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Refining the estimation of kiwifruit size from linear fruit dimensions
Scientia Horticulturae 262 (2020) 108878
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Refining the estimation of kiwifruit size from linear fruit dimensions a,b,
H. Yuan
c
b
c
d
*, P. Martin , C. Fullerton , N. Gould , A. Hall , J. Burdon
b
T
a
Institute of Agro-Products Processing Science and Technology, Sichuan Academy of Agricultural Sciences, Chengdu, 610066, China The New Zealand Institute for Plant and Food Research Limited, Private Bag 92169, Auckland Mail Centre, 1142, Auckland, New Zealand The New Zealand Institute for Plant and Food Research Limited, 412 No 1 Road, RD 2, Te Puke, 3182, New Zealand d The New Zealand Institute for Plant and Food Research Limited, Private Bag 11600, Palmerston North, 4442, New Zealand b c
ARTICLE INFO
ABSTRACT
Keywords: Actinidia Fruit growth Development Maturity Softening Density
The ability to predict kiwifruit yield and size profile allows fine tuning of on-orchard vine management processes to maximise fruit growth rate. Fruit growth curves for kiwifruit are usually determined from repeated measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters, leading to models for estimating fruit weight. A reduction in estimated fruit weight has been reported for fruit late in development. In this paper, the relationship between fruit dimensions and fruit size (weight and/or volume) has been investigated for Actinidia chinensis var. deliciosa ‘Hayward’ and A. chinensis var. chinensis ‘Zesy002’ (marketed as Zespri® SunGold Kiwifruit; commonly called Gold3). It was hypothesised that a change in the relative dimensions of the fruit might occur as the fruit softened. It was found that the three linear dimensions of the fruit used in the estimation of fruit weight did decrease slightly as the fruit changed to rapid softening, with a larger change in ‘Zesy002’ than ‘Hayward’. A consistent gradual increase in fruit density occurred for both cultivars over the period fruit were monitored, even after the cessation of dry matter accumulation. It is suggested that the apparent late season ‘shrinkage’ of kiwifruit when monitoring fruit growth on the vine is largely an artefact of the application of a single model based on linear fruit dimensions at all stages of development. This approach disregards other changes in the fruit shape and softening associated changes that affect fruit density. Inclusion of a fruit age element into the equations estimating fruit size improves the accuracy, but without defining specifically the aspect of biology driving the changing relationship.
1. Introduction The profitability of a fruit industry is dependent on fruit yield and the fruit size profile. For kiwifruit, in general terms, larger fruit size tends to relate to better financial returns for the grower. The ability to predict fruit yield and size profile as soon as possible during fruit growth allows optimisation of on-orchard vine management processes to maximise the fruit growth rate. Accurate fruit size estimates also assist with planning for marketing the crop. Kiwifruit can be harvested at a range of maturities to meet specific market windows (Burdon, 2018). However, in so doing, the impact on fruit size must be considered, since harvesting early whilst the fruit are still growing may affect the final yield and thus the financial return for the grower. Hence in New Zealand a payment model exists to encourage early harvests so that growers are not penalised financially for providing fruit for early season supply (Currie et al., 1999). Numerous aspects of fruit growth associated with horticultural practices used to increase fruit size have been reported, including
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enhanced pollination (Ainalidou et al., 2015), spring vine girdling (Currie et al., 2018), crop load adjustment, biostimulants (Woolley and Cruz-Castillo, 2006), and in some countries direct use of plant growth regulators such as CPPU. These have been associated with fruit growth and the specific cell division and cell expansion phases of specific tissue zones (Hopping, 1976). A further component of yield is the amount of carbohydrate ending up in the fruit, which can affect final consumer acceptability of the fruit (Patterson and Currie, 2011). Consumer acceptability of Actinidia chinensis var. deliciosa ‘Hayward’ kiwifruit depends largely on the ripe fruit soluble solids content (rSSC; McMath et al., 1992; Crisosto and Crisosto, 2001; Burdon et al., 2004). To this end, girdling has become a common practice in New Zealand to increase the carbohydrate supply to the fruit (Patterson and Currie, 2011). Fruit growth curves for kiwifruit are usually determined from repeated measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters for a cohort of fruit that remain on the vine. This approach has led to models for estimating fruit weight from LD1D2
Corresponding author at: Institute of Agro-Products Processing Science and Technology, Sichuan Academy of Agricultural Sciences, Chengdu, 610066, China. E-mail address: [email protected] (H. Yuan).
https://doi.org/10.1016/j.scienta.2019.108878 Received 28 May 2019; Received in revised form 19 September 2019; Accepted 20 September 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved.
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measurements for ‘Hayward’ (Snelgar et al., 1992) and Actinidia chinensis var. chinensis ‘Hort16A’ fruit (Minchin et al., 2003). These data are often analysed looking for a future prediction of fruit size from measurements made as early as possible in the season (Hall et al., 1996; Minchin et al., 2003). However, it is also recognised that some aspects of fruit growth are dependent on the climate at the time, i.e. late season fruit growth (Woodward, 2009). While late season growth can add a significant amount to fruit size, it appears that fruit that are growing strongly when harvested may be more susceptible to chilling injury in storage (Burdon et al., 2014a, 2016). The observations on fruit growth in ‘Hort16A’ have suggested that fruit growth ceases and the fruit size (mass) can then decrease by up to 7% (Minchin et al., 2003). This has led to discussion about causal factors and possible water imbalance of fruit late in the growth period and loss of potential yield. Preliminary observations (Burdon unpublished data) showed that the amount of ‘shrinkage’ occurring was highly unlikely and was more likely to be an artefact of the fruit size estimation method. However, the suggestion of shrinkage raised the question as to what was occurring in the fruit such that the relationship between dimensions and fruit size had changed, and continued to change, the longer fruit remained on the vine. In this paper, the relationship between fruit dimension and fruit size (weight and/or volume) has been investigated for the two main commercial kiwifruit cultivars grown in New Zealand; ‘Hayward’ and A. chinensis var. chinensis ‘Zesy002’ (marketed as Zespri® SunGold Kiwifruit; commonly called Gold3). It was hypothesised that a change in the relative dimensions of the fruit might occur as the fruit softened. Fruit remaining attached to the vine were measured weekly for their three linear dimensions (LD1D2), from which an estimated fruit weight has been calculated. Also, every week a number of fruit were detached, had their dimensions measured, along with fruit weight and fruit volume, using the Archimedes principle. These fruit were then measured for firmness, both whole fruit and individual tissue zones, along with the commonly measured fruit compositional attributes used to monitor fruit development: seed colour, fruit SSC, dry matter and flesh colour.
pericarp, inner pericarp and core), seed coat colour, flesh colour, SSC and dry matter. 2.3. Fruit assessment methodology 2.3.1. On-vine estimation of fruit weight Fruit weight was estimated by calculation from calliper measurements in mm for L, D1 and D2. The equations for the estimation of fruit weight from the measurements were from Snelgar et al. (1992) for ‘Hayward’ fruit (fruit weight; FW = 0.454*(L*D1*D2)1.05, and from unpublished data from PFR for ‘Zesy002’ (FW = 0.515*(L*D1*D2)1.03). 2.3.2. Off-vine estimation of fruit weight and measurement of fruit volume and density Each fruit was measured with callipers for L, D1 and D2 and fruit weight estimated using the equations provided for on-vine fruit weight estimations. Fruit were then weighed and then weighed in water (Archimedes principle). The water displacement method gave the fruit volume and dividing the fruit weight (mass) by this value gave the fruit density (kg L−1). 2.3.3. Compression firmness measurement The whole fruit was compressed a distance of 2 mm by flat plate applied at 5 mm s−1. The force recorded after 2 mm displacement was taken as the firmness value. Firmness was measured as kg force and data converted to N, where 1 kgf = 9.81 N. 2.3.4. Standard penetrometer firmness measurement Fruit firmness was measured using a Fruit Texture Analyser (Güss, model GS14, South Africa) fitted with a 7.9-mm Effegi™ penetrometer probe after removal of skin and flesh to a depth of approximately 1 mm. The probe was driven into the flesh at 5 mm s−1 to a depth of 7.9 mm, and the maximum force recorded as the firmness value. Firmness was measured twice at the equator of each fruit, with the two measurements taken at 90° to each other. Firmness was measured as kg force and data converted to N, where 1 kgf = 9.81 N.
2. Materials and methods
2.3.5. Fruit core, outer pericarp and inner pericarp firmness measurement The fruit core firmness was measured after removal of approximately 15 mm of the fruit at the stem end. The measurement was made with a 4-mm diameter probe driven into the core at 5 mm s−1 to a depth of 6 mm, and the maximum force recorded as the firmness value. After the core measurement had been made, the inner and outer pericarp were measured at two positions, 90° to each other, using the same probe and setting as for the core. The internal structure of a kiwifruit is illustrated in Fig. 11B. Firmness was measured as kg force and data converted to N, where 1 kgf = 9.81 N.
2.1. Fruit The two cultivars of kiwifruit investigated in this study were ‘Hayward’, the most commonly grown green-fleshed cultivar globally, and the yellow-fleshed cultivar ‘Zesy002’. Fruit were harvested from five vines of each of the two cultivars grown adjacent to each other in a single orchard block at the New Zealand Institute for Plant and Food Research Limited (PFR) orchard in Te Puke, Bay of Plenty, New Zealand (37°49’S, 176°19’E). The Bay of Plenty region in New Zealand is characterised by good winter chilling, warm springs, and mild summers and autumns (Snelgar et al., 2010). The soil is deep, free draining and of volcanic origin. Rainfall averages 1600 mm per annum, distributed throughout the year.
2.3.6. Soluble solids content An average SSC was determined for individual fruit by measuring samples of juice from the stylar and stem ends of the fruit separately using a hand-held refractometer (Master Series, 0–30%, Atago) or, in riper fruit, a digital refractometer (0–50%, ‘pocket’ PAL-1, Atago), and the two values being averaged (Harman and Hopkirk, 1982).
2.2. Fruit assessments On- and off-vine measurements of the fruit of both cultivars were made at weekly intervals from the end of February. For each cultivar, 20 fruit were tagged at the start of the trial and measured weekly whilst on the vine for three linear measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters. For ‘Hayward’, all 20 fruit were retained for the full period of observation. For ‘Zesy002’, one fruit dropped in the middle of May and was eliminated from the dataset. In addition, each week, a 30 fruit sample was harvested and measured for the following: fruit weight, three linear dimensions, volume by water displacement, whole fruit compression firmness, whole fruit firmness (standard penetrometer measure), tissue zone firmness (outer
2.3.7. Dry matter The dry matter of fruit at harvest was determined by drying a 2-mm transverse slice from the middle of the fruit at 65 °C for approximately 24 h. 2.3.8. Seed coat colour Seed coat colour was scored by eye and is reported as the percentage of seeds that were dark or black. 2.3.9. Flesh colour Flesh colour (°hue) was measured using a Minolta CR300 Chroma 2
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Meter after removal of skin and flesh to a depth of approximately 2 mm. Flesh colour was measured twice at the equator of each fruit, with two measurements taken at 90° to each other.
fruit. The nett result on the estimation of fruit size for ‘Zesy002’ was a reduction in estimated fruit weight of up to ∼ 12.6%. The occurrence of > 10% loss in fruit size would be expected to show up in the appearance of the fruit, for example with shrivel. There was no obvious change in fruit appearance, such as shrivel, which would suggest shrinkage of the fruit had occurred whilst on the vine.
2.4. Data analysis Graphics and line fitting to data were created using Origin v8.5 (OriginLab Corporation, One Roundhouse Plaza, Northampton, MA01060, USA). The preliminary growth data on the vine were calculated from the existing models, but then the off-vine data for dimensions and fruit weights were used to re-determine the best fit models for both cultivars. A range of models were fitted relating FW to fruit dimensions (LDD = LxD1xD2), with and without an effect of fruit age (days after mid bloom), by nonlinear least-squares regression using SAS PROC NLIN v.9.4 of the SAS System for Windows (Copyright ©2008, SAS Institute Inc., Cary, NC). Models dependent on fruit dimensions only included linear (FW = m*LDD + c) or power functions (FW = m*(LDD)p), where multiplier m, and either constant c or power p were parameters to be fitted. The effect of fruit age was included by making the multiplier m in these equations a linear or power function of fruit age (days after mid bloom).
3.2. Off-vine measurement and estimation of fruit weight, fruit volume and density For both cultivars, there were strong linear associations between the measured fruit weight and the estimated weight from fruit dimensions (‘Hayward’ r = 0.997, R2 = 0.994; ‘Zesy002’ r = 0.973, R2 = 0.942) and also with the volume as determined by water displacement (‘Hayward’ r = 0.998, R2 = 0.996; ‘Zesy002’ r = 0.999, R2 = 0.998) (Fig. 2). The estimated fruit weight from dimensions was shown for both cultivars to under-estimate the fruit weight of the larger fruit later in the harvest season relative to the smaller fruit earlier in the season. This can be seen as the deviation from the y = x, or 1:1, line presented in Fig. 2, in which the slope of the linear relationship was < 1 for both cultivars (slopes: ‘Hayward’ 0.892; ‘Zesy002’ 0.813). Likewise, the relationship between measured fruit weight and volume diverged slightly from the 1:1 relationship, with a relatively lower volume for the heavier fruit compared with the lighter fruit (slopes: ‘Hayward’ 0.920; ‘Zesy002’ 0.883). The three linear fruit dimensions have usually been used to directly estimate fruit weight, although the use of dimensions alone would actually lead more directly to an estimate of fruit volume. There were strong relationships between the estimated (from same LDD equations used for weight) and measured volume for both cultivars (‘Hayward’: r = 0.998, R2 = 0.995; ‘Zesy002’: r = 0.981, R2 = 0.963; Fig. 3). The relationships were close to parallel to the 1:1 line for the whole range of fruit sizes measured (slopes: ‘Hayward’ 0.977; ‘Zesy002’ 0.928). However, while for ‘Hayward’ the data were close to the 1:1 relationship, the preliminary model used for fruit of ‘Zesy002’ seems to require a further off-set. Some of the difference from the 1:1 line may be the
3. Results 3.1. On-vine measurement of fruit dimensions and estimation of fruit weight The calliper measurements of the length and diameter dimensions of both cultivars of fruit showed consistent gradual change with time on the vine (Fig. 1). ‘Hayward’ fruit dimensions increased until late May, followed by a slight decline in all dimensions from the start of June. The decline in dimensions was associated with a decline in the estimated fruit weight of up to ∼ 3.2% by the end of June. A similar but more marked change occurred in the dimensions and estimated fruit weight of the ‘Zesy002’ fruit. The ‘Zesy002’ fruit increased in all dimensions until mid-April after which time there was a more marked decrease in all dimensions than seen in the ‘Hayward’
Fig. 1. Linear dimensions (A, C; length ( ), minimum (◼) and maximum ( ) diameters) and estimated fruit weight calculated from dimensions (B, D) of Actinidia chinensis var. deliciosa ‘Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during development on the vine. Each value is the mean ± s.e.m. of 20 fruit for ‘Hayward’ and 19 fruit for ‘Zesy002’ (note — repeated measurements on the same fruit).
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Fig. 2. Relationships between measured fruit weight and the estimated fruit weight from calliper measurements (A, C) and fruit volume measured by water displacement (B, D) for Actinidia chinensis var. deliciosa Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during development. Measurements made on fruit off the vine. Each value is the mean of 30 fruit.
result of the estimation models having been optimised for fruit weight rather than volume. Using a single model based on the three linear measurements of fruit size to provide an estimate of fruit weight assumes that the model accounts for changes in both fruit size and density that occur during fruit development. The ability of the model to account for both fruit growth and changing density has not been discussed previously. If the estimation of fruit size is considered as an estimation of fruit volume rather than fruit mass, then consideration of density should be added into the model. For detached fruit, there were consistent differences in the fruit weight and fruit volume measurements over time with fruit development (Fig. 4). The greater variability in week to week measurements compared with Fig. 1 reflects the fruit at each week being independent samples, although from the same vines, as compared with the same fruit being measured repeatedly on the vine in Fig. 1. The density of both ‘Hayward’ and ‘Zesy002’ fruit increased throughout the period of fruit development observed. ‘Hayward’ fruit increased in density from ∼ 1.017 kg L−1 to ∼ 1.042 kg L−1 over a period of ∼ 18 weeks. For ‘Zesy002’, the fruit increased in density from ∼ 1.045 kg L−1 to ∼ 1.066 kg L−1 over a 15 week period (Fig. 4).
While the changes in density during fruit maturation appear small, it is clear that when added to the estimate of fruit weight by simply multiplying by density, the relationship between actual and predicted fruit weight was improved (Fig. 5 compared with Fig. 2). In particular, the addition of density into the estimate removed the slight deviation from the 1:1 in the later harvested fruit (slopes: ‘Hayward’ 0.972; ‘Zesy002’ 0.929). Overall, whilst the ‘Hayward’ fit is good, it appears that the preliminary model for ‘Zesy002’ can be improved. 3.3. Improved models for estimation of fruit weight from three linear dimensions Given the obvious limitations of the preliminary model for estimation of fruit weight from fruit dimensions alone for ‘Zesy002’, the data have been re-evaluated for an improved model. This re-evaluation has included a fruit age element (days after mid-bloom; DAMB), which takes into consideration the changes associated with fruit density, softening and shape, without having to measure these characteristics. The best model for expressing ‘Zesy002’ FW in terms of fruit dimensions (LDD), with no dependence on fruit age, was Fig. 3. Relationships between fruit volume measured by water displacement and fruit volume estimated from calliper measurements for Actinidia chinensis var. deliciosa ‘Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during development. Measurements made on fruit off the vine. Each value is the mean of 30 fruit.
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Fig. 4. Fruit weight (◼) and volume ( ) (A, C) and the calculated density (B, D) of Actinidia chinensis var. deliciosa ‘Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during fruit development. Measurements made on fruit off the vine. Each value is the mean ± s.e.m. of 30 fruit.
Fig. 5. Relationship between fruit weight and the estimated fruit weight (EstFW) from three linear measurements of fruit size and including a component for fruit density for Actinidia chinensis var. deliciosa ‘Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during fruit development. Measurements made on fruit off the vine. Each value is the mean of 30 fruit.
FW = m x LDDp
(Fig. 6). Having improved the model for the estimation of ‘Zesy002’ fruit weight by incorporating a fruit age component, the off-vine data for ‘Hayward’ fruit were re-worked in a similar way. The best model for expressing ‘Hayward’ FW in terms of fruit dimensions (LDD), with no dependence on fruit age, was
with multiplier m = 0.467 and power p = 1.05 (error sum of squares (SSE)=6008, R2 = 0.938). If fruit age was included in the model, then the fitted value for the power p was not significantly different from one (P > 0.05). The model chosen in this case was therefore simply
FW = m x LDDp
FW = m x LDD
with multiplier m = 0.378 and power p = 1.09 (SSE=5071, R2 = 0.970). While the form of this equation is the same as that of Snelgar et al. (1992), the fitted parameter values are significantly different. If fruit age was included in the model then the power p in the equation above reduced to p = 1.05, with the best fit being with multiplier m dependent on fruit age (DAMB) according to
with the best fit being with multiplier m dependent on fruit age (DAMB) according to m = 0.536 + 0.000394 x DAMB (SSE = 3543, R2 = 0.963). With fruit age included as a predictor of fresh weight, the SSE was reduced by over 40%. For the range of dates for which data were collected here, the fruit age effect meant the multiplier ranged from m = 0.583 on 6 March (120 DAMB) to m = 0.627 on 26 June (232 DAMB). This had a large effect on the calculated fresh weights such that there was now little difference between actual weights and the estimated weights when age was included
m = 0.447 + 0.000112 x DAMB (SSE = 4500, R2 = 0.973). With fruit age included as a predictor of fresh weight, the SSE was reduced by about 11%. For the range of dates for which data were collected, the fruit age effect meant the multiplier 5
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Fig. 6. Revised estimation of fruit weight of Actinidia chinensis var. deliciosa ‘Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit from three linear dimensions (LDD) measured during fruit development and supplemented by fruit age. Each value is the mean of 30 fruit.
ranged from m = 0.583 on 6 March (92 DAMB) to m = 0.627 on 2 August (241 DAMB). Inclusion of fruit age had a much smaller effect on the estimate of fruit weight from dimensions for ‘Hayward’ than it did for ‘Zesy002’. Re-working the dimensions data from Fig. 1 with the new models including fruit age, it is possible to see that the inclusion of fruit age reduces the apparent shrinkage of the fruit late in development for both ‘Hayward’ and ‘Zesy002’ fruit (Fig. 7), although the reduction in estimated size is not eliminated completely.
3.4.4. Fruit firmness In ‘Hayward’, there was a slow decrease in fruit firmness until early June at which time firmness was ∼ 74.6 N and the softening rate increased slightly resulting in fruit at ∼ 46.1 N at the final assessment at the start of July (Fig. 8). In ‘Zesy002’, the slow softening phase lasted until mid-May when fruit were ∼ 65.7 N. Thereafter, softening was faster, with fruit being at 10 N firmness, or less, by late June. 3.4.5. Flesh colour There was little change in the flesh colour of the ‘Hayward’ fruit (Fig. 8). The flesh colour changed over a period of 15 weeks from ∼ 115°h in mid-March to ∼ 113°h at the start of July. In contrast with ‘Hayward’, the flesh colour of ‘Zesy002’ changed rapidly. There was a complete loss of green colour over a period of ∼ 9 weeks, starting in mid-March and degreening was complete (flesh colour < 103°h) by mid-May.
3.4. On-vine monitoring of fruit development 3.4.1. Seed colour The seed coats for both cultivars darkened over a period of 3–4 weeks (Fig. 8), between 21 March and 18 April for ‘Hayward’ and between 28 February and 21 March for ‘Zesy002’.
3.4.6. Fruit growth Fruit growth had ceased at the end of May for ‘Hayward’ and by mid-April for ‘Zesy002’ (Fig. 1). There then followed a marked decline in the estimated fruit size by early June for ‘Hayward’ and late-April for ‘Zesy002’.
3.4.2. Dry matter The accumulation of dry matter in ‘Hayward’ fruit slowed at the end of March, but continued to increase slowly until the end of May (Fig. 8). The accumulation of dry matter in ‘Zesy002’ fruit also slowed at the end of March, but with a slow increase only until mid-May.
3.5. Firmness assessments of fruit during development
3.4.3. Soluble solids content The SSC of ‘Hayward’ fruit was stable at 4–5% until late April, with an increase thereafter at increasing rate of accumulation until early June (Fig. 8). In ‘Zesy002’, there was no clear stable initial phase at the start of monitoring, but with a steady slow increase in SSC from ∼ 4% at the end of February to ∼ 6% in mid-April. From mid-April, the rate of accumulation increased and continued at this faster rate until late May when the accumulation rate slowed.
There were marked changes in the fruit softening parameters over the periods 23–30 May for ‘Hayward’ and 2–9 May for ‘Zesy002’ (Fig. 9). The change in ‘Hayward’ fruit was less distinct than in ‘Zesy002’. While there was a very distinct change from slow to fast softening with a decrease of 7.8 N over the period 2–9 May for ‘Zesy002’, in ‘Hayward’ there was possibly only a slower change in the softening rate over a couple of weeks. No explanation is offered for the sudden increase in compression firmness seen towards the end of April Fig. 7. Estimated fruit weight calculated from linear dimensions of length, minimum and maximum diameters with (◼) and without ( ) a component for fruit age for Actinidia chinensis var. deliciosa Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during development on the vine. Each value is the mean ± s.e.m. of 20 fruit for ‘Hayward’ and 19 fruit for ‘Zesy002’ (note — repeated measurements on the same fruit).
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The degree of shrinkage was associated with the degree of softening, being greater in ‘Zesy002’ fruit than ‘Hayward’ fruit. Overall, the rates of softening and ‘shrinkage’ were slower in ‘Hayward’ than in ‘Zesy002’ fruit. The association between a change in the fruit linear dimensions and softening appeared to be closer to the compression measurement of fruit firmness than penetrometer in ‘Hayward’, with a change in fruit compression being noticeable earlier than a change in penetrometer measurement for firmness. In ‘Zesy002’ fruit, the change to a reduction in linear dimensions occurred before any noticeable change in whole fruit softening, possibly more associated with the inner pericarp softening that commenced earlier. Also, in ‘Hayward’ fruit, there was a clearer change to rapid softening for the compression measurement compared with the penetrometer. Of the individual tissue zone measurements of firmness, the change in softening rate of the inner pericarp occurred earlier than other tissue zones, or measures of whole fruit firmness, and was in closer synchrony with the change to a reduction in fruit dimensions. 4. Discussion The on-vine monitoring of fruit growth from three linear measurements demonstrated the reduction in estimated fruit size described previously for ‘Hort16A’ fruit late in development (Minchin et al., 2003). The under-estimate of fruit size from linear dimensions recorded for ‘Zesy002’ fruit was considerably greater than for ‘Hayward’ fruit. The off-vine measurements of the fruit suggested a consistent gradual increase in fruit density over the whole period that fruit were monitored. The under-estimate of fruit size late in the season occurred at a time when fruit had started to soften rapidly, possibly contributing to the increase in fruit density irrespective of any other water or carbohydrate flows in or out of the fruit. The inclusion of a factor for fruit density, or simply fruit age, into the estimation of fruit weight from the three linear dimensions improved the accuracy of the estimate, in particular for the late season fruit. These findings are now discussed in more detail. There was a clear association between the timing of the period of reduction in fruit size and a change to rapid softening in the fruit. There were reductions in the dimensions of the fruit occurring, for each of the three dimensions; L, D1 and D2. The obvious suggestion that as the fruit softened the calliper measurement was able to deform the fruit was eliminated by a single experienced operator taking care not to compress the fruit when taking measurements. It had initially been thought that the reduction in the fruit dimensions may have been associated with some form of ‘relaxation’ within the fruit structure, resulting from a change in fruit texture that allowed some shape change in the fruit. One aspect of shape change could be associated with a ‘rounding up’ of the fruit and elimination of some of the angularity that may exist in the cross sectional shape of the unripe fruit. While kiwifruit are often regarded as being rounded in shape, when unripe, they can have significant angularity in their cross sectional shape (see Fig. 11), this is beyond the differences in minimum and maximum diameters that are associated with ‘flat’ fruit (Watson and Gould, 1993). Given that both minimum and maximum diameters were being measured, a loss from the maximum would be expected to result in an increase in the minimum. Likewise, any reduction in fruit length may be reflected in an increase in diameters. However, the current measurements do not fully describe the shape of the fruit and it is possible that the shape of the fruit may change once the fruit have started to soften, accounting for a slight reduction in the measured dimensions, but not in volume. One possibility is that the shape of the fruit “shoulders”, which is not taken into account if fruit weight is estimated from just L, D1 and D2, changes as fruit age and soften. In Section 3.3 it was found that the multiplier needed to calculate ‘Zesy002’ fruit weight from LDD increased by about 7.5% (0.583 to 0.627) between 6 March and 26 June.
Fig. 8. Soluble solids content (SSC, ◼), fruit firmness (FF, ), dry matter content (DM, ), incidence of black seeds (BS, ) and flesh colour (FC, ) changes in Actinidia chinensis var. deliciosa Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during fruit development on the vine. Each value is the mean ± s.e.m. of 30 fruit.
for both cultivars at the same time. The relative change in tissue zone softening differed between the two cultivars and is most easily seen in the normalised data in Fig. 9. In ‘Hayward’ fruit, the order in which the whole fruit or tissue zones changed from slow to fast softening was first observed in the inner and outer pericarp, then in the core and by compression, and finally by standard penetrometer. The initial changes in both inner and outer pericarp occurred 1–2 weeks before being detected by whole fruit compression test. The change in the core softening rate coincided with the overall change measured by compression test. The current standard method of assessing fruit firmness by penetrometer appears to be the last method by which a change in softening rate can be detected, with the change detected 1–2 weeks later than by compression test. In ‘Zesy002’ fruit, the inner pericarp changed softening rate first, about 1–2 weeks before the outer pericarp or by compression or standard penetrometer, which registered a change at about the same time. The core was the last tissue zone to show a changed softening rate, occurring 1–2 weeks after the change detected by compression and penetrometer. 3.6. Associations between the fruit development and size estimation The major aspect of fruit development measured that coincided with the ‘shrinkage’ of the fruit determined from on-vine calliper measurement for both cultivars was fruit softening (Fig. 10). 7
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Fig. 9. Compression firmness (◼), fruit firmness ( ), core firmness ( ), outer pericarp firmness ( ) and inner pericarp firmness ( ) changes in Actinidia chinensis var. deliciosa Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during fruit development on the vine. Values presented as raw data (A, C) or normalised where the harvest value = 100 (B, D). Each value is the mean ± s.e.m. of 30 fruit.
Given that fruit density increased by approximately 2% (1.045 to 1.066) over the same period, the remaining 5.5% of the change in multiplier must have been caused by changes in fruit shape. Approximating the shape of the fruit at any time by a superellipsoid, it was calculated how much the shoulder size would need to change to get the required 5.5% change in fruit volume without changing L, D1, or D2. A superellipsoid describes ‘Zesy002’ kiwifruit shape well, and the change in shoulder shape needed to change the relationship between L*D1*D2 by 5.5% is relatively small (Fig. 12). Currently there is no further explanation for the softening associated reduction in dimensions that are measured whilst monitoring fruit growth on the vine. Future approaches could include taking direct measurements of fruit volume in the orchard and making more detailed descriptions of fruit shape changes throughout development. Fruit growth is often referred to in terms of nett material changes
due to xylem and phloem inflow relative to losses from transpiration (Hall et al., 2013). It has been shown in ‘Hort16A’ kiwifruit that the fruit may deform whilst attached to the vine where a vine is unable to maintain water supply to the fruit to match transpiration losses in a high water vapour pressure deficit environment (Thorp et al., 2007; Clearwater et al., 2012). An alternative circumstance is that if the inflow to the fruit is disrupted, irrespective of the environment, it may be possible for there to be a lack of water to maintain fruit size or growth. These, and other, factors associated with fruit growth in kiwifruit have been discussed recently in the context of a biophysical model of fruit development (Hall et al., 2013). A further consideration is the change in fruit texture, which was identified by a compression measurement earlier than by the standard penetrometer. Similarly, a difference in compression and penetrometer data was found recently in describing kiwifruit texture following Fig. 10. Relationship between estimated fruit weight ( ) and fruit firmness measured by penetrometer (◼) and whole fruit compression ( ) in Actinidia chinensis var. deliciosa Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during fruit development. Fruit weight estimates from the new models is described in Section 3.3 and Fig. 7. Each value is the mean ± s.e.m. of 20 fruit (weight) or 30 fruit (firmness).
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Fig. 11. Actinidia chinensis var. deliciosa ‘Hayward’ kiwifruit showing aspects of angularity in cross sectional shape when unripe: A) whole fruit, B) cross section at the fruit equator, C) a ‘flat’ fruit (based on the minimum: maximum diameter ratio).
Fig. 12. A difference in longitudinal cross-sectional shape equivalent to a 5.5% change in volume (A) and overlaid on a Actinidia chinensis var. chinensis ‘Zesy002’ kiwifruit (B). A similar side-on view of a Actinidia chinensis var. deliciosa ‘Hayward’ fruit (C).
controlled atmosphere storage which was associated with the relative softening of the inner and outer pericarp tissues (Li et al., 2017). In the ‘Zesy002’ fruit, for which the greatest change in dimensions was recorded as the fruit softened rapidly, the compression measurement showed a change in softening rate about one week before being detected by the penetrometer. In addition, the degree of reduction in fruit dimensions appears linked to the degree of softening. In ‘Zesy002’ a 12.6% reduction in estimated fruit size from the maximum was associated with softening to ≤ 10 N, whereas in ‘Hayward’ a 3.2% reduction in estimated fruit size was associated with softening to 46 N. While the association between decreasing dimension measurements and an increased degree of fruit softening is clear, it may not be the softening per se that is important, rather the fact that the change in tissue zone textures make it possible for the relative dimensions of the fruit to change as the fruit soften. The investigation into the relationship between fruit dimensions and actual measurements of fruit weight and volume revealed a systematic inaccuracy in the application of a single model for fruit weight estimation to all stages of fruit development. The method was underestimating the fruit weight in the later harvested, larger, fruit. The fruit density continued to increase slowly even after the fruit had stopped accumulating carbohydrate (dry matter) and whilst the fruit was softening. This continued increase in density can be accounted for by changes in the fruit other than simple loss of water. There are two aspects to fruit density that appear relevant: the inflow of carbohydrates, which tends to cease with the termination of growth (Burdon et al., 2014a, 2016), and, thereafter, the change to rapid softening. Unlike apples, which may contain up to 25% air spaces between the cells (Reeve, 1953; Hall et al., 2016), kiwifruit contain lesser amounts of air,
and this air content decreases as the fruit soften (Harker and Hallett, 1994). This reduction in intercellular air space in kiwifruit may be associated with the increased cell wall swelling that occurs as the fruit soften (Redgwell et al., 1997; Burdon et al., 2014b). The cell wall swelling, and increased water holding capacity, may cause a reduction in the inter-cellular air space within the kiwifruit, and thereby increase the density of the fruit. In ‘Hayward’ fruit, the outer pericarp cell walls have been reported to increase in thickness by 3–4 fold during ripening (Hallett et al., 1992). Also, during ripening there is a progression in the development of the inner pericarp, which contains large thin walled cells that develop into a mucilaginous matrix with little intercellular air space. Adding fruit density as a factor into the estimation of fruit weight from the three linear dimensions improved the fruit size estimation in the later, larger, fruit. Consequently, there are several aspects of fruit development that are associated with the changing relationship between fruit linear dimensions and the fruit weight. To actually measure these and incorporate them into a model predicting fruit weight would be time consuming and defeat the goal of a rapid, non-destructive method for estimating fruit growth. Therefore, the incorporation of fruit age from flowering into the prediction equations for each cultivar provides for improved accuracy by adding an element associated with these late season aspects of fruit development, but without actually measuring them. In conclusion, while the three linear dimensions of the fruit used in the estimation of fruit weight may decrease slightly and be associated with fruit softening, it is difficult to envisage a decrease in the fresh weight of the fruit by over 10% as the fruit ripen. Instead, it is more likely that there is a subtle change in the shape of the fruit affecting 9
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those dimensions measured, yet not resulting in a major decrease in the fruit volume. It is suggested that the late season ‘shrinkage’ of kiwifruit when monitoring fruit growth on the vine is largely an artefact of the application of a single model based on linear fruit dimensions at all stages of development. This approach disregards other changes in the fruit shape and softening associated changes that affect fruit density. Inclusion of a fruit age element into the equations estimating fruit size improves the accuracy, but without defining specifically the aspect of biology driving the changing relationship.
Crisosto, C.H., Crisosto, G.M., 2001. Understanding consumer acceptance of early harvested’ Hayward’ kiwifruit. Postharvest Biol. Technol. 22, 205–213. Currie, M.B., Patterson, K.J., Snelgar, W.P., Blattmann, P., 2018. Girdling kiwifruit vines for commercial advantage: opportunbities and risks. Acta Hortic. 1218, 405–412. Currie, M., Richardson, A., Snelgar, B., 1999. Financial importance of late season fruit growth. New Zealand Kiwifruit J. (Mar/Apr), 10–11. Hall, A.J., McPherson, H.G., Crawford, R.A., Seager, N.G., 1996. Using early-season measurements to estimate fruit volume at harvest in kiwifruit. New Zealand J. Crop Hortic. Sci. 24, 379–391. Hall, A.J., Minchin, P.E.H., Clearwater, M.J., Genard, M., 2013. A biophysical model of kiwifruit (Actinidia deliciosa) berry development. J. Exp. Bot. 64 (18), 5473–5483. Hall, M.I., Hallett, I.C., Johnston, J.W., 2016. X-ray micro-computed tomography analysis of apple texture and structure. Acta Hortic. 1120, 535–538. Hallett, I.C., MacRae, E.A., Wegrzyn, T.F., 1992. Changes in kiwifruit cell wall ultrastructure and cell packing during postharvest ripening. Int. J. Plant Sci. 153, 49–60. Harker, F.J., Hallett, I.C., 1994. Physiological and mechanical properties of kiwifruit tissue associated with texture change during cool storage. J. Am. Soc. Hortic. Sci. 119 (5), 987–993. Harman, J.E., Hopkirk, G., 1982. Kiwifruit Testing for Maturity: Use of Index. AgLink Publication HPP213. Ministry of Agriculture and Fisheries, Wellington, New Zealand. Hopping, M.E., 1976. Structure and development of fruit and seeds in Chinese gooseberry (Actinidia chinensis Planch.). New Zealand J. Bot. 14, 63–68. Li, H., Billing, D., Pidakala, P., Burdon, J., 2017. Textural changes in ‘Hayward’ kiwifruit during and after storage in controlled atmospheres. Sci. Hortic. 222, 40–45. McMath, K.L., Paterson, V.J., Young, H., Ball, R.D., MacRae, E.A., 1992. Factors affecting the sensory perception of sweetness and acidity in kiwifruit. Acta Hortic. 297, 489–500. Minchin, P.E.H., Richardson, A.C., Patterson, K.J., Martin, P.J., 2003. Prediction of final weight for Actinidia chinensis ‘Hort16A’ fruit. New Zealand J. Crop Hortic. Sci. 31, 147–157. Patterson, K.J., Currie, M.B., 2011. Optimising kiwifruit vine performance for high productivity and superior fruit taste. Acta Hortic. 913, 257–268. Redgwell, R.J., MacRae, E., Hallett, I., Fischer, M., Perry, J., Harker, R., 1997. In vivo and in vitro swelling of cell walls during fruit ripening. Planta 203, 162–173. Reeve, R.M., 1953. Histological investigation of texture in apples. II Structure and intercellular spaces. J. Food Sci. 18, 604–617. Snelgar, W.P., Manson, P.J., Martin, P.J., 1992. Influence of time of shading on flowering and yield of kiwifruit vines. J. Hortic. Sci. 67, 481–487. Snelgar, B., Thorp, G., Spinelli, R., Brun, S., 2010. We don’t know how lucky we are? New Zealand Kiwifruit J. (July/Aug), 14–18. Thorp, T.G., Clearwater, M.J., Barnett, A.M., Martin, P.J., Blattmann, P.J., Currie, M.B., 2007. ’Hort16A’ fruit beak end softening and shrivelling in California. Acta Hortic. 753, 389–396. Watson, M., Gould, K.S., 1993. The development of fruit shape in kiwifruit: growth characteristics and positional differences. J. Hortic. Sci. 68, 185–194. Woodward, T., 2009. Making an informed harvest decision. New Zealand Kiwifruit J. (Mar/Apr), 26–31. Woolley, D., Cruz-Castillo, J.G., 2006. Stimulation of fruit growth of green and gold kiwifruit. Acta Hortic. 727, 291–293.
Acknowledgements The authors gratefully acknowledge: Dr Yongqing Zhu from Institute of Agro-Products Processing Science and Technology, Sichuan Academy of Agricultural Sciences, and the New Zealand-China Joint Kiwifruit Laboratory for financial support to Huaiyu Yuan. This work was undertaken as part of the New Zealand-China Joint Kiwifruit Laboratory under the Science and Technology Support Program of Sichuan Province (2018NZ0010); the finance department of Sichuan Province Innovation Support Project (2016TSCY-009) and the New Zealand Institute for Plant and Food Research Limited Strategic Science Investment Fund ‘Premium Kiwifruit’ research programme. References Ainalidou, A., Karamanoli, K., Menkissoglu-Spioudi, U., Diamantidis, G., Matsi, T., 2015. CPPU treatment and pollination: their combined effect on kiwifruit growth and quality. Sci. Hortic. 193, 147–154. Burdon, J., 2018. Kiwifruit biology: the commercial implications of fruit maturation. Hortic. Rev. 46, 385–421. Burdon, J., McLeod, D., Lallu, N., Gamble, J., Petley, M., Gunson, A., 2004. Consumer evaluation of ‘Hayward’ kiwifruit of different at-harvest dry matter contents. Postharvest Biol. Technol. 34, 245–255. Burdon, J., Pidakala, P., Martin, P., McAtee, P.A., Boldingh, H.L., Hall, A., Schaffer, R.J., 2014a. Postharvest performance of the yellow-fleshed’ Hort16A’ kiwifruit in relation to fruit maturation. Postharvest Biol. Technol. 92, 98–106. Burdon, J., Punter, M., Billing, D., Pidakala, P., Kerr, K., 2014b. Shrivel development in kiwifruit. Postharvest Biol. Technol. 87, 1–5. Burdon, J., Pidakala, P., Martin, P., Billing, D., Boldingh, H., 2016. Fruit maturation and the soluble solids harvest index for ‘Hayward’ kiwifruit. Sci. Hortic. 213, 193–198. Clearwater, M.J., Luo, Z., Ong, S.E.C., Blattmann, P., Thorp, T.G., 2012. Vascular functioning and the water balance of ripening kiwifruit (Actinidia chinensis) berries. J. Exp. Bot. 63 (5), 1835–1847.
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Refining the estimation of kiwifruit size from linear fruit dimensions a,b,
H. Yuan
c
b
c
d
*, P. Martin , C. Fullerton , N. Gould , A. Hall , J. Burdon
b
T
a
Institute of Agro-Products Processing Science and Technology, Sichuan Academy of Agricultural Sciences, Chengdu, 610066, China The New Zealand Institute for Plant and Food Research Limited, Private Bag 92169, Auckland Mail Centre, 1142, Auckland, New Zealand The New Zealand Institute for Plant and Food Research Limited, 412 No 1 Road, RD 2, Te Puke, 3182, New Zealand d The New Zealand Institute for Plant and Food Research Limited, Private Bag 11600, Palmerston North, 4442, New Zealand b c
ARTICLE INFO
ABSTRACT
Keywords: Actinidia Fruit growth Development Maturity Softening Density
The ability to predict kiwifruit yield and size profile allows fine tuning of on-orchard vine management processes to maximise fruit growth rate. Fruit growth curves for kiwifruit are usually determined from repeated measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters, leading to models for estimating fruit weight. A reduction in estimated fruit weight has been reported for fruit late in development. In this paper, the relationship between fruit dimensions and fruit size (weight and/or volume) has been investigated for Actinidia chinensis var. deliciosa ‘Hayward’ and A. chinensis var. chinensis ‘Zesy002’ (marketed as Zespri® SunGold Kiwifruit; commonly called Gold3). It was hypothesised that a change in the relative dimensions of the fruit might occur as the fruit softened. It was found that the three linear dimensions of the fruit used in the estimation of fruit weight did decrease slightly as the fruit changed to rapid softening, with a larger change in ‘Zesy002’ than ‘Hayward’. A consistent gradual increase in fruit density occurred for both cultivars over the period fruit were monitored, even after the cessation of dry matter accumulation. It is suggested that the apparent late season ‘shrinkage’ of kiwifruit when monitoring fruit growth on the vine is largely an artefact of the application of a single model based on linear fruit dimensions at all stages of development. This approach disregards other changes in the fruit shape and softening associated changes that affect fruit density. Inclusion of a fruit age element into the equations estimating fruit size improves the accuracy, but without defining specifically the aspect of biology driving the changing relationship.
1. Introduction The profitability of a fruit industry is dependent on fruit yield and the fruit size profile. For kiwifruit, in general terms, larger fruit size tends to relate to better financial returns for the grower. The ability to predict fruit yield and size profile as soon as possible during fruit growth allows optimisation of on-orchard vine management processes to maximise the fruit growth rate. Accurate fruit size estimates also assist with planning for marketing the crop. Kiwifruit can be harvested at a range of maturities to meet specific market windows (Burdon, 2018). However, in so doing, the impact on fruit size must be considered, since harvesting early whilst the fruit are still growing may affect the final yield and thus the financial return for the grower. Hence in New Zealand a payment model exists to encourage early harvests so that growers are not penalised financially for providing fruit for early season supply (Currie et al., 1999). Numerous aspects of fruit growth associated with horticultural practices used to increase fruit size have been reported, including
⁎
enhanced pollination (Ainalidou et al., 2015), spring vine girdling (Currie et al., 2018), crop load adjustment, biostimulants (Woolley and Cruz-Castillo, 2006), and in some countries direct use of plant growth regulators such as CPPU. These have been associated with fruit growth and the specific cell division and cell expansion phases of specific tissue zones (Hopping, 1976). A further component of yield is the amount of carbohydrate ending up in the fruit, which can affect final consumer acceptability of the fruit (Patterson and Currie, 2011). Consumer acceptability of Actinidia chinensis var. deliciosa ‘Hayward’ kiwifruit depends largely on the ripe fruit soluble solids content (rSSC; McMath et al., 1992; Crisosto and Crisosto, 2001; Burdon et al., 2004). To this end, girdling has become a common practice in New Zealand to increase the carbohydrate supply to the fruit (Patterson and Currie, 2011). Fruit growth curves for kiwifruit are usually determined from repeated measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters for a cohort of fruit that remain on the vine. This approach has led to models for estimating fruit weight from LD1D2
Corresponding author at: Institute of Agro-Products Processing Science and Technology, Sichuan Academy of Agricultural Sciences, Chengdu, 610066, China. E-mail address: [email protected] (H. Yuan).
https://doi.org/10.1016/j.scienta.2019.108878 Received 28 May 2019; Received in revised form 19 September 2019; Accepted 20 September 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved.
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measurements for ‘Hayward’ (Snelgar et al., 1992) and Actinidia chinensis var. chinensis ‘Hort16A’ fruit (Minchin et al., 2003). These data are often analysed looking for a future prediction of fruit size from measurements made as early as possible in the season (Hall et al., 1996; Minchin et al., 2003). However, it is also recognised that some aspects of fruit growth are dependent on the climate at the time, i.e. late season fruit growth (Woodward, 2009). While late season growth can add a significant amount to fruit size, it appears that fruit that are growing strongly when harvested may be more susceptible to chilling injury in storage (Burdon et al., 2014a, 2016). The observations on fruit growth in ‘Hort16A’ have suggested that fruit growth ceases and the fruit size (mass) can then decrease by up to 7% (Minchin et al., 2003). This has led to discussion about causal factors and possible water imbalance of fruit late in the growth period and loss of potential yield. Preliminary observations (Burdon unpublished data) showed that the amount of ‘shrinkage’ occurring was highly unlikely and was more likely to be an artefact of the fruit size estimation method. However, the suggestion of shrinkage raised the question as to what was occurring in the fruit such that the relationship between dimensions and fruit size had changed, and continued to change, the longer fruit remained on the vine. In this paper, the relationship between fruit dimension and fruit size (weight and/or volume) has been investigated for the two main commercial kiwifruit cultivars grown in New Zealand; ‘Hayward’ and A. chinensis var. chinensis ‘Zesy002’ (marketed as Zespri® SunGold Kiwifruit; commonly called Gold3). It was hypothesised that a change in the relative dimensions of the fruit might occur as the fruit softened. Fruit remaining attached to the vine were measured weekly for their three linear dimensions (LD1D2), from which an estimated fruit weight has been calculated. Also, every week a number of fruit were detached, had their dimensions measured, along with fruit weight and fruit volume, using the Archimedes principle. These fruit were then measured for firmness, both whole fruit and individual tissue zones, along with the commonly measured fruit compositional attributes used to monitor fruit development: seed colour, fruit SSC, dry matter and flesh colour.
pericarp, inner pericarp and core), seed coat colour, flesh colour, SSC and dry matter. 2.3. Fruit assessment methodology 2.3.1. On-vine estimation of fruit weight Fruit weight was estimated by calculation from calliper measurements in mm for L, D1 and D2. The equations for the estimation of fruit weight from the measurements were from Snelgar et al. (1992) for ‘Hayward’ fruit (fruit weight; FW = 0.454*(L*D1*D2)1.05, and from unpublished data from PFR for ‘Zesy002’ (FW = 0.515*(L*D1*D2)1.03). 2.3.2. Off-vine estimation of fruit weight and measurement of fruit volume and density Each fruit was measured with callipers for L, D1 and D2 and fruit weight estimated using the equations provided for on-vine fruit weight estimations. Fruit were then weighed and then weighed in water (Archimedes principle). The water displacement method gave the fruit volume and dividing the fruit weight (mass) by this value gave the fruit density (kg L−1). 2.3.3. Compression firmness measurement The whole fruit was compressed a distance of 2 mm by flat plate applied at 5 mm s−1. The force recorded after 2 mm displacement was taken as the firmness value. Firmness was measured as kg force and data converted to N, where 1 kgf = 9.81 N. 2.3.4. Standard penetrometer firmness measurement Fruit firmness was measured using a Fruit Texture Analyser (Güss, model GS14, South Africa) fitted with a 7.9-mm Effegi™ penetrometer probe after removal of skin and flesh to a depth of approximately 1 mm. The probe was driven into the flesh at 5 mm s−1 to a depth of 7.9 mm, and the maximum force recorded as the firmness value. Firmness was measured twice at the equator of each fruit, with the two measurements taken at 90° to each other. Firmness was measured as kg force and data converted to N, where 1 kgf = 9.81 N.
2. Materials and methods
2.3.5. Fruit core, outer pericarp and inner pericarp firmness measurement The fruit core firmness was measured after removal of approximately 15 mm of the fruit at the stem end. The measurement was made with a 4-mm diameter probe driven into the core at 5 mm s−1 to a depth of 6 mm, and the maximum force recorded as the firmness value. After the core measurement had been made, the inner and outer pericarp were measured at two positions, 90° to each other, using the same probe and setting as for the core. The internal structure of a kiwifruit is illustrated in Fig. 11B. Firmness was measured as kg force and data converted to N, where 1 kgf = 9.81 N.
2.1. Fruit The two cultivars of kiwifruit investigated in this study were ‘Hayward’, the most commonly grown green-fleshed cultivar globally, and the yellow-fleshed cultivar ‘Zesy002’. Fruit were harvested from five vines of each of the two cultivars grown adjacent to each other in a single orchard block at the New Zealand Institute for Plant and Food Research Limited (PFR) orchard in Te Puke, Bay of Plenty, New Zealand (37°49’S, 176°19’E). The Bay of Plenty region in New Zealand is characterised by good winter chilling, warm springs, and mild summers and autumns (Snelgar et al., 2010). The soil is deep, free draining and of volcanic origin. Rainfall averages 1600 mm per annum, distributed throughout the year.
2.3.6. Soluble solids content An average SSC was determined for individual fruit by measuring samples of juice from the stylar and stem ends of the fruit separately using a hand-held refractometer (Master Series, 0–30%, Atago) or, in riper fruit, a digital refractometer (0–50%, ‘pocket’ PAL-1, Atago), and the two values being averaged (Harman and Hopkirk, 1982).
2.2. Fruit assessments On- and off-vine measurements of the fruit of both cultivars were made at weekly intervals from the end of February. For each cultivar, 20 fruit were tagged at the start of the trial and measured weekly whilst on the vine for three linear measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters. For ‘Hayward’, all 20 fruit were retained for the full period of observation. For ‘Zesy002’, one fruit dropped in the middle of May and was eliminated from the dataset. In addition, each week, a 30 fruit sample was harvested and measured for the following: fruit weight, three linear dimensions, volume by water displacement, whole fruit compression firmness, whole fruit firmness (standard penetrometer measure), tissue zone firmness (outer
2.3.7. Dry matter The dry matter of fruit at harvest was determined by drying a 2-mm transverse slice from the middle of the fruit at 65 °C for approximately 24 h. 2.3.8. Seed coat colour Seed coat colour was scored by eye and is reported as the percentage of seeds that were dark or black. 2.3.9. Flesh colour Flesh colour (°hue) was measured using a Minolta CR300 Chroma 2
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Meter after removal of skin and flesh to a depth of approximately 2 mm. Flesh colour was measured twice at the equator of each fruit, with two measurements taken at 90° to each other.
fruit. The nett result on the estimation of fruit size for ‘Zesy002’ was a reduction in estimated fruit weight of up to ∼ 12.6%. The occurrence of > 10% loss in fruit size would be expected to show up in the appearance of the fruit, for example with shrivel. There was no obvious change in fruit appearance, such as shrivel, which would suggest shrinkage of the fruit had occurred whilst on the vine.
2.4. Data analysis Graphics and line fitting to data were created using Origin v8.5 (OriginLab Corporation, One Roundhouse Plaza, Northampton, MA01060, USA). The preliminary growth data on the vine were calculated from the existing models, but then the off-vine data for dimensions and fruit weights were used to re-determine the best fit models for both cultivars. A range of models were fitted relating FW to fruit dimensions (LDD = LxD1xD2), with and without an effect of fruit age (days after mid bloom), by nonlinear least-squares regression using SAS PROC NLIN v.9.4 of the SAS System for Windows (Copyright ©2008, SAS Institute Inc., Cary, NC). Models dependent on fruit dimensions only included linear (FW = m*LDD + c) or power functions (FW = m*(LDD)p), where multiplier m, and either constant c or power p were parameters to be fitted. The effect of fruit age was included by making the multiplier m in these equations a linear or power function of fruit age (days after mid bloom).
3.2. Off-vine measurement and estimation of fruit weight, fruit volume and density For both cultivars, there were strong linear associations between the measured fruit weight and the estimated weight from fruit dimensions (‘Hayward’ r = 0.997, R2 = 0.994; ‘Zesy002’ r = 0.973, R2 = 0.942) and also with the volume as determined by water displacement (‘Hayward’ r = 0.998, R2 = 0.996; ‘Zesy002’ r = 0.999, R2 = 0.998) (Fig. 2). The estimated fruit weight from dimensions was shown for both cultivars to under-estimate the fruit weight of the larger fruit later in the harvest season relative to the smaller fruit earlier in the season. This can be seen as the deviation from the y = x, or 1:1, line presented in Fig. 2, in which the slope of the linear relationship was < 1 for both cultivars (slopes: ‘Hayward’ 0.892; ‘Zesy002’ 0.813). Likewise, the relationship between measured fruit weight and volume diverged slightly from the 1:1 relationship, with a relatively lower volume for the heavier fruit compared with the lighter fruit (slopes: ‘Hayward’ 0.920; ‘Zesy002’ 0.883). The three linear fruit dimensions have usually been used to directly estimate fruit weight, although the use of dimensions alone would actually lead more directly to an estimate of fruit volume. There were strong relationships between the estimated (from same LDD equations used for weight) and measured volume for both cultivars (‘Hayward’: r = 0.998, R2 = 0.995; ‘Zesy002’: r = 0.981, R2 = 0.963; Fig. 3). The relationships were close to parallel to the 1:1 line for the whole range of fruit sizes measured (slopes: ‘Hayward’ 0.977; ‘Zesy002’ 0.928). However, while for ‘Hayward’ the data were close to the 1:1 relationship, the preliminary model used for fruit of ‘Zesy002’ seems to require a further off-set. Some of the difference from the 1:1 line may be the
3. Results 3.1. On-vine measurement of fruit dimensions and estimation of fruit weight The calliper measurements of the length and diameter dimensions of both cultivars of fruit showed consistent gradual change with time on the vine (Fig. 1). ‘Hayward’ fruit dimensions increased until late May, followed by a slight decline in all dimensions from the start of June. The decline in dimensions was associated with a decline in the estimated fruit weight of up to ∼ 3.2% by the end of June. A similar but more marked change occurred in the dimensions and estimated fruit weight of the ‘Zesy002’ fruit. The ‘Zesy002’ fruit increased in all dimensions until mid-April after which time there was a more marked decrease in all dimensions than seen in the ‘Hayward’
Fig. 1. Linear dimensions (A, C; length ( ), minimum (◼) and maximum ( ) diameters) and estimated fruit weight calculated from dimensions (B, D) of Actinidia chinensis var. deliciosa ‘Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during development on the vine. Each value is the mean ± s.e.m. of 20 fruit for ‘Hayward’ and 19 fruit for ‘Zesy002’ (note — repeated measurements on the same fruit).
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Fig. 2. Relationships between measured fruit weight and the estimated fruit weight from calliper measurements (A, C) and fruit volume measured by water displacement (B, D) for Actinidia chinensis var. deliciosa Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during development. Measurements made on fruit off the vine. Each value is the mean of 30 fruit.
result of the estimation models having been optimised for fruit weight rather than volume. Using a single model based on the three linear measurements of fruit size to provide an estimate of fruit weight assumes that the model accounts for changes in both fruit size and density that occur during fruit development. The ability of the model to account for both fruit growth and changing density has not been discussed previously. If the estimation of fruit size is considered as an estimation of fruit volume rather than fruit mass, then consideration of density should be added into the model. For detached fruit, there were consistent differences in the fruit weight and fruit volume measurements over time with fruit development (Fig. 4). The greater variability in week to week measurements compared with Fig. 1 reflects the fruit at each week being independent samples, although from the same vines, as compared with the same fruit being measured repeatedly on the vine in Fig. 1. The density of both ‘Hayward’ and ‘Zesy002’ fruit increased throughout the period of fruit development observed. ‘Hayward’ fruit increased in density from ∼ 1.017 kg L−1 to ∼ 1.042 kg L−1 over a period of ∼ 18 weeks. For ‘Zesy002’, the fruit increased in density from ∼ 1.045 kg L−1 to ∼ 1.066 kg L−1 over a 15 week period (Fig. 4).
While the changes in density during fruit maturation appear small, it is clear that when added to the estimate of fruit weight by simply multiplying by density, the relationship between actual and predicted fruit weight was improved (Fig. 5 compared with Fig. 2). In particular, the addition of density into the estimate removed the slight deviation from the 1:1 in the later harvested fruit (slopes: ‘Hayward’ 0.972; ‘Zesy002’ 0.929). Overall, whilst the ‘Hayward’ fit is good, it appears that the preliminary model for ‘Zesy002’ can be improved. 3.3. Improved models for estimation of fruit weight from three linear dimensions Given the obvious limitations of the preliminary model for estimation of fruit weight from fruit dimensions alone for ‘Zesy002’, the data have been re-evaluated for an improved model. This re-evaluation has included a fruit age element (days after mid-bloom; DAMB), which takes into consideration the changes associated with fruit density, softening and shape, without having to measure these characteristics. The best model for expressing ‘Zesy002’ FW in terms of fruit dimensions (LDD), with no dependence on fruit age, was Fig. 3. Relationships between fruit volume measured by water displacement and fruit volume estimated from calliper measurements for Actinidia chinensis var. deliciosa ‘Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during development. Measurements made on fruit off the vine. Each value is the mean of 30 fruit.
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Fig. 4. Fruit weight (◼) and volume ( ) (A, C) and the calculated density (B, D) of Actinidia chinensis var. deliciosa ‘Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during fruit development. Measurements made on fruit off the vine. Each value is the mean ± s.e.m. of 30 fruit.
Fig. 5. Relationship between fruit weight and the estimated fruit weight (EstFW) from three linear measurements of fruit size and including a component for fruit density for Actinidia chinensis var. deliciosa ‘Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during fruit development. Measurements made on fruit off the vine. Each value is the mean of 30 fruit.
FW = m x LDDp
(Fig. 6). Having improved the model for the estimation of ‘Zesy002’ fruit weight by incorporating a fruit age component, the off-vine data for ‘Hayward’ fruit were re-worked in a similar way. The best model for expressing ‘Hayward’ FW in terms of fruit dimensions (LDD), with no dependence on fruit age, was
with multiplier m = 0.467 and power p = 1.05 (error sum of squares (SSE)=6008, R2 = 0.938). If fruit age was included in the model, then the fitted value for the power p was not significantly different from one (P > 0.05). The model chosen in this case was therefore simply
FW = m x LDDp
FW = m x LDD
with multiplier m = 0.378 and power p = 1.09 (SSE=5071, R2 = 0.970). While the form of this equation is the same as that of Snelgar et al. (1992), the fitted parameter values are significantly different. If fruit age was included in the model then the power p in the equation above reduced to p = 1.05, with the best fit being with multiplier m dependent on fruit age (DAMB) according to
with the best fit being with multiplier m dependent on fruit age (DAMB) according to m = 0.536 + 0.000394 x DAMB (SSE = 3543, R2 = 0.963). With fruit age included as a predictor of fresh weight, the SSE was reduced by over 40%. For the range of dates for which data were collected here, the fruit age effect meant the multiplier ranged from m = 0.583 on 6 March (120 DAMB) to m = 0.627 on 26 June (232 DAMB). This had a large effect on the calculated fresh weights such that there was now little difference between actual weights and the estimated weights when age was included
m = 0.447 + 0.000112 x DAMB (SSE = 4500, R2 = 0.973). With fruit age included as a predictor of fresh weight, the SSE was reduced by about 11%. For the range of dates for which data were collected, the fruit age effect meant the multiplier 5
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Fig. 6. Revised estimation of fruit weight of Actinidia chinensis var. deliciosa ‘Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit from three linear dimensions (LDD) measured during fruit development and supplemented by fruit age. Each value is the mean of 30 fruit.
ranged from m = 0.583 on 6 March (92 DAMB) to m = 0.627 on 2 August (241 DAMB). Inclusion of fruit age had a much smaller effect on the estimate of fruit weight from dimensions for ‘Hayward’ than it did for ‘Zesy002’. Re-working the dimensions data from Fig. 1 with the new models including fruit age, it is possible to see that the inclusion of fruit age reduces the apparent shrinkage of the fruit late in development for both ‘Hayward’ and ‘Zesy002’ fruit (Fig. 7), although the reduction in estimated size is not eliminated completely.
3.4.4. Fruit firmness In ‘Hayward’, there was a slow decrease in fruit firmness until early June at which time firmness was ∼ 74.6 N and the softening rate increased slightly resulting in fruit at ∼ 46.1 N at the final assessment at the start of July (Fig. 8). In ‘Zesy002’, the slow softening phase lasted until mid-May when fruit were ∼ 65.7 N. Thereafter, softening was faster, with fruit being at 10 N firmness, or less, by late June. 3.4.5. Flesh colour There was little change in the flesh colour of the ‘Hayward’ fruit (Fig. 8). The flesh colour changed over a period of 15 weeks from ∼ 115°h in mid-March to ∼ 113°h at the start of July. In contrast with ‘Hayward’, the flesh colour of ‘Zesy002’ changed rapidly. There was a complete loss of green colour over a period of ∼ 9 weeks, starting in mid-March and degreening was complete (flesh colour < 103°h) by mid-May.
3.4. On-vine monitoring of fruit development 3.4.1. Seed colour The seed coats for both cultivars darkened over a period of 3–4 weeks (Fig. 8), between 21 March and 18 April for ‘Hayward’ and between 28 February and 21 March for ‘Zesy002’.
3.4.6. Fruit growth Fruit growth had ceased at the end of May for ‘Hayward’ and by mid-April for ‘Zesy002’ (Fig. 1). There then followed a marked decline in the estimated fruit size by early June for ‘Hayward’ and late-April for ‘Zesy002’.
3.4.2. Dry matter The accumulation of dry matter in ‘Hayward’ fruit slowed at the end of March, but continued to increase slowly until the end of May (Fig. 8). The accumulation of dry matter in ‘Zesy002’ fruit also slowed at the end of March, but with a slow increase only until mid-May.
3.5. Firmness assessments of fruit during development
3.4.3. Soluble solids content The SSC of ‘Hayward’ fruit was stable at 4–5% until late April, with an increase thereafter at increasing rate of accumulation until early June (Fig. 8). In ‘Zesy002’, there was no clear stable initial phase at the start of monitoring, but with a steady slow increase in SSC from ∼ 4% at the end of February to ∼ 6% in mid-April. From mid-April, the rate of accumulation increased and continued at this faster rate until late May when the accumulation rate slowed.
There were marked changes in the fruit softening parameters over the periods 23–30 May for ‘Hayward’ and 2–9 May for ‘Zesy002’ (Fig. 9). The change in ‘Hayward’ fruit was less distinct than in ‘Zesy002’. While there was a very distinct change from slow to fast softening with a decrease of 7.8 N over the period 2–9 May for ‘Zesy002’, in ‘Hayward’ there was possibly only a slower change in the softening rate over a couple of weeks. No explanation is offered for the sudden increase in compression firmness seen towards the end of April Fig. 7. Estimated fruit weight calculated from linear dimensions of length, minimum and maximum diameters with (◼) and without ( ) a component for fruit age for Actinidia chinensis var. deliciosa Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during development on the vine. Each value is the mean ± s.e.m. of 20 fruit for ‘Hayward’ and 19 fruit for ‘Zesy002’ (note — repeated measurements on the same fruit).
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The degree of shrinkage was associated with the degree of softening, being greater in ‘Zesy002’ fruit than ‘Hayward’ fruit. Overall, the rates of softening and ‘shrinkage’ were slower in ‘Hayward’ than in ‘Zesy002’ fruit. The association between a change in the fruit linear dimensions and softening appeared to be closer to the compression measurement of fruit firmness than penetrometer in ‘Hayward’, with a change in fruit compression being noticeable earlier than a change in penetrometer measurement for firmness. In ‘Zesy002’ fruit, the change to a reduction in linear dimensions occurred before any noticeable change in whole fruit softening, possibly more associated with the inner pericarp softening that commenced earlier. Also, in ‘Hayward’ fruit, there was a clearer change to rapid softening for the compression measurement compared with the penetrometer. Of the individual tissue zone measurements of firmness, the change in softening rate of the inner pericarp occurred earlier than other tissue zones, or measures of whole fruit firmness, and was in closer synchrony with the change to a reduction in fruit dimensions. 4. Discussion The on-vine monitoring of fruit growth from three linear measurements demonstrated the reduction in estimated fruit size described previously for ‘Hort16A’ fruit late in development (Minchin et al., 2003). The under-estimate of fruit size from linear dimensions recorded for ‘Zesy002’ fruit was considerably greater than for ‘Hayward’ fruit. The off-vine measurements of the fruit suggested a consistent gradual increase in fruit density over the whole period that fruit were monitored. The under-estimate of fruit size late in the season occurred at a time when fruit had started to soften rapidly, possibly contributing to the increase in fruit density irrespective of any other water or carbohydrate flows in or out of the fruit. The inclusion of a factor for fruit density, or simply fruit age, into the estimation of fruit weight from the three linear dimensions improved the accuracy of the estimate, in particular for the late season fruit. These findings are now discussed in more detail. There was a clear association between the timing of the period of reduction in fruit size and a change to rapid softening in the fruit. There were reductions in the dimensions of the fruit occurring, for each of the three dimensions; L, D1 and D2. The obvious suggestion that as the fruit softened the calliper measurement was able to deform the fruit was eliminated by a single experienced operator taking care not to compress the fruit when taking measurements. It had initially been thought that the reduction in the fruit dimensions may have been associated with some form of ‘relaxation’ within the fruit structure, resulting from a change in fruit texture that allowed some shape change in the fruit. One aspect of shape change could be associated with a ‘rounding up’ of the fruit and elimination of some of the angularity that may exist in the cross sectional shape of the unripe fruit. While kiwifruit are often regarded as being rounded in shape, when unripe, they can have significant angularity in their cross sectional shape (see Fig. 11), this is beyond the differences in minimum and maximum diameters that are associated with ‘flat’ fruit (Watson and Gould, 1993). Given that both minimum and maximum diameters were being measured, a loss from the maximum would be expected to result in an increase in the minimum. Likewise, any reduction in fruit length may be reflected in an increase in diameters. However, the current measurements do not fully describe the shape of the fruit and it is possible that the shape of the fruit may change once the fruit have started to soften, accounting for a slight reduction in the measured dimensions, but not in volume. One possibility is that the shape of the fruit “shoulders”, which is not taken into account if fruit weight is estimated from just L, D1 and D2, changes as fruit age and soften. In Section 3.3 it was found that the multiplier needed to calculate ‘Zesy002’ fruit weight from LDD increased by about 7.5% (0.583 to 0.627) between 6 March and 26 June.
Fig. 8. Soluble solids content (SSC, ◼), fruit firmness (FF, ), dry matter content (DM, ), incidence of black seeds (BS, ) and flesh colour (FC, ) changes in Actinidia chinensis var. deliciosa Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during fruit development on the vine. Each value is the mean ± s.e.m. of 30 fruit.
for both cultivars at the same time. The relative change in tissue zone softening differed between the two cultivars and is most easily seen in the normalised data in Fig. 9. In ‘Hayward’ fruit, the order in which the whole fruit or tissue zones changed from slow to fast softening was first observed in the inner and outer pericarp, then in the core and by compression, and finally by standard penetrometer. The initial changes in both inner and outer pericarp occurred 1–2 weeks before being detected by whole fruit compression test. The change in the core softening rate coincided with the overall change measured by compression test. The current standard method of assessing fruit firmness by penetrometer appears to be the last method by which a change in softening rate can be detected, with the change detected 1–2 weeks later than by compression test. In ‘Zesy002’ fruit, the inner pericarp changed softening rate first, about 1–2 weeks before the outer pericarp or by compression or standard penetrometer, which registered a change at about the same time. The core was the last tissue zone to show a changed softening rate, occurring 1–2 weeks after the change detected by compression and penetrometer. 3.6. Associations between the fruit development and size estimation The major aspect of fruit development measured that coincided with the ‘shrinkage’ of the fruit determined from on-vine calliper measurement for both cultivars was fruit softening (Fig. 10). 7
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Fig. 9. Compression firmness (◼), fruit firmness ( ), core firmness ( ), outer pericarp firmness ( ) and inner pericarp firmness ( ) changes in Actinidia chinensis var. deliciosa Hayward’ (A, B) and A. chinensis var. chinensis ‘Zesy002’ (C, D) fruit during fruit development on the vine. Values presented as raw data (A, C) or normalised where the harvest value = 100 (B, D). Each value is the mean ± s.e.m. of 30 fruit.
Given that fruit density increased by approximately 2% (1.045 to 1.066) over the same period, the remaining 5.5% of the change in multiplier must have been caused by changes in fruit shape. Approximating the shape of the fruit at any time by a superellipsoid, it was calculated how much the shoulder size would need to change to get the required 5.5% change in fruit volume without changing L, D1, or D2. A superellipsoid describes ‘Zesy002’ kiwifruit shape well, and the change in shoulder shape needed to change the relationship between L*D1*D2 by 5.5% is relatively small (Fig. 12). Currently there is no further explanation for the softening associated reduction in dimensions that are measured whilst monitoring fruit growth on the vine. Future approaches could include taking direct measurements of fruit volume in the orchard and making more detailed descriptions of fruit shape changes throughout development. Fruit growth is often referred to in terms of nett material changes
due to xylem and phloem inflow relative to losses from transpiration (Hall et al., 2013). It has been shown in ‘Hort16A’ kiwifruit that the fruit may deform whilst attached to the vine where a vine is unable to maintain water supply to the fruit to match transpiration losses in a high water vapour pressure deficit environment (Thorp et al., 2007; Clearwater et al., 2012). An alternative circumstance is that if the inflow to the fruit is disrupted, irrespective of the environment, it may be possible for there to be a lack of water to maintain fruit size or growth. These, and other, factors associated with fruit growth in kiwifruit have been discussed recently in the context of a biophysical model of fruit development (Hall et al., 2013). A further consideration is the change in fruit texture, which was identified by a compression measurement earlier than by the standard penetrometer. Similarly, a difference in compression and penetrometer data was found recently in describing kiwifruit texture following Fig. 10. Relationship between estimated fruit weight ( ) and fruit firmness measured by penetrometer (◼) and whole fruit compression ( ) in Actinidia chinensis var. deliciosa Hayward’ (A) and A. chinensis var. chinensis ‘Zesy002’ (B) fruit during fruit development. Fruit weight estimates from the new models is described in Section 3.3 and Fig. 7. Each value is the mean ± s.e.m. of 20 fruit (weight) or 30 fruit (firmness).
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Fig. 11. Actinidia chinensis var. deliciosa ‘Hayward’ kiwifruit showing aspects of angularity in cross sectional shape when unripe: A) whole fruit, B) cross section at the fruit equator, C) a ‘flat’ fruit (based on the minimum: maximum diameter ratio).
Fig. 12. A difference in longitudinal cross-sectional shape equivalent to a 5.5% change in volume (A) and overlaid on a Actinidia chinensis var. chinensis ‘Zesy002’ kiwifruit (B). A similar side-on view of a Actinidia chinensis var. deliciosa ‘Hayward’ fruit (C).
controlled atmosphere storage which was associated with the relative softening of the inner and outer pericarp tissues (Li et al., 2017). In the ‘Zesy002’ fruit, for which the greatest change in dimensions was recorded as the fruit softened rapidly, the compression measurement showed a change in softening rate about one week before being detected by the penetrometer. In addition, the degree of reduction in fruit dimensions appears linked to the degree of softening. In ‘Zesy002’ a 12.6% reduction in estimated fruit size from the maximum was associated with softening to ≤ 10 N, whereas in ‘Hayward’ a 3.2% reduction in estimated fruit size was associated with softening to 46 N. While the association between decreasing dimension measurements and an increased degree of fruit softening is clear, it may not be the softening per se that is important, rather the fact that the change in tissue zone textures make it possible for the relative dimensions of the fruit to change as the fruit soften. The investigation into the relationship between fruit dimensions and actual measurements of fruit weight and volume revealed a systematic inaccuracy in the application of a single model for fruit weight estimation to all stages of fruit development. The method was underestimating the fruit weight in the later harvested, larger, fruit. The fruit density continued to increase slowly even after the fruit had stopped accumulating carbohydrate (dry matter) and whilst the fruit was softening. This continued increase in density can be accounted for by changes in the fruit other than simple loss of water. There are two aspects to fruit density that appear relevant: the inflow of carbohydrates, which tends to cease with the termination of growth (Burdon et al., 2014a, 2016), and, thereafter, the change to rapid softening. Unlike apples, which may contain up to 25% air spaces between the cells (Reeve, 1953; Hall et al., 2016), kiwifruit contain lesser amounts of air,
and this air content decreases as the fruit soften (Harker and Hallett, 1994). This reduction in intercellular air space in kiwifruit may be associated with the increased cell wall swelling that occurs as the fruit soften (Redgwell et al., 1997; Burdon et al., 2014b). The cell wall swelling, and increased water holding capacity, may cause a reduction in the inter-cellular air space within the kiwifruit, and thereby increase the density of the fruit. In ‘Hayward’ fruit, the outer pericarp cell walls have been reported to increase in thickness by 3–4 fold during ripening (Hallett et al., 1992). Also, during ripening there is a progression in the development of the inner pericarp, which contains large thin walled cells that develop into a mucilaginous matrix with little intercellular air space. Adding fruit density as a factor into the estimation of fruit weight from the three linear dimensions improved the fruit size estimation in the later, larger, fruit. Consequently, there are several aspects of fruit development that are associated with the changing relationship between fruit linear dimensions and the fruit weight. To actually measure these and incorporate them into a model predicting fruit weight would be time consuming and defeat the goal of a rapid, non-destructive method for estimating fruit growth. Therefore, the incorporation of fruit age from flowering into the prediction equations for each cultivar provides for improved accuracy by adding an element associated with these late season aspects of fruit development, but without actually measuring them. In conclusion, while the three linear dimensions of the fruit used in the estimation of fruit weight may decrease slightly and be associated with fruit softening, it is difficult to envisage a decrease in the fresh weight of the fruit by over 10% as the fruit ripen. Instead, it is more likely that there is a subtle change in the shape of the fruit affecting 9
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those dimensions measured, yet not resulting in a major decrease in the fruit volume. It is suggested that the late season ‘shrinkage’ of kiwifruit when monitoring fruit growth on the vine is largely an artefact of the application of a single model based on linear fruit dimensions at all stages of development. This approach disregards other changes in the fruit shape and softening associated changes that affect fruit density. Inclusion of a fruit age element into the equations estimating fruit size improves the accuracy, but without defining specifically the aspect of biology driving the changing relationship.
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Acknowledgements The authors gratefully acknowledge: Dr Yongqing Zhu from Institute of Agro-Products Processing Science and Technology, Sichuan Academy of Agricultural Sciences, and the New Zealand-China Joint Kiwifruit Laboratory for financial support to Huaiyu Yuan. This work was undertaken as part of the New Zealand-China Joint Kiwifruit Laboratory under the Science and Technology Support Program of Sichuan Province (2018NZ0010); the finance department of Sichuan Province Innovation Support Project (2016TSCY-009) and the New Zealand Institute for Plant and Food Research Limited Strategic Science Investment Fund ‘Premium Kiwifruit’ research programme. References Ainalidou, A., Karamanoli, K., Menkissoglu-Spioudi, U., Diamantidis, G., Matsi, T., 2015. CPPU treatment and pollination: their combined effect on kiwifruit growth and quality. Sci. Hortic. 193, 147–154. Burdon, J., 2018. Kiwifruit biology: the commercial implications of fruit maturation. Hortic. Rev. 46, 385–421. Burdon, J., McLeod, D., Lallu, N., Gamble, J., Petley, M., Gunson, A., 2004. Consumer evaluation of ‘Hayward’ kiwifruit of different at-harvest dry matter contents. Postharvest Biol. Technol. 34, 245–255. Burdon, J., Pidakala, P., Martin, P., McAtee, P.A., Boldingh, H.L., Hall, A., Schaffer, R.J., 2014a. Postharvest performance of the yellow-fleshed’ Hort16A’ kiwifruit in relation to fruit maturation. Postharvest Biol. Technol. 92, 98–106. Burdon, J., Punter, M., Billing, D., Pidakala, P., Kerr, K., 2014b. Shrivel development in kiwifruit. Postharvest Biol. Technol. 87, 1–5. Burdon, J., Pidakala, P., Martin, P., Billing, D., Boldingh, H., 2016. Fruit maturation and the soluble solids harvest index for ‘Hayward’ kiwifruit. Sci. Hortic. 213, 193–198. Clearwater, M.J., Luo, Z., Ong, S.E.C., Blattmann, P., Thorp, T.G., 2012. Vascular functioning and the water balance of ripening kiwifruit (Actinidia chinensis) berries. J. Exp. Bot. 63 (5), 1835–1847.
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