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Mango fruit calcium levels and the effect of postharvest calcium infiltration at different maturities
Scientia Horticulturae 91 (2001) 81±99
Mango fruit calcium levels and the effect of postharvest calcium in®ltration at different maturities D.C. Joycea,*, A.J. Shorterb, P.D. Hockingsc a
Postharvest Laboratory, Silsoe College, Cran®eld University, Silsoe, Bedfordshire MK45 4DT, UK b CSIRO Plant Industry, 120 Meiers Road, Long Pocket, Queensland 4068, Australia c SmithKline Beecham Pharmaceuticals, The Frythe, Welwyn, Hertfordshire AL6 9AR, UK Accepted 15 February 2001
Abstract Calcium concentrations in `Kensington' and `Sensation' mango fruits were measured throughout fruit development on the tree. Flesh calcium concentrations fell from 2.1 to 0.8 mg/g d.w. for `Kensington' and 1.6 to 0.8 mg/g d.w. for `Sensation' as the fruits grew into maturity. Cuticle thickness in both cultivars varied only slightly during growth. Increases in cell wall thickness and in cell length and breadth were similar for both cultivars. Cell wall thickness was greatest in the outer ¯esh, while cell size increased most in the inner ¯esh of the fruit. Calcium concentration pro®les were determined in `Kensington' fruit harvested at normal commercial maturity for untreated fruit and fruit vacuum in®ltrated ( 33 kPa) with 4% (w/v) calcium chloride. Similar pro®les were obtained for both cases. The skin, outer ¯esh, middle ¯esh and inner ¯esh had sequentially decreasing calcium concentrations. Concentrations ranged between 0.371 mg/g d.w. (skin) and 0.095 mg/g d.w. (inner ¯esh) for untreated ripened fruit. Corresponding concentrations for calciumtreated ripened fruits were 0.547±0.086 mg/g d.w. Calcium-treated fruits exhibited no differences in colour or ®rmness changes and weight loss during shelf life as compared to control fruit. Some lenticel damage was observed as a result of calcium in®ltration. Shelf life studies were undertaken on control and vacuum in®ltrated `Kensington' and `Sensation' mango fruits harvested at early ( 3 weeks), middle, and late (3 weeks) stages of maturity and on `Irwin' and `Palmer' mangoes harvested at middle maturity, i.e. normal commercial harvest time. Calcium levels in skin tissue of only `Kensington' and `Palmer' mangoes were slightly higher for calcium-treated than for untreated fruits. Calcium levels in ¯esh tissue were not increased by vacuum in®ltration of calcium into any of
*
Corresponding author. Tel.: 44-1525-863000; fax: 44-1525-863001. E-mail address: d.joyce@cran®eld.ac.uk (D.C. Joyce). 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 4 7 - 3
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the cultivars or maturities. Treatment with 4% (w/v) calcium chloride did not extend the shelf life of fruits of any of the four cultivars. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Calcium; Fruit; Mango; Maturity
1. Introduction Low fruit calcium levels have been associated with reduced postharvest life and physiological disorders (Wills et al., 1998). For example, low levels have been correlated with physiological disorders of avocados (Chaplin and Scott, 1980), papaya (Qiu et al., 1995), apples (Conway et al., 1992) and mangoes (Van Eeden, 1992). Delayed ripening in response to increased fruit calcium levels has been obtained with apples (Klein and Lurie, 1994) and avocados (Wills and Tirmazi, 1982). Calcium treatment has been shown to decrease respiration, reduce ethylene production and to delay the onset of ripening in apples (Ferguson, 1984), avocados (Tingwa and Young, 1974; Rensburg and Engelbrecht, 1986; Wills and Sirivatanapa, 1988; Yuen et al., 1994) and mangoes (Tirmazi and Wills, 1981; Mootoo, 1991; Van Eeden, 1992; Yuniarti and Suhardi, 1992). Surface injury, however, is a negative effect sometimes observed when the calcium concentration of mango fruit is increased by postharvest in®ltration with calcium salts (Tirmazi and Wills, 1981; Van Eeden, 1992; Shorter and Joyce, 1998). In addition, attempts to increase fruit calcium levels either by preharvest spray application (mangoes (McKenzie, 1994), papaya (Qiu et al., 1995)) or by postharvest treatment (avocadoes (Yuen et al., 1994), mangoes (Wills et al., 1988; Van Eeden, 1992)) have been dif®cult in so far as success has been limited. However, calcium chloride dips in either 4% (Tirmazi and Wills, 1981) or 8% (Mootoo, 1991; Yuniarti and Suhardi, 1992) CaCl2 have been reported to increase the time to ripening of green mature mangoes. Factors that in¯uence calcium content include nutrient balance and fruit size (Van Eeden, 1992; Qiu et al., 1995). With regard to postharvest in®ltration of calcium into mango fruit, Van Eeden (1992) found that there was little movement beyond the surface layer. On the other hand, Mootoo (1991) recorded increased levels in both skin and ¯esh layers. Fruit maturity can have a profound in¯uence on calcium uptake. For instance, apples harvested 2 weeks before normal harvest time (maturity) and vacuum in®ltrated with 8% calcium chloride had double the calcium levels of untreated fruits (Conway et al., 1992). In contrast, fruits treated in the same manner but at normal harvest time had calcium levels ®ve times higher than untreated control fruits. With regard to the concentration of calcium, Yuen et al. (1994) reported that vacuum in®ltration with 4% (w/v) calcium chloride of avocado fruits harvested 2 weeks prior to normal harvest time increased the time to ripen whilst causing negligible injury to the fruit. However,
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treatment with a higher 8% (w/v) calcium solution failed to extend shelf life and caused injury to the fruit. The ®rst objective of this work was to describe the calcium status of `Kensington' and `Sensation' mango fruits during fruit development up to commercial harvest maturity. The second objective was to determine calcium concentration pro®les in normal `Kensington' fruit and those subjected to vacuum in®ltration of calcium. The third objective was to investigate the response to calcium applied as a postharvest treatment to each of four mango cultivars, two of which were treated at three maturity stages. 2. Materials and methods 2.1. Experiment 1: calcium status during fruit development `Kensington' and `Sensation' mango fruits (Mangifera indica L.) were harvested at regular intervals during the 1993±1994 summer fruiting season commencing at an early fruit development stage (fruitlet; November) through to normal commercial maturity (February) from near Nambour (26.388S, 152.588E) in south east Queensland. Selected trees were tagged in each orchard for repeated harvests during the fruit growth period. Initial selection of 10 trees at each site was based on uniformity of plant size and fruitlet development. At approximately weekly intervals (Table 1), 10 fruits of each cultivar (1 fruit per tree) were Table 1 Harvest times (1993±1994) and days prior to commercial harvest for examining fruit development (fruit width and calcium concentrations) and cellular studies with `Sensation' and `Kensington' cultivars Harvest time
`Sensation' Development studies
15 November 22 November 29 November 13 December 27 December 10 January 24 January 29 January 4 February 21 February a
±a 87 80 66 52 38 28 ± 17 0
±: no sampling on this date.
`Kensington' Cellular studies ± 87 80 66 52 38 28 ± 17 0
Development studies ± 64 57 43 29 15 5 0 ± ±
Cellular studies 71 57 43 29 15 5 0 ± ±
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removed for growth analysis. Additionally, 10 fruits from the same trees were collected at fortnightly intervals for calcium analysis. Fruits were transported to the Brisbane laboratory where they were weighed n 10 and measured for external size n 10, seed dimensions n 10 and then prepared for calcium analysis n 10. Dry matter (DM) content was also determined n 5. Calcium concentrations in fruits were measured for skin and ¯esh tissue samples separately. Approximately, 10 g fresh weight of skin or ¯esh tissue was placed in an 8 cm diameter glass Petri dish. Petri plates were lined with `Glad Bake', a non-stick baking paper to prevent adhesion of fruit tissue to the container surfaces. Tissue was dried in a convection oven at 608C to constant weight and then stored in paper bags kept in a sealed plastic bag containing silica gel. At the end of sequential sampling, one mature fruit from each tagged tree was harvested for shelf life assessment n 10. In addition, `Kensington Pride' and `Sensation' mango fruits were weighed and their dimensions were measured (e.g. fruit width; n 10). Shelf life assessments involved weighing fruit daily in order to calculate weight (water) loss as a proportion (%) of initial fresh weight. Subjective assessments were made daily of fruit hand ®rmness (1 hard, 3 sprung, 5 slightly soft, 7 soft ripe, and 9 oversoft) and skin colour (1 100% green, 3 75% green, 5 50% green/yellow, 7 75% yellow and 9 100% yellow) (Shorter and Joyce, 1998). End of shelf life for each individual fruit was determined as being when the fruit had reached stage 9 ®rmness (oversoft) recorded for 3 successive days. 2.2. Experiment 2: calcium pro®les Mature green (14:36 0:53% DM) `Kensington' mangoes were harvested in November 1993 from a farm near Ayr (19.348S, 147.248E) in central Queensland. They were ¯own to Brisbane within 24 h of harvest. The 40 fruits were then randomly sorted into calcium in®ltration treatment units at the laboratory. Twenty untreated control ( calcium) fruits were dipped into Sportak fungicide (0.55 ml/l; a.i. 450 g prochloraz /l) for 30 s and then allowed to drain and air dry. Ten control fruits were then put aside for calcium analysis on green mature fruit and 10 control fruits were kept for shelf life evaluation at 228C followed by calcium analysis on the ripened fruit. At the same time, the remaining 20 fruits were treated with calcium at reduced pressure ( 33 kPa; Shorter and Joyce, 1998). The 4% CaCl2 (w/v) solution, containing 0.025% Agral 600, was applied in a 15 l capacity glass vacuum desiccator. The desiccator was connected to a Speedivac high vacuum pump (Edwards High Vacuum, Manor Royal, Crawley, England) and a negative pressure gauge (Dobbie AS-1349 (0±100 kPa), BEP Engineering Products Pty., Brisbane, Australia) to maintain and monitor the reduced pressure, respectively. For in®ltration, fruits were placed into the desiccator and kept submerged for 0.5 min. The vacuum ( 33 kPa) in the
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headspace over the solution was then drawn for 4.5 min. Upon restoration of atmospheric pressure, fruits were left submerged for further 5 min (Wills and Tirmazi, 1979). Finally, fruits were removed from the desiccator and rinsed under deionised water to remove calcium from the surface. Calcium-treated fruits were then dipped into Sportak fungicide for 30 s, and allowed to drain and air dry. Ten green fruits were taken for calcium analysis, and the remaining 10 fruits were kept for shelf life assessments and subsequent calcium analysis on the ripened fruit. Sectioning of fruit for calcium analysis involved taking transverse core samples of fruit tissue with a 10 mm diameter cork borer. These cores were divided with a scapel blade into skin (10 mm diameter 1 mm thick), outer ¯esh (next to skin; 10 mm diameter 10 mm thick), middle ¯esh (10 mm diameter 10 mm thick) and inner ¯esh (next to seed; 10 mm diameter 10 mm thick). A total of 10 g fresh weight of tissue was obtained for each section type. Skin and ¯esh samples were then dried and stored as described in experiment 1. 2.3. Experiment 3: fruit maturity Mango fruits were harvested in December±March 1993±1994 from a farm near Nambour (26.388S, 152.588E). Fruits were picked early in the morning with 2± 5 cm long stems attached, and transported to the Brisbane laboratory by air conditioned car. Four mango cultivars were used in this experiment: `Kensington Pride', `Sensation', `Irwin' and `Palmer'. Harvest times (maturities) for `Kensington Pride' and `Sensation' fruits were early maturity (3 weeks prior to normal commercial harvest; for `Kensington' on 17 January and `Sensation' on 7 February), middle maturity (normal commercial harvest; 14% DM n 5 (O'Hare, 1995); for `Kensington' on 7 February and `Sensation' on 28 February), and late maturity (3 weeks after commercial harvest; for `Kensington' on 28 February and `Sensation' on 21 March). `Irwin' and `Palmer' fruits were only harvested at middle (i.e. normal commercial harvest) maturity, for `Irwin' on 2 February (DM 13:55 1:20% n 5) and `Palmer' on 7 March (DM 14:21 0:49% n 5). `Palmer' fruits were collected from two separate sites on the farm. For each mango cultivar there were 10 individual replicate fruit for each treatment viz. untreated, calcium-treated by maturity (early, middle and late). On arrival at the laboratory, the fruit stem was cut-off adjacent to the skin. Fruits were immediately inverted and placed on wire racks for 30 min to de-sap. Physical dimensions of width, length, diameter, and volume for all the fruits were then measured before treatments were applied. The DM contents were determined on subsamples of ®ve fruits per sample lot. Untreated (control) fruits were dipped in Sportak fungicide for 30 s, drained and allowed to air dry for about 30 min before being re-weighed. Calcium-treated
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fruits were placed into a 4% (w/v) CaCl2 (commercial grade 78±80% CaCl2 ¯ake) plus 0.025% Agral1 600 solution and vacuum in®ltrated in a 15 l desiccator. They were subsequently handled as described in experiment 2. These fruits were rinsed with deionised water. At the end of shelf life (stage 9 (oversoft)), fruit tissues were sampled for calcium analysis by taking cores from the equatorial region of the fruit with a 10 mm diameter cork borer. Skin, outer ¯esh, middle ¯esh and inner ¯esh samples were obtained as described in experiment 2. During shelf life evaluation at 228C, fruits were scored daily for calcium injury/ lenticel damage (1 none, 3 slight (3±5%), 5 moderate (5±10%), 7 severe (10±20%), and 9 very severe (>20%)). 2.4. Experiment design and analysis Fruits were arranged in completely randomised designs in mango fruit trays for shelf life assessment at 228C. Data were analysed by ANOVA using MSTAT-C biometrics software (MSTAT Development Team, 1988). 2.5. Calcium analysis Samples of oven-dried mango fruit tissues were ground to a powder in a mortar and pestle with liquid nitrogen and then further ground in a Shatterbox1 (Spex Industries, Metuchen, USA) grinder. Sub-samples of approximately 0.4±0.5 g of skin or ¯esh tissue were dry-ashed in a muf¯e furnace at 5008C for 12 h. The resultant powder was dissolved in 5 ml of aqueous 1 M HCl. Twenty ®ve millilitres of releasing agent (1000 mg Sr/ml) solution was then added and the mixture made up to a total volume of 50 ml with Millipore Q1 ultra pure DI water. Calcium analysis was with a Varian AA-875 series atomic absorption spectrophotometer. Results were expressed as mg Ca/g dry weight (d.w.) of sample. 2.6. Microscopy and magnetic resonance imaging Excised mango skin and outer, middle and inner ¯esh tissue was ®xed in 3% (v/v) glutaraldehyde (pH 7, 0.1 M phosphate buffer) and dehydrated through a 30, 50, 70, 90 and 100% (v/v) ethanol series, with two changes at 100%. In®ltration and embedding was with Historesin1. Four micrometre thick sections were cut with glass knives, mounted on glass slides and stained with either 1% (w/v) safranin or sudan black B (saturated in 70% (v/v) ethanol) for cell wall and cuticle observation, respectively. Measurements of cell dimensions were made using a calibrated eyepiece graticule (micrometre) in an Olympus BH2 light microscope.
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Magnetic resonance images of longitudinal slices through mangoes were acquired on a Spectrospin 40 cm horizontal bore magnet operating at 190 MHz and interfaced to a Bruker MSL 200 console (Joyce et al., 1993). Spin-echo images were obtained using a multi-slice, multi-echo sequence. 3. Results and discussion 3.1. Fruit development Fruit width (Figs. 1A and 2A) and fresh weight (Figs. 1B and 2B) increments during growth and maturation were generally similar for both `Kensington' and `Sensation' cultivars. Flesh calcium concentrations in fruits of both cultivars tended to decline markedly with growth and maturation (Figs. 1C and 2C). However, when expressed on a whole fruit basis, fruit calcium content usually increased (Figs. 1D and 2D). Changes in cuticle thickness for both `Kensington' and `Sensation' mango fruits followed no consistent trends during development (Figs. 3A and 4A). Cell wall thickness increased near the outer fruit surface (Figs. 3B and 4B) for both cultivars. Cell dimensions (length and breadth) in the outer ¯esh tended not to increase as rapidly during fruit development as those in the middle and, slightly more so, inner regions of the mesocarp nearest the seed (Figs. 3C and D, and 4C and D). The tendency for larger cells in the inner ¯esh progressing to smaller cells in the outer ¯esh is re¯ected in less free water near the skin. This trend can be seen as comparatively darker outer ¯esh regions in the magnetic resonance image (Fig. 5). Less free water may be associated with relatively greater cell wall material and, possibly, more air therein (Figs. 3B and 4B). 3.2. Fruit calcium pro®les Flesh calcium concentrations proximally, distally and across the skin to the inner ¯esh were similar for both untreated and calcium-treated fruits (Table 2). A lower calcium concentration was measured for the skin of untreated ripened fruit compared to calcium-treated ripened fruit. However, no such difference was recorded for mature green fruit. Calcium levels in the skin samples were signi®cantly higher than those in the ¯esh. Higher calcium levels in skin compared to pulp have also been reported for mangoes by Van Eeden (1992). That there were no signi®cant differences among ¯esh pro®les between treatments suggests that calcium failed to penetrate into the fruit under vacuum.
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Fig. 1. Fruit width (A), fruit fresh weight (B), ¯esh calcium concentration (C) and fruit calcium content (D) for `Kensington' mangoes during growth and maturation. Data are means S:E: (n 10 fruit).
3.3. Fruit maturity and calcium in®ltration Differences in DM content between early, middle and late maturity mangoes (Table 3) were generally related to increasing physical dimensions (Table 4) of fruit at harvest, especially for the `Kensington' cultivar. Length, width, diameter and volume of `Kensington' mangoes increased through normal commercial
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Fig. 2. Fruit width (A), fruit fresh weight (B), ¯esh calcium concentration (C) and fruit calcium content (D) for `Sensation' mangoes during growth and maturation. Data are means S:E: (n 10 fruit).
maturity to late maturity. For `Sensation' mangoes, commercial harvest coincided with maximum fruit size. `Irwin' and `Palmer' mango fruits were both larger at commercial harvest than either `Kensington' or `Sensation' fruit. In®ltration with 4% (w/v) CaCl2 at about 33 kPa (250 mm Hg) has been shown to increase the shelf life of `Kensington' mangoes (Tirmazi and Wills, 1981). However, in the present study, no signi®cant bene®cial effect was observed
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Fig. 3. Cuticle thickness (A) and outer (*), middle (*) and inner (!) ¯esh cell wall thickness (B), cell length (C) and cell breadth (D) for `Kensington' mango fruit during growth and maturation. Data are means S:E: (n 10 fruit).
for calcium-treated `Kensington', `Sensation', `Irwin' or `Palmer' fruits (Table 5). Untreated and calcium-treated early maturity fruits of both `Kensington' and `Sensation' cultivars had longer shelf lives than fruits picked at either middle (commercial harvest) or late maturity. Weight loss (%) of harvested untreated and calcium-treated fruit was similar across all cultivars and maturities (data not
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Fig. 4. Cuticle thickness (A) and outer (*), middle (*) and inner (!) ¯esh cell wall thickness (B), cell length (C) and cell breadth (D) for `Sensation' mango fruit during growth and maturation. Data are means S:E: (n 10 fruit).
presented). Calcium in®ltration of mangoes did not affect ®rmness or colour (data not presented), as fruit of all cultivars showed similar trends in fruit softening and skin colour change from green to yellow. Conway et al. (1992) suggested that the pathway for postharvest calcium uptake by in®ltration could be via the lenticels. Moreover, high postharvest
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Fig. 5. Transverse magnetic resonance images (multi-echo (top), T2 map (middle) and 1H density (bottom)) of a `Kensington' mango fruit.
calcium treatment concentrations (e.g. 6% CaCl2) applied to mangoes can result in skin damage around lenticels (Van Eeden, 1992; Shorter and Joyce, 1998). In the present experiment, especially with early and, to a lesser extent, late maturity `Kensington' fruit and also with early maturity `Sensation' fruit, calcium injury was manifested as lenticel blackening (Fig. 6). For the `Irwin' cultivar, a high level of lenticel spotting was evident on both untreated and calcium-treated fruit (Fig. 7). In this experiment, a signi®cant increase in skin calcium concentration was measured in calcium-treated versus untreated `Kensington' mangoes (Table 6). Relatively high calcium levels were also consistently observed in skin tissue for all four cultivars, regardless of treatment and compared to ¯esh.
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Table 2 Calcium concentration (mg/g d.w.) pro®les in mature green and ripened `Kensington' fruit either untreated (control) or treated with 4% (w/v) calcium chloride by vacuum ( 33 kPa) in®ltrationa Tissue sample
Untreated unripened
Untreated ripened
Calcium-treated unripened
Calcium-treated ripened
Row mean
Proximal flesh Distal flesh Skin Outer flesh Middle flesh Inner flesh
0.094 0.071 0.532 0.123 0.055 0.050
0.088 0.052 0.371 0.052 0.052 0.095
0.086 0.068 0.487 0.104 0.104 0.070
0.085 0.066 0.547 0.068 0.068 0.086
0.088 0.064 0.484 0.088 0.088 0.075
Column mean
0.154
c c a c c c
c c b c c c
0.118
0.148
c c a c c c
c c a c c c
0.157
0.144
a
Within the table, means followed by the same letter are not signi®cantly different to each other (l.s.d. at P < 0:05, n 6).
Calcium concentration in the ¯esh of calcium-treated fruit was not increased by vacuum in®ltration. Treatment interactions were not signi®cant for either skin and pulp tissue or harvest maturities of the one cultivar (Table 7). Yuen et al. (1994) found that avocado maturity did not in¯uence calcium uptake. Small but not signi®cant variations between untreated and calcium-treated fruits were obtained for `Kensington', `Sensation' and `Palmer' cultivars. As mentioned above, `Irwin' had signi®cant lenticel injury in general. However, injury tended to be slightly more in the calcium-treated fruits. Rates of water loss between untreated and calcium-treated mango fruit and between maturities of the same cultivar were not signi®cantly different (data not presented). Table 3 DM (%) for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sites. Data are means of ®ve replicates S:E:M:a Cultivar
Maturity
DM (%), mean S:E:
`Kensington' `Kensington' `Kensington' `Sensation' `Sensation' `Sensation' `Irwin' `Palmer', site 1 `Palmer', site 2
Early Middle Late Early Middle Late Middle Middle Middle
12:57 0:99 14:58 0:74 15:91 0:15 12:56 0:21 14:69 0:34 16:98 0:66 13:55 1:20 14:21 0:49 13:04 0:52
a
b ab a c b a
Within cultivars those means followed by the same letter are not signi®cantly different to each other (l.s.d. at P < 0:05, n 5).
Table 4 Physical dimensions for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sites. Data are means of 10 replicates S:E:M:a Cultivar maturity
Mean S:E: Length (mm)
Width (mm)
Diameter (mm)
Volume (cm3)
`Kensington' Early Middle Late
103 1:1 b 107 1:3 b 122 2:3 a
73:9 0:6 c 79:8 0:9 b 95:5 1:3 a
77:5 0:7 c 82:5 0:8 b 98:0 0:8 a
326 7:7 c 378 11 b 616 16 a
`Sensation' Early Middle Late
100 1:1 b 104 1:1 a 97:2 0:7 c
77:6 0:7 c 83:0 0:8 a 79:7 0:5 b
82:0 0:8 b 88:0 0:9 a 83:4 0:5 b
352 9:4 b 434 13 a 363 5:4 b
`Irwin' Middle
116 1:3
79:2 0:6
82:8 0:5
420 7:5
`Palmer' Middle, site 1 Middle, site 2
133 1:3 135 1:3
89:5 1 86:9 0:8
93:4 0:8 91:5 0:8
614 15 614 16
a Within cultivars those means followed by the same letter are not signi®cantly different to each other (l.s.d. at P < 0:05, n 10).
Table 5 Shelf life (days) at 228C of untreated and calcium-treated mango fruit for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sites. Data are means of 10 replicates S:E:M:a Cultivar maturity
Mean S:E:
Row means
Untreated
Calcium-treated
`Kensington' Early Middle Late
15:70 0:2 a 13:70 0:5 b 06:60 0:2 d
15:50 0:3 a 12:10 0:5 c 06:40 0:2 d
15:6 12.9 6.5
`Sensation' Early Middle Late
17:50 0:8 a 12:80 0:7 b 09:10 0:3 c
18:40 0:3 a 13:60 0:3 b 08:80 0:3 c
17.95 12.8 8.95
`Irwin' Middle
14:20 0:9
13:70 0:6
13.95
`Palmer' Middle, site 1 Middle, site 2
10:30 0:7 08:90 0:4
11:20 0:5 08:60 0:3
10:75 8.75
Column means
12.08
12.03
a
Within cultivars, signi®cant differences between means are shown by different lower case letters (l.s.d. at P < 0:05, n 10).
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Fig. 6. Development of skin disorders (lenticel injury) on untreated (control (*)) and calcium vacuum in®ltration-treated (*), early (A), middle (B) and late (C) harvest `Kensington' mango fruit and early (D), middle (E) and late (F) harvest `Sensation' mango fruit. Data are means S:E: (n 10 fruit).
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Fig. 7. Development of skin disorders (lenticel injury) on untreated (control (*)) and calcium vacuum in®ltrated-treated (*), `Irwin' (A) and `Palmer' (B) mango fruit, picked at normal commercial harvest. Data are means S:E: (n 10 fruit).
In conclusion, fruit width and weight growth patterns were generally similar for `Kensington' and `Sensation' mango fruits, as were trends in ¯esh calcium concentration and content. Decreasing ¯esh calcium concentrations during fruit growth can be attributed to cell expansion. Nonetheless, total ¯esh calcium content increases through most of mango fruit development. Mango fruit calcium concentrations decrease from the skin through the outer and middle to the inner mesocarp. This trend in the ¯esh is attributable to smaller cells with thicker cell walls in the outer mesocarp. Magnetic resonance images of mango mesocarp
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Table 6 Calcium concentration (mg/g d.w.) in mango skin and ¯esh samples from untreated and calciumtreated mango fruit for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sitesa Cultivar maturity
Untreated skin
Calcium-treated skin
Untreated flesh
Calcium-treated flesh
Row means
`Kensington' Early Middle Late
1.64 bc 1.72 abc 1.34 c
2.14 a 2.08 ab 1.95 ab
0.56 d 0.40 d 0.25 d
0.67 d 0.48 d 0.49 d
1.25 1.17 1.01
`Sensation' Early Middle Late
2.17 a 2.16 a 2.27 a
2.70 a 2.53 a 2.75 a
0.71 b 0.59 b 0.63 b
0.92 b 0.63 b 0.61 b
1.63 1.48 2.51
`Irwin' Middle
2.33 a
2.37 a
0.64 b
0.80 b
1.54
`Palmer' Middle, site 1 Middle, site 2
1.34 b 1.60 b
1.84 a 2.30 a
0.41 c 0.36 c
0.44 c 0.51 c
1.01 1.16
Column means
1.84
2.29
0.51
0.62
a
Within cultivars, signi®cant differences between means are shown by different lower case letters (l.s.d. at P < 0:05, n 10).
re¯ect this trend, with there being relatively more proton (water) signal near the seed. Extension of shelf life as a consequence of in®ltration of calcium into mango fruit (Tirmazi and Wills, 1981; Mootoo, 1991) was not obtained in these experiments. Signi®cant calcium uptake was only recorded in the skin of `Kensington' mangoes at three different maturities, but was not obtained with `Sensation', `Irwin' or `Palmer' fruit. Higher calcium in the skin of `Kensington' mangoes, particularly early and late season fruits, were correlated with symptoms of calcium injury. Accordingly, calcium in®ltration of mangoes as a postharvest treatment does not appear to offer an advantage because of the dif®culty of in®ltrating fruit and the attendant risk of calcium injury. Although these present results disagree with those reported by Tirmazi and Wills (1981) and Mootoo (1991), they con®rm those obtained by Van Eeden (1992). Contradictory ®ndings can suggest that factors such as inherent nutrient levels in fruit, varietal effects and/or maturity interact to in¯uence calcium uptake and response. Another approach is required if calcium levels in the fruit ¯esh are to be increased consistently. Calcium foliar sprays have been suggested as one alternative way to increase fruit calcium. However, they have been reported as unsuccessful when applied to `Sensation' mango trees (McKenzie, 1994).
Table 7 Treatment (calcium) interactions between the different cultivars (Table 6) harvested at different stages of maturity `Kensington'
`Sensation'
`Irwin'
`Palmer'
Factor A 0:0000 (skin/flesh) Factor B 0:0030 (calcium) A B 0:0842 Factor C 0:1270 (early, middle and late maturity) A C 0:6825 B C 0:6947
Factor A 0:0000 (skin/flesh) Factor B 0:0339 (calcium) A B 0:1221 Factor C 0:6088 (early, middle and late maturity) A C 0:6611 B C 0:8175
Factor A 0:0000 (skin/flesh) Factor B 0:4769 (calcium) A B 0:6618
Site 1, factor A 0:0000 (skin/flesh) Factor B 0:0458 (calcium) A B 0:0745
Site 2, factor A 0:0000 (skin/flesh) Factor B 0:0011 (calcium) A B 0:0124
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References Chaplin, G.R., Scott, K.J., 1980. Association of calcium in chilling injury susceptibility of stored avocados. Hortscience 15, 514±515. Conway, S.C., Sams, C.E., McGuire, R.G., Kelman, A., 1992. Calcium treatment of apples and potatoes to reduce postharvest decay. Plant Disease 76, 329±334. Ferguson, I.B., 1984. Calcium in plant senescence and fruit ripening. Plant Cell Environ. 7, 477± 489. Joyce, D.C., Hockings, P.D., Mazucco, R.A., Shorter, A.J., Brereton, I.M., 1993. Heat treatment injury of mango fruit revealed by nondestructive magnetic resonance imaging. Postharvest Biol. Technol. 3, 305±311. Klein, J.D., Lurie, S., 1994. Time, temperature, and calcium interact in scald reduction and ®rmness retention in heated apples. Hortscience 29, 194±195. McKenzie, C.B., 1994. Preliminary results of calcium and potassium uptake from foliar sprays on Sensation mango. South African Mango Growers' Association Yearbook, Vol. 14, pp. 24±25. Mootoo, A., 1991. Effect of postharvest calcium chloride dips on ripening changes in `Julie' mangoes. Trop. Sci. 31, 243±248. MSTAT Development Team, 1988. A Microcomputer Program for the Design, Management, and Analysis of Agronomic Research Experiments. Michigan State University, East Lansing, MI. O'Hare, T.J., 1995. Effect of ripening temperature on quality and compositional changes of mango (Mangifera indica L.) cv. Kensington. Aust. J. Exp. Agric. 35, 259±263. Qiu, Y., Nishina, M.S., Paull, R.E., 1995. Papaya fruit growth, calcium uptake, and fruit ripening. J. Am. Soc. Hort. Sci. 120, 246±253. Rensburg, E.V., Engelbrecht, A.H.P., 1986. Effect of calcium salts on susceptibility to browning of avocado fruit. J. Food Sci. 51, 1067±1070. Shorter, A.J., Joyce, D.C., 1998. Effect of partial pressure in®ltration of calcium into `Kensington' mango fruit. Aust. J. Exp. Agric. 38, 287±294. Tingwa, P.O., Young, R.E., 1974. The effect of calcium on the ripening of avocado (Persea americana Mill.) fruits. J. Am. Soc. Hort. Sci. 99, 540±542. Tirmazi, S.I.H., Wills, R.B.H., 1981. Retardation of ripening of mangoes by postharvest application of calcium. Trop. Agric. 58, 137±141. Van Eeden, S.J., 1992. Calcium in®ltration as a possible postharvest treatment to increase storage potential of mango fruit. South African Mango Growers' Association Yearbook, Vol. 12, pp. 26± 27. Wills, R.B.H., Sirivatanapa, S., 1988. Evaluation of postharvest in®ltration of calcium to delay the ripening of avocados. Aust. J. Exp. Agric. 28, 801±804. Wills, R.B.H., Tirmazi, S.I.H., 1979. Effect of calcium and other minerals on ripening of tomatoes. Aust. J. Plant Physiol. 6, 221±227. Wills, R.B.H., Tirmazi, S.I.H., 1982. Inhibition of ripening of avocados with calcium. Sci. Hortic. 16, 323±330. Wills, R.B.H., Yuen, C.M.C., Sabari, Laksmi, S.D.L., Suyanti, 1988. Effect of calcium in®ltration on delayed ripening of three mango cultivars in Indonesia. ASEAN Food J. 4, 67±68. Wills, R., McGlasson, B., Graham, D., Joyce, D., 1998. Postharvest: An Introduction to the Physiology and Handling of Fruit, Vegetables and Ornamentals. UNSW Press, Sydney, 262 pp. Yuen, C.M.C., Caf®n, N., Boonyakiat, D., 1994. Effect of calcium in®ltration on ripening of avocados at different maturities. Aust. J. Exp. Agric. 34, 123±126. Yuniarti, Suhardi, 1992. Ripening retardation of Arumanis mango. ASEAN Food J. 7, 207±208.
Mango fruit calcium levels and the effect of postharvest calcium in®ltration at different maturities D.C. Joycea,*, A.J. Shorterb, P.D. Hockingsc a
Postharvest Laboratory, Silsoe College, Cran®eld University, Silsoe, Bedfordshire MK45 4DT, UK b CSIRO Plant Industry, 120 Meiers Road, Long Pocket, Queensland 4068, Australia c SmithKline Beecham Pharmaceuticals, The Frythe, Welwyn, Hertfordshire AL6 9AR, UK Accepted 15 February 2001
Abstract Calcium concentrations in `Kensington' and `Sensation' mango fruits were measured throughout fruit development on the tree. Flesh calcium concentrations fell from 2.1 to 0.8 mg/g d.w. for `Kensington' and 1.6 to 0.8 mg/g d.w. for `Sensation' as the fruits grew into maturity. Cuticle thickness in both cultivars varied only slightly during growth. Increases in cell wall thickness and in cell length and breadth were similar for both cultivars. Cell wall thickness was greatest in the outer ¯esh, while cell size increased most in the inner ¯esh of the fruit. Calcium concentration pro®les were determined in `Kensington' fruit harvested at normal commercial maturity for untreated fruit and fruit vacuum in®ltrated ( 33 kPa) with 4% (w/v) calcium chloride. Similar pro®les were obtained for both cases. The skin, outer ¯esh, middle ¯esh and inner ¯esh had sequentially decreasing calcium concentrations. Concentrations ranged between 0.371 mg/g d.w. (skin) and 0.095 mg/g d.w. (inner ¯esh) for untreated ripened fruit. Corresponding concentrations for calciumtreated ripened fruits were 0.547±0.086 mg/g d.w. Calcium-treated fruits exhibited no differences in colour or ®rmness changes and weight loss during shelf life as compared to control fruit. Some lenticel damage was observed as a result of calcium in®ltration. Shelf life studies were undertaken on control and vacuum in®ltrated `Kensington' and `Sensation' mango fruits harvested at early ( 3 weeks), middle, and late (3 weeks) stages of maturity and on `Irwin' and `Palmer' mangoes harvested at middle maturity, i.e. normal commercial harvest time. Calcium levels in skin tissue of only `Kensington' and `Palmer' mangoes were slightly higher for calcium-treated than for untreated fruits. Calcium levels in ¯esh tissue were not increased by vacuum in®ltration of calcium into any of
*
Corresponding author. Tel.: 44-1525-863000; fax: 44-1525-863001. E-mail address: d.joyce@cran®eld.ac.uk (D.C. Joyce). 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 4 7 - 3
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the cultivars or maturities. Treatment with 4% (w/v) calcium chloride did not extend the shelf life of fruits of any of the four cultivars. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Calcium; Fruit; Mango; Maturity
1. Introduction Low fruit calcium levels have been associated with reduced postharvest life and physiological disorders (Wills et al., 1998). For example, low levels have been correlated with physiological disorders of avocados (Chaplin and Scott, 1980), papaya (Qiu et al., 1995), apples (Conway et al., 1992) and mangoes (Van Eeden, 1992). Delayed ripening in response to increased fruit calcium levels has been obtained with apples (Klein and Lurie, 1994) and avocados (Wills and Tirmazi, 1982). Calcium treatment has been shown to decrease respiration, reduce ethylene production and to delay the onset of ripening in apples (Ferguson, 1984), avocados (Tingwa and Young, 1974; Rensburg and Engelbrecht, 1986; Wills and Sirivatanapa, 1988; Yuen et al., 1994) and mangoes (Tirmazi and Wills, 1981; Mootoo, 1991; Van Eeden, 1992; Yuniarti and Suhardi, 1992). Surface injury, however, is a negative effect sometimes observed when the calcium concentration of mango fruit is increased by postharvest in®ltration with calcium salts (Tirmazi and Wills, 1981; Van Eeden, 1992; Shorter and Joyce, 1998). In addition, attempts to increase fruit calcium levels either by preharvest spray application (mangoes (McKenzie, 1994), papaya (Qiu et al., 1995)) or by postharvest treatment (avocadoes (Yuen et al., 1994), mangoes (Wills et al., 1988; Van Eeden, 1992)) have been dif®cult in so far as success has been limited. However, calcium chloride dips in either 4% (Tirmazi and Wills, 1981) or 8% (Mootoo, 1991; Yuniarti and Suhardi, 1992) CaCl2 have been reported to increase the time to ripening of green mature mangoes. Factors that in¯uence calcium content include nutrient balance and fruit size (Van Eeden, 1992; Qiu et al., 1995). With regard to postharvest in®ltration of calcium into mango fruit, Van Eeden (1992) found that there was little movement beyond the surface layer. On the other hand, Mootoo (1991) recorded increased levels in both skin and ¯esh layers. Fruit maturity can have a profound in¯uence on calcium uptake. For instance, apples harvested 2 weeks before normal harvest time (maturity) and vacuum in®ltrated with 8% calcium chloride had double the calcium levels of untreated fruits (Conway et al., 1992). In contrast, fruits treated in the same manner but at normal harvest time had calcium levels ®ve times higher than untreated control fruits. With regard to the concentration of calcium, Yuen et al. (1994) reported that vacuum in®ltration with 4% (w/v) calcium chloride of avocado fruits harvested 2 weeks prior to normal harvest time increased the time to ripen whilst causing negligible injury to the fruit. However,
D.C. Joyce et al. / Scientia Horticulturae 91 (2001) 81±99
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treatment with a higher 8% (w/v) calcium solution failed to extend shelf life and caused injury to the fruit. The ®rst objective of this work was to describe the calcium status of `Kensington' and `Sensation' mango fruits during fruit development up to commercial harvest maturity. The second objective was to determine calcium concentration pro®les in normal `Kensington' fruit and those subjected to vacuum in®ltration of calcium. The third objective was to investigate the response to calcium applied as a postharvest treatment to each of four mango cultivars, two of which were treated at three maturity stages. 2. Materials and methods 2.1. Experiment 1: calcium status during fruit development `Kensington' and `Sensation' mango fruits (Mangifera indica L.) were harvested at regular intervals during the 1993±1994 summer fruiting season commencing at an early fruit development stage (fruitlet; November) through to normal commercial maturity (February) from near Nambour (26.388S, 152.588E) in south east Queensland. Selected trees were tagged in each orchard for repeated harvests during the fruit growth period. Initial selection of 10 trees at each site was based on uniformity of plant size and fruitlet development. At approximately weekly intervals (Table 1), 10 fruits of each cultivar (1 fruit per tree) were Table 1 Harvest times (1993±1994) and days prior to commercial harvest for examining fruit development (fruit width and calcium concentrations) and cellular studies with `Sensation' and `Kensington' cultivars Harvest time
`Sensation' Development studies
15 November 22 November 29 November 13 December 27 December 10 January 24 January 29 January 4 February 21 February a
±a 87 80 66 52 38 28 ± 17 0
±: no sampling on this date.
`Kensington' Cellular studies ± 87 80 66 52 38 28 ± 17 0
Development studies ± 64 57 43 29 15 5 0 ± ±
Cellular studies 71 57 43 29 15 5 0 ± ±
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removed for growth analysis. Additionally, 10 fruits from the same trees were collected at fortnightly intervals for calcium analysis. Fruits were transported to the Brisbane laboratory where they were weighed n 10 and measured for external size n 10, seed dimensions n 10 and then prepared for calcium analysis n 10. Dry matter (DM) content was also determined n 5. Calcium concentrations in fruits were measured for skin and ¯esh tissue samples separately. Approximately, 10 g fresh weight of skin or ¯esh tissue was placed in an 8 cm diameter glass Petri dish. Petri plates were lined with `Glad Bake', a non-stick baking paper to prevent adhesion of fruit tissue to the container surfaces. Tissue was dried in a convection oven at 608C to constant weight and then stored in paper bags kept in a sealed plastic bag containing silica gel. At the end of sequential sampling, one mature fruit from each tagged tree was harvested for shelf life assessment n 10. In addition, `Kensington Pride' and `Sensation' mango fruits were weighed and their dimensions were measured (e.g. fruit width; n 10). Shelf life assessments involved weighing fruit daily in order to calculate weight (water) loss as a proportion (%) of initial fresh weight. Subjective assessments were made daily of fruit hand ®rmness (1 hard, 3 sprung, 5 slightly soft, 7 soft ripe, and 9 oversoft) and skin colour (1 100% green, 3 75% green, 5 50% green/yellow, 7 75% yellow and 9 100% yellow) (Shorter and Joyce, 1998). End of shelf life for each individual fruit was determined as being when the fruit had reached stage 9 ®rmness (oversoft) recorded for 3 successive days. 2.2. Experiment 2: calcium pro®les Mature green (14:36 0:53% DM) `Kensington' mangoes were harvested in November 1993 from a farm near Ayr (19.348S, 147.248E) in central Queensland. They were ¯own to Brisbane within 24 h of harvest. The 40 fruits were then randomly sorted into calcium in®ltration treatment units at the laboratory. Twenty untreated control ( calcium) fruits were dipped into Sportak fungicide (0.55 ml/l; a.i. 450 g prochloraz /l) for 30 s and then allowed to drain and air dry. Ten control fruits were then put aside for calcium analysis on green mature fruit and 10 control fruits were kept for shelf life evaluation at 228C followed by calcium analysis on the ripened fruit. At the same time, the remaining 20 fruits were treated with calcium at reduced pressure ( 33 kPa; Shorter and Joyce, 1998). The 4% CaCl2 (w/v) solution, containing 0.025% Agral 600, was applied in a 15 l capacity glass vacuum desiccator. The desiccator was connected to a Speedivac high vacuum pump (Edwards High Vacuum, Manor Royal, Crawley, England) and a negative pressure gauge (Dobbie AS-1349 (0±100 kPa), BEP Engineering Products Pty., Brisbane, Australia) to maintain and monitor the reduced pressure, respectively. For in®ltration, fruits were placed into the desiccator and kept submerged for 0.5 min. The vacuum ( 33 kPa) in the
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headspace over the solution was then drawn for 4.5 min. Upon restoration of atmospheric pressure, fruits were left submerged for further 5 min (Wills and Tirmazi, 1979). Finally, fruits were removed from the desiccator and rinsed under deionised water to remove calcium from the surface. Calcium-treated fruits were then dipped into Sportak fungicide for 30 s, and allowed to drain and air dry. Ten green fruits were taken for calcium analysis, and the remaining 10 fruits were kept for shelf life assessments and subsequent calcium analysis on the ripened fruit. Sectioning of fruit for calcium analysis involved taking transverse core samples of fruit tissue with a 10 mm diameter cork borer. These cores were divided with a scapel blade into skin (10 mm diameter 1 mm thick), outer ¯esh (next to skin; 10 mm diameter 10 mm thick), middle ¯esh (10 mm diameter 10 mm thick) and inner ¯esh (next to seed; 10 mm diameter 10 mm thick). A total of 10 g fresh weight of tissue was obtained for each section type. Skin and ¯esh samples were then dried and stored as described in experiment 1. 2.3. Experiment 3: fruit maturity Mango fruits were harvested in December±March 1993±1994 from a farm near Nambour (26.388S, 152.588E). Fruits were picked early in the morning with 2± 5 cm long stems attached, and transported to the Brisbane laboratory by air conditioned car. Four mango cultivars were used in this experiment: `Kensington Pride', `Sensation', `Irwin' and `Palmer'. Harvest times (maturities) for `Kensington Pride' and `Sensation' fruits were early maturity (3 weeks prior to normal commercial harvest; for `Kensington' on 17 January and `Sensation' on 7 February), middle maturity (normal commercial harvest; 14% DM n 5 (O'Hare, 1995); for `Kensington' on 7 February and `Sensation' on 28 February), and late maturity (3 weeks after commercial harvest; for `Kensington' on 28 February and `Sensation' on 21 March). `Irwin' and `Palmer' fruits were only harvested at middle (i.e. normal commercial harvest) maturity, for `Irwin' on 2 February (DM 13:55 1:20% n 5) and `Palmer' on 7 March (DM 14:21 0:49% n 5). `Palmer' fruits were collected from two separate sites on the farm. For each mango cultivar there were 10 individual replicate fruit for each treatment viz. untreated, calcium-treated by maturity (early, middle and late). On arrival at the laboratory, the fruit stem was cut-off adjacent to the skin. Fruits were immediately inverted and placed on wire racks for 30 min to de-sap. Physical dimensions of width, length, diameter, and volume for all the fruits were then measured before treatments were applied. The DM contents were determined on subsamples of ®ve fruits per sample lot. Untreated (control) fruits were dipped in Sportak fungicide for 30 s, drained and allowed to air dry for about 30 min before being re-weighed. Calcium-treated
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fruits were placed into a 4% (w/v) CaCl2 (commercial grade 78±80% CaCl2 ¯ake) plus 0.025% Agral1 600 solution and vacuum in®ltrated in a 15 l desiccator. They were subsequently handled as described in experiment 2. These fruits were rinsed with deionised water. At the end of shelf life (stage 9 (oversoft)), fruit tissues were sampled for calcium analysis by taking cores from the equatorial region of the fruit with a 10 mm diameter cork borer. Skin, outer ¯esh, middle ¯esh and inner ¯esh samples were obtained as described in experiment 2. During shelf life evaluation at 228C, fruits were scored daily for calcium injury/ lenticel damage (1 none, 3 slight (3±5%), 5 moderate (5±10%), 7 severe (10±20%), and 9 very severe (>20%)). 2.4. Experiment design and analysis Fruits were arranged in completely randomised designs in mango fruit trays for shelf life assessment at 228C. Data were analysed by ANOVA using MSTAT-C biometrics software (MSTAT Development Team, 1988). 2.5. Calcium analysis Samples of oven-dried mango fruit tissues were ground to a powder in a mortar and pestle with liquid nitrogen and then further ground in a Shatterbox1 (Spex Industries, Metuchen, USA) grinder. Sub-samples of approximately 0.4±0.5 g of skin or ¯esh tissue were dry-ashed in a muf¯e furnace at 5008C for 12 h. The resultant powder was dissolved in 5 ml of aqueous 1 M HCl. Twenty ®ve millilitres of releasing agent (1000 mg Sr/ml) solution was then added and the mixture made up to a total volume of 50 ml with Millipore Q1 ultra pure DI water. Calcium analysis was with a Varian AA-875 series atomic absorption spectrophotometer. Results were expressed as mg Ca/g dry weight (d.w.) of sample. 2.6. Microscopy and magnetic resonance imaging Excised mango skin and outer, middle and inner ¯esh tissue was ®xed in 3% (v/v) glutaraldehyde (pH 7, 0.1 M phosphate buffer) and dehydrated through a 30, 50, 70, 90 and 100% (v/v) ethanol series, with two changes at 100%. In®ltration and embedding was with Historesin1. Four micrometre thick sections were cut with glass knives, mounted on glass slides and stained with either 1% (w/v) safranin or sudan black B (saturated in 70% (v/v) ethanol) for cell wall and cuticle observation, respectively. Measurements of cell dimensions were made using a calibrated eyepiece graticule (micrometre) in an Olympus BH2 light microscope.
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Magnetic resonance images of longitudinal slices through mangoes were acquired on a Spectrospin 40 cm horizontal bore magnet operating at 190 MHz and interfaced to a Bruker MSL 200 console (Joyce et al., 1993). Spin-echo images were obtained using a multi-slice, multi-echo sequence. 3. Results and discussion 3.1. Fruit development Fruit width (Figs. 1A and 2A) and fresh weight (Figs. 1B and 2B) increments during growth and maturation were generally similar for both `Kensington' and `Sensation' cultivars. Flesh calcium concentrations in fruits of both cultivars tended to decline markedly with growth and maturation (Figs. 1C and 2C). However, when expressed on a whole fruit basis, fruit calcium content usually increased (Figs. 1D and 2D). Changes in cuticle thickness for both `Kensington' and `Sensation' mango fruits followed no consistent trends during development (Figs. 3A and 4A). Cell wall thickness increased near the outer fruit surface (Figs. 3B and 4B) for both cultivars. Cell dimensions (length and breadth) in the outer ¯esh tended not to increase as rapidly during fruit development as those in the middle and, slightly more so, inner regions of the mesocarp nearest the seed (Figs. 3C and D, and 4C and D). The tendency for larger cells in the inner ¯esh progressing to smaller cells in the outer ¯esh is re¯ected in less free water near the skin. This trend can be seen as comparatively darker outer ¯esh regions in the magnetic resonance image (Fig. 5). Less free water may be associated with relatively greater cell wall material and, possibly, more air therein (Figs. 3B and 4B). 3.2. Fruit calcium pro®les Flesh calcium concentrations proximally, distally and across the skin to the inner ¯esh were similar for both untreated and calcium-treated fruits (Table 2). A lower calcium concentration was measured for the skin of untreated ripened fruit compared to calcium-treated ripened fruit. However, no such difference was recorded for mature green fruit. Calcium levels in the skin samples were signi®cantly higher than those in the ¯esh. Higher calcium levels in skin compared to pulp have also been reported for mangoes by Van Eeden (1992). That there were no signi®cant differences among ¯esh pro®les between treatments suggests that calcium failed to penetrate into the fruit under vacuum.
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Fig. 1. Fruit width (A), fruit fresh weight (B), ¯esh calcium concentration (C) and fruit calcium content (D) for `Kensington' mangoes during growth and maturation. Data are means S:E: (n 10 fruit).
3.3. Fruit maturity and calcium in®ltration Differences in DM content between early, middle and late maturity mangoes (Table 3) were generally related to increasing physical dimensions (Table 4) of fruit at harvest, especially for the `Kensington' cultivar. Length, width, diameter and volume of `Kensington' mangoes increased through normal commercial
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89
Fig. 2. Fruit width (A), fruit fresh weight (B), ¯esh calcium concentration (C) and fruit calcium content (D) for `Sensation' mangoes during growth and maturation. Data are means S:E: (n 10 fruit).
maturity to late maturity. For `Sensation' mangoes, commercial harvest coincided with maximum fruit size. `Irwin' and `Palmer' mango fruits were both larger at commercial harvest than either `Kensington' or `Sensation' fruit. In®ltration with 4% (w/v) CaCl2 at about 33 kPa (250 mm Hg) has been shown to increase the shelf life of `Kensington' mangoes (Tirmazi and Wills, 1981). However, in the present study, no signi®cant bene®cial effect was observed
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Fig. 3. Cuticle thickness (A) and outer (*), middle (*) and inner (!) ¯esh cell wall thickness (B), cell length (C) and cell breadth (D) for `Kensington' mango fruit during growth and maturation. Data are means S:E: (n 10 fruit).
for calcium-treated `Kensington', `Sensation', `Irwin' or `Palmer' fruits (Table 5). Untreated and calcium-treated early maturity fruits of both `Kensington' and `Sensation' cultivars had longer shelf lives than fruits picked at either middle (commercial harvest) or late maturity. Weight loss (%) of harvested untreated and calcium-treated fruit was similar across all cultivars and maturities (data not
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91
Fig. 4. Cuticle thickness (A) and outer (*), middle (*) and inner (!) ¯esh cell wall thickness (B), cell length (C) and cell breadth (D) for `Sensation' mango fruit during growth and maturation. Data are means S:E: (n 10 fruit).
presented). Calcium in®ltration of mangoes did not affect ®rmness or colour (data not presented), as fruit of all cultivars showed similar trends in fruit softening and skin colour change from green to yellow. Conway et al. (1992) suggested that the pathway for postharvest calcium uptake by in®ltration could be via the lenticels. Moreover, high postharvest
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Fig. 5. Transverse magnetic resonance images (multi-echo (top), T2 map (middle) and 1H density (bottom)) of a `Kensington' mango fruit.
calcium treatment concentrations (e.g. 6% CaCl2) applied to mangoes can result in skin damage around lenticels (Van Eeden, 1992; Shorter and Joyce, 1998). In the present experiment, especially with early and, to a lesser extent, late maturity `Kensington' fruit and also with early maturity `Sensation' fruit, calcium injury was manifested as lenticel blackening (Fig. 6). For the `Irwin' cultivar, a high level of lenticel spotting was evident on both untreated and calcium-treated fruit (Fig. 7). In this experiment, a signi®cant increase in skin calcium concentration was measured in calcium-treated versus untreated `Kensington' mangoes (Table 6). Relatively high calcium levels were also consistently observed in skin tissue for all four cultivars, regardless of treatment and compared to ¯esh.
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Table 2 Calcium concentration (mg/g d.w.) pro®les in mature green and ripened `Kensington' fruit either untreated (control) or treated with 4% (w/v) calcium chloride by vacuum ( 33 kPa) in®ltrationa Tissue sample
Untreated unripened
Untreated ripened
Calcium-treated unripened
Calcium-treated ripened
Row mean
Proximal flesh Distal flesh Skin Outer flesh Middle flesh Inner flesh
0.094 0.071 0.532 0.123 0.055 0.050
0.088 0.052 0.371 0.052 0.052 0.095
0.086 0.068 0.487 0.104 0.104 0.070
0.085 0.066 0.547 0.068 0.068 0.086
0.088 0.064 0.484 0.088 0.088 0.075
Column mean
0.154
c c a c c c
c c b c c c
0.118
0.148
c c a c c c
c c a c c c
0.157
0.144
a
Within the table, means followed by the same letter are not signi®cantly different to each other (l.s.d. at P < 0:05, n 6).
Calcium concentration in the ¯esh of calcium-treated fruit was not increased by vacuum in®ltration. Treatment interactions were not signi®cant for either skin and pulp tissue or harvest maturities of the one cultivar (Table 7). Yuen et al. (1994) found that avocado maturity did not in¯uence calcium uptake. Small but not signi®cant variations between untreated and calcium-treated fruits were obtained for `Kensington', `Sensation' and `Palmer' cultivars. As mentioned above, `Irwin' had signi®cant lenticel injury in general. However, injury tended to be slightly more in the calcium-treated fruits. Rates of water loss between untreated and calcium-treated mango fruit and between maturities of the same cultivar were not signi®cantly different (data not presented). Table 3 DM (%) for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sites. Data are means of ®ve replicates S:E:M:a Cultivar
Maturity
DM (%), mean S:E:
`Kensington' `Kensington' `Kensington' `Sensation' `Sensation' `Sensation' `Irwin' `Palmer', site 1 `Palmer', site 2
Early Middle Late Early Middle Late Middle Middle Middle
12:57 0:99 14:58 0:74 15:91 0:15 12:56 0:21 14:69 0:34 16:98 0:66 13:55 1:20 14:21 0:49 13:04 0:52
a
b ab a c b a
Within cultivars those means followed by the same letter are not signi®cantly different to each other (l.s.d. at P < 0:05, n 5).
Table 4 Physical dimensions for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sites. Data are means of 10 replicates S:E:M:a Cultivar maturity
Mean S:E: Length (mm)
Width (mm)
Diameter (mm)
Volume (cm3)
`Kensington' Early Middle Late
103 1:1 b 107 1:3 b 122 2:3 a
73:9 0:6 c 79:8 0:9 b 95:5 1:3 a
77:5 0:7 c 82:5 0:8 b 98:0 0:8 a
326 7:7 c 378 11 b 616 16 a
`Sensation' Early Middle Late
100 1:1 b 104 1:1 a 97:2 0:7 c
77:6 0:7 c 83:0 0:8 a 79:7 0:5 b
82:0 0:8 b 88:0 0:9 a 83:4 0:5 b
352 9:4 b 434 13 a 363 5:4 b
`Irwin' Middle
116 1:3
79:2 0:6
82:8 0:5
420 7:5
`Palmer' Middle, site 1 Middle, site 2
133 1:3 135 1:3
89:5 1 86:9 0:8
93:4 0:8 91:5 0:8
614 15 614 16
a Within cultivars those means followed by the same letter are not signi®cantly different to each other (l.s.d. at P < 0:05, n 10).
Table 5 Shelf life (days) at 228C of untreated and calcium-treated mango fruit for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sites. Data are means of 10 replicates S:E:M:a Cultivar maturity
Mean S:E:
Row means
Untreated
Calcium-treated
`Kensington' Early Middle Late
15:70 0:2 a 13:70 0:5 b 06:60 0:2 d
15:50 0:3 a 12:10 0:5 c 06:40 0:2 d
15:6 12.9 6.5
`Sensation' Early Middle Late
17:50 0:8 a 12:80 0:7 b 09:10 0:3 c
18:40 0:3 a 13:60 0:3 b 08:80 0:3 c
17.95 12.8 8.95
`Irwin' Middle
14:20 0:9
13:70 0:6
13.95
`Palmer' Middle, site 1 Middle, site 2
10:30 0:7 08:90 0:4
11:20 0:5 08:60 0:3
10:75 8.75
Column means
12.08
12.03
a
Within cultivars, signi®cant differences between means are shown by different lower case letters (l.s.d. at P < 0:05, n 10).
D.C. Joyce et al. / Scientia Horticulturae 91 (2001) 81±99
95
Fig. 6. Development of skin disorders (lenticel injury) on untreated (control (*)) and calcium vacuum in®ltration-treated (*), early (A), middle (B) and late (C) harvest `Kensington' mango fruit and early (D), middle (E) and late (F) harvest `Sensation' mango fruit. Data are means S:E: (n 10 fruit).
96
D.C. Joyce et al. / Scientia Horticulturae 91 (2001) 81±99
Fig. 7. Development of skin disorders (lenticel injury) on untreated (control (*)) and calcium vacuum in®ltrated-treated (*), `Irwin' (A) and `Palmer' (B) mango fruit, picked at normal commercial harvest. Data are means S:E: (n 10 fruit).
In conclusion, fruit width and weight growth patterns were generally similar for `Kensington' and `Sensation' mango fruits, as were trends in ¯esh calcium concentration and content. Decreasing ¯esh calcium concentrations during fruit growth can be attributed to cell expansion. Nonetheless, total ¯esh calcium content increases through most of mango fruit development. Mango fruit calcium concentrations decrease from the skin through the outer and middle to the inner mesocarp. This trend in the ¯esh is attributable to smaller cells with thicker cell walls in the outer mesocarp. Magnetic resonance images of mango mesocarp
D.C. Joyce et al. / Scientia Horticulturae 91 (2001) 81±99
97
Table 6 Calcium concentration (mg/g d.w.) in mango skin and ¯esh samples from untreated and calciumtreated mango fruit for four mango cultivars, two of which were harvested at three different maturities and one of which was harvested from two sitesa Cultivar maturity
Untreated skin
Calcium-treated skin
Untreated flesh
Calcium-treated flesh
Row means
`Kensington' Early Middle Late
1.64 bc 1.72 abc 1.34 c
2.14 a 2.08 ab 1.95 ab
0.56 d 0.40 d 0.25 d
0.67 d 0.48 d 0.49 d
1.25 1.17 1.01
`Sensation' Early Middle Late
2.17 a 2.16 a 2.27 a
2.70 a 2.53 a 2.75 a
0.71 b 0.59 b 0.63 b
0.92 b 0.63 b 0.61 b
1.63 1.48 2.51
`Irwin' Middle
2.33 a
2.37 a
0.64 b
0.80 b
1.54
`Palmer' Middle, site 1 Middle, site 2
1.34 b 1.60 b
1.84 a 2.30 a
0.41 c 0.36 c
0.44 c 0.51 c
1.01 1.16
Column means
1.84
2.29
0.51
0.62
a
Within cultivars, signi®cant differences between means are shown by different lower case letters (l.s.d. at P < 0:05, n 10).
re¯ect this trend, with there being relatively more proton (water) signal near the seed. Extension of shelf life as a consequence of in®ltration of calcium into mango fruit (Tirmazi and Wills, 1981; Mootoo, 1991) was not obtained in these experiments. Signi®cant calcium uptake was only recorded in the skin of `Kensington' mangoes at three different maturities, but was not obtained with `Sensation', `Irwin' or `Palmer' fruit. Higher calcium in the skin of `Kensington' mangoes, particularly early and late season fruits, were correlated with symptoms of calcium injury. Accordingly, calcium in®ltration of mangoes as a postharvest treatment does not appear to offer an advantage because of the dif®culty of in®ltrating fruit and the attendant risk of calcium injury. Although these present results disagree with those reported by Tirmazi and Wills (1981) and Mootoo (1991), they con®rm those obtained by Van Eeden (1992). Contradictory ®ndings can suggest that factors such as inherent nutrient levels in fruit, varietal effects and/or maturity interact to in¯uence calcium uptake and response. Another approach is required if calcium levels in the fruit ¯esh are to be increased consistently. Calcium foliar sprays have been suggested as one alternative way to increase fruit calcium. However, they have been reported as unsuccessful when applied to `Sensation' mango trees (McKenzie, 1994).
Table 7 Treatment (calcium) interactions between the different cultivars (Table 6) harvested at different stages of maturity `Kensington'
`Sensation'
`Irwin'
`Palmer'
Factor A 0:0000 (skin/flesh) Factor B 0:0030 (calcium) A B 0:0842 Factor C 0:1270 (early, middle and late maturity) A C 0:6825 B C 0:6947
Factor A 0:0000 (skin/flesh) Factor B 0:0339 (calcium) A B 0:1221 Factor C 0:6088 (early, middle and late maturity) A C 0:6611 B C 0:8175
Factor A 0:0000 (skin/flesh) Factor B 0:4769 (calcium) A B 0:6618
Site 1, factor A 0:0000 (skin/flesh) Factor B 0:0458 (calcium) A B 0:0745
Site 2, factor A 0:0000 (skin/flesh) Factor B 0:0011 (calcium) A B 0:0124
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