Pollination effects on fruit mineral composition, seeds and cropping characteristics of ‘Braeburn’ apple trees

SCIENTIA HORTICULTUM ELSEVIER

Scientia Horticulturae

66 ( 1996) 169- 180

Pollination effects on fruit mineral composition, seeds and cropping characteristics of ‘Braebum’ apple trees Richard K. Volz ay*, D. Stuart Tusk

a, Ian B. Ferguson b

aHorticultural and Food Research Institute of New Zealand, Hawkes Bay Research Centre, Private Bay 1401, Havelock North, New Zealand b Horticultural and Food Research Institute of New Zealand, Mt. Albert Research Centre, Private Bag 92169, Auckland, New Zealand Accepted 5 May 19%

Abstract ‘Braebum’ apple (Mulus domestica Borkh) trees were supplementary hand-pollinated or partially netted over a 3-4 week period during early and mid-blossom and compared with trees pollinated only by bees. Trees were later hand-thinned to a moderate or heavy level. In addition, individual fruit from flower clusters which had reached king petal fall in the first or fourth week of bloom were compared on supplementary hand and bee-only pollination treatments which had been moderately thinned. Partially netting trees reduced, while supplementary pollination increased, initial and final set on spur and terminal sites, seed number and final fruit Ca concentrations. Final set on l-year axillary sites was increased by partial netting but reduced by supplementary pollination. Final yield, fruit Mg and K concentrations and average fruit size were unaffected by pollination treatment while hand-thinning level had no effect on fruit distributions, cropping or fruit mineral concentrations. For fruiting sites in trees and fruit of known blossom dates, initial fruit set and seed number per fruit were greater for later flowering sites but fruit Ca concentrations and bourse leaf areas were less than those from early flowering sites. Supplementary hand-pollination increased initial fruit set, seed numbers and fruit Ca concentrations on early flowering sites but had little effect on late flowering sites. These results show that poor pollination during the early blossom period can reduce fruit Ca concentrations for the ‘Braebum’ cultivar. This effect may occur by reducing seed numbers in fruit as well as by altering fruit distributions on the tree. Keywords:

Malus domestica; Flowering

* Corresponding

date; Calcium; Spur leaves; Fruit set

author.

0304-4238/96/$15.00 Copyright PII SO304-4238(96)00934-X

0 1996 Elsevier Science B.V. All rights reserved.

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1. Introduction

Low fruit calcium (Cal concentrations can predispose apples to storage disorders (Ferguson and Watkins, 1989). High Ca concentrations are required in fruit at harvest so Ca sprays are routinely applied throughout fruit development. However, most mineral input into fruit occurs from the tree, and factors affecting this pathway are only beginning to be understood. Crop load (Ferguson and Watkins, 1992; Volz et al., 1993), fruiting position within the tree canopy (Ferguson and Triggs, 1990; Volz et al., 1994) and leaves subtending the fruiting spur (Jones and Samuelson, 1983; Volz et al., 1996) can all directly affect the accumulation of Ca into apple fruit. Fruit size can affect fruit mineral nutrition indirectly, with larger fruit having lower Ca concentrations (Perring and Jackson, 1975). Although it is well known that seed number can affect fruit size and shape in many crops, including apple, several recent studies have indicated that seed numbers also may directly affect fruit Ca accumulation. Low seed numbers in apple fruit have been associated with low fruit Ca concentrations (Bramlage et al., 1990; Tomala and Dilley, 1990). Fruit Ca concentrations were reduced after seed numbers were lowered using exogenous plant growth regulators at bloom (Bangerth, 1976) or immediately after it (Greene et al., 1982). In self-sterile apple cultivars, compatible pollen from suitable pollinizers is required to be deposited on the stigmatic surface of the flower so that ovule fertilisation and seed and fruit development can occur. Poor pollination therefore can reduce the number of seeds which develop in fruit (Wertheim, 1991; Brookfield et al., 1996) as well as influence the cropping behaviour of apple trees. Where bee activity and/or pollen availability is limited over a significant part of the flowering period, pollination of flower clusters which open at this time may be reduced resulting in loss of total tree productivity (Williams and Smith, 1967; Maggs et al., 1971). The apple cultivar ‘Braebum’ is susceptible to Ca-related disorders, is self-sterile and often begins flowering earlier than other commercial cultivars which act as pollinizers (unpublished data). Usually its long blossom duration (up to 5 weeks) in New Zealand overlaps sufficiently with the bloom period of pollinizers to allow considerable fruit setting. Nevertheless, this asynchronous flowering pattern may result in poor fruit set and fruit of low seed numbers from early flowering clusters, leading to altered fruit distribution on the tree. We hypothesise that cross-pollination in ‘Braebum’ may limit fruit mineral concentrations and this might occur through various means, directly through the effect of seeds and/or indirectly by affecting the cropping patterns of trees. To test the hypothesis, we experimentally imposed different levels of pollination on whole ‘Braebum’ trees during early blossom and on individual flower clusters opening at different times during bloom. 2. Materials and methods 2.1. Trees

Three rows in a five-row block of apple trees (M&us domestica Borkh. cultivar ‘Braebum’) (4 year) on MM. 106 rootstock planted at the HortResearch Hawkes Bay

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Research Orchard, Hawkes Bay, New Zealand, were chosen for the study in 1992. The trees were planted at 4.5 X 2.5 m such that 13.5 m was the maximum distance to the closest ‘Gala’ pollinizers (planted in parallel rows on each side of the ‘Braebum’ block). ‘Braebum’ trees flowered over a period of 34 days (14 September to 18 October), with full bloom on 1 October. ‘Gala’ trees began flowering 19 days after the first ‘Braebum’ flowers opened (3 October) and continued to flower for a further 22 days. These trees were not chemically thinned but otherwise were managed according to standard commercial practice, including 12 applications of Ca sprays throughout the growing season. 2.2. Experimental Five pollination/fruit thinning treatments were each imposed on eight three-tree plots. Hand-thinning treatments were included to enable assessment of possible interactions between crop load and pollination intensity. The treatments were as follows. (1) HPMT - High pollination intensity (HP)/moderate thinning (MT). All open flowers were hand-pollinated three times weekly during bloom. The only suitable pollen source available at the beginning of ‘Braebum’ bloom was a small amount of Malus floribunda cultivar ‘Profusion’. ‘Profusion’ pollen produces similar fruit set, seed number and fruit size for ‘Braebum’ as ‘Gala’ and ‘Granny Smith’ pollen (unpublished data). Therefore during the first 13 days of ‘Braebum’ bloom (14 September to 27 September) we used ‘Profusion’ pollen, and thereafter ‘Granny Smith’ pollen (27 September to 12 October). Crop load was reduced to a ‘commercial’ load by hand-thinning spur sites on 11-14 November to one to two fruit and spacing all axillary sites approximately 0.2 m apart. (2) HPHT - High pollination intensity (HP)/heavy thinning (HT). Flowers were hand-pollinated as for (1) but fruit were selectively thinned on 1 1- 14 November so that one fruit per site remained on spurs and terminals while spacing axillary fruit approximately 0.75 m apart. (3) LPMT - Low pollination intensity (LPI/moderate thinning (MT). Trees were completely covered with white hail netting (15% shade) to exclude insects from 13 September to 6 October. These covers were opened 1 h per day two to three times per week to allow a low level of insect pollination. Hand-thinning was carried out as for (1). (4) MPMT - Medium pollination intensity (MP)/moderate thinning (MT). Trees were only pollinated by insects. Hand-thinning was carried out as for (1). (5) MPHT - Medium pollination intensity(MP)/heavy thinning (HT). Trees were only pollinated by insects. Hand-thinning was carried out as for (2). In order to determine more precisely relationships amongst seeds, fruit calcium and flowering date, at 7 and 28 days after first flowering 18 flower clusters at king petal fall in each plot were selected at random and tagged for HPMT and MPMT treatments (144 flower clusters per flowering date and pollination treatment). All chosen sites were l-2 m from the ground located on the outside of the tree canopy. At each tagged site, all fruit were removed except for the largest lateral fruit 17 days after tagging. 2.3. Cropping Immediately before flowering, spur plus terminal and axillary flower buds were counted separately and basal branch cross-sectional area (BCA) measured on two

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bottom tier branches per tree (six branches per plot). Fruits were counted on each branch 26 days after full bloom (dafb) (initial fruit set), 36 dafb (fruit set before hand-thinning), and 50 dafb (final fruit set after hand-thinning). The proportion of total fruit on spur plus terminal sites on each branch was calculated. Fruit set was also calculated as fruit number/flower bud number X 100, and fruit density calculated as fruit no/BCA. Fruit were selectively harvested according to commercial colour standards (> 40% skin red blush coverage) on 22 March, 4 April and 16 April and strip-picked on 22 April. All fruit were weighed individually on an electronic weight grader and average fruit size, yield and fruit number per plot were calculated from these data. Fruit number was recorded at each tagged site 17 days after tagging (initial set) and 2 days before harvest (final set after thinning). Fruit from these tagged sites were picked immediately before commercial harvest and weighed (5-16 fruit per plot). Primary and bourse leaf areas for each tagged site were measured using a Licor 3100 Leaf Area Meter. 2.4. Seeds and fruit minerals Five fruit of similar size (179-197 g) were sampled from each plot from the first and third selective harvest (40 fruit per treatment per harvest). Seeds were counted and two longitudinal wedges (approx. 3 g FW in total) from opposite sides of each fruit (excluding seeds and core but including skin) were taken (as representative of the whole fruit) and Ca, Mg and K analysed by nitric/perchloric acid digestion and atomic absorption spectrophotometry (Turner et al., 1977). For fruit from the tagged sites seeds were counted and weighed and minerals analysed as described above. 2.5. Statistical analysis The whole tree experiment comparing effects of pollination intensity/thinning treatments was organised as a randomised block design with all treatments represented equally in each of the three tree rows. Effects of harvest date or flowering date were included as a split plot where appropriate. Statistical analysis was carried out using the SAS General Linear Model procedure to calculate least squared means and LSDs. Orthogonal contrasts were used to determine the separate effects of pollination and thinning.

3. Results 3.1. Cropping (whole trees) Exclusion of bees from trees by tenting during bloom (LP) reduced initial fruit set and fruit density for spur plus terminal positions and across all fruiting sites as compared with other treatments (Table 1). Axillary fruit density, but not set, was lower for LP

R.K. Vok et al./ Scientia Horticulturae Table 1 Fruit density ‘Braebum’

and fruit set before

fruit thinning

for different

66 (1996) 169-l 80

pollination

173

and hand-thinning

treatments

for

apple (whole trees)

Treatment

HPMTd HPHT LPMT MPMT MPHT LSD ( P = 0.05) F significance Treatment Contrasts HP vs. MP HP vs. LP MP vs. LP HT ’ vs. MT

Fruit density a (cm-*)

Fruit set b (o/o)

S+TC

Ax

Total

S+T

Ax

Total

31 31 12 24 24 7

18 18 15 19 21 6

49 50 27 45 43 12

389 354 Ill 278 254 147

247 238 413 213 215 358

318 260 145 227 212 123

NS

NS

NS NS NS NS

NS * NS NS

NS

..I

***

**

NS

*

***

NS NS *

‘**

***

NS

NS

NS

l

*

*at

a**

*

NS

Pronortion of S +T fruit (%) 63 66 45 55 54 7

*

l

*

*** *** ** NS

a Fruit no./branch cross-sectional area. b Fruit no./flower bud X 100. ’ S + T, spur plus l-year terminal clusters; Ax, 1-year axillary clusters. d HP, supplementary hand-pollination; MP, bee-only pollination; LP, net coverage; HT, heavy thinning; MT, moderate thinning. ’ Thinning contrast excludes net coverage treatment. ** *I* respectively. . , . NS, Significant at 0.05 2 P > 0.01, 0.01 2 P > 0.001, P I 0.001, non-significant, l

trees than bee-only pollination (MP) trees. Supplementary hand pollination (HP) resulted in greatest initial fruit set and density on spur plus terminal sites, but fruit set and density for axillary and total sites were the same as MP. The proportion of total fruit on spur plus terminal sites measured immediately after bloom was greatest for HP, intermediate for MP and lowest for LP. Fruit set immediately before thinning showed similar but less distinctive trends (data not shown). HP also resulted in highest final fruit set and density measured after hand-thinning on spur plus terminal sites, but MP and LP had similar final fruit set and density (Table 2). In contrast, final fruit set and density on axillaries was lowest for HP, highest for LP and intermediate for MP treatments. There was no effect of pollination treatment on final fruit density across all sites, although final set was greater for HP and LP than MP. Fruit thinning intensity also affected tree fruiting patterns, but to a much lesser extent than pollination treatments. Final fruit set of spur and terminals and across all sites was lower for HT than MT; however, fruit density was not affected. The proportion of total fruit on spur plus terminal sites after thinning was greatest for HP, intermediate for MP and lowest on LP, while hand-thinning had no effect. There was no effect of treatment on fruit number or yield per tree (data not shown), with 71-76 kg per tree (331-373 fruit per tree) harvested across all treatments. MPHT had larger fruit (P = 0.01) than either of the supplementary hand-pollination treatments

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Table 2 Fruit density and fruit set after thinning for different pollination apple (whole trees) Treatment

HPMT d HPHT LPMT MPMT MPHT LSD (P = 0.05) F significance Treatment Contrasts HPvs.MP

Fruit density a (cm- 2,

and hand-thinning

tteatments

Fruit set b (o/o)

Proportion of S +T fruit (%)

S+TC

Ax

Total

S+T

Ax

Total

6.9 6.2 5.1 5.5 4.9 1.0

1.1 2.1 3.2 2.6 2.2 0.8

8.0 8.3 8.4 8.2 7.1 1.3

72 61 58 58 47 11

11 17 34 23 16 5

44 37 41 39 29 8

85 75 62 70 70 7

NS

***

***

*’

***

**

***

***

**

NS

***

HpvsJp

**

a**

NS

***

MP vs. LP HT e vs. MT

NS NS

* NS

NS NS

NS **

**

for ‘Braebum’

*

l

**

*t*

NS

a**

*** NS

* **

* NS

a Fruit no./branch cross-sectional area. b Fruit no./flower bud X 100. ’ S +T, spur plus l-year terminal clusters; Ax, l-year axillary clusters. d HP, supplementary hand-pollination; MP, bee-only pollination; LP, net coverage; HT. heavy thinning; MT, moderate thinning. ’ Thinning contrast excludes net coverage treatment. * * **t respectively. . > 7 NS, significant at 0.05 2 P > 0.01, 0.01 2 P > 0.001, P 5 0.001, non-significant, l

(average weight 221 g vs. 206-207 g), while other treatments were intermediate in size. Where appropriate treatments were combined, neither pollination nor hand-thinning affected fruit size (data not shown). Table 3 Fruit set and size, and spur leaf area as influenced ‘Braebum’ apple (tagged sites)

by blossom date within a tree and pollination

Pollination treatment

Blossom date

Fruit no. per cluster Initial

Final

HPMT a

2 1 September 12 October 2 1 September 12 October

2.8 3.5 1.4 3.3 0.4

MPMT LSD ( P = 0.05) F significance Pollination (P) Blossom date (B) PXB

** *** *** l

treatment for

Fruit weight(g)

Leaf area (cm*) Primary

Bourse

0.50 0.29 0.33 0.32 0.16

170 162 178 174 24

5.3 5.0 5.9 6.6 3.5

138 42 196 34 69

NS NS NS

NS NS NS

NS NS NS

NS **

a HP, supplementary hand-pollination; MP, bee-only pollination; * * * Significant at P < 0.001; NS, not significant (P > 0.05).

MT, moderate

thinning.

l

NS

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3.2. Cropping and spur leaves (tagged sites)

Supplementary pollination increased initial fruit set for early blooming sites but not for late blooming sites in a tree (Table 3). However, initial set was greater for later than earlier flowering sites for both pollination treatments. Final fruit set, primary spur leaf area and fruit weight were not influenced by hand-pollination or blossom date. Final bourse leaf area was much greater for earlier than later flowering sites but was not influenced by pollination treatment. 3.3. Fruit minerals and seeds (whole trees) Across all treatments, fruit Ca concentration (16 April) increased by 0.4 mg per 100 g FW, while seed number per fruit decreased by 1.l seeds from the first to the third harvests (Table 4). Fruit Mg concentrations were slightly higher at the first than the third harvest, while this was reversed for K concentrations.

Table 4 Fruit mineral concentrations and seed number as affected by pollination and cropload treatments and harvest date for ‘Braebum’ apple __ (whole trees). Analyses were carried out on fruit of the same size class (179- 197 g) Harvest date

Treatment

Fruit mineral concentration

Seed no. per fruit

(mg per 100 g FW

HPMT ’ HPHT LPMT MPMT MPHT HPMT HPHT LPMT MPMT MPHT

22 March

16 April

LSD (P = 0.05) F significance Harvest date (HI Treatment(T) Contrasts HXT HP vs. MP HP vs. LP MP vs. LP HT b vs. MT

Ca

Mg

K

4.7 5.2 3.8 4.2 4.2 5.2 5.4 4.0 4.7 4.5 0.7

3.7 4.0 3.8 3.7 3.8 3.8 3.7 3.6 3.5 3.4 0.3

104 101 101 101 97 107 102 106 103 101 7

**

6.3 6.5 4.9 5.9 6.2 5.0 5.7 3.7 4.5 5.2 1.1

*** *a*

NS *** **t ** NS

NS **

NS *

NS NS NS

NS NS *

NS NS *** *** NS

it HP, supplementary hand-pollination; MP, bee-only pollination; LP, net coverage; HT. heavy thinning; MT, moderate thinning. b Thinning contrast excludes low pollination treatment. * * *t and NS: Significant at 0.05 2 P > 0.001, 0.01 2 P > 0.001, P I 0.001 and non-significant, 9 7 respectively. l

l

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Horticuiturae 66 (19%) 169-180

Table 5 Correlation coefficients (r) relating fruit calcium concentration and seed number to several cropping measurements for individual plots for ‘Braebum’ apple (n = 40). Ca concentration and seed number averaged over both harvest dates Variable

Ca concentration

Seed no. per fruit

0.48 *

Spur and terminal fiwit set Initial Pre-thinning Final

0.35 * 0.55 * 0.42 *

0.33 = 0.49 * * 0.28 NS

Spur and terminal fruit density Initial Pre-thinning Final

0.50 * * 0.49 * 0.40 *

0.41 * * 0.44 * * 0.20 NS

Proportion of spur and terminal fruit Initial Pre-thinning Final

0.65 * * 0.44 * ’ 0.44 ’ *

0.61 * * * 0.09 NS 0.16 NS

l

l

l

l

l

Seed no per fiwit

*3 * 9 * * * and NS: 0.05 > P > 0.01, 0.01 5 P < 0.001, P < 0.001 and not significant, respectively. l

Pollination treatment had a large effect on fruit Ca concentrations. HP resulted in the highest, LP the lowest, and MP had intermediate Ca concentrations pooled across both harvest dates. There was no consistent difference between thinning treatments. Seed number per fruit followed a similar trend as for Ca, although there was no significant difference between HP and MP. Fruit Mg and R concen~ations were also greater for the HP than MP. Heavier thinning reduced fruit K concentrations slightly. For individual plots, fruit: Ca concentrations were positively correlated with seed number per fruit for data averaged across both harvest dates (Table 5). However, fruit Ca was not related to seed number on an individual fruit basis (P = 0.85, n = 200). For whole plots, fruit Ca concentrations and seed numbers also increased with increasing initial fruit set and density on spur plus terminal sites and with increasing proportion of fruit on these sites. For fruit Ca, but not seed number, these relationships also were signi~c~t at the ~s~-~i~ing fruit set ~~ssment. Relationships between fruit Ca and axillary or total fruit density were usually not significant (data not shown). Fruit Mg and K concentrations were not related to tree fruiting characteristics, nor seed number per fruit on a whole plot or individual fruit basis (data not shown).

3.4. Fruit minerals and see& (tagged sites) For data pooled over both ~llina~on reagents, fruit Ca c~ncen~ations were higher for the earlier blossom date (Table 6). This was mainly due to high fruit Ca concentrations from early blossom sites which had been hand-pollinated, as indicated by the significant (P = 0.02) pollination X blossom date interaction. Fruit from late blossom

Table 6 Fruit miner& concentrations at commerciat harvest as affwted by blossom date witbii a tree and pollination latent for ‘Braebum” apple (tagged sites) Pollination treatment

Blossum date

Frnit mineral concentration (mg per 100 g FW) Ca

Mg

K

HPMT a

21 September 12 October 2 1 September 12 October

6.6 5.1 5.7 5.4 0.8

4.0 4.0 3.7 4.0 0.3

101 128 97 128 9

NS ** 1

NS NS NS

NS *** NS

MPMT LSD FP = 0.05) F significance

Poilination (P) Blossom date (31 PXB

Seed no. per fruit

5.9 7.2 2.7 6.9 1.2 * *** **

a HP, supplementary hand-pollination; MP, bee-only pollination; LP, net coverage; HT, heavy thinning; MT, moderate thinning. tilt * ff and NS: significant at 0.05 2 P > 0.01, 0.01 2 P > 0.001, P s 0.001 and not significant, I T respectively.

sites, whether bee-only or hand ~~~~nated, and those from early opening bee-only ~llinated flowers had similar Ca concen~ations. Seed numbers were greater for fruit from the late blossom date. Hand ~llination increased seed number for fruit from early flowers by two to three seeds. However, there was no effect of hand pollination on seed number for fruit from late flowers. Seed weight showed a similar trend (data not shown). Fruit Mg concen~ations were similar for both ~llination treatments and flowering dates. Fruit K concentrations were greater for late opening flowers than early opening but was not affected by pollination treatment. For individual fruit from tagged sites across both pollination treatments and blossom dates fn = 180), fruit Ca declined (P < 0.001) with increasing fruit weight (r = -0.36) and increased with increasing primary spur leaf area ir = 0.35) but was not related to seed number, total seed weight or bourse leaf area (P > O.OS).Fruit K concentration increased (P < O.OOl), with screwing seed number (r = 0.44), but declined with increasing bourse leaf area (r = -0.36). Fruit Mg concen~ation also increased with increasing spur leaf area (r = -0.36, P < 0.001). When correlation analysis of tagged sites was confined to those from one blossom date, there was also no relationship found between individual fruit mineral concentra~on and seed number or total seed weight t P = 0.44-0.67).

4. Discussion

Different ~llination intensities during the early-mid blossom period (14 September12 October) did not change overall final fruit density, final fruit numbers or yield per

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tree at commercial harvest, but rather altered distribution of fruit on clusters which blossomed at different times. Heavy pollination (supplementary by hand) increased the proportion of total fruit on early opening spur and terminal sites while poorer pollination ~netting) decreased this prounion. Improving early ~lIination, when the ~ounl of compatible pollen ‘naturally’ available for cross-pollinating was probably very low, directly increased initial fruit set on early opening spur and terminal sites (Tables 1 and 3). However improving early ~llination also decreased final fruit set and density of later flowering l-year axillaries (Table 2). This may have resulted from better developed fruit from successfully pollinated early flowering sites outcompeting smaller fruit from later flowering sites for assimilates (Denne, 1963; Stephenson, 1981; Bangerth and Ho, 1984). Generally pollination effects on seed number per fruit followed the same patterns as that observed for set of spur and terminal fruit (Table 1, Table 3, Table 4, Table 5) indicating that fruit set and seed number probably both provide similar measures of ~llination effectiveness. Other studies have shown also that where greater ~IIination intensities have improved fruit set, seed numbers in these fruit were also higher (DeGrandi-Hoffman et al., 1990; Lezec et al., 1990; Wertheim, 1991).

Pollination intensity positively affected Ca concentrations of fruit sampled from whole trees at commercial harvest, the effect being independent of the influence of fruit size (Table 4). Al~ough minerals (and seeds) were determined from fruit samples selectively colour-picked at two harvest dates and not at random, we would argue that they represent au accurate measure of the overall fruit mineral composition from each plot. These results support another study, which found that ‘Braebum’ fruit on trees at a large distance (> 25 m) from a suitable pollinizer had lower fruit Ca contents than trees closer to the pollinizer, and when such trees were hand-pollinated, fruit Ca concentrations were increased (Brookfield et al., 1996). In this particular study, ~llination intensity was likely to have been reduced over the whole flowering period due to the greater distance from the pollinizer. The present study shows that poor pollination intensity over the early blossom period in relatively narrow blocks (C 14 m between pollinizer and ‘Braebum’ tree) also can limit the Ca status of ‘Braeburn’ fruit. Where seed number in individual fmit from early flowering spurs was increased by hand-pollination, final fruit Ca concentrations also were increased (Table 6). This supports the hypothesis that seeds directly affect Ca influx into fruit (Bramlage et al., 1990; Tomala and Dilley, 1990). However, this effect of seeds on Ca movement would not appear particularly strong relative to other tree factors affecting fruit Ca concentrations. Firstly, seed number was lower in fruit harvested from the first compared with the third pick but fruit Ca concentrations actually increased (Table 4). Secondly, Ca concen~ations of fruit from early blossoming sites were greater than those from late blossoming sites, despite having one to four less seeds per fruit (Table 6). It is therefore not surprising that relationships could not be found between fruit Ca and seed number for individual fruit. Similarly, Brookfield et al. (19%) also did not find a correlation

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between seed number and individual fruit in the population as a whole which contrasts with results of Bramlage et al. (1990). A major factor obscuring the effect of seeds on fruit Ca in the present study could be the local leaves subtending the fruit, which are known to have a positive effect on fruit Ca status (Jones and Samuelson, 1983) and have been correlated with fruit Ca concentrations for individual spurs (Ferree and Palmer, 1982; Proctor and Palmer, 1991; Volz et al., 1994). Final bourse leaf area was considerably reduced for late compared with early flowering clusters (Table 3), possibly negating positive effects of their higher seed numbers on fruit Ca. Conversely, fruit Ca concentrations were greatest from early flowering spurs which had been hand-pollinated, where both seed number and bourse leaf area were relatively high. A positive effect of seed number on fruit Ca influx may only occur where leaf area is not limiting this process. Although crop load and fruit size can have major effects on fruit mineral concentrations (Perring and Jackson, 1975; Ferguson and Watkins, 1992; Volz et al., 19931, it is unlikely that these were the means by which pollination affected fruit Ca. Overall fruit densities throughout fruit development and final crop loads and mean fruit sizes were generally similar for all pollination treatments. Nevertheless, less effective pollination during early blossom may not have reduced fruit Ca concentrations by simply reducing seed numbers in the majority of fruit. We found between-plot variability in fruit Ca concentrations to be more strongly correlated positively with initial proportion of fruit on spur and terminal sites than seed number per fruit (Table 5). Poor pollination may diminish seed number in fruit on early flowering (spur and terminal) sites with high spur leaf areas while also favouring an increase in the proportion of fruit on later flowering (axillary) sites with low spur leaf areas, thereby increasing the proportion of fruit with low Ca concentrations. Similarly, the negative effect of light cropping on Ca concentrations of fruit of any one size might also occur through increasing the proportion of such fruit which set on spurs with lower leaf area (Volz et al., 1993). In conclusion, we have shown that pollination effectiveness during the early blossom period of ‘Braebum’ apple trees can influence fruit positioning and distribution on trees, seed numbers and fruit Ca concentrations, without affecting final crop load. Growers of ‘Braebum’ should consider planting suitable pollinizers, even in relatively narrow blocks, so that fruit mineral concentrations can be optimised.

Acknowledgements

We would like to thank Judith Bowen, Murray Oliver and Wendy Cashmore for technical assistance and the New Zealand Apple and Pear Marketing Board and the New Zealand Foundation for Science and Technology for financial assistance.

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