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Mineral accumulation in apple fruit as affected by spur leaves
SCIENTIA HORlICULTuRR Scientia Horticulturae 65 (1996) 151-161
Mineral accumulation in apple fruit as affected by spur leaves R.K. Volz a7* , D.S. Tush
a, I.B. Ferguson b
a The Horticulture and Food Research Institute of New Zealand, Havelock North Research Centre, Private Bag 1401, Havelock North, New Zealand b Mt Albert Research Centre, Private Bag 92 169, Auckland, New Zealand
Accepted 2 1 November 1995
Abstract The mineral composition of apple (Mulus domesrica Borkh.) fruit from spurs with artificially adjusted spur leaf areas was examined throughout fruit development. Primary leaf and/or bourse shoot removal at bloom or petal fall reduced Ca content during the growing season on ‘Gala’, ‘Golden Delicious’ and ‘Royal Gala’. Bourse shoot removal 75 days after full bloom had a similar but less pronounced effect on final fruit Ca content for ‘Royal Gala’. Primary leaves assisted movement of Ca into fruit earlier and were more efficient on a per leaf area basis throughout fruit growth than bourse leaves. Fruit Mg contents showed similar but less consistent responses to spur leaf removal compared with Ca. Fruit K contents and fruit fresh weight were similar from spurs whose primary spur or bourse shoots/leaves had been removed. Keywords: Apple; Calcium; Magnesium; Potassium; Fruit; Leaves
1. Introduction The mineral status of apple fruit is critical to the postharvest storage life of the fruit. This is particularly the case in the relationship between Ca concentrations in the fruit flesh and the development of specific disorders such as bitter pit (Ferguson and Watkins, 1989). Since it is clearly beneficial to achieve high levels of Ca in fruit at maturity, our knowledge of the factors influencing the movement of Ca and other minerals into the fruit during development becomes an important issue.
l
Corresponding author.
0304-4238/%/$15.00
0 1996 Elsevier Science B.V. All rights reserved
PII 0304-4238(95)00870-5
152
R.K. Volt et al./Scientia
Horticulturae 65 (1996) 151-161
Factors pertaining to the locational and structural characteristics of fruiting sites within the tree canopy affect the accumulation of mineral nutrients by developing apple fruit. Some of these are relatively gross; for instance, fruit in the upper parts of the canopy tend to have lower Ca concentrations than those in the lower parts (Jackson et al., 1971; Haynes and Goh, 1980; Ferguson and Triggs, 1990). Others relate more to age and characteristics of the leaves on the fruiting wood or spur. In a recent study, we found that terminal fruiting sites tend to have fruit with higher Ca contents than fruit from l-year laterals or 2-year spurs (Volz et al., 1994). Older fruiting spurs may produce fruit that are less susceptible to bitter pit (Schumacher et al., 1980). Some differences in fruit quality and mineral composition associated with wood age may be related to differences in fruit size (e.g. Jackson, 1970). However, it is also likely that differences in fruit Ca content relative to different fruiting positions are due to the influence of leaves associated with the fruiting wood. There is increasing evidence that a greater leaf area (of both primary and bourse leaves) on the fruiting wood can be linked to higher Ca contents of the subtending fruit (Ferree and Palmer, 1982; Jones and Samuelson, 1983; Proctor and Palmer, 1991). In analysing the effects of crop loading on mineral contents of apple fruit, we also found that a lower leaf area on fruiting spurs from light cropping trees may be the reason for lower Ca contents in fruit from such trees (Volz et al., 1993). The distribution of carbon assimilate from different leaf types to fruit on spurs during fruit growth is dependent upon leaf type and the stage of leaf relative to fruit development (Tustin and Lai, 1990). However, little work has been carried out that investigates the temporal nature of the leaf effect on mineral input into fruit, and in particular in relation to the different leaf types on the fruiting spur. Therefore we have investigated more closely the relationship between leaf area on fruiting sites and mineral accumulation in apple fruit during fruit development by manipulating leaf area experimentally.
2. Materials and methods 2.1. Trees Trees used in the study were trained as centre-leader pyramids and managed according to standard commercial practice other than that there were no Ca sprays applied. All spurs selected in these experiments were on the outer part of the canopy, well exposed and between 1 and 2 m from the ground. 2.2. Bow-se leaf area reduction (1988) All 2-year flowering spurs whose development neared king flower anthesis were thinned to retain only the king bloom on ten ‘Royal Gala’ (3 years) trees on MM 106 rootstock growing at the HortResearch Orchard, Havelock North. At anthesis, these flowers were hand-pollinated with fresh ‘Braebum’ apple pollen. At petal fall, 20 thinned spurs per tree were selected randomly for each of the following treatments,
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which modified fruiting spur bourse leaf area at two stages of growth: 1. Control, with unmodified bourse leaf area. 2. Bourse shoot removed at 5 dafb (days after full bloom) but with the first expanded leaf retained. The single leaf was retained to minimise effects on fruit growth rate and weight due to lower leaf area. Bourse regrowths were removed at fortnightly intervals. 3. Every second leaf was removed from the bourse as it developed, beginning with the second leaf, from 5 dafb over the following 77 days. 4. Every second leaf was removed beginning with the second leaf at 75 dafb immediately following December fruit drop. 5. Every bourse leaf except the first expanded leaf was removed at 75 dafb. Spurs with intact fruit were sampled at 48, 95 and 134 dafb (commercial harvest). Two spurs per tree (20 per treatment) were collected for the first and third sample periods but only one spur per tree (ten per treatment) was taken in mid-season. 2.3. Interaction of primary and bow-se leaves 2.3.1.
‘Gala’ and ‘Golden Delicious’ (1989)
Ten ‘Gala’ (8 years) and 15 ‘Golden Delicious’ (10 years) trees on M 793 rootstock were selected at the HortResearch Orchard, Appleby. Thirty flowering 2-year spurs on each ‘Gala’ tree were tagged at pink bud and 24 2-year spurs on each ‘Golden Delicious’ tree were tagged at king full bloom. All lateral flowers were removed from each spur. Immediately after tagging, on each tree, half of the tagged spurs were randomly allocated to one of two treatments: (1) bourse shoot/bud removed, (2) bourse shoot/bud removed, 50% primary leaves removed. 2.3.2. ‘Royal Gala’ (1990) Thirty ‘Royal Gala’ trees were selected from the block used in the previous year at Appleby. Twenty four flowering 2-year spurs were tagged on each tree at the king full bloom stage and all lateral flowers removed. Immediately after tagging, six spurs per tree were randomly allocated to one of four leaf removal treatments: (1) bourse shoot/bud removed, (2) bourse shoot/bud removed, 50% primary leaves removed, (3) 50% primary leaves removed, (4) no leaf/shoot removal (control). All bourse shoots were actively growing when spurs were tagged. For ‘Gala’ and ‘Royal Gala’, bourses were 0.5-2 cm and for ‘Golden Delicious’ l-5 cm long at the time of removal. In both experiments all bourse regrowths from treatments (1) and (2) were removed if necessary at l-2 weekly intervals, bourse regrowth length always being less than 2 cm. Fruit were harvested at 6-15-day intervals over the following 40 days for all cultivars in both years. For ‘Royal Gala’, further samples were taken at 63 and 134 (commercial harvest) dafb. The first ‘Gala’ sample was taken at king full bloom and for ‘Golden Delicious’ and ‘Royal Gala’ the first sample was at 5 dafb. At each sample date 20 ‘Gala’ and 15 ‘Golden Delicious’ tagged spurs were harvested for each treatment at random from throughout the block. For ‘Royal Gala’ the 30 trees were divided into five plots of six trees. At each sample date three tagged spurs per treatment were harvested at
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random from each plot. This allowed variation in rates of mineral uptake within treatments to be determined. 2.4. Measurements In all experiments, fresh weights of individual fruit were measured. Individual whole fruit were analysed for minerals except for those fruit sampled from 70 dafb (early Dec.), whereupon longitudinal wedges were taken from each fruit as above. The longitudinal wedges were taken as representative samples of whole fruit. The tissues were digested in nitric/perchloric acids and analysed for Ca, Mg and K by atomic absorption spectrophotometry (Turner et al., 1977). Immediately after their collection, spurs were subdivided into primary and bourse leaf groups and the leaf areas measured using a LiCor Li-3100 Area Meter or a Delta T Area Meter (II). Rates of mineral uptake into fruit were calculated as : (M2-Ml)/(d2-dl) where M 2 = mineral content at time d2 and M 1 = mineral content at time dl. 2.5. Statistical analysis All experiments were organised as completely randomised designs. However, the ‘Royal Gala’ experiment in 1990 was organised as a randomised block design with treatments factorially arranged. Analysis of variance was used to test for significant effects. Fresh weight was used as the covariate at all sampling dates except at the final sampling date for ‘Royal Gala’ in 1990, where fruit weight was affected by treatment.
3. Results K content were generally similar for all leaf removal treatments in all experiments and therefore K data are not presented. 3.1. Bow-se leaf area reduction
Maximum bourse leaf area of 310 cm’ was reached by 44 dafb on untreated spurs (data not shown). Removal of every second leaf from the developing bourse shoot progressively from petal fall, or removal of every second bourse leaf 75 dafb, reduced final bourse leaf area to 130-165 cm’, while removal of all but one leaf at either time reduced bourse leaf area to 20 cm2. Primary leaf area and fruit size were unaffected by bourse modification at all sampling date (dates not shown). Mean fruit weights at each harvest date ranged from 15-17 g, 75-84 g and 131-142 g at 48, 95 and 134 dafb, respectively. Removal of the bourse shoot at 5 dafb leaving one bourse leaf per spur reduced fruit Ca content by approximately 20% at 48 and 95 dafb compared with the control and this difference increased to 30% at fruit maturity (Fig. 1). Removal of bourse leaves 75 dafb
R.K. Volz et al./Scientia
Horticulturae
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Fig. 1. Effect of bourse leaf removal at different times of the season on fruit Ca content during fruit development for ‘Royal Gala’ apple (1988). Bar = LSD (P = 0.05) for comparisons among treatments at any one sampling date.
had little effect on Ca content of fruit sampled 20 days later. However by fruit maturity, removal of all but one leaf at this stage reduced Ca content by 15% compared with the control. Removal of 50% of the bourse leaves at either timing reduced Ca content to a level intermediate between the severe treatment (one leaf left) carried out at the same 13
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Bourse leaf area (cm? Fig. 2. Relationship between fmal bourse leaf area and fmal fruit Ca (open symbols) and Mg content (closed symbols) as influenced by timing of boune leaf removal for ‘Royal Gala’ apple (1988). Upper bar-LSD (P = 0.05) for Ca, Lower bar-LSD (P = 0.05) for Mg.
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Table 1 Estimated rate of Ca uptake into ‘Royal Gala’ fruit due to bourse leaves during different periods of fruit development (1988). (Calculated from data in Fig. I). Total data are values for controls (100% bourse leaf area), other data are values for spurs whose bourse leaves were removed from petal fall. Fruit Ca content assumed to be 0.05 g at 5 datb (see Fig. 3(B)) Interval (dafb)
5-48 (43 days) 48-95 (47 days) 95- 134 (39 days)
Final Ca content (mg per fruit) Total
Other
Bourse
Increase due to bourse
2.8 11.3 12.3
2.3 9.4 8.8
0.5 1.9 1.6
0.45 1.4 1.6
Rate of Ca uptake (pgday-‘)
10 30 41
time and control. At fruit maturity fruit Ca content decreased with reducing bourse leaf area for spurs modified at 75 dafb (7.9 pug crnm2) but this effect was much less than that for spurs modified 5 dafb (12 pg cme2 1 (Fig. 2). From the data in Figs. 1 and 2, we have estimated the rate of Ca uptake into fruit which is dependent upon bourse leaf area in the early (5-48 dafb), mid (48-95 dafb) and late (95-134 dafb) stages of fruit development (Table 1). The greatest effect of bourse leaves on Ca accumulation rate occurred in the later rather than in the early or middle periods of fruit development. This was not just a function of increasing total leaf area, as expressing this estimated rate on a leaf area basis showed the same trend (data not shown). Magnesium contents were not affected by any treatment at 48 or 95 dafb (data not shown). However by the last sample date, Mg content responded to changes in bourse leaf area similar to that described for Ca although less in magnitude (Fig. 2). Magnesium content decreased with reducing bourse leaf area and this response was greater for the earlier than the later timing of leaf removal (3.9 g cme2 vs 2.8 ,ug cmv2). 3.2. Interaction of primary and bow-se leaves In 1989 partial removal of primary leaves from deboursed spurs reduced primary leaf area by 45-48% for ‘Gala’ and ‘Golden Delicious’, but fruit size was not affected (data not shown). Fruit Ca content increased steadily after anthesis for both treatments and cultivars (Fig. 3(A), (B)). Spurs with 100% primary leaves had higher (P < 0.05) fruit Ca content after 13-34 dafh than those with some primary leaves removed. Variation in rate of Ca uptake within treatments and thus statistical significance of differences in rates between treatments were not able to be determined in this experiment. Nevertheless for both cultivars average rate of Ca uptake slowed considerably from 16-21 dafb for fruit from which some primary leaves had been removed (Fig. 3(C), CD)). Fruit Mg increased steadily over the sampling period but there was little difference in content or rate of uptake between the treatments (data not shown). In 1990 partial removal of ‘Royal Gala’ primary leaves at king full bloom successfully reduced primary leaf area from 22-26 cm2 to 1l- 13 cm2, at all sampling dates
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Golden Delicious
0
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10
Is
20
25
Days after bloom
30
35
01 40
0
5
10
15
20
25
30
35
40
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Fig. 3. Fruit Ca accumulation during early fruit development as affected by primary leaves removed at full bloom for ‘Gala’ and ‘Golden Delicious’ apple (1989) (solid square = +leaves, open circle = -leaves). Bar = LSD (P = 0.05). For (A) and (B) the absence of LSD at any one sampling date indicates non-significant difference between treatments.
(data not shown). For those spurs with bourse shoots attached, bourse leaf area increased steadily (5.1 cm* day- ‘1 from anthesis until 41 dafb when bourse growth was at 210 cm* (data not shown). Thereafter bourse leaf area grew more slowly reaching 240 cm* at the final harvest. There was no effect of primary leaf area removal on bourse shoot development. There was also no effect of leaf or bourse shoot removal on fruit weight, except at commercial harvest (P = O.OOl>when spurs without bourse shoots and 50% of primary leaves had a lower fruit weight compared with the other three treatments (104 g vs 119 g). There was no interaction between primary leaf and bourse shoot removal treatments except at 22 dafb. Therefore, main factor averages are presented at each date (Fig. 4). Increase in fruit Ca content was curvilinear to 20 dafb, and then linear over the remainder of the growing season (Fig. 4(A), (B)). Removal of 50% of primary leaves (Fig. 4(A)) or the bourse shoot (Fig. 4(B)) reduced fruit Ca content at all dates (P < 0.05) after 22 dafb, such that fruit Ca was lowered by approximately 15% ( - 50% primary leaves) and 30% ( - bourse shoot) at commercial harvest. Similarly, rates of Ca uptake were lower for the 50% primary leaf treatments than 100% primary leaf
R.K. Volz et al./ Scientia Horriculturae 65 (1996) 151-161
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Primary leaves
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Fig. 4. Fruit Ca accumulation throughout fruit development as affected by primary leaf removal (50%) and bourse shoot removal at bloom for ‘Royal Gala’ apple (1990) (solid square = + primary leaves or + bourse shoot, open circle = - 50% primary leaves or - bourse shoot). Bar = LSD (P = 0.05). The absence of LSD at any one sampling date indicates non-significant difference between treatments.
Table 2 Estimated rate of Ca uptake per leaf area into ‘Royal Gala’ fruit due to primary or bourse leaves throughout fruit development (1990). Leaf areas were estimated as 50% (primary) or 100% (bourse) of control spur values midway through each interval. Rates of Ca uptake due to 50% primary leaf area or 100% bourse leaf area calculated from data in Figs. 4(C) and 4(D), respectively Internal
Leaf area (cm’ )
(datb)
50%
5-12 12-22 22-41 41-63 63-134
primary
100% bourse
12 13 13 12 I1
43 87 161 214 229
Rate of Ca uptake
primary
Increase due to 100% bourse
(pgday-‘)
(pgday-‘)
1.0 10.6 12.9 11.5 9.3
0.3 4.7 27.0 9.9 28.7
Increase due to 50%
Primary (pg day-’
0.08 0.82 0.99 0.96 0.80
cm-* >
Bourse (pgday-’
0.007 0.05 0.17 0.05 0.13
cm-*)
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treatments, differences being significant (P < 0.05) at 17 and 32 dafb (Fig. 4(C)). Rates of Ca uptake were lower for treatments without, compared with those with bourses, at 32 and 99 dafb (Fig. 4(D)). Generally, primary leaves began their effect on fruit Ca uptake early in fruit development and it was maintained throughout the season. In contrast bourse leaves exerted a greater effect on fruit Ca uptake rate later in the season, although the effect was somewhat variable. Expressed on a per leaf area basis, primary leaves had a much greater effect on rate of Ca uptake throughout fruit development and this effect was noticeable earlier than for bourse leaves (Table 2). At 22 datb, a significant interaction (P = 0.03) between bourse and primary leaf treatments indicated that the reduction in Ca observed for both leaf types was due to only the most severe leaf removal treatment (386 pg per fruit vs 533 pg per fruit average for three other treatments). Fruit Mg content increased during development for all treatments (data not shown). However it was not affected by bourse shoot or primary leaf removal except at commercial harvest (P = 0.013) when spurs that had the lowest leaf area had lower Mg content (4 vs 5 mg per fruit). 4. Discussion The positive effect of leaves associated with fruiting wood on Ca uptake into apple fruit is thought to occur from soon after anthesis (Jones and Samuelson, 1983). This is supported by our results, where we detected this influence from 13 dafb for ‘Gala’ and ‘Golden Delicious’ fruit (Fig. 31, although later (at 22 dafb) for ‘Royal Gala’ fruit (Fig. 4). This difference between cultivars in the initial expression of the leaf effect may be related to differences in fruit size at this early stage of development. At 13 dafb, ‘Gala’ and ‘Golden Delicious’ fruit weighed 70-90 g, whereas at 12 dafb, ‘Royal Gala’ fruit were only 50 g. This suggests that leaf area may only limit Ca inflow into fruit once fruit reach a certain size or stage of development after bloom. This concept is supported with the finding that at an early stage of fruit growth, there appears to be a maximum threshold leaf area above which fruit Ca input does not increase in response to further increases in leaf area. For ‘Royal Gala’ at 22 da& there was little effect of increasing leaf area.on Ca input when either primary leaf area was at 100% or the bourse shoot was intact and only when both leaf types were reduced. At later growth stages, rate of Ca accumulation became more sensitive to the partial absence of primary (Fig. 4(B)), or bourse leaves (Table 11, or complete absence of the bourse shoot (Fig. 4(D)) removed at bloom. If we assume that spur and bourse leaves are in some way positively influencing Ca accumulation by developing fruit, then our work indicates that there are several differences between these leaf types in this function. Firstly, our data support the hypothesis that primary leaves are relatively more effective on a per leaf area basis than bourse leaves in aiding Ca movement into fruit (Volz et al., 1994). For ‘Royal Gala’, addition of 240 cm’ of bourse leaf area increased final fruit Ca content by 2.6 mg whereas the addition of only 10 cm* of primary leaf area increased final Ca content by 1.1 mg. Assuming a linear relationship between primary or bourse leaf area and final Ca content (Fig. 2, Volz et al., 1994), this translates into leaf ‘efficiencies’ of 108 and 9 ,ug
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Ca cme2 for primary and bourse leaves, respectively. This compares favourably with other data for ‘Cox’ (48 vs 5 pg Ca cm -2 for primary and bourse leaves, respectively), ‘Golden Delicious’ (29 vs 6 pg Ca cm -2), both calculated from Proctor and Palmer (1991) and ‘Royal Gala’ (41 vs 13 pg Ca cmM2) calculated from Volz et al. (1994). Primary leaves would also seem to begin their effect earlier in fruit development (Fig. 4) than the developing bourse, maintaining a high ‘efficiency’ throughout fruit development (Table 2). For ‘Royal Gala’ at least, bourse leaves provided greater assistance to fruit later in the season (Table 1, Fig. 4(D)). That primary and bourse leaf types exert their effects on Ca uptake into fruit at different stages of development is supported by results of Proctor and Palmer (1991) for ‘Golden Delicious’, where primary leaves and/or bourse shoots were removed at different times from 0 to 8 weeks after full bloom. The positive effect of leaves on final fruit Ca content decreased with later primary leaf removal (i.e. early input of Ca was prevented) , whereas there was no influence of timing of bourse shoot removal (presumably since the positive effect of bourse leaves occurred after 8 weeks from bloom). The effect of the bourse on fruit Ca would be expected to increase with time as a function of increasing shoot growth (and therefore leaf area) up to shoot termination. However, this does not explain the means by which the bourse shoot exerted a stronger effect on fruit Ca accumulation rate in the later part of the season than that in mid-season, despite bourse growth having terminated before this time (Tables 1 and 2). The higher efficiencies and earlier effects of primary leaves are suggestive of a much more effective means of assisting fruit Ca input compared with bourse leaves. Such differences may be related to the capacity of the two leaf types to withdraw xylem water from the fruit in response to evaporative demand (Lang and Volz, 1993). Leaf removal reduced final Mg content for ‘Royal Gala’ in 1988 (to a lesser extent than occurred for Ca), but not for any cultivar in 1989 and only where fruit size was reduced in 1990. Removal of primary and/or bourse leaves before bloom reduced Mg input into ‘Golden Delicious’ fruit (Ferree and Palmer, 1982) while fruit Mg accumulation was slightly greater for spurs with long than those with short bourses for ‘Golden Delicious’ and ‘Cox’ (Jones and Samuelson, 1983). In contrast leaf effects on K accumulation into fruit were minimal (Ferree and Palmer, 1982; Jones and Samuelson, 1983). The relative effect of leaves on the input of these different minerals into fruit (Ca > Mg > K) is inversely related to their mobility in the phloem. Leaf effects on Mg accumulation may only occur when transport to fruit via the phloem becomes limiting. Although we are unclear as to the exact mechanism by which leaves assist Ca and Mg into nearby fruit, our experiments confirm that both primary and bourse leaf types are important in influencing Ca flow. If leaf area is to be used in predicting final fruit Ca status, area of both leaf types would need to be taken into account.
Acknowledgements
We wish to thank Judith Bowen, Greg Lupton and Wendy Cashmore for technical assistance, and the New Zealand Foundation for Science and Technology and the New Zealand Apple & Pear Marketing Board for financial assistance.
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References Ferguson, LB. and Triggs, C.M., 1990. Sampling factors affecting the use of mineral analysis of apple fruit for the prediction of bitter pit. NZ J. Crop Hart. Sci., 18: 147-152. Ferguson, I.B. and Watkins, C.B., 1989. Bitter pit in apple fruit. Hort. Rev., 11: 289-355. Ferree, D.C. and Palmer, J.W., 1982. Effect of spur defoliation and ringing during bloom on fruiting, fruit mineral level, and net photosynthesis of ‘Golden Delicious’ apple. J. Am. Sot. Hort. Sci., 107: 1182-l 186. Haynes, R.J. and Goh, K.M., 1980. Variation in the nutrient content of leaves and fruit with season and crown position for two apple varieties. Aust. J. Agric. Res., 31: 739-748. Jackson, J.E., 1970. Individual fruit size in relation to age of bearing wood on ‘Laxton’s Superb’ apple trees. Rep. East Malhng Res. Sm 1969, pp. 83-85. Jackson, J.E., Sharples, R.O. and Palmer, J.W., 1971. The influence of shade and within-tree position on apple fruit size, colour and storage quality. J. Hart. Sci., 46: 277-287. Jones, H.G. and Samuelson, T.J., 1983. Calcium uptake by developing apple fruits. II. The role of spur leaves. J. Hart. Sci., 58: 183-190. Lang, A. and Volz, R.K., 1993. Leaf area, xylem cycling and Ca status in apples. Acta Hortic., 343: 56-58. Proctor, J.T. and Palmer, J.W., 1991. The role of spur and bourse leaves of three apple cultivars on fruit set and growth and calcium content. J. Hort. Sci., 66: 275-282. Schumacher, R., Frankhauser, F. and Stadler, W., 1980. Influence of shoot growth, average fruit weight and daminozide on bitter pit. In: D. Atkinson, J.E. Jackson, R.O. Sharples and W.M. Wailer (Editor), Mineral Nutrition of Fruit Trees. Butterworths, London, pp. 83-91. Turner, N.A., Ferguson, I.B. and Sharples, R.O., 1977. Sampling and analysis for determining the relationship of calcium concentration to bitter pit in apple fruit. NZ J. Agric. Res., 20: 525-532. Tustin, D.S. and Lai, R., 1990. Source-sink dynamics in developing fruiting spurs of apple. In: E. Raldini et al. (Editors), XX1 11 International Horticultural Congress. Tecnoprint, Bologna, Italy, p. 611. Volz, R.K., Ferguson, I.B., Bowen, J.H. and Watkins, C.B., 1993. Crop load effects on mineral nutrition, maturity, fruiting and tree growth of ‘Cox’s Orange Pippin’ apple. J Hort. Sci., 68: 127-137. Volz, R.K., Ferguson, I.B., Hewett, E.W. and Woolley, D.J., 1994. Wood age and leaf area influences fruit size and mineral composition of apple fruit. J. Hort. Sci., 69: 385-395.
Mineral accumulation in apple fruit as affected by spur leaves R.K. Volz a7* , D.S. Tush
a, I.B. Ferguson b
a The Horticulture and Food Research Institute of New Zealand, Havelock North Research Centre, Private Bag 1401, Havelock North, New Zealand b Mt Albert Research Centre, Private Bag 92 169, Auckland, New Zealand
Accepted 2 1 November 1995
Abstract The mineral composition of apple (Mulus domesrica Borkh.) fruit from spurs with artificially adjusted spur leaf areas was examined throughout fruit development. Primary leaf and/or bourse shoot removal at bloom or petal fall reduced Ca content during the growing season on ‘Gala’, ‘Golden Delicious’ and ‘Royal Gala’. Bourse shoot removal 75 days after full bloom had a similar but less pronounced effect on final fruit Ca content for ‘Royal Gala’. Primary leaves assisted movement of Ca into fruit earlier and were more efficient on a per leaf area basis throughout fruit growth than bourse leaves. Fruit Mg contents showed similar but less consistent responses to spur leaf removal compared with Ca. Fruit K contents and fruit fresh weight were similar from spurs whose primary spur or bourse shoots/leaves had been removed. Keywords: Apple; Calcium; Magnesium; Potassium; Fruit; Leaves
1. Introduction The mineral status of apple fruit is critical to the postharvest storage life of the fruit. This is particularly the case in the relationship between Ca concentrations in the fruit flesh and the development of specific disorders such as bitter pit (Ferguson and Watkins, 1989). Since it is clearly beneficial to achieve high levels of Ca in fruit at maturity, our knowledge of the factors influencing the movement of Ca and other minerals into the fruit during development becomes an important issue.
l
Corresponding author.
0304-4238/%/$15.00
0 1996 Elsevier Science B.V. All rights reserved
PII 0304-4238(95)00870-5
152
R.K. Volt et al./Scientia
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Factors pertaining to the locational and structural characteristics of fruiting sites within the tree canopy affect the accumulation of mineral nutrients by developing apple fruit. Some of these are relatively gross; for instance, fruit in the upper parts of the canopy tend to have lower Ca concentrations than those in the lower parts (Jackson et al., 1971; Haynes and Goh, 1980; Ferguson and Triggs, 1990). Others relate more to age and characteristics of the leaves on the fruiting wood or spur. In a recent study, we found that terminal fruiting sites tend to have fruit with higher Ca contents than fruit from l-year laterals or 2-year spurs (Volz et al., 1994). Older fruiting spurs may produce fruit that are less susceptible to bitter pit (Schumacher et al., 1980). Some differences in fruit quality and mineral composition associated with wood age may be related to differences in fruit size (e.g. Jackson, 1970). However, it is also likely that differences in fruit Ca content relative to different fruiting positions are due to the influence of leaves associated with the fruiting wood. There is increasing evidence that a greater leaf area (of both primary and bourse leaves) on the fruiting wood can be linked to higher Ca contents of the subtending fruit (Ferree and Palmer, 1982; Jones and Samuelson, 1983; Proctor and Palmer, 1991). In analysing the effects of crop loading on mineral contents of apple fruit, we also found that a lower leaf area on fruiting spurs from light cropping trees may be the reason for lower Ca contents in fruit from such trees (Volz et al., 1993). The distribution of carbon assimilate from different leaf types to fruit on spurs during fruit growth is dependent upon leaf type and the stage of leaf relative to fruit development (Tustin and Lai, 1990). However, little work has been carried out that investigates the temporal nature of the leaf effect on mineral input into fruit, and in particular in relation to the different leaf types on the fruiting spur. Therefore we have investigated more closely the relationship between leaf area on fruiting sites and mineral accumulation in apple fruit during fruit development by manipulating leaf area experimentally.
2. Materials and methods 2.1. Trees Trees used in the study were trained as centre-leader pyramids and managed according to standard commercial practice other than that there were no Ca sprays applied. All spurs selected in these experiments were on the outer part of the canopy, well exposed and between 1 and 2 m from the ground. 2.2. Bow-se leaf area reduction (1988) All 2-year flowering spurs whose development neared king flower anthesis were thinned to retain only the king bloom on ten ‘Royal Gala’ (3 years) trees on MM 106 rootstock growing at the HortResearch Orchard, Havelock North. At anthesis, these flowers were hand-pollinated with fresh ‘Braebum’ apple pollen. At petal fall, 20 thinned spurs per tree were selected randomly for each of the following treatments,
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which modified fruiting spur bourse leaf area at two stages of growth: 1. Control, with unmodified bourse leaf area. 2. Bourse shoot removed at 5 dafb (days after full bloom) but with the first expanded leaf retained. The single leaf was retained to minimise effects on fruit growth rate and weight due to lower leaf area. Bourse regrowths were removed at fortnightly intervals. 3. Every second leaf was removed from the bourse as it developed, beginning with the second leaf, from 5 dafb over the following 77 days. 4. Every second leaf was removed beginning with the second leaf at 75 dafb immediately following December fruit drop. 5. Every bourse leaf except the first expanded leaf was removed at 75 dafb. Spurs with intact fruit were sampled at 48, 95 and 134 dafb (commercial harvest). Two spurs per tree (20 per treatment) were collected for the first and third sample periods but only one spur per tree (ten per treatment) was taken in mid-season. 2.3. Interaction of primary and bow-se leaves 2.3.1.
‘Gala’ and ‘Golden Delicious’ (1989)
Ten ‘Gala’ (8 years) and 15 ‘Golden Delicious’ (10 years) trees on M 793 rootstock were selected at the HortResearch Orchard, Appleby. Thirty flowering 2-year spurs on each ‘Gala’ tree were tagged at pink bud and 24 2-year spurs on each ‘Golden Delicious’ tree were tagged at king full bloom. All lateral flowers were removed from each spur. Immediately after tagging, on each tree, half of the tagged spurs were randomly allocated to one of two treatments: (1) bourse shoot/bud removed, (2) bourse shoot/bud removed, 50% primary leaves removed. 2.3.2. ‘Royal Gala’ (1990) Thirty ‘Royal Gala’ trees were selected from the block used in the previous year at Appleby. Twenty four flowering 2-year spurs were tagged on each tree at the king full bloom stage and all lateral flowers removed. Immediately after tagging, six spurs per tree were randomly allocated to one of four leaf removal treatments: (1) bourse shoot/bud removed, (2) bourse shoot/bud removed, 50% primary leaves removed, (3) 50% primary leaves removed, (4) no leaf/shoot removal (control). All bourse shoots were actively growing when spurs were tagged. For ‘Gala’ and ‘Royal Gala’, bourses were 0.5-2 cm and for ‘Golden Delicious’ l-5 cm long at the time of removal. In both experiments all bourse regrowths from treatments (1) and (2) were removed if necessary at l-2 weekly intervals, bourse regrowth length always being less than 2 cm. Fruit were harvested at 6-15-day intervals over the following 40 days for all cultivars in both years. For ‘Royal Gala’, further samples were taken at 63 and 134 (commercial harvest) dafb. The first ‘Gala’ sample was taken at king full bloom and for ‘Golden Delicious’ and ‘Royal Gala’ the first sample was at 5 dafb. At each sample date 20 ‘Gala’ and 15 ‘Golden Delicious’ tagged spurs were harvested for each treatment at random from throughout the block. For ‘Royal Gala’ the 30 trees were divided into five plots of six trees. At each sample date three tagged spurs per treatment were harvested at
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random from each plot. This allowed variation in rates of mineral uptake within treatments to be determined. 2.4. Measurements In all experiments, fresh weights of individual fruit were measured. Individual whole fruit were analysed for minerals except for those fruit sampled from 70 dafb (early Dec.), whereupon longitudinal wedges were taken from each fruit as above. The longitudinal wedges were taken as representative samples of whole fruit. The tissues were digested in nitric/perchloric acids and analysed for Ca, Mg and K by atomic absorption spectrophotometry (Turner et al., 1977). Immediately after their collection, spurs were subdivided into primary and bourse leaf groups and the leaf areas measured using a LiCor Li-3100 Area Meter or a Delta T Area Meter (II). Rates of mineral uptake into fruit were calculated as : (M2-Ml)/(d2-dl) where M 2 = mineral content at time d2 and M 1 = mineral content at time dl. 2.5. Statistical analysis All experiments were organised as completely randomised designs. However, the ‘Royal Gala’ experiment in 1990 was organised as a randomised block design with treatments factorially arranged. Analysis of variance was used to test for significant effects. Fresh weight was used as the covariate at all sampling dates except at the final sampling date for ‘Royal Gala’ in 1990, where fruit weight was affected by treatment.
3. Results K content were generally similar for all leaf removal treatments in all experiments and therefore K data are not presented. 3.1. Bow-se leaf area reduction
Maximum bourse leaf area of 310 cm’ was reached by 44 dafb on untreated spurs (data not shown). Removal of every second leaf from the developing bourse shoot progressively from petal fall, or removal of every second bourse leaf 75 dafb, reduced final bourse leaf area to 130-165 cm’, while removal of all but one leaf at either time reduced bourse leaf area to 20 cm2. Primary leaf area and fruit size were unaffected by bourse modification at all sampling date (dates not shown). Mean fruit weights at each harvest date ranged from 15-17 g, 75-84 g and 131-142 g at 48, 95 and 134 dafb, respectively. Removal of the bourse shoot at 5 dafb leaving one bourse leaf per spur reduced fruit Ca content by approximately 20% at 48 and 95 dafb compared with the control and this difference increased to 30% at fruit maturity (Fig. 1). Removal of bourse leaves 75 dafb
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.
-uUatrcated *.-.* Ewywfwdldremmd,5Qfb
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140
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Fig. 1. Effect of bourse leaf removal at different times of the season on fruit Ca content during fruit development for ‘Royal Gala’ apple (1988). Bar = LSD (P = 0.05) for comparisons among treatments at any one sampling date.
had little effect on Ca content of fruit sampled 20 days later. However by fruit maturity, removal of all but one leaf at this stage reduced Ca content by 15% compared with the control. Removal of 50% of the bourse leaves at either timing reduced Ca content to a level intermediate between the severe treatment (one leaf left) carried out at the same 13
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Bourse leaf area (cm? Fig. 2. Relationship between fmal bourse leaf area and fmal fruit Ca (open symbols) and Mg content (closed symbols) as influenced by timing of boune leaf removal for ‘Royal Gala’ apple (1988). Upper bar-LSD (P = 0.05) for Ca, Lower bar-LSD (P = 0.05) for Mg.
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Table 1 Estimated rate of Ca uptake into ‘Royal Gala’ fruit due to bourse leaves during different periods of fruit development (1988). (Calculated from data in Fig. I). Total data are values for controls (100% bourse leaf area), other data are values for spurs whose bourse leaves were removed from petal fall. Fruit Ca content assumed to be 0.05 g at 5 datb (see Fig. 3(B)) Interval (dafb)
5-48 (43 days) 48-95 (47 days) 95- 134 (39 days)
Final Ca content (mg per fruit) Total
Other
Bourse
Increase due to bourse
2.8 11.3 12.3
2.3 9.4 8.8
0.5 1.9 1.6
0.45 1.4 1.6
Rate of Ca uptake (pgday-‘)
10 30 41
time and control. At fruit maturity fruit Ca content decreased with reducing bourse leaf area for spurs modified at 75 dafb (7.9 pug crnm2) but this effect was much less than that for spurs modified 5 dafb (12 pg cme2 1 (Fig. 2). From the data in Figs. 1 and 2, we have estimated the rate of Ca uptake into fruit which is dependent upon bourse leaf area in the early (5-48 dafb), mid (48-95 dafb) and late (95-134 dafb) stages of fruit development (Table 1). The greatest effect of bourse leaves on Ca accumulation rate occurred in the later rather than in the early or middle periods of fruit development. This was not just a function of increasing total leaf area, as expressing this estimated rate on a leaf area basis showed the same trend (data not shown). Magnesium contents were not affected by any treatment at 48 or 95 dafb (data not shown). However by the last sample date, Mg content responded to changes in bourse leaf area similar to that described for Ca although less in magnitude (Fig. 2). Magnesium content decreased with reducing bourse leaf area and this response was greater for the earlier than the later timing of leaf removal (3.9 g cme2 vs 2.8 ,ug cmv2). 3.2. Interaction of primary and bow-se leaves In 1989 partial removal of primary leaves from deboursed spurs reduced primary leaf area by 45-48% for ‘Gala’ and ‘Golden Delicious’, but fruit size was not affected (data not shown). Fruit Ca content increased steadily after anthesis for both treatments and cultivars (Fig. 3(A), (B)). Spurs with 100% primary leaves had higher (P < 0.05) fruit Ca content after 13-34 dafh than those with some primary leaves removed. Variation in rate of Ca uptake within treatments and thus statistical significance of differences in rates between treatments were not able to be determined in this experiment. Nevertheless for both cultivars average rate of Ca uptake slowed considerably from 16-21 dafb for fruit from which some primary leaves had been removed (Fig. 3(C), CD)). Fruit Mg increased steadily over the sampling period but there was little difference in content or rate of uptake between the treatments (data not shown). In 1990 partial removal of ‘Royal Gala’ primary leaves at king full bloom successfully reduced primary leaf area from 22-26 cm2 to 1l- 13 cm2, at all sampling dates
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Golden Delicious
0
5
10
Is
20
25
Days after bloom
30
35
01 40
0
5
10
15
20
25
30
35
40
i
Days after bloom
Fig. 3. Fruit Ca accumulation during early fruit development as affected by primary leaves removed at full bloom for ‘Gala’ and ‘Golden Delicious’ apple (1989) (solid square = +leaves, open circle = -leaves). Bar = LSD (P = 0.05). For (A) and (B) the absence of LSD at any one sampling date indicates non-significant difference between treatments.
(data not shown). For those spurs with bourse shoots attached, bourse leaf area increased steadily (5.1 cm* day- ‘1 from anthesis until 41 dafb when bourse growth was at 210 cm* (data not shown). Thereafter bourse leaf area grew more slowly reaching 240 cm* at the final harvest. There was no effect of primary leaf area removal on bourse shoot development. There was also no effect of leaf or bourse shoot removal on fruit weight, except at commercial harvest (P = O.OOl>when spurs without bourse shoots and 50% of primary leaves had a lower fruit weight compared with the other three treatments (104 g vs 119 g). There was no interaction between primary leaf and bourse shoot removal treatments except at 22 dafb. Therefore, main factor averages are presented at each date (Fig. 4). Increase in fruit Ca content was curvilinear to 20 dafb, and then linear over the remainder of the growing season (Fig. 4(A), (B)). Removal of 50% of primary leaves (Fig. 4(A)) or the bourse shoot (Fig. 4(B)) reduced fruit Ca content at all dates (P < 0.05) after 22 dafb, such that fruit Ca was lowered by approximately 15% ( - 50% primary leaves) and 30% ( - bourse shoot) at commercial harvest. Similarly, rates of Ca uptake were lower for the 50% primary leaf treatments than 100% primary leaf
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Primary leaves
Bourse shoot 12.
1B
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140
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20
40
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00
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l20
140
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Days after full bloom
Fig. 4. Fruit Ca accumulation throughout fruit development as affected by primary leaf removal (50%) and bourse shoot removal at bloom for ‘Royal Gala’ apple (1990) (solid square = + primary leaves or + bourse shoot, open circle = - 50% primary leaves or - bourse shoot). Bar = LSD (P = 0.05). The absence of LSD at any one sampling date indicates non-significant difference between treatments.
Table 2 Estimated rate of Ca uptake per leaf area into ‘Royal Gala’ fruit due to primary or bourse leaves throughout fruit development (1990). Leaf areas were estimated as 50% (primary) or 100% (bourse) of control spur values midway through each interval. Rates of Ca uptake due to 50% primary leaf area or 100% bourse leaf area calculated from data in Figs. 4(C) and 4(D), respectively Internal
Leaf area (cm’ )
(datb)
50%
5-12 12-22 22-41 41-63 63-134
primary
100% bourse
12 13 13 12 I1
43 87 161 214 229
Rate of Ca uptake
primary
Increase due to 100% bourse
(pgday-‘)
(pgday-‘)
1.0 10.6 12.9 11.5 9.3
0.3 4.7 27.0 9.9 28.7
Increase due to 50%
Primary (pg day-’
0.08 0.82 0.99 0.96 0.80
cm-* >
Bourse (pgday-’
0.007 0.05 0.17 0.05 0.13
cm-*)
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treatments, differences being significant (P < 0.05) at 17 and 32 dafb (Fig. 4(C)). Rates of Ca uptake were lower for treatments without, compared with those with bourses, at 32 and 99 dafb (Fig. 4(D)). Generally, primary leaves began their effect on fruit Ca uptake early in fruit development and it was maintained throughout the season. In contrast bourse leaves exerted a greater effect on fruit Ca uptake rate later in the season, although the effect was somewhat variable. Expressed on a per leaf area basis, primary leaves had a much greater effect on rate of Ca uptake throughout fruit development and this effect was noticeable earlier than for bourse leaves (Table 2). At 22 datb, a significant interaction (P = 0.03) between bourse and primary leaf treatments indicated that the reduction in Ca observed for both leaf types was due to only the most severe leaf removal treatment (386 pg per fruit vs 533 pg per fruit average for three other treatments). Fruit Mg content increased during development for all treatments (data not shown). However it was not affected by bourse shoot or primary leaf removal except at commercial harvest (P = 0.013) when spurs that had the lowest leaf area had lower Mg content (4 vs 5 mg per fruit). 4. Discussion The positive effect of leaves associated with fruiting wood on Ca uptake into apple fruit is thought to occur from soon after anthesis (Jones and Samuelson, 1983). This is supported by our results, where we detected this influence from 13 dafb for ‘Gala’ and ‘Golden Delicious’ fruit (Fig. 31, although later (at 22 dafb) for ‘Royal Gala’ fruit (Fig. 4). This difference between cultivars in the initial expression of the leaf effect may be related to differences in fruit size at this early stage of development. At 13 dafb, ‘Gala’ and ‘Golden Delicious’ fruit weighed 70-90 g, whereas at 12 dafb, ‘Royal Gala’ fruit were only 50 g. This suggests that leaf area may only limit Ca inflow into fruit once fruit reach a certain size or stage of development after bloom. This concept is supported with the finding that at an early stage of fruit growth, there appears to be a maximum threshold leaf area above which fruit Ca input does not increase in response to further increases in leaf area. For ‘Royal Gala’ at 22 da& there was little effect of increasing leaf area.on Ca input when either primary leaf area was at 100% or the bourse shoot was intact and only when both leaf types were reduced. At later growth stages, rate of Ca accumulation became more sensitive to the partial absence of primary (Fig. 4(B)), or bourse leaves (Table 11, or complete absence of the bourse shoot (Fig. 4(D)) removed at bloom. If we assume that spur and bourse leaves are in some way positively influencing Ca accumulation by developing fruit, then our work indicates that there are several differences between these leaf types in this function. Firstly, our data support the hypothesis that primary leaves are relatively more effective on a per leaf area basis than bourse leaves in aiding Ca movement into fruit (Volz et al., 1994). For ‘Royal Gala’, addition of 240 cm’ of bourse leaf area increased final fruit Ca content by 2.6 mg whereas the addition of only 10 cm* of primary leaf area increased final Ca content by 1.1 mg. Assuming a linear relationship between primary or bourse leaf area and final Ca content (Fig. 2, Volz et al., 1994), this translates into leaf ‘efficiencies’ of 108 and 9 ,ug
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Ca cme2 for primary and bourse leaves, respectively. This compares favourably with other data for ‘Cox’ (48 vs 5 pg Ca cm -2 for primary and bourse leaves, respectively), ‘Golden Delicious’ (29 vs 6 pg Ca cm -2), both calculated from Proctor and Palmer (1991) and ‘Royal Gala’ (41 vs 13 pg Ca cmM2) calculated from Volz et al. (1994). Primary leaves would also seem to begin their effect earlier in fruit development (Fig. 4) than the developing bourse, maintaining a high ‘efficiency’ throughout fruit development (Table 2). For ‘Royal Gala’ at least, bourse leaves provided greater assistance to fruit later in the season (Table 1, Fig. 4(D)). That primary and bourse leaf types exert their effects on Ca uptake into fruit at different stages of development is supported by results of Proctor and Palmer (1991) for ‘Golden Delicious’, where primary leaves and/or bourse shoots were removed at different times from 0 to 8 weeks after full bloom. The positive effect of leaves on final fruit Ca content decreased with later primary leaf removal (i.e. early input of Ca was prevented) , whereas there was no influence of timing of bourse shoot removal (presumably since the positive effect of bourse leaves occurred after 8 weeks from bloom). The effect of the bourse on fruit Ca would be expected to increase with time as a function of increasing shoot growth (and therefore leaf area) up to shoot termination. However, this does not explain the means by which the bourse shoot exerted a stronger effect on fruit Ca accumulation rate in the later part of the season than that in mid-season, despite bourse growth having terminated before this time (Tables 1 and 2). The higher efficiencies and earlier effects of primary leaves are suggestive of a much more effective means of assisting fruit Ca input compared with bourse leaves. Such differences may be related to the capacity of the two leaf types to withdraw xylem water from the fruit in response to evaporative demand (Lang and Volz, 1993). Leaf removal reduced final Mg content for ‘Royal Gala’ in 1988 (to a lesser extent than occurred for Ca), but not for any cultivar in 1989 and only where fruit size was reduced in 1990. Removal of primary and/or bourse leaves before bloom reduced Mg input into ‘Golden Delicious’ fruit (Ferree and Palmer, 1982) while fruit Mg accumulation was slightly greater for spurs with long than those with short bourses for ‘Golden Delicious’ and ‘Cox’ (Jones and Samuelson, 1983). In contrast leaf effects on K accumulation into fruit were minimal (Ferree and Palmer, 1982; Jones and Samuelson, 1983). The relative effect of leaves on the input of these different minerals into fruit (Ca > Mg > K) is inversely related to their mobility in the phloem. Leaf effects on Mg accumulation may only occur when transport to fruit via the phloem becomes limiting. Although we are unclear as to the exact mechanism by which leaves assist Ca and Mg into nearby fruit, our experiments confirm that both primary and bourse leaf types are important in influencing Ca flow. If leaf area is to be used in predicting final fruit Ca status, area of both leaf types would need to be taken into account.
Acknowledgements
We wish to thank Judith Bowen, Greg Lupton and Wendy Cashmore for technical assistance, and the New Zealand Foundation for Science and Technology and the New Zealand Apple & Pear Marketing Board for financial assistance.
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References Ferguson, LB. and Triggs, C.M., 1990. Sampling factors affecting the use of mineral analysis of apple fruit for the prediction of bitter pit. NZ J. Crop Hart. Sci., 18: 147-152. Ferguson, I.B. and Watkins, C.B., 1989. Bitter pit in apple fruit. Hort. Rev., 11: 289-355. Ferree, D.C. and Palmer, J.W., 1982. Effect of spur defoliation and ringing during bloom on fruiting, fruit mineral level, and net photosynthesis of ‘Golden Delicious’ apple. J. Am. Sot. Hort. Sci., 107: 1182-l 186. Haynes, R.J. and Goh, K.M., 1980. Variation in the nutrient content of leaves and fruit with season and crown position for two apple varieties. Aust. J. Agric. Res., 31: 739-748. Jackson, J.E., 1970. Individual fruit size in relation to age of bearing wood on ‘Laxton’s Superb’ apple trees. Rep. East Malhng Res. Sm 1969, pp. 83-85. Jackson, J.E., Sharples, R.O. and Palmer, J.W., 1971. The influence of shade and within-tree position on apple fruit size, colour and storage quality. J. Hart. Sci., 46: 277-287. Jones, H.G. and Samuelson, T.J., 1983. Calcium uptake by developing apple fruits. II. The role of spur leaves. J. Hart. Sci., 58: 183-190. Lang, A. and Volz, R.K., 1993. Leaf area, xylem cycling and Ca status in apples. Acta Hortic., 343: 56-58. Proctor, J.T. and Palmer, J.W., 1991. The role of spur and bourse leaves of three apple cultivars on fruit set and growth and calcium content. J. Hort. Sci., 66: 275-282. Schumacher, R., Frankhauser, F. and Stadler, W., 1980. Influence of shoot growth, average fruit weight and daminozide on bitter pit. In: D. Atkinson, J.E. Jackson, R.O. Sharples and W.M. Wailer (Editor), Mineral Nutrition of Fruit Trees. Butterworths, London, pp. 83-91. Turner, N.A., Ferguson, I.B. and Sharples, R.O., 1977. Sampling and analysis for determining the relationship of calcium concentration to bitter pit in apple fruit. NZ J. Agric. Res., 20: 525-532. Tustin, D.S. and Lai, R., 1990. Source-sink dynamics in developing fruiting spurs of apple. In: E. Raldini et al. (Editors), XX1 11 International Horticultural Congress. Tecnoprint, Bologna, Italy, p. 611. Volz, R.K., Ferguson, I.B., Bowen, J.H. and Watkins, C.B., 1993. Crop load effects on mineral nutrition, maturity, fruiting and tree growth of ‘Cox’s Orange Pippin’ apple. J Hort. Sci., 68: 127-137. Volz, R.K., Ferguson, I.B., Hewett, E.W. and Woolley, D.J., 1994. Wood age and leaf area influences fruit size and mineral composition of apple fruit. J. Hort. Sci., 69: 385-395.