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Softening of ‘Hayward’ kiwifruit on the vine and in storage: The effects of temperature
Scientia Horticulturae 220 (2017) 176–182
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Softening of ‘Hayward’ kiwifruit on the vine and in storage: The effects of temperature
MARK
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J. Burdona, , P. Pidakalaa, P. Martinb, D. Billinga a b
The New Zealand Institute for Plant & Food Research Ltd, Private Bag 92169, Auckland Mail Centre, 1142, Auckland, New Zealand The New Zealand Institute for Plant & Food Research Ltd, 412 No 1 Road, RD 2, Te Puke, 3182, New Zealand
A R T I C L E I N F O
A B S T R A C T
Keywords: Actinidia deliciosa Ripening Softening Storage Temperature
Observations were made on the softening of ‘Hayward’ kiwifruit on the vine, in storage and in response to low temperatures. The pattern of ‘Hayward’ softening on the vine and in storage was sigmoidal, as is typical for kiwifruit. On the vine, the initial slow softening rate of ‘Hayward’ fruit was consistent across seasons, and similar to other cultivars, at ∼2.9 N/week or less. The rate of rapid softening differed among cultivars at 5.9–14.7 N/ week. In storage, the initial slow phase of softening was seen only in less mature fruit harvested before any increase in softening rate had occurred on the vine. Maturing ‘Hayward’ fruit developed a capacity to soften in response to low temperatures 1–2 weeks before the on-vine period of faster softening commenced. There appear to be three aspects of fruit softening to consider in response to temperature: induction of softening at low (e.g. 8–10 °C) but not chilling temperatures, slowing of the biochemical reaction rates that cause softening by lower temperatures (e.g. 4 °C) and chilling by low temperatures close to 0 °C. These findings are discussed in the context of implementing research and the potential commercial impacts from a better understanding of temperature effects on kiwifruit softening.
1. Introduction The international success of kiwifruit in global trade is largely based on the good postharvest performance of the ‘Hayward’ cultivar, which can be stored for prolonged periods close to 0 °C for distribution to global markets. In commercial trade, kiwifruit tend to be harvested at a mature but unripe state. When harvested, the fruit is firm and starchy and can withstand the physical and temperature management of commercial handling and storage. Between harvest and reaching the consumer, the fruit must ripen to an acceptable eating state, for which a major component is the change in the fruit structure to a juicy, melting texture. Understanding the way the fruit softens is critical to commercial success, both in retarding softening to prolong storage, and also for ripening early season fruit in a controlled manner (Lallu et al., 1989; Crisosto, 1997). Fruit firmness, as measured by a penetrometer, is the main commercial quality criterion, with postharvest handling practices, including bin storage, grading and export, all dependent on the fruit not being too soft. Although often referred to as climacteric (Kader, 2002; Antunes, 2007), kiwifruit is not a typical climacteric fruit. The majority of the ripening, and in particular softening, occurs before any marked increase in ethylene production (Kim et al., 1999). However, the ethylene action
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Corresponding author. E-mail address: [email protected] (J. Burdon).
http://dx.doi.org/10.1016/j.scienta.2017.04.004 Received 1 October 2016; Received in revised form 29 March 2017; Accepted 5 April 2017 0304-4238/ © 2017 Elsevier B.V. All rights reserved.
inhibitor 1-methylcyclopropene has been shown to slow the softening of kiwifruit (Regiroli and Vriends, 2007). Ethylene production tends to increase after the fruit have reached eating softness (Kim et al., 1999), or if firmer, the ethylene production is associated with a rot or disorder such as chilling damage (Yano and Hasegawa, 1993; Feng et al., 2003). The softening pattern of kiwifruit has long been described as sigmoidal, with an initial slow phase (Phase 1), a rapid second phase (Phase 2), and a final slow phase (Phase 3) when about eating ripe (firmness < 10 N; MacRae and Redgwell, 1992). A fourth phase has also been proposed, but occurs at a firmness below normal commercial or eating ripe, and is associated with fruit that are over-ripe (Atkinson et al., 2011). Schröder and Atkinson (2006) reviewed the cell wall changes associated with textural changes of fruit softening. The fruit firmness determined for the softening pattern is that of the outer pericarp, measured by a penetrometer. However, the fruit comprises three main tissue zones, the inner and outer pericarp and the core. Little attention is paid to inner pericarp and core, although both may have a significant impact on the state of the fruit. As a wider range of genetic material becomes commercialised or considered for commercialisation, tissue zone differences are becoming apparent. In particular, the eating quality of a fruit may be affected by excessively hard cores in otherwise ripe fruit (Jeffery et al., 2013). Generally, hard
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orchard, Te Puke in 2014. For comparative purposes, ‘Hayward’ fruit were also obtained from this orchard block in addition to the fruit obtained from the commercial orchard and main ‘Hayward’ block at the research orchard.
cores tend not to be an issue in ‘Hayward’ kiwifruit, although recent suggestions that cores may remain firm in otherwise ripe fruit may possibly be due to attempts to ripen early season fruit by temperature management without ethylene, and attempting to ripen 1-MCP-treated fruit shortly after treatment. For ‘Hayward’ kiwifruit, within the commercial harvest window the fruit soften only slowly on the vine, whereas some of the newer released cultivars, may soften rapidly earlier in the season. Thus firmness becomes a significant criterion for consideration at harvest, alongside flesh colour and the traditional measure of soluble solids content (SSC; Harman, 1981). In ‘Hayward’ kiwifruit grown in New Zealand, a rapid softening phase is usually only seen on-vine if fruit are left un-harvested well after the commercial harvest window. There is also a suggestion, from field observations (Burdon and Lallu, 2011), vines grown in controlled environments (Seager et al., 1996) and postharvest treatments (on ‘Sanuki Gold’, Mworia et al., 2012), that low temperatures may induce softening in kiwifruit. Recently it has been suggested that low temperatures enhance ethylene sensitivity in the cultivar ‘Rainbow Red’ (Murakami et al., 2014). It has long been recognised that early harvested fruit may remain firm if held at room temperatures and ethylene treatments may be needed to facilitate ripening (Lallu et al., 1989). The use of temperature management through refrigeration to retard softening is a common approach for prolonging the storage life of all fresh produce, including kiwifruit. In the storage temperature range 0–5 °C, there is more rapid softening at higher temperature; also Phase 3 of softening occurs at a lower firmness at higher temperatures (Patterson et al., 2003). However, the use of refrigeration may damage the fruit, and temperature management should be to cool the fruit as soon as possible, and to the lowest safe temperature that does not induce chilling damage. Rapid cooling and lower storage temperatures tend to exacerbate chilling damage whereas slower cooling and higher storage temperatures limit chilling damage but may not be so effective in slowing softening, thereby reducing the storage potential. These general statements about fresh produce handling may have exceptions, and ‘Hayward’ kiwifruit is a good example where cooling may be delayed for 48–72 h in a process termed curing to minimise the occurrence of stem end rots caused by Botrytis cinerea (Manning et al., 2010). In this paper, observations have been made on the softening of ‘Hayward’ kiwifruit on the vine over several seasons and contrasted with the softening of more recently released cultivars. In storage, the softening pattern of fruit of different maturities have been characterised. In addition, the changing capacity with maturation of ‘Hayward’ kiwifruit to soften in response to low temperature has been investigated for both short-term and long-term effects on fruit firmness. These findings are discussed in the context of potential commercial impacts from a better understanding of temperature effects on kiwifruit softening.
2.2. Treatments/data samples 2.2.1. On-vine softening The on-vine softening of ‘Hayward’ kiwifruit in 2009–2012 and for five cultivars in 2014 was assessed from weekly harvests of 30 fruit measured destructively for fruit firmness. 2.2.2. Softening in storage In 2010, ‘Hayward’ fruit harvested at weekly intervals were placed into commercial packaging (three bulk loose fill packs of ∼10 kg containing ∼100 fruit each) and placed on the day of harvest into a coolstore operating at 0 °C. Samples of thirty fruit, 10 per box, were measured for firmness at two-week intervals throughout storage up to 20 weeks. In 2014, a single harvest of ‘Hayward’ fruit was placed into commercial single layer trays with a polyliner (three trays each with 30 fruit) and placed on the day of harvest into a coolstore operating at 0 °C. Twenty fruit samples were measured for firmness and core firmness after 6 days and 8, 12 and 20 weeks of storage. 2.2.3. Development of the capacity to respond to temperature In 2012, in conjunction with the weekly monitoring of fruit firmness on the vine, harvested ‘Hayward’ fruit were exposed to temperatures of 0, 4, 8, 10, 12, 14 and 16 °C for 1 week after which firmness was measured on a 20 fruit sample from each temperature. 2.2.4. Temperature response In 2012, 2013 and 2014, ‘Hayward’ fruit in commercial single layer trays with a polyliner were placed in different holding temperatures between 0 °C and 16 °C on the day of harvest. Three trays of 30 fruit were placed at each temperature. Fruit firmness was measured on 30 fruit samples after holding for 1, 2 and 3 weeks at constant temperature. In 2013, the same procedure was applied to ‘Zesh004’ fruit to check for consistency of effect between cultivars. In 2014, the impact of 2, 4 or 6 days at 4, 8 or 16 °C before transfer to 0 °C on the softening of ‘Hayward’ kiwifruit stored at 0 °C for up to 20 weeks was determined. Three trays each of 30 fruit were placed at each temperature on the day of harvest. A further three trays of fruit were placed directly into storage at 0 °C on the day of harvest. Fruit firmness was measured on 20 fruit samples after 6 days, 8 weeks, 12 weeks and 20 weeks. 2.3. Assessment methodology 2.3.1. Firmness Fruit firmness was measured using a Fruit Texture Analyser (GÜSS, model GS14, South Africa) fitted with a 7.9-mm diameter 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 to a displacement 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 kgf and data converted to N, where 1 kgf = 9.81 N. Core firmness was measured using a 4-mm diameter probe driven into the core at 5 mm/s after first cutting the fruit transversely 10–15 mm from the stem end and discarding the fruit piece with the picking scar.
2. Materials and methods 2.1. Fruit Actinidia chinensis var. deliciosa ‘Hayward’ fruit were sourced from the same two vines on a commercial orchard in Te Puke, Bay of Plenty, New Zealand (Te Puke: 37°49′S, 176°19′E) from 2009 to 2012. In 2013 and 2014, fruit were sourced from vines at the PFR research orchard in Te Puke. A. chinensis var. chinensis ‘Zesy002’ (marketed as Zespri® SunGold Kiwifruit, commonly called Gold3), A. chinensis var. chinensis ‘Zesy003’ (marketed as Zespri® Charm Kiwifruit, commonly called Gold9), A. chinensis var. chinensis x A. chinensis var. deliciosa ‘Zesh004’ (marketed as Zespri® Sweet Green Kiwifruit, commonly called Green14), A. chinensis var. deliciosa ‘Tomua’, and ‘Hayward’ cultivars were harvested from 3- or 4- vine plots on a single orchard block at the PFR research
2.3.2. Soluble solids content At harvest, SSC was determined separately for the stylar and stem ends of the fruit and averaged using a hand-held refractometer (Master Series, 0–30%, Atago). After fruit ripening, juice was measured using a digital refractometer (Pocket PAL-1, 0–50%, Atago). 177
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2.3.3. Seed coat colour Seed coat colour was scored by eye and is reported as the percentage of seeds that were dark/black.
Table 1 Rate of on-vine softening of ‘Hayward’ kiwifruit in 2009–2012. Linear fits and their correlation (r) and regression (R2) coefficients to data in Fig. 1 edited to the linear slow softening phase (Phase 1) only.
2.3.4. Fresh weight Fruit fresh weight (FW) was estimated by calculation from calliper measurements in mm of fruit length (L), and the minimum (D1) and maximum (D2) diameters (0.454 × (L × D1 × D2)1.05); Snelgar et al., 1992).
Harvest
Softening rate
Correlation
Regression
Season 2009 2010 2011 2012
(N/week) 3.24 2.94 3.53 2.45
(r) −0.931 −0.970 −0.982 −0.979
(R2) 0.85 0.94 0.96 0.95
2.4. Data analysis Trends and patterns within the data are described graphically using Origin v8.5 (OriginLab Corporation, One Roundhouse Plaza, Northampton, MA01060, USA). Linear fitted lines, Pearson’s correlation coefficients and coefficients of regression of the linear fits were calculated. Statistical separation among sample means was by analysis of variance (ANOVA) using GenStat Release 14.2 [(PC/Windows 7) Copyright 2011, VSN International Ltd, UK]. 3. Results and discussion 3.1. On-vine softening of ‘Hayward’ kiwifruit ‘Hayward’ fruit showed a consistent slow linear softening phase of 2.5–3.5 N/week whilst on the vine for the four seasons monitored between 2009 and 2012 (Fig. 1, Table 1). Despite similar rates, there were between-season differences of up to ∼10 N in fruit firmness at a given time in the year. For all seasons, there was a quite uniform slow rate of softening during the normal harvest window, up to the end of May/early June. In 2012, fruit were specifically left on the vine after the normal harvest window to see the full development of on-vine softening. There was a rapid softening phase of up to 21 N/week from mid-June, which resulted in fruit at ∼25 N at the end of June.
Fig. 2. Fruit firmness of five cultivars of kiwifruit (‘Zesy002’, ‘Zesy003’, Zesh004’, ‘Tomua’ and ‘Hayward’) during fruit development. Vines grown at the same site, harvested in 2014. Each value is the mean of 30 fruit. Table 2 On-vine fruit softening rates of five kiwifruit cultivars (‘Zesy002’, ‘Zesy003’, ‘Zesh004’, ‘Tomua’, ‘Hayward’) grown at the same site in 2014. Linear fits and their correlation (r) and regression (R2) coefficients to selected data for Phase 1 (Slow) and Phase 2 (Rapid) of softening from Fig. 2.
3.2. On-vine softening of five cultivars of kiwifruit In 2014, the on-vine softening of a range of cultivars, including ‘Hayward’, was compared for vines grown on the same site. At the start of fruit monitoring at the end of February, the fruit firmness differed from ∼78.5 N in ‘Zesy003’ to ∼117.7 N in ‘Hayward’ and ‘Zesh004’. Fruit from all five cultivars showed a similar pattern of slow softening for a period of weeks followed by more rapid softening (Fig. 2, Table 2), the change in softening rate occurred between late April and mid-May, when fruit firmness was between 68.7 and 83.4 N. For ‘Hayward’, there was little noticeable change in softening rate, and in contrast with the
Cultivar
‘Zesy002’ ‘Zesy003’ ‘Zesh004’ ‘Tomua’ ‘Hayward’
Phase 1 (Slow)
Phase 2 (Rapid) 2
Rate (N/wk)
r
R
2.26 0.98 1.96 1.77 3.04
−0.955 −0.867 −0.912 −0.941 −0.988
0.90 0.72 0.82 0.87 0.97
Rate (N/wk)
r
R2
13.54 10.69 14.32 6.87 5.79
−0.999 −0.998 −0.999 −0.985 −0.987
0.99 0.99 0.99 0.96 0.97
data for 2012, the fruit were still at ∼47 N at the end of June. In ‘Hort16A’, it has been observed that the change to rapid fruit softening on the vine occurred when the fruit were at 40–60 N, and not at a fixed firmness value (Burdon, unpublished data). ‘Zesy002’ and ‘Zesy003’ were the first cultivars to commence rapid fruit softening in late April, followed by ‘Zesh004’ in mid-May. ‘Tomua’ and ‘Hayward’ differed from the other cultivars with a slower rate of rapid softening (Table 2), commencing in early May for ‘Tomua’ and late May for ‘Hayward’. The change from slow to rapid softening in ‘Hayward’ was less distinct than for the other cultivars. Even with this limited data set, it is possible to discriminate among the cultivars for the earlier, faster softening A. chinensis var. chinensis cultivars (‘Zesy002’ and ‘Zesy003’) and the later, slower A. chinensis var. deliciosa cultivars (‘Tomua’ and ‘Hayward’). The change to rapid softening in the hybrid between the cultivars var. chinensis and var. deliciosa (‘Zesh004’) occurred later than, but to a similar rate as, the var. chinensis cultivars. To some extent, the differences among cultivars in the timing of the change to rapid softening can be associated with the flowering window
Fig. 1. Fruit firmness of ‘Hayward’ kiwifruit on the vine during fruit development. Fruit from the same 2 vines on a single orchard measured during the 2009–2012 harvest seasons. Each value is the mean of 30 fruit.
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Table 3 Softening during storage at 0 °C of ‘Hayward’ kiwifruit harvested at 120–197 days after mid-bloom (DAMB) in 2010. Duration of Phase 1 and the softening rates in Phase 1 (slow) and Phase 2 (rapid) calculated from linear fits to the data in Fig. 3. Harvest
Duration of Phase 1
Softening rate (N/week)
(DAMB)
(Weeks)
Phase 1 (Slow)
Phase 2 (Rapid)
120 141 169 176 197
8 6 4 4 0
2.6 3.2 4.7 5.2 naa
9.8 10.6 10.1 10.8 9.1
a
na not applicable − no slow phase.
context for the developmental state of the fruit at each harvest. The seeds had all darkened by early April and fruit growth had ceased by late April. There was a consistent trend for an increasing rate of soluble solids accumulation throughout fruit development from early April until the last harvest on 1 June. The firmness at which softening stabilised in storage at Phase 3 appears dependent on maturity at harvest, being firmer at ∼8.8 N for fruit harvested at 197 DAMB and softer at ∼4.9 N for fruit harvested at 141–169 DAMB. The interpretation of this final firmness value is affected by the development of chilling injury in some of the early harvested fruit. This can affect softening such that chill damaged fruit remain relatively firm late into storage, but when they soften, they soften rapidly and continue to do so until over-soft (< 4 N). The data show that a wide range of softening patterns can be achieved from fruit of a single genotype depending on maturity. It is also known that the softening pattern can be affected by other factors: faster softening at higher storage temperature or with ethylene treatment and slower softening when stored in controlled or modified atmosphere conditions (Burdon and Lallu, 2011). Also, separate measurements of outer pericarp and core firmness during storage show these tissue zones in ‘Hayward’ kiwifruit to have different softening patterns (Fig. 4). While the outer pericarp softening is similar to the standard penetrometer measurement, being an exponential decay, or Phases 2 and 3 of the previously described sigmoidal, the core softening is more linear. For ‘Hayward’ kiwifruit, the different ripening patterns of the outer pericarp and core do not lead to a problem with fruit quality. The use of ethylene to ripen fruit harvested early in the season (Lallu et al., 1989) ensures that all tissue zones soften at the same time (data not shown) and more mature fruit tend to be stored allowing softening to occur
Fig. 3. ‘Hayward’ kiwifruit softening patterns on the vine (A) and in storage at 0 °C (B). Fruit harvested at weekly intervals up to 197 days after mid-bloom (DAMB) in 2010. Also shown in A are the fruit soluble solids content (SSC), incidence of black seeds and fresh weight as indicators of fruit development. Each value is the mean of 30 fruit (A) or 20 fruit (B), ± s.e.m.
of the five cultivars, for which mid-bloom occurs within a 3–4 week period. ‘Zesy003’ was the earliest to mid-bloom, followed by ‘Zesy002’, ‘Zesh004’, ‘Tomua’ and ‘Hayward’, in that order. The seasonal difference between 2012 and 2014 in ‘Hayward’ fruit firmness at the end of June was marked, with fruit in 2012 being ∼25 N, whereas in 2014 the fruit firmness was still ∼50 N. 3.3. Softening of ‘Hayward’ kiwifruit in storage There was little or no association between the on vine softening rate of ‘Hayward’ fruit and the softening of fruit in coolstore at 0 °C (Fig. 3). While there was little change in the rate of softening on the vine, at ∼2.9 N/week, there were marked differences in the softening patterns of the fruit from different harvests when in storage. In storage, Phase 1 (slow) softening started at similar rates (2–3 N/ week) to the on-vine softening from early harvests, but slowly increased in later harvested fruit up to 5.9 N/week (Table 3). The duration of the slow Phase 1 softening period was reduced in fruit from later harvests. The accuracy of the calculated rates, and the duration of Phase 1, is limited by only sampling the fruit at 2-weekly intervals. Phase 2 (rapid) rates of softening in storage were consistent among harvests within the range 8.8–10.8 N/week (Table 2). As with the rate and duration of Phase1, the accuracy was limited for identifying the change-over point for Phase 1 to Phase 2 and also from Phase 2 to Phase 3. The fruit seed colour, growth data and SSC in Fig. 3A provide more
Fig. 4. Softening pattern of the outer pericarp ( ; standard firmness measurement with a 7.9 mm diameter penetrometer) and core ( ; 4 mm diameter penetrometer) of ‘Hayward’ kiwifruit stored at 0 °C for up to 20 weeks in 2014. Each value is the mean of 20 fruit ± s.e.m.
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Fig. 6. Firmness of ‘Hayward’ kiwifruit measured at harvest (At-h) and after one week at 16, 14, 12, 10, 8, 4 or 0 °C. Fruit were harvested on 21 May 2012, 190 days after midbloom, when soluble solids content was 9.4%. Each value is the mean of 20 fruit. Values not sharing a common letter differ at P = 0.05.
softening at 10 °C was greater than for earlier harvested fruit, with a firmness loss of 20–30 N. Statistical analysis of the temperature response of fruit harvested on 21 May 2012 is shown in Fig. 6, in which a greater number of temperatures have been included than presented in Fig. 5. The temperature response suggests that increased softening may be induced as temperatures decreased from 16 °C to about 10 °C, but that lower temperatures slowed softening, possibly on the basis of both simple biochemical reaction rates and also possibly by more specific temperature-regulated effects, as suggested by the marked difference in the softening of fruit at 4 °C and 0 °C. The changes in softening rates are beyond any simple temperature-firmness coefficient as used to adjust firmness values for fruit measured at different temperatures (Jeffery and Banks, 1994). The marked change in capacity to respond to temperature occurred in fruit that had not shown any change in softening rate on the vine, and 1–2 weeks before on-vine softening started to accelerate. However, this change in capacity to respond to temperature was associated with the time at which fruit growth ceased (Fig. 5). Taking the earlier finding of the development with maturation of the capacity to respond to temperature (Figs. 5 and 6), a more detailed trial showed the degree of softening that can be induced over a short time-frame of 3 weeks at a range of temperatures (Fig. 7) or over a longer timeframe in storage at 0 °C following up to 6 days at a range of temperatures between 4 and 16 °C (Fig. 8). The short term effect of temperature on ‘Hayward’ fruit softening seen in 2012 was repeated in 2013 and compared with another cultivar, ‘Zesh004’ (Fig. 7). While similar trends were seen for both cultivars, there were differences in both the degree of softening, and also the separation of softening rates among the treatment temperatures. In ‘Hayward’, most rapid softening occurred at 8 °C, then 4 °C with 12 °C intermediate between 8 and 0 °C. In ‘Zesh004’, most rapid softening occurred at 8 °C and 12 °C and then 4 °C. Softening at 4, 8 and 12 °C were all significantly faster than at 16 °C or 0 °C. The apparently greater softening in ‘Zesh004’ than ‘Hayward’ may merely reflect a difference in the at-harvest maturity and thus capacity to respond. It does, however, indicate some degree of commonality in response of different kiwifruit cultivars to temperature. Also, it has been shown in ‘Hort16A’ kiwifruit that there is similarly an increased capacity to soften in response to ethylene which occurs before any change in softening rate on the vine (Burdon et al., 2014). Whether there are any commonalities in the metabolic control of these softening responses remains to be determined. This also concurs with a finding that low temperature may modulate softening in an ethylene independent manner (Mworia et al., 2012).
Fig. 5. Pattern of ‘Hayward’ kiwifruit softening on the vine (A; FF) and softening in response to temperature (1 week at 0–16 °C) off the vine (B). Also shown in A are the fruit soluble solids content (SSC), incidence of black seeds (BS) and estimated fruit weight (FW) as indicators of fruit development and maturation. Each value is the mean of 30 fruit (only 15 for FW; A) or 20 fruit (B).
prior to marketing and consumption. 3.4. Softening of harvested ‘Hayward’ kiwifruit in response to temperature In the 2012 season, sampling from the vine was continued past the commercial harvest and through June to observe the more rapid fruit softening on the vine (Fig. 5A). The rate of slow softening on the vine was similar to previous years at ∼2 N/week (Fig. 1). The prolonged period of sampling resulted in observation of a softening rate of up to 20.6 N/week towards the end of June. The response of harvested fruit to temperatures between 16 °C and 0 °C was consistent between harvests, with fruit held at 10 °C being softest one week after harvest (Fig. 5A and B). While the pattern of softening in response to different temperatures was consistent, the magnitude of change in firmness increased markedly between 14 May 2015 and 21 May 2015 (Fig. 5B). Fruit harvested on 16 April 2012 showed little softening during one week after harvest, irrespective of temperature. For the next few harvests, it was apparent that fruit held at 10 °C tended to soften slightly more than the fruit held at other temperatures, with fruit at 16 °C, 4 °C or 0 °C being approximately the same firmness after one week, and little changed from harvest. Fruit from 21 May 2012, and later, showed several differences from earlier harvested fruit. The fruit held at 16 °C softened more markedly, fruit held at 0 °C were little changed from at-harvest firmness and the 180
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softening at 8 °C for ‘Hayward’ fruit shown in Fig. 7. The firmness of ‘Hayward’ fruit held at 8 °C for 6 days appears anomalous alongside the other treatments (Fig. 8), yet the softening rate and firmness is in fact in line with the softening seen at 8 °C in previous years (2013; Fig. 7). 4. Summary The pattern of ‘Hayward’ kiwifruit softening on the vine and in storage was sigmoidal. On the vine, the initial slow softening rate of ‘Hayward’ fruit was consistent among seasons, and similar to other cultivars, at ∼2.9 N/week or less. The rate of rapid softening differed among cultivars at 5.9–14.7 N/week, with A. chinensis var. chinensis cultivars showing more rapid softening than A. chinensis var. deliciosa cultivars. In storage, the initial slow phase of softening was only seen in less mature fruit harvested before any increase in softening rate on the vine. It is also known that storage temperature and atmosphere (oxygen, carbon dioxide and ethylene) will affect the rate of softening of the main phase of rapid softening (Burdon and Lallu, 2011). ‘Hayward’ fruit developed with maturation a capacity to soften in response to low temperatures, occurring 1–2 weeks before faster softening occurred on the vine. The change in softening rate was associated with cessation of fruit growth and accumulation of carbohydrate. There may be three aspects of fruit softening to consider in response to temperature: induction of softening at low (e.g. 8–10 °C) but not chilling temperatures, slowing of biochemical reaction rates that affect softening by lower temperatures (e.g. 4 °C) and chilling by low temperatures close to 0 °C. The findings from this work have implications for both researchers and commercial operators. In conducting research, understanding the physiological state of the fruit at harvest is relevant to any findings on the postharvest responses of the fruit. Commercially, temperature management and the response of fruit to temperature is a major component of the commercial handling of kiwifruit. A better understanding of cultivar-specific responses of fruit to temperature raises the possibility that general approaches taken to manage fruit performance, such as through slow or fast cooling regimes, may be better tailored to match the fruit’s capacity to respond to, or withstand, specific temperatures. More precise temperature management may improve the use of conditioning temperatures for flesh degreening (De Silva et al., 2007; Burdon and Lallu, 2011), ripening of early harvested fruit (Lallu et al., 1989), curing in controlled refrigerated conditions (Tonini et al., 1999) or the storage of early harvested fruit or fruit of chilling sensitive cultivars.
Fig. 7. Softening of ‘Hayward’ (A) and ‘Zesh004’ (B) kiwifruit in response to holding at 0, 4, 8, 12 and 16 °C after harvest in 2013. Soluble solids content at harvest 8.1% and 10.3%, respectively. Each value is the mean of 30 fruit. Bars are LSD at P = 0.05 among individual treatments.
Acknowledgements The authors gratefully acknowledge the assistance of Murray Judd for access to the commercial ‘Hayward’ orchard used in 2009–2012. This work was conducted as part of the Plant & Food Research Core Kiwifruit programme ‘Premium kiwifruit: Growing the future’. References Antunes, M., 2007. The role of ethylene in kiwifruit ripening and senescence. Stewart Postharvest Rev. 3, 1–8. Atkinson, R.G., Gunaseelan, K., Wang, M.Y., Luo, L., Wang, T.C., Norling, C.L., Johnston, S.L., Maddumage, R., Schroder, R., Schaffer, R.J., 2011. Dissecting the role of climacteric ethylene in kiwifruit (Actinidia chinensis) ripening using a 1aminocyclopropane-1-carboxylic acid oxidase knockdown line. J. Exp. Bot. 62 (11), 3821–3835. Burdon, J., Lallu, N., 2011. Kiwifruit (Actinidia spp.). In: Yahia, E.M. (Ed.), Postharvest Biology and Technology of Tropical and Subtropical Fruit. Volume 3, Cocona to Mango. Woodhead Publishing Limited, UK, pp. 326–360. Burdon, J., Pidakala, P., Martin, P., McAtee, P.A., Boldingh, H.L., Hall, A., Schaffer, R.J., 2014. Postharvest performance of the yellow-fleshed ‘Hort16A’ kiwifruit in relation to fruit maturation. Postharvest Biol. Technol. 92, 98–106. Crisosto, C.H., 1997. Final preconditioning guidelines for kiwifruit shippers. Cent. Val. Postharvest Newslett. 6, 1–4. De Silva, N., Hall, A.J., Burdon, J., Lallu, N., Connolly, P., Amos, N., 2007. Modelling the
Fig. 8. Softening of ‘Hayward’ kiwifruit during storage for up to 20 weeks at 0 °C following holding at 4, 8 or 16 °C for 2, 4 or 6 days, or placing directly to 0 °C in 2014. Each value is the mean of 20 fruit. Bars are LSD at P = 0.05 among individual treatments.
The holding of fruit for up to 6 days at 4, 8 or 16 °C affected softening in subsequent storage at 0 °C (Fig. 8). Throughout storage, the fruit that had been held at 16 °C tended to be firmest and the fruit held at 8 °C the softest. This concurs with the short term effect of accelerated 181
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J. Burdon et al. effect of holding temperature on flesh de-greening of ‘Hort16A’ kiwifruit. Acta Hortic. 753, 769–775. Feng, J., Maguire, K.M., MacKay, B.R., 2003. Factors affecting ethylene production of Hayward kiwifruit. Acta Hortic. 610, 203–209. Harman, J.E., 1981. Kiwifruit maturity. Orchardist N. Z. 54, 126–127 (130). Jeffery, P.B., Banks, N.H., 1994. Firmness-temperature coefficient of kiwifruit. N. Z. J. Crop Hortic. Sci. 22, 97–101. Jeffery, P.B., East, A.R., Jabbar, A., Heyes, J.A., Bollen, F., 2013. Characterisation, measurement and factors influencing columella softening in kiwifruit. Acta Hortic. 1012 1497–1491. Kader, A.A., 2002. Biology and technology: an overview. Postharvest Technology of Horticultural Crops. University of California Agriculture and Natural Resources Publicationpp. 3311. Kim, H., Hewett, E., Lallu, N., 1999. The role of ethylene in kiwifruit softening. Acta Hortic. 498, 255–262. Lallu, N., Searle, A.N., MacRae, E.A., 1989. An investigation of ripening and handling strategies for early season kiwifruit (Actinidia deliciosa cv. Hayward). J. Sci. Food Agric. 47, 387–400. MacRae, E., Redgwell, R., 1992. Softening in kiwifruit. Postharvest News Inf. 3, 49N–52N. Manning, M.A., Pak, H.A., Beresford, R.M., 2010. Non-fungicidal control of Botrytis storage rot in New Zealand kiwifruit through pre- and postharvest crop management. In: Prusky, D., Gullino, M.L. (Eds.), Postharvest Pathology, Plant Pathology in the
21st Century Vol. 2 Springer Science + Business Media B.V. 2010. Murakami, S., Ikoma, Y., Yano, M., 2014. Low temperature increases ethylene sensitivity in Actinidia chinensis ‘Rainbow Red’ kiwifruit. J. Japan. Soc. Hortic. Sci. 83 (4), 322–326. Mworia, E.G., Yoshikawa, T., Salikon, N., Oda, C., Asiche, W.O., Yokotani, N., Abe, D., Ushijima, K., Nakano, R., Kubo, Y., 2012. Low-temperature-modulated fruit ripening is independent of ethylene in ‘Sanuki Gold’ kiwifruit. J. Exp. Bot. 63 (2), 963–971. Patterson, K., Burdon, J., Lallu, N., 2003. ‘Hort16A’ kiwifruit: progress and issues with commercialisation. Acta Hortic. 610, 267–273. Regiroli, G., Vriends, P., 2007. SmartFreshSM (1-methylcyclopropene) benefits for kiwifruit. Acta Hortic. 753, 745–753. Schröder, R., Atkinson, R.G., 2006. Kiwifruit cell walls: towards an understanding of softening? N. Z. J. For. Sci. 36, 112–129. Seager, N.G., Warrington, I.J., Hewett, E.W., 1996. Maturation of kiwifruit grown at different temperatures in controlled environments. J. Hortic. Sci. 71 (4), 639–652. 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 (4), 481–487. Tonini, G., Barberini, K., Bassi, F., Proni, R., 1999. Effects of new curing and controlled atmosphere storage technology on Botrytis rots and flesh firmness in kiwifruit. Acta Hortic. 498, 285–291. Yano, M., Hasegawa, Y., 1993. Ethylene production in harvested kiwifruit with special reference to ripe rot. J. Japan Soc. Hortic. Sci. 62, 443–449.
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Softening of ‘Hayward’ kiwifruit on the vine and in storage: The effects of temperature
MARK
⁎
J. Burdona, , P. Pidakalaa, P. Martinb, D. Billinga a b
The New Zealand Institute for Plant & Food Research Ltd, Private Bag 92169, Auckland Mail Centre, 1142, Auckland, New Zealand The New Zealand Institute for Plant & Food Research Ltd, 412 No 1 Road, RD 2, Te Puke, 3182, New Zealand
A R T I C L E I N F O
A B S T R A C T
Keywords: Actinidia deliciosa Ripening Softening Storage Temperature
Observations were made on the softening of ‘Hayward’ kiwifruit on the vine, in storage and in response to low temperatures. The pattern of ‘Hayward’ softening on the vine and in storage was sigmoidal, as is typical for kiwifruit. On the vine, the initial slow softening rate of ‘Hayward’ fruit was consistent across seasons, and similar to other cultivars, at ∼2.9 N/week or less. The rate of rapid softening differed among cultivars at 5.9–14.7 N/ week. In storage, the initial slow phase of softening was seen only in less mature fruit harvested before any increase in softening rate had occurred on the vine. Maturing ‘Hayward’ fruit developed a capacity to soften in response to low temperatures 1–2 weeks before the on-vine period of faster softening commenced. There appear to be three aspects of fruit softening to consider in response to temperature: induction of softening at low (e.g. 8–10 °C) but not chilling temperatures, slowing of the biochemical reaction rates that cause softening by lower temperatures (e.g. 4 °C) and chilling by low temperatures close to 0 °C. These findings are discussed in the context of implementing research and the potential commercial impacts from a better understanding of temperature effects on kiwifruit softening.
1. Introduction The international success of kiwifruit in global trade is largely based on the good postharvest performance of the ‘Hayward’ cultivar, which can be stored for prolonged periods close to 0 °C for distribution to global markets. In commercial trade, kiwifruit tend to be harvested at a mature but unripe state. When harvested, the fruit is firm and starchy and can withstand the physical and temperature management of commercial handling and storage. Between harvest and reaching the consumer, the fruit must ripen to an acceptable eating state, for which a major component is the change in the fruit structure to a juicy, melting texture. Understanding the way the fruit softens is critical to commercial success, both in retarding softening to prolong storage, and also for ripening early season fruit in a controlled manner (Lallu et al., 1989; Crisosto, 1997). Fruit firmness, as measured by a penetrometer, is the main commercial quality criterion, with postharvest handling practices, including bin storage, grading and export, all dependent on the fruit not being too soft. Although often referred to as climacteric (Kader, 2002; Antunes, 2007), kiwifruit is not a typical climacteric fruit. The majority of the ripening, and in particular softening, occurs before any marked increase in ethylene production (Kim et al., 1999). However, the ethylene action
⁎
Corresponding author. E-mail address: [email protected] (J. Burdon).
http://dx.doi.org/10.1016/j.scienta.2017.04.004 Received 1 October 2016; Received in revised form 29 March 2017; Accepted 5 April 2017 0304-4238/ © 2017 Elsevier B.V. All rights reserved.
inhibitor 1-methylcyclopropene has been shown to slow the softening of kiwifruit (Regiroli and Vriends, 2007). Ethylene production tends to increase after the fruit have reached eating softness (Kim et al., 1999), or if firmer, the ethylene production is associated with a rot or disorder such as chilling damage (Yano and Hasegawa, 1993; Feng et al., 2003). The softening pattern of kiwifruit has long been described as sigmoidal, with an initial slow phase (Phase 1), a rapid second phase (Phase 2), and a final slow phase (Phase 3) when about eating ripe (firmness < 10 N; MacRae and Redgwell, 1992). A fourth phase has also been proposed, but occurs at a firmness below normal commercial or eating ripe, and is associated with fruit that are over-ripe (Atkinson et al., 2011). Schröder and Atkinson (2006) reviewed the cell wall changes associated with textural changes of fruit softening. The fruit firmness determined for the softening pattern is that of the outer pericarp, measured by a penetrometer. However, the fruit comprises three main tissue zones, the inner and outer pericarp and the core. Little attention is paid to inner pericarp and core, although both may have a significant impact on the state of the fruit. As a wider range of genetic material becomes commercialised or considered for commercialisation, tissue zone differences are becoming apparent. In particular, the eating quality of a fruit may be affected by excessively hard cores in otherwise ripe fruit (Jeffery et al., 2013). Generally, hard
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orchard, Te Puke in 2014. For comparative purposes, ‘Hayward’ fruit were also obtained from this orchard block in addition to the fruit obtained from the commercial orchard and main ‘Hayward’ block at the research orchard.
cores tend not to be an issue in ‘Hayward’ kiwifruit, although recent suggestions that cores may remain firm in otherwise ripe fruit may possibly be due to attempts to ripen early season fruit by temperature management without ethylene, and attempting to ripen 1-MCP-treated fruit shortly after treatment. For ‘Hayward’ kiwifruit, within the commercial harvest window the fruit soften only slowly on the vine, whereas some of the newer released cultivars, may soften rapidly earlier in the season. Thus firmness becomes a significant criterion for consideration at harvest, alongside flesh colour and the traditional measure of soluble solids content (SSC; Harman, 1981). In ‘Hayward’ kiwifruit grown in New Zealand, a rapid softening phase is usually only seen on-vine if fruit are left un-harvested well after the commercial harvest window. There is also a suggestion, from field observations (Burdon and Lallu, 2011), vines grown in controlled environments (Seager et al., 1996) and postharvest treatments (on ‘Sanuki Gold’, Mworia et al., 2012), that low temperatures may induce softening in kiwifruit. Recently it has been suggested that low temperatures enhance ethylene sensitivity in the cultivar ‘Rainbow Red’ (Murakami et al., 2014). It has long been recognised that early harvested fruit may remain firm if held at room temperatures and ethylene treatments may be needed to facilitate ripening (Lallu et al., 1989). The use of temperature management through refrigeration to retard softening is a common approach for prolonging the storage life of all fresh produce, including kiwifruit. In the storage temperature range 0–5 °C, there is more rapid softening at higher temperature; also Phase 3 of softening occurs at a lower firmness at higher temperatures (Patterson et al., 2003). However, the use of refrigeration may damage the fruit, and temperature management should be to cool the fruit as soon as possible, and to the lowest safe temperature that does not induce chilling damage. Rapid cooling and lower storage temperatures tend to exacerbate chilling damage whereas slower cooling and higher storage temperatures limit chilling damage but may not be so effective in slowing softening, thereby reducing the storage potential. These general statements about fresh produce handling may have exceptions, and ‘Hayward’ kiwifruit is a good example where cooling may be delayed for 48–72 h in a process termed curing to minimise the occurrence of stem end rots caused by Botrytis cinerea (Manning et al., 2010). In this paper, observations have been made on the softening of ‘Hayward’ kiwifruit on the vine over several seasons and contrasted with the softening of more recently released cultivars. In storage, the softening pattern of fruit of different maturities have been characterised. In addition, the changing capacity with maturation of ‘Hayward’ kiwifruit to soften in response to low temperature has been investigated for both short-term and long-term effects on fruit firmness. These findings are discussed in the context of potential commercial impacts from a better understanding of temperature effects on kiwifruit softening.
2.2. Treatments/data samples 2.2.1. On-vine softening The on-vine softening of ‘Hayward’ kiwifruit in 2009–2012 and for five cultivars in 2014 was assessed from weekly harvests of 30 fruit measured destructively for fruit firmness. 2.2.2. Softening in storage In 2010, ‘Hayward’ fruit harvested at weekly intervals were placed into commercial packaging (three bulk loose fill packs of ∼10 kg containing ∼100 fruit each) and placed on the day of harvest into a coolstore operating at 0 °C. Samples of thirty fruit, 10 per box, were measured for firmness at two-week intervals throughout storage up to 20 weeks. In 2014, a single harvest of ‘Hayward’ fruit was placed into commercial single layer trays with a polyliner (three trays each with 30 fruit) and placed on the day of harvest into a coolstore operating at 0 °C. Twenty fruit samples were measured for firmness and core firmness after 6 days and 8, 12 and 20 weeks of storage. 2.2.3. Development of the capacity to respond to temperature In 2012, in conjunction with the weekly monitoring of fruit firmness on the vine, harvested ‘Hayward’ fruit were exposed to temperatures of 0, 4, 8, 10, 12, 14 and 16 °C for 1 week after which firmness was measured on a 20 fruit sample from each temperature. 2.2.4. Temperature response In 2012, 2013 and 2014, ‘Hayward’ fruit in commercial single layer trays with a polyliner were placed in different holding temperatures between 0 °C and 16 °C on the day of harvest. Three trays of 30 fruit were placed at each temperature. Fruit firmness was measured on 30 fruit samples after holding for 1, 2 and 3 weeks at constant temperature. In 2013, the same procedure was applied to ‘Zesh004’ fruit to check for consistency of effect between cultivars. In 2014, the impact of 2, 4 or 6 days at 4, 8 or 16 °C before transfer to 0 °C on the softening of ‘Hayward’ kiwifruit stored at 0 °C for up to 20 weeks was determined. Three trays each of 30 fruit were placed at each temperature on the day of harvest. A further three trays of fruit were placed directly into storage at 0 °C on the day of harvest. Fruit firmness was measured on 20 fruit samples after 6 days, 8 weeks, 12 weeks and 20 weeks. 2.3. Assessment methodology 2.3.1. Firmness Fruit firmness was measured using a Fruit Texture Analyser (GÜSS, model GS14, South Africa) fitted with a 7.9-mm diameter 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 to a displacement 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 kgf and data converted to N, where 1 kgf = 9.81 N. Core firmness was measured using a 4-mm diameter probe driven into the core at 5 mm/s after first cutting the fruit transversely 10–15 mm from the stem end and discarding the fruit piece with the picking scar.
2. Materials and methods 2.1. Fruit Actinidia chinensis var. deliciosa ‘Hayward’ fruit were sourced from the same two vines on a commercial orchard in Te Puke, Bay of Plenty, New Zealand (Te Puke: 37°49′S, 176°19′E) from 2009 to 2012. In 2013 and 2014, fruit were sourced from vines at the PFR research orchard in Te Puke. A. chinensis var. chinensis ‘Zesy002’ (marketed as Zespri® SunGold Kiwifruit, commonly called Gold3), A. chinensis var. chinensis ‘Zesy003’ (marketed as Zespri® Charm Kiwifruit, commonly called Gold9), A. chinensis var. chinensis x A. chinensis var. deliciosa ‘Zesh004’ (marketed as Zespri® Sweet Green Kiwifruit, commonly called Green14), A. chinensis var. deliciosa ‘Tomua’, and ‘Hayward’ cultivars were harvested from 3- or 4- vine plots on a single orchard block at the PFR research
2.3.2. Soluble solids content At harvest, SSC was determined separately for the stylar and stem ends of the fruit and averaged using a hand-held refractometer (Master Series, 0–30%, Atago). After fruit ripening, juice was measured using a digital refractometer (Pocket PAL-1, 0–50%, Atago). 177
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2.3.3. Seed coat colour Seed coat colour was scored by eye and is reported as the percentage of seeds that were dark/black.
Table 1 Rate of on-vine softening of ‘Hayward’ kiwifruit in 2009–2012. Linear fits and their correlation (r) and regression (R2) coefficients to data in Fig. 1 edited to the linear slow softening phase (Phase 1) only.
2.3.4. Fresh weight Fruit fresh weight (FW) was estimated by calculation from calliper measurements in mm of fruit length (L), and the minimum (D1) and maximum (D2) diameters (0.454 × (L × D1 × D2)1.05); Snelgar et al., 1992).
Harvest
Softening rate
Correlation
Regression
Season 2009 2010 2011 2012
(N/week) 3.24 2.94 3.53 2.45
(r) −0.931 −0.970 −0.982 −0.979
(R2) 0.85 0.94 0.96 0.95
2.4. Data analysis Trends and patterns within the data are described graphically using Origin v8.5 (OriginLab Corporation, One Roundhouse Plaza, Northampton, MA01060, USA). Linear fitted lines, Pearson’s correlation coefficients and coefficients of regression of the linear fits were calculated. Statistical separation among sample means was by analysis of variance (ANOVA) using GenStat Release 14.2 [(PC/Windows 7) Copyright 2011, VSN International Ltd, UK]. 3. Results and discussion 3.1. On-vine softening of ‘Hayward’ kiwifruit ‘Hayward’ fruit showed a consistent slow linear softening phase of 2.5–3.5 N/week whilst on the vine for the four seasons monitored between 2009 and 2012 (Fig. 1, Table 1). Despite similar rates, there were between-season differences of up to ∼10 N in fruit firmness at a given time in the year. For all seasons, there was a quite uniform slow rate of softening during the normal harvest window, up to the end of May/early June. In 2012, fruit were specifically left on the vine after the normal harvest window to see the full development of on-vine softening. There was a rapid softening phase of up to 21 N/week from mid-June, which resulted in fruit at ∼25 N at the end of June.
Fig. 2. Fruit firmness of five cultivars of kiwifruit (‘Zesy002’, ‘Zesy003’, Zesh004’, ‘Tomua’ and ‘Hayward’) during fruit development. Vines grown at the same site, harvested in 2014. Each value is the mean of 30 fruit. Table 2 On-vine fruit softening rates of five kiwifruit cultivars (‘Zesy002’, ‘Zesy003’, ‘Zesh004’, ‘Tomua’, ‘Hayward’) grown at the same site in 2014. Linear fits and their correlation (r) and regression (R2) coefficients to selected data for Phase 1 (Slow) and Phase 2 (Rapid) of softening from Fig. 2.
3.2. On-vine softening of five cultivars of kiwifruit In 2014, the on-vine softening of a range of cultivars, including ‘Hayward’, was compared for vines grown on the same site. At the start of fruit monitoring at the end of February, the fruit firmness differed from ∼78.5 N in ‘Zesy003’ to ∼117.7 N in ‘Hayward’ and ‘Zesh004’. Fruit from all five cultivars showed a similar pattern of slow softening for a period of weeks followed by more rapid softening (Fig. 2, Table 2), the change in softening rate occurred between late April and mid-May, when fruit firmness was between 68.7 and 83.4 N. For ‘Hayward’, there was little noticeable change in softening rate, and in contrast with the
Cultivar
‘Zesy002’ ‘Zesy003’ ‘Zesh004’ ‘Tomua’ ‘Hayward’
Phase 1 (Slow)
Phase 2 (Rapid) 2
Rate (N/wk)
r
R
2.26 0.98 1.96 1.77 3.04
−0.955 −0.867 −0.912 −0.941 −0.988
0.90 0.72 0.82 0.87 0.97
Rate (N/wk)
r
R2
13.54 10.69 14.32 6.87 5.79
−0.999 −0.998 −0.999 −0.985 −0.987
0.99 0.99 0.99 0.96 0.97
data for 2012, the fruit were still at ∼47 N at the end of June. In ‘Hort16A’, it has been observed that the change to rapid fruit softening on the vine occurred when the fruit were at 40–60 N, and not at a fixed firmness value (Burdon, unpublished data). ‘Zesy002’ and ‘Zesy003’ were the first cultivars to commence rapid fruit softening in late April, followed by ‘Zesh004’ in mid-May. ‘Tomua’ and ‘Hayward’ differed from the other cultivars with a slower rate of rapid softening (Table 2), commencing in early May for ‘Tomua’ and late May for ‘Hayward’. The change from slow to rapid softening in ‘Hayward’ was less distinct than for the other cultivars. Even with this limited data set, it is possible to discriminate among the cultivars for the earlier, faster softening A. chinensis var. chinensis cultivars (‘Zesy002’ and ‘Zesy003’) and the later, slower A. chinensis var. deliciosa cultivars (‘Tomua’ and ‘Hayward’). The change to rapid softening in the hybrid between the cultivars var. chinensis and var. deliciosa (‘Zesh004’) occurred later than, but to a similar rate as, the var. chinensis cultivars. To some extent, the differences among cultivars in the timing of the change to rapid softening can be associated with the flowering window
Fig. 1. Fruit firmness of ‘Hayward’ kiwifruit on the vine during fruit development. Fruit from the same 2 vines on a single orchard measured during the 2009–2012 harvest seasons. Each value is the mean of 30 fruit.
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Table 3 Softening during storage at 0 °C of ‘Hayward’ kiwifruit harvested at 120–197 days after mid-bloom (DAMB) in 2010. Duration of Phase 1 and the softening rates in Phase 1 (slow) and Phase 2 (rapid) calculated from linear fits to the data in Fig. 3. Harvest
Duration of Phase 1
Softening rate (N/week)
(DAMB)
(Weeks)
Phase 1 (Slow)
Phase 2 (Rapid)
120 141 169 176 197
8 6 4 4 0
2.6 3.2 4.7 5.2 naa
9.8 10.6 10.1 10.8 9.1
a
na not applicable − no slow phase.
context for the developmental state of the fruit at each harvest. The seeds had all darkened by early April and fruit growth had ceased by late April. There was a consistent trend for an increasing rate of soluble solids accumulation throughout fruit development from early April until the last harvest on 1 June. The firmness at which softening stabilised in storage at Phase 3 appears dependent on maturity at harvest, being firmer at ∼8.8 N for fruit harvested at 197 DAMB and softer at ∼4.9 N for fruit harvested at 141–169 DAMB. The interpretation of this final firmness value is affected by the development of chilling injury in some of the early harvested fruit. This can affect softening such that chill damaged fruit remain relatively firm late into storage, but when they soften, they soften rapidly and continue to do so until over-soft (< 4 N). The data show that a wide range of softening patterns can be achieved from fruit of a single genotype depending on maturity. It is also known that the softening pattern can be affected by other factors: faster softening at higher storage temperature or with ethylene treatment and slower softening when stored in controlled or modified atmosphere conditions (Burdon and Lallu, 2011). Also, separate measurements of outer pericarp and core firmness during storage show these tissue zones in ‘Hayward’ kiwifruit to have different softening patterns (Fig. 4). While the outer pericarp softening is similar to the standard penetrometer measurement, being an exponential decay, or Phases 2 and 3 of the previously described sigmoidal, the core softening is more linear. For ‘Hayward’ kiwifruit, the different ripening patterns of the outer pericarp and core do not lead to a problem with fruit quality. The use of ethylene to ripen fruit harvested early in the season (Lallu et al., 1989) ensures that all tissue zones soften at the same time (data not shown) and more mature fruit tend to be stored allowing softening to occur
Fig. 3. ‘Hayward’ kiwifruit softening patterns on the vine (A) and in storage at 0 °C (B). Fruit harvested at weekly intervals up to 197 days after mid-bloom (DAMB) in 2010. Also shown in A are the fruit soluble solids content (SSC), incidence of black seeds and fresh weight as indicators of fruit development. Each value is the mean of 30 fruit (A) or 20 fruit (B), ± s.e.m.
of the five cultivars, for which mid-bloom occurs within a 3–4 week period. ‘Zesy003’ was the earliest to mid-bloom, followed by ‘Zesy002’, ‘Zesh004’, ‘Tomua’ and ‘Hayward’, in that order. The seasonal difference between 2012 and 2014 in ‘Hayward’ fruit firmness at the end of June was marked, with fruit in 2012 being ∼25 N, whereas in 2014 the fruit firmness was still ∼50 N. 3.3. Softening of ‘Hayward’ kiwifruit in storage There was little or no association between the on vine softening rate of ‘Hayward’ fruit and the softening of fruit in coolstore at 0 °C (Fig. 3). While there was little change in the rate of softening on the vine, at ∼2.9 N/week, there were marked differences in the softening patterns of the fruit from different harvests when in storage. In storage, Phase 1 (slow) softening started at similar rates (2–3 N/ week) to the on-vine softening from early harvests, but slowly increased in later harvested fruit up to 5.9 N/week (Table 3). The duration of the slow Phase 1 softening period was reduced in fruit from later harvests. The accuracy of the calculated rates, and the duration of Phase 1, is limited by only sampling the fruit at 2-weekly intervals. Phase 2 (rapid) rates of softening in storage were consistent among harvests within the range 8.8–10.8 N/week (Table 2). As with the rate and duration of Phase1, the accuracy was limited for identifying the change-over point for Phase 1 to Phase 2 and also from Phase 2 to Phase 3. The fruit seed colour, growth data and SSC in Fig. 3A provide more
Fig. 4. Softening pattern of the outer pericarp ( ; standard firmness measurement with a 7.9 mm diameter penetrometer) and core ( ; 4 mm diameter penetrometer) of ‘Hayward’ kiwifruit stored at 0 °C for up to 20 weeks in 2014. Each value is the mean of 20 fruit ± s.e.m.
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Fig. 6. Firmness of ‘Hayward’ kiwifruit measured at harvest (At-h) and after one week at 16, 14, 12, 10, 8, 4 or 0 °C. Fruit were harvested on 21 May 2012, 190 days after midbloom, when soluble solids content was 9.4%. Each value is the mean of 20 fruit. Values not sharing a common letter differ at P = 0.05.
softening at 10 °C was greater than for earlier harvested fruit, with a firmness loss of 20–30 N. Statistical analysis of the temperature response of fruit harvested on 21 May 2012 is shown in Fig. 6, in which a greater number of temperatures have been included than presented in Fig. 5. The temperature response suggests that increased softening may be induced as temperatures decreased from 16 °C to about 10 °C, but that lower temperatures slowed softening, possibly on the basis of both simple biochemical reaction rates and also possibly by more specific temperature-regulated effects, as suggested by the marked difference in the softening of fruit at 4 °C and 0 °C. The changes in softening rates are beyond any simple temperature-firmness coefficient as used to adjust firmness values for fruit measured at different temperatures (Jeffery and Banks, 1994). The marked change in capacity to respond to temperature occurred in fruit that had not shown any change in softening rate on the vine, and 1–2 weeks before on-vine softening started to accelerate. However, this change in capacity to respond to temperature was associated with the time at which fruit growth ceased (Fig. 5). Taking the earlier finding of the development with maturation of the capacity to respond to temperature (Figs. 5 and 6), a more detailed trial showed the degree of softening that can be induced over a short time-frame of 3 weeks at a range of temperatures (Fig. 7) or over a longer timeframe in storage at 0 °C following up to 6 days at a range of temperatures between 4 and 16 °C (Fig. 8). The short term effect of temperature on ‘Hayward’ fruit softening seen in 2012 was repeated in 2013 and compared with another cultivar, ‘Zesh004’ (Fig. 7). While similar trends were seen for both cultivars, there were differences in both the degree of softening, and also the separation of softening rates among the treatment temperatures. In ‘Hayward’, most rapid softening occurred at 8 °C, then 4 °C with 12 °C intermediate between 8 and 0 °C. In ‘Zesh004’, most rapid softening occurred at 8 °C and 12 °C and then 4 °C. Softening at 4, 8 and 12 °C were all significantly faster than at 16 °C or 0 °C. The apparently greater softening in ‘Zesh004’ than ‘Hayward’ may merely reflect a difference in the at-harvest maturity and thus capacity to respond. It does, however, indicate some degree of commonality in response of different kiwifruit cultivars to temperature. Also, it has been shown in ‘Hort16A’ kiwifruit that there is similarly an increased capacity to soften in response to ethylene which occurs before any change in softening rate on the vine (Burdon et al., 2014). Whether there are any commonalities in the metabolic control of these softening responses remains to be determined. This also concurs with a finding that low temperature may modulate softening in an ethylene independent manner (Mworia et al., 2012).
Fig. 5. Pattern of ‘Hayward’ kiwifruit softening on the vine (A; FF) and softening in response to temperature (1 week at 0–16 °C) off the vine (B). Also shown in A are the fruit soluble solids content (SSC), incidence of black seeds (BS) and estimated fruit weight (FW) as indicators of fruit development and maturation. Each value is the mean of 30 fruit (only 15 for FW; A) or 20 fruit (B).
prior to marketing and consumption. 3.4. Softening of harvested ‘Hayward’ kiwifruit in response to temperature In the 2012 season, sampling from the vine was continued past the commercial harvest and through June to observe the more rapid fruit softening on the vine (Fig. 5A). The rate of slow softening on the vine was similar to previous years at ∼2 N/week (Fig. 1). The prolonged period of sampling resulted in observation of a softening rate of up to 20.6 N/week towards the end of June. The response of harvested fruit to temperatures between 16 °C and 0 °C was consistent between harvests, with fruit held at 10 °C being softest one week after harvest (Fig. 5A and B). While the pattern of softening in response to different temperatures was consistent, the magnitude of change in firmness increased markedly between 14 May 2015 and 21 May 2015 (Fig. 5B). Fruit harvested on 16 April 2012 showed little softening during one week after harvest, irrespective of temperature. For the next few harvests, it was apparent that fruit held at 10 °C tended to soften slightly more than the fruit held at other temperatures, with fruit at 16 °C, 4 °C or 0 °C being approximately the same firmness after one week, and little changed from harvest. Fruit from 21 May 2012, and later, showed several differences from earlier harvested fruit. The fruit held at 16 °C softened more markedly, fruit held at 0 °C were little changed from at-harvest firmness and the 180
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softening at 8 °C for ‘Hayward’ fruit shown in Fig. 7. The firmness of ‘Hayward’ fruit held at 8 °C for 6 days appears anomalous alongside the other treatments (Fig. 8), yet the softening rate and firmness is in fact in line with the softening seen at 8 °C in previous years (2013; Fig. 7). 4. Summary The pattern of ‘Hayward’ kiwifruit softening on the vine and in storage was sigmoidal. On the vine, the initial slow softening rate of ‘Hayward’ fruit was consistent among seasons, and similar to other cultivars, at ∼2.9 N/week or less. The rate of rapid softening differed among cultivars at 5.9–14.7 N/week, with A. chinensis var. chinensis cultivars showing more rapid softening than A. chinensis var. deliciosa cultivars. In storage, the initial slow phase of softening was only seen in less mature fruit harvested before any increase in softening rate on the vine. It is also known that storage temperature and atmosphere (oxygen, carbon dioxide and ethylene) will affect the rate of softening of the main phase of rapid softening (Burdon and Lallu, 2011). ‘Hayward’ fruit developed with maturation a capacity to soften in response to low temperatures, occurring 1–2 weeks before faster softening occurred on the vine. The change in softening rate was associated with cessation of fruit growth and accumulation of carbohydrate. There may be three aspects of fruit softening to consider in response to temperature: induction of softening at low (e.g. 8–10 °C) but not chilling temperatures, slowing of biochemical reaction rates that affect softening by lower temperatures (e.g. 4 °C) and chilling by low temperatures close to 0 °C. The findings from this work have implications for both researchers and commercial operators. In conducting research, understanding the physiological state of the fruit at harvest is relevant to any findings on the postharvest responses of the fruit. Commercially, temperature management and the response of fruit to temperature is a major component of the commercial handling of kiwifruit. A better understanding of cultivar-specific responses of fruit to temperature raises the possibility that general approaches taken to manage fruit performance, such as through slow or fast cooling regimes, may be better tailored to match the fruit’s capacity to respond to, or withstand, specific temperatures. More precise temperature management may improve the use of conditioning temperatures for flesh degreening (De Silva et al., 2007; Burdon and Lallu, 2011), ripening of early harvested fruit (Lallu et al., 1989), curing in controlled refrigerated conditions (Tonini et al., 1999) or the storage of early harvested fruit or fruit of chilling sensitive cultivars.
Fig. 7. Softening of ‘Hayward’ (A) and ‘Zesh004’ (B) kiwifruit in response to holding at 0, 4, 8, 12 and 16 °C after harvest in 2013. Soluble solids content at harvest 8.1% and 10.3%, respectively. Each value is the mean of 30 fruit. Bars are LSD at P = 0.05 among individual treatments.
Acknowledgements The authors gratefully acknowledge the assistance of Murray Judd for access to the commercial ‘Hayward’ orchard used in 2009–2012. This work was conducted as part of the Plant & Food Research Core Kiwifruit programme ‘Premium kiwifruit: Growing the future’. References Antunes, M., 2007. The role of ethylene in kiwifruit ripening and senescence. Stewart Postharvest Rev. 3, 1–8. Atkinson, R.G., Gunaseelan, K., Wang, M.Y., Luo, L., Wang, T.C., Norling, C.L., Johnston, S.L., Maddumage, R., Schroder, R., Schaffer, R.J., 2011. Dissecting the role of climacteric ethylene in kiwifruit (Actinidia chinensis) ripening using a 1aminocyclopropane-1-carboxylic acid oxidase knockdown line. J. Exp. Bot. 62 (11), 3821–3835. Burdon, J., Lallu, N., 2011. Kiwifruit (Actinidia spp.). In: Yahia, E.M. (Ed.), Postharvest Biology and Technology of Tropical and Subtropical Fruit. Volume 3, Cocona to Mango. Woodhead Publishing Limited, UK, pp. 326–360. Burdon, J., Pidakala, P., Martin, P., McAtee, P.A., Boldingh, H.L., Hall, A., Schaffer, R.J., 2014. Postharvest performance of the yellow-fleshed ‘Hort16A’ kiwifruit in relation to fruit maturation. Postharvest Biol. Technol. 92, 98–106. Crisosto, C.H., 1997. Final preconditioning guidelines for kiwifruit shippers. Cent. Val. Postharvest Newslett. 6, 1–4. De Silva, N., Hall, A.J., Burdon, J., Lallu, N., Connolly, P., Amos, N., 2007. Modelling the
Fig. 8. Softening of ‘Hayward’ kiwifruit during storage for up to 20 weeks at 0 °C following holding at 4, 8 or 16 °C for 2, 4 or 6 days, or placing directly to 0 °C in 2014. Each value is the mean of 20 fruit. Bars are LSD at P = 0.05 among individual treatments.
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