). A sample of 15 fruits from each experimental vine was then harvested using the sampling scheme developed by Miles et al. (1996). Each fruit was weighed and packed into single-layer trays with polyliners, as used commercially, which were then placed in a cool store at 08C for 12 weeks. A second set of 15 fruits was sampled according the same method and assessed immediately after harvest for dry matter content. Fruits were weighed, sliced and oven-dried at 608C, then weighed again to determine percent dry matter content. In 1998 and 1999, vines were segregated into leader and fruiting zones for fruit sampling. The leader zone extended 0.6±0.7 m out from each side of the leader,
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while the fruiting zone included all fruits beyond this point to the edge of the fruiting canopy. Sampled fruits were sorted and stored according to their position on the vine. Fifteen fruits were sampled from each zone. Remaining fruits were harvested and weight graded according to standard pack-house procedures. All fruits were harvested on the same day. For SSC measurement, fruits were removed from the cool store then ripened by exposure to 50 ml ethylene in a 468l container for 24 h. They were then left to equilibrate at room temperature to eating-ripeness (approximately 13 days). SSC was measured on a combined sample from stalk and blossom end of each fruit using an Atago digital PR-1 refractometer. 3. Results 3.1. Canopy composition After 3 years, leader pruned vines had a greater proportion of 2-year-old wood than vines pruned by conventional cane replacement, and a tendency to produce more spurs (short shoots, with internodes <15 mm long). One-year-old canes were signi®cantly thinner on leader pruned vines, and a greater percentage of the canes had terminated their extension-growth before the end of the growing season (Table 1). Winter bud densities were similar after pruning for conventional and leader pruned vines with 39.9 and 39.5 buds per square meter, respectively. Bud- break began on approximately the same date (early September) in each year on vines in both treatments. Eighteen months after the start of the experiment, estimated leaf area from quadrate measurements showed that leader pruned vines had approximately 20% less leaves (40.5 per square meter of cf. 51.9) but this had no signi®cant effect on canopy depth, or area of individual leaves. Table 1 Canopy composition after winter pruning of vines managed either by conventional or leader pruninga Canopy composition
Conventional
Leader pruned
LSD (P 0:05 )
Ratio of 2:1-year-old wood No of spurs per vine Basal diameter of 1-year-old canes Percentage of canes self terminated
1:80 11.4 15.0 22.8
1:17 15.1 13.7 35.1
± 3.9 0.6 9.7
a
After the transition year, data collected for the remaining 2 years of the trial were similar (results from 1998 are shown).
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Table 2 Differences in characteristics of leaves in shaded positions on the vine compared with those in exposed (well-lit) positionsa
Leaf length (cm) Leaf breadth (cm) Leaf area (cm2) Petiole length (cm) Relative chlorophyll content (SPAD) Fresh weight(g) of leaf disc (0.8 cm2) a
Shaded
Exposed
LSD (P 0:05 )
11.95 13.47 148.1 11.49 56.98 0.296
13.48 16.56 204.0 8.99 56.29 0.397
0.66 0.97 18.6 1.21 1.44 0.016
Combined data from conventional and leader pruned vines.
For both treatments, leaves in shaded positions, in the innermost parts of the canopy were smaller than those in more exposed positions and had longer petioles. Leaf discs from shaded leaves also weighed less. There were no signi®cant differences in the relative amount of chlorophyll present in the leaves (SPAD measurements), and this was consistent in both pruning methods (Table 2). 3.2. Fruit production Crop yields varied each year and were particularly low in 1999 because inadequate winter chilling resulted in poor bud-break and ¯owering. The numbers of fruits per vine were slightly greater on leader pruned vines for 2 of the 3 years of this experiment. Average fruit weights on leader pruned vines were consistently greater, and in the third year (1999) this translated into an improvement in yield (Table 3). That year, it was found that there were signi®cantly (P 0:05) more fruits on leader pruned vines that weighed 120 g or more than on the conventional vines (Fig. 1). Also, fruits from the upper part of the canopy (leader zone) weighed signi®cantly more than fruits from Table 3 Crop load, fruit weight and yield on vines maintained by either conventional or leader pruning management practice Yield (kg)
1996 1997 1998 1999
LSD (P 0:05 )
Conventional
Leader pruned
69.8 76.1 78.7 51.1
83.7 79.0 59.8
7.4 6.6 7.9
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Fig. 1. Distribution of fruit weights at harvest for leader pruned (*) and conventional (*) vines for the 1999 season.
lower down the canes, and this was consistent for both the pruning treatments (Table 4). 3.3. Fruit quality In the ®rst year of the experiment there were no signi®cant differences between treatments in dry matter and soluble-solids content of the fruit. Mean dry matter at harvest in 1997 was 14:6 0:3 for the conventionally pruned vines and 14:9 0:5 for leader pruned vines, while the mean SSC at harvest for both treatments was 7:0 0:3 Brix. In year 2 (1998), when fruits were sorted according to their position on the vine, and stored for 12 weeks at 08C, fruits from the leader (upper) zone appeared to have higher soluble-solids content than fruits from the fruiting (lower) zone and this was con®rmed the following year for both the pruning treatments (Table 5). Table 4 Positional differences fruit size from the upper and lower zones of vines managed by either cane replacement or leader pruning Fruit weight (g)
Conventional Leader pruned LSD P 0.05
LSD (P 0:05 )
Upper zone
Lower zone
121.3 127.1 3.7
116.6 118.3 3.7
3.8 3.8
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Table 5 Positional effects on soluble-solids content of stored fruit harvested from the leader zone (upper) and fruiting zone (lower) of kiwifruit vines managed by either conventional or leader pruning techniques Conventional Upper
Lower
Soluble-solids content (Brix8) 1998 12.8 12.6 1999 13.5 13.0
Leader pruned LSD (P 0:05 )
Upper
Lower
LSD (P 0:05 )
0.2 0.3
13.3 14.3
12.7 13.3
0.2 0.3
4. Discussion 4.1. Fruit yield and quality Leader pruning of kiwifruit vines led to more open canopies and resulted in a change in canopy architecture, so that fruit yield and soluble solids concentration were improved. The differences between treatments in fruit quality were most pronounced for fruit harvested from positions close to the leader. Fruit in the leader zone might have been exposed to higher light levels through the growing season due to the constant removal of vegetative shoots along the centre of the vine. Biasi et al. (1993) note that in kiwifruit, both leaves and fruits need exposure to light for high fruit quality. 4.2. Resource allocation Spatial difference in fruit attributes were reported by Smith et al. (1994), who also found that fruits with the highest quality occurred in positions close to the leader. They state that vines with a denser leaf canopy will have a greater proportion of fruits with superior characteristics. Our hypothesis is that the distribution of leaves over the whole canopy is a more important factor than leaf area index per se. Estimation of leaf area index from extrapolation of quadrat measurements suggest that leader pruned vines had approximately 20% less leaf area, and yet, these vines had higher yields. It is possible that continual removal of vegetative shoots from the centre of leader pruned vines changed the radiation environment in this area so that photosynthetic capacity of the inner leaves was improved. As noted by Buwalda (1994), variation in radiation interception in both time and space within a plant canopy, can affect fruit size distribution through variation in supply of assimilated carbon. Although photosynthesis was not measured in this trial, other research has shown that leaves growing in the shade have lower rates of assimilation (Smith and Buwalda, 1994). While leaves are able to acclimate to low light levels, their contribution to overall canopy
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photosynthesis is possibly reduced. Indirect evidence of lower light on the inner parts of the canopy were seen in this work with the increase in petiole length in shaded leaves. Alternatively, there may have been a change in the leader pruned vines in carbohydrate partitioning. Famiani et al. (1997) showed that the assimilate can be easily translocated within the plant to support fruit growth on shoots with a small leaf:fruit ratio, thus fruits located in areas of the vine that have few leaves can be supplied with assimilate from some distance away. Snelgar and Thorp (1988) and Lai et al. (1988) also found that the photo-assimilate was freely translocatable in kiwifruit vines and reported that laterals ®ve to six nodes away from a fruit can supply the carbohydrate required for growth. However, recent work by Piller et al. (2000), using girdled canes, indicated that the assimilate may move less freely down the cane, from the base to the apex, than in the reverse direction. Thus positioning more leaves at the ends of the canes may enhance growth of fruits near the leader. A direct effect of leader pruning is that although cane number is reduced the number of leaves per fruiting lateral further down the canes is increased. Work by Amano et al. (1998) showed that shoot elongation during fruit growth adversely affects the distribution of photosynthate into fruits, whilst the results of Greer (1999) imply that competition between fruit and vegetative sinks has a regulatory role on photo-assimilation of leaves. Leaves that emerged in spring with axillary ¯owers and fruits had both a higher initial rate of photosynthesis and an earlier time of maximal capacity than leaves not subtending fruits (Greer, 1999). Therefore cultural practices such as leader pruning which remove vegetative growth and retain self terminating shoots may be an effective means to enhance assimilate uptake by fruits. Our results are consistent with observations that removal of excessive vigour improves productivity and fruit quality in kiwifruit vines (Sale and Lyford, 1990). At the present time, returns to growers are based on fruit numbers and size, with maximum pro®t being gained through production of large fruits. Vines managed by leader pruning produced larger fruits. Cost bene®t analysis after 3 years showed that annual returns to growers using this system could be increased by up to NZ$ 5,400/ha. 4.3. Effects on vine health The long-term effects of leader pruning on vine health are unknown. Reductions in leaf potassium concentration were measured in our work during the transition year to a leader pruning system (Miller et al., 1997), but this did not recur in subsequent years. Work by Buwalda (1991) suggests that altering leaf/fruit ratios may effect root turn-over, and that insuf®cient resource allocation to the roots will reduce performance of the entire vine. There were no visual indications in our
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work over 3 years that the vines were under stress. This suggests that overall carbohydrate demand by the vines was being satis®ed. It is thought that assimilate can be transferred within the vine to areas of high demand (Lai et al., 1988), but the rate at which a vine can adjust to altered demands by individual components has not been determined, and deserves further investigation. Our work would suggest that the combined effects of a pruning strategy are cumulative over a number of seasons and can result in improved fruit yield and quality. Provided that leaf:fruit ratios are maintained on a whole vine basis at two to four leaves per fruit (Famiani et al., 1997), then we believe that leader pruning is a sustainable management strategy that growers can use to improve pro®tability of the orchard. Acknowledgements We thank Barbara Dow for the help in statistical analyses, and also several colleagues for providing constructive comments during preparation of this manuscript. The research was funded by the Foundation for Research, Science and Technology. References Amano, S., Yui, T., Yamada, H., Mizutani, F., Kadoya, K., 1998. Effect of growth habit of bearing shoot on the distribution of 13C-photosynthates in kiwifruit vines. J. Jpn. Soc. Hortic. Sci. 67, 875±879. Biasi, R., Costa, G., Manson, P.J., 1993. Light in¯uences on kiwifruit (Actinidia deliciosa) quality. Acta Hortic. 379, 245±251. Buwalda, J.G., 1991. Root growth of kiwifruit vines and the impact of canopy manipulations. Dev. Agric. Manage. For. Ecol. 24, 431±441. Buwalda, J.G., 1994. The impact of canopy growth and temporal changes in radiation on the dynamics of canopy carbon acquisition for kiwifruit (Actinidia deliciosa) vines during spring. Env. Exp. Bot. 34, 145±151. Famiani, F., Antognozzi, E., Boco, M., Tombesi, A., Battistelli, A., Moscatello, S., Spaccino, L., 1997. Effects of altered source±sink relationships on fruit development and quality in Actinidia deliciosa. Acta Hortic. 444, 355±360. Henzell, R.F., Briscoe, M., Gravett, I., 1990. Little things can mean a lot. In: New Zealand Kiwifruit Natural Resourses Conference. Special Publication No. 3, 15±16. Rotorua, June 24±25. Greer, D.H., 1999. Seasonal and daily changes in carbon acquisition of kiwifruit leaves with and without axillary fruit. N.Z. J. Crop Hortic. Sci. 27, 23±31. Harman, J.E., 1981. Kiwifruit maturity. The orchardist N.Z. 54 (126-127), 130. Lai, R., Woolley, D.J., Lawes, G.S., 1988. Patterns of assimilate transport from leaves to fruit within a kiwifruit (Actinidia deliciosa) lateral. J. Hortic. Sci. 63, 725±730. Miles, D.B., Smith, G.S., Miller, S.A., 1996. Within plant sampling procedures Ð fruit variation in kiwifruit vines. Ann. Bot. 78, 289±294. Miller, S.A., Broom, F.D., Thorp, T.G., Barnett, A.M., 1997. Kiwifruit canopy management Ð transition to leader pruning. The Orchardist N.Z. September, pp. 39±41.
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Piller, G.J., Greaves, A.J., Mowat, A.D., Meekings, J.S., 2000. Effects of fruit:leaf spatial organisation on fruit maturation in kiwifruit. Ann. Bot., (in press). Sale, P.R., 1990. Kiwifruit growing. GP Books, Wellington, New Zealand. Sale, P.R., Lyford, P.B., 1990. Cultural, management and harvesting practices for kiwifruit in New Zealand. In: Warrington, I.J., Weston, G.C. (Eds.), Kiwifruit Science and Management. Richards Publisher, Auckland, pp. 247±296. Smith, G.S., Buwalda, J.G., 1994. Kiwifruit. In: Schaffer, B., Andersen, P.C. (Eds.), Environmental Physiology of Fruit Crops. CRC Press, Boca Raton, FL. pp. 135±163. Smith, G.S., Gravett, I.M., Edwards, C.M., Curtis, J.P., Buwalda, J.G., 1994. Spatial analysis of the canopy of kiwifruit vines as it relates to the physical, chemical and postharvest attributes of the fruit. Ann. Bot. 73, 99±111. Snelgar, W.P., Thorp, T.G., 1988. Leaf area, fruit weight and productivity in kiwifruit. Sci. Hortic. 36, 241±249. Snelgar, W.P., Hopkirk, G., Seelye, R.J., Martin, P.J., Manson, P.J., 1998. Relationship between canopy density and fruit quality of kiwifruit. N.Z. J. Crop Hortic. Sci. 26, 223±232. Wilson, G., 1989. Self-terminating canes are inferior. N.Z. Kiwifruit, September 6. Volz, R.K., Gibbs, H.M., Lupton, G.B., 1991. Variation in fruitfulness among kiwifruit replacement canes. Acta Hortic. 297, 443±449.