HORTSCIENCE 43(5):1377–1381. 2008.

Turgor Closely Relates to Postharvest
Fruit Softening and Can Be a UsefulIndex to Select a Parent for Producing
Cultivars with Good Storage
Potential in Apple
Hiroshi Iwanami1, Shigeki Moriya, Nobuhiro Kotoda,
and Kazuyuki Abe
Apple Breeding and Physiology Research Team, National Institute of Fruit
Tree Science, Shimo-kuriyagawa, Morioka 020-0123, Japan
Additional index words. Malus ·domestica, cultivar difference, firmness, mealiness, shelf life
Abstract. Changes in turgor and flesh firmness during storage at 20 8C were investigatedusing 27 apple (Malus ·domestica Borkh.) cultivars for 2 years. Flesh firmness wasmeasured using a penetrometer, and turgor was determined using a thermocouple
psychrometer. Firmness and turgor of fruit decreased during storage. The cultivars withlittle softening during storage had low rates of reduction in turgor. The softening rates
in mealy cultivars were high, but there were cultivars with low rates of turgor reduction.
When the rates of reduction in turgor after harvest were low, the mealy cultivars of the
fruit tended to develop severe mealiness during storage. Therefore, a low rate of reduction in turgor could contribute to cultivars with both good shelf life and severe mealiness.
The reduction rates of turgor in progeny cultivars were nearly identical to the mean
reduction rates of turgor of their parents. This suggests that a cultivar with a low
reduction rate of turgor, although it can be mealy, has the potential to produce a progeny
with a low reduction rate of turgor.

Softening of fruit after harvest is a serious
problem in the apple industry, and the mechanisms of softening have been thoroughly
investigated. In microscopic studies, as apple
fruit softened, the middle lamella degraded,
and cells separated when failure occurred in
the parenchyma tissue (Glenn and Poovaiah,
1990; Lapsley et al., 1992). When fruit werefractured in tensile tests, the fractures in freshand firm fruit showed that cells were broken
predominantly across the equator (cell fracture) of the fruit, whereas in stored and soft
fruit, cells were separated at the middle
lamella without any damage (cell-to-celldebonding) (Harker et al., 2002). Mealiness,
which is characterized by texture deterioration, resulting in soft and dry fruit, is a typicalcondition of the tissue showing cell-to-celldebonding. The way that cells separate or
break open is determined by the strength ofthe cell wall relative to the strength ofadhesion between neighboring cells, and bothmight be expected to decline as fruit softens(Harker et al., 1997). Mealy fruit tend to losecell-to-cell adhesion but maintain cell wallstrength; as a result, it is difficult to ruptureindividual cells and release their content
when chewing (Harker and Hallett, 1992).

Received for publication 12 Mar. 2008. Acceptedfor publication 28 Apr. 2008.
1
To whom reprint requests should be addressed;
e-mail hiwanami@affrc.go.jp

HORTSCIENCE VOL. 43(5) AUGUST 2008

Cell turgor has a major function in determining tissue strength and changes in turgor
are an integral part of fruit softening (Harker
et al., 1997). A positive relationship between
cell turgor and tissue puncture firmness wasobserved using four cultivars with different
softening rates (Tong et al., 1999). Fruit
treated with a solution of calcium chloride
had higher turgor and lower internal air space
and were firmer than nontreated fruit after
storage (Saftner et al., 1998). When turgor
was modified by soaking the tissue in several
solutions of mannitol, the fruit tissue failed
by cell rupture at high turgor and by cell
debonding at low turgor, and failure force
under compression was reduced as turgor
decreased (Lin and Pitt, 1986). These results
suggest that a reduction in turgor could
reduce firmness. On the other hand, mealy
fruit had higher turgor than nonmealy fruit(Harker and Hallett, 1992). This high turgor
could induce cells to expand and becomerounded; as a result, the internal air space
could increase, the cell contact area could
decrease, and cell adhesion could be reduced(De Smedt et al., 1998; Harker and Hallett,1992; Hatfield and Knee, 1988). Therefore,high turgor could contribute to both fruit
firmness and mealiness.
Mealiness is a sensory texture attributeand a convenient method for evaluating
mealiness objectively is not yet available.
Because mealiness is characterized by weakstrength of adhesion between neighboring
cells, Motomura et al. (2000) proposed aquantitative method for measuring the degreeof cell separation by shaking tissue discs in a
sucrose solution. This method can only detect
the contribution of cell separation to tissue
failure. Using this method, Iwanami et al.
(2008) revealed that the occurrence anddevelopment of mealiness (cell separation)
were considered to involve major genes andthat apple genotypes could be divided into
two types, mealy and nonmealy.
Previous studies concerned with the mechanism of softening have been undertakenwith a single or a few cultivars without any
distinction between mealy and nonmealy
genotypes. Moreover, although a reductionin cell turgor may contribute to loss offirmness during storage, turgor was measured
only one or two times during storage, andobservations of the relationships between
firmness and turgor have been limited to thetime immediately after storage. Therefore, theobjectives of this study were to reveal the
relationships between change in turgor andsoftening during storage considering theoccurrence of mealiness.
Materials and Methods
Fruit materials and storage conditions.
Twenty-seven apple cultivars from an
orchard at the Apple Research Station at
the National Institute of Fruit Tree Science(NIFTS), Japan, were selected on the basis ofdiversity of the harvest date. The cultivars
were used as parents in the NIFTS applebreeding programs. Twenty fruit were picked
randomly from a tree when most of the fruit
were determined to be mature on the basisof ground color, texture, flavor, and starchstaining from 19 and 26 of the 27 cultivars in2006 and 2007, respectively (Table 1). After
weighing, the fruit were stored in a chamber
controlled at 20 C and 85% relative humidity (RH). The next morning, four of the stored
fruit were removed from the chamber tomeasure the fruit characteristics as harvest
values. The measurements were performed
with four of the remaining fruit at 5, 10, 20,
and 30 d after the first sampling.
Measurement of flesh firmness, mealiness,and turgor. Flesh firmness was measured
using a penetrometer (FT327; McCormick
Fruit Technology, Yakima, WA) mounted ona drill press and fitted with an 11.1-mm probe
on the pared surfaces of the sunny and shadysides of each fruit.
Mealiness was measured according to
Iwanami et al. (2005). Ten discs (10 mm indiameter, 5 mm in thickness) were takenfrom 10 mm below the skin of the equatorial
region of all sides of each fruit with a cork
borer, infiltrated with a 12% sucrose solution
under a vacuum, filtered with gauze, andweighed. The discs were transferred to a test
tube (diameter 30 mm) containing 10 mL of
the sucrose solution and shaken for 8 h (20 C,
180 rpm, 3-cm wide shake; PLUS SHAKER
EP-1, TAITEC, Tokyo). The degree of mealiness was calculated as (Wi – Ws)/Wi ·100(%), where Wi and Ws = weight of discs
before and after shaking, respectively.

1377

Table 1. Parentage and fruit characteristics at harvest in 27 cultivars evaluated in this study.
Abbreviationin Figure 1
CNT
ASP

Harvestdate

Cultivar
Parentage
Chinatsu
Akane · Earliblaze
American Summer
Unknown
Pearmain
Sansa
SNS
Gala · Akane
14 Sept.
Kiou
KIO
Orin · Senshu
Akane
AKN
Jonathan · Worcester Pearmain
14 Sept.
Tsugaru
TGR
Golden Delicious · Jonathan
21 Sept.
McIntoshMC
Unknown
21 Sept.
Hatsuaki
HTA
Jonathan · Golden Delicious
28 Sept.
Royal Gala
GAL
Kidd’s Orange · Golden Delicious
SilkenSLK
Honeygold · Sunrise
28 Sept.
SenshuSSU
Toko (Golden Delicious · Indo) · Fuji
5 Oct.
MacounMCN
McIntosh · Jersey Black
5 Oct.
Red Gold
RG
Golden Delicious · Richared Delicious
JonathanJT
Unknown
12 Oct.
KitaroKIT
Hatsuaki · Fuji
12 Oct.
Jonagold
JG
Golden Delicious · Jonathan
19 Oct.
Starking DeliciousSD
Sport of Delicious
19 Oct.
Golden Delicious
GD
Unknown
26 Oct.
MegumiMG
Ralls Janet · Jonathan
KotaroKOT
Hatsuak · Fuji
26 Oct.
Shinano Gold
SGGolden Delicious · Senshu
2 Nov.
Orin
OR
Golden Delicious · Indo
9 Nov.
Fuji
FJ
Ralls Janet · Delicious
9 Nov.
Indo
IND
Unknown
16 Nov.
Ralls Janet
RJ
Unknown
16 Nov.
BraeburnBB
Lady Hamilton · unknown
Pink Lady
PLGolden Delicious · Lady Williamsz
Starch staining by iodine solution was rated from 0 = 0% staining to 5 = 100% staining.

The turgor was determined using a thermocouple psychrometer (SC-10A; Decagon
Devices, Pullman, WA) in a room controlled
at 20 C. Because the accuracy of a measurement with the instrument can also be affectedby low RH, the room was controlled at 75%
RH. Two discs (14.5 mm in diameter, 4 mm
in thickness) were taken from 10 mm below
the skin of the equatorial region of each fruitwith a cork borer and placed on the bottom of
the psychrometer sampling cup immediately
after excision. The surface of the discs was
wiped off with tissue paper. After measuring
the water potential of the individual discs, eachsample cup was sealed and frozen at –80 C
and thawed at 20 C. When temperatureequilibrium was achieved, the osmotic potential (yS) was measured for the same sample.
Data were obtained as microvolts and converted to MPa based on a calibration curveusing NaCl standards. The turgor potential
was calculated by subtracting the yS from thewater potential.
Results
Changes in firmness, turgor, and degree of
mealiness in the flesh during storage variedamong cultivars. According to the relationships among change in firmness, turgor, andmealiness, the cultivars were divided into fivegroups: 1) fruit that softened rapidly within
10 d of harvest, turgor continued to decrease
throughout storage, and mealiness occurredand developed at an extreme rate for 20 d(Fig. 1A); 2) fruit that softened within 20 d,turgor decreased rapidly within 20 d, but
no further decrease was observed during the

1378

2006
Firmness(N)

StarchindexzHarvestdate
16 Aug.
30 Aug.

2007
Firmness
(N)
72.1
65.8

Starchindexz
2.7
2.6

53.2

1.7

67.2
45.5
50.4
58.2

1.3
1.3
2.9
0.6

61.3
64.2
76.3

2.9
1.9
3.2

6 Sept.
6 Sept.
13 Sept.
13 Sept.
20 Sept.
20 Sept.
27 Sept.
27 Sept.
4 Oct.

63.7
82.2
69.7
54.9
56.8
64.1
68.4
71.2
59.7

2.7
3.9
1.1
2.1
2.4
1.6
0.9
3.7
1.7

72.9
66.8
65.9
59.8
59.2

2.5
1.8
1.3
2.6
1.9

75.7
68.2
65.9
76.3
90.9
75.4

2.1
1.6
2.0
1.8
3.0
1.0

4 Oct.
11 Oct.
11 Oct.
18 Oct18 Oct.
25 Oct.
25 Oct.
1 Nov.
1 Nov.
8 Nov
8 Nov.
15 Nov.
15 Nov.
22 Nov.
22 Nov.

70.6
68.5
60.2
77.3
54.2
70.6
57.9
74.8
68.1
65.1
67.5
84.3
70.3
92.1
93.3

4.0
2.3
4.0
2.5
2.1
1.9
2.5
1.8
1.7
2.1
1.6
3.0
1.4
1.3
2.5

rest of the storage period, and mealinessoccurred and developed (Fig. 1B); 3) fruitthat softened within 20 d, turgor decreased
within 20 d, but no more decrease was observed during the rest of the storage period,and mealiness occurred but did not develop(Fig. 1C); 4) fruit that softened relatively
slowly, turgor continued to decrease throughout storage, and slight mealiness occurred at
the end of the storage period (Fig. 1D); and 5)
fruit that did not soften significantly, turgor
continued to decrease throughout storage,
and mealiness hardly occurred (Fig. 1E).
Considering the occurrence and developmentof mealiness exclusively, distinct segregation
was observed at 30 d of storage between thecultivars in groups A–B and C–E in Figure 1.
The cultivars in groups A–B can be defined
as mealy.
Although the measurements of firmness at
harvest in the cultivars in group D were above
70 N and higher than those in groups C and E,
except for ‘Pink Lady,’ the measurements ofturgor at harvest in the cultivars in group D
were not always higher than those in groups
C and E (Fig. 1). Moreover, although themeasurements of firmness at 30 d of storage
in the cultivars in group A were below 40 N
and lower than those in groups D and E, themeasurements of turgor at 30 d of storage inthe cultivars in group A were from 0.1 to 0.2
MPa and almost the same as those in groups
D and E. Therefore, the value of turgor itself
did not seem to determine the measurementof firmness. Although the degrees of reduction
in turgor and firmness during storage variedamong cultivars, the periods of the reduction
in firmness were almost the same as those of

the reduction in turgor, and softening seemedto accompany the reduction in turgor in mostcultivars, except for those in group A. In thecultivars in Figure 1A, firmness decreased
within 10 d with the decrease in turgor, but thedecrease in firmness slowed down after 10
d despite the continuous decrease in turgor.
Because softening occurred with the
reduction of turgor for at least 10 d in almost
all cultivars, the genotypic difference andrelationship between softening and the reduction of turgor were evaluated by comparing
the rates of the reduction of firmness andturgor for 10 d. The rates of the reduction of
firmness and turgor were obtained by applying a linear regression to the changes in firmness and turgor, respectively. Individual fruit
data for each sampling, not mean value ofeach sampling fruit, were used for the application of the linear regression. In the cultivars
in group E in Figure 1 (nonmealy, nonsoftening), the softening rates were all low, and the
reduction rates of turgor were also low (Fig.
2). On the other hand, the softening rates of
the cultivars in groups A and B (mealy) were
all high, but the reduction rates of turgor
varied widely. The softening rates of ‘Starking Delicious’ and ‘Macoun’ were high, but
the reduction rates of turgor were nearly
identical to those of the nonsoftening group
cultivars Fuji or Senshu. The softening ratesof the cultivars in groups C and D (nonmealy,
softening) were intermediate, and the reduction rates of turgor were higher thanthose of the cultivars in group E. The softening rates were significantly correlated withthe reduction rates of turgor in nonmealy
cultivars.
HORTSCIENCE VOL. 43(5) AUGUST 2008

Fig. 1. Changes in flesh firmness, turgor, and degree of mealiness in 27 apple cultivars during storage at 20 C, 85% relative humidity. The data were collected in2007 for all cultivars except ‘Macoun’. (A) Fruit that softened rapidly for 10 d; turgor continued to decrease throughout storage; and extreme mealinessoccurred and continued to develop for 20 d. (B) Fruit that softened within 20 d; turgor decreased rapidly for 10 or 20 d; and mealiness occurred and developed.
(C) Fruit that softened for 20 d; turgor decreased for 20 d; and mealiness occurred but did not develop. (D) Fruit that softened relatively slowly; turgor
continued to decrease throughout storage; and some mealiness occurred late in storage. (E) Fruit that did not decrease significantly; turgor continued todecrease throughout storage; and mealiness was slight. Vertical bars represent SE of four fruit replications.

The measurements of turgor were notalways higher or lower in mealy cultivars
than nonmealy cultivars at harvest or at theend of storage (Fig. 1), indicating that thevalue of turgor itself also did not determine
the occurrence of mealiness. However, thedegrees of mealiness at 30 d of storage were
influenced by the reduction rates of turgor.
When the reduction of turgor after harvest
was slow, mealy cultivars tended to develop
severe mealiness during storage. Significant
negative relationships were observed between the degrees of mealiness at 30 d ofstorage and the reduction rates of turgor inmealy cultivars (Fig. 3).
Twelve cultivars (Sansa, Kiou, Tsugaru,
Hatsuaki, Red Gold, Kitaro, Jonagold,
Megumi, Kotaro, Shinano Gold, Orin, andFuji) have resulted from the crosses between
the cultivars used in this study, assuming thatthree sport cultivars, Starking Delicious,
Richared Delicious, and Royal Gala, are thesame as the original cultivars Delicious andGala. The reduction rates of turgor in the 12
cultivars were significantly correlated withthe mean reduction rates of turgor in thoseparent cultivars (Fig. 4A). The regression
coefficient was 1.01 ± 0.26, indicating thatthe value of the reduction rate of turgor in aprogeny cultivar was close to the mean valueof those in its parents. Significant relationHORTSCIENCE VOL. 43(5) AUGUST 2008

ships were also observed between the softening rates of the progeny cultivars and thoseof their parents (Fig. 4B), but the regression
coefficient (1.33 ± 0.52) was far from 1.0 because there were outliers (‘Jonagold’, ‘ShinanoGold’, and ‘Fuji’) and the effect of the outliers on the regression line was very large.
Discussion
Softening was closely related withchanges in turgor because the reduction in
turgor slowed during storage accompanied
slow softening in most cultivars (Fig. 1). The
value of turgor itself, however, did not seemto determine the measurement of firmnessand occurrence of mealiness. Tong et al.
(1999) compared puncture firmness with cell
turgor after 6 months of cold storage andindicated that the fruit of ‘Honeycrisp’ and‘Delicious’ were firmer and had higher turgor
than those of ‘Honeygold’ and ‘Macoun’.
Therefore, they suggested that there werepositive relationships between firmness andturgor. However, the fruit of ‘Honeygold’after 3 months of cold storage were as firm as
those of ‘Delicious’ and firmer than those of‘Macoun’ but had as low turgor as that of‘Macoun’. Therefore, the relationship between the values of firmness and turgor could
depend on the storage period.

The softening rates and reduction rates ofturgor during 10 d of storage at 20 C were
continuously distributed among the cultivars
tested (Fig. 2). Softening is a process primarily associated with changes in the cell wallsof parenchyma cells, which result in tissue
failure (Harker et al., 1997). As fruit soften,
tissue failure predominantly occurs first bycell fracture, then by cell rupture, and finallyby cell-to-cell debonding (Harker et al.,2002). Harker et al. (2006) observed that fruitof ‘Pacific Rose’ did not soften during longstorage and the failure modes of the fruit weremainly cell fracture both at harvest and after
storage. The fruit of ‘Jazz’ softened by 10%
to 20% of firmness during storage, showingthat the failure modes of the fruit were mainly
cell fracture at harvest but cell rupture exclusively after storage. In our study, although asignificant positive relationship was observed
between both rates in nonmealy cultivars, thecultivars in which the reduction rates ofturgor were from 0.010 to 0.014 MPad–1
had a relatively wide range of softening rates
from 0.3 to 2.5 Nd–1 (Fig. 2). Our results,
therefore, suggested that tissue failure could
occur by cell fracture exclusively and by cell
rupture in nonmealy fruit with a reduction
rate of turgor lower than 0.010 MPad–1 andhigher than 0.014 MPad–1, respectively. The
range between 0.010 and 0.014 MPad–1 can1379

Fig. 2. Relationship between the reduction rates of turgor and the softening rates in 27 apple cultivars
stored at 20 C. The letters with each symbol are an abbreviation of the cultivar. The cultivars weredivided into three groups according to the relationship between change in firmness and mealinessas shown in Figure 1. The mealy cultivar, nonmealy and softening cultivar, and nonmealy andnonsoftening cultivar belong to groups A to B, C to D, and E in Figure 1, respectively. The solid line
represents the regression equation calculated using data of nonmealy cultivars.

Fig. 3. Relationship between the reduction rate ofturgor and the degree of mealiness at 30 d ofstorage in 13 mealy apple cultivars stored at
20 C. The letters with each symbol are theabbreviation of the cultivar. The solid line
represents the regression equation.

be defined as a period of transition, and bothcell fracture and rupture may occur at various
ratios depending on fruit conditions.
The softening rates of mealy cultivars
were all high and over 2 Nd–1 in mostcultivars, although the reduction rates ofturgor varied widely (Fig. 2). The highsoftening rates were consistent with theresults by Iwanami et al. (2005), indicatingthat the softening rates of mealy cultivars
were all high irrespective of the degree of
mealiness. Harker et al. (2006) also observedthat fruit that softened markedly during1380

Fig. 4. Relationship between the mean value ofparents and the value of their progeny cultivar(A) in the reduction rate of turgor and (B) in thesoftening rate of 12 apple cultivars. The valuesof the progeny and parent cultivar are averagedfor 2 years when applicable. The solid line
represents the regression equation.

storage failed by cell-to-cell debonding.
What was rather surprising in our study isthat there were cultivars with a low reduction
rate of turgor. It seems, however, natural for

mealy cultivars to retain relatively highturgor after harvest because high turgor could
cause cells to swell, resulting in a reductionof the cell-to-cell contact area (Harker andHallett, 1992). In fact, a negative relationshipwas observed between the degree of mealiness at 30 d of storage and the reduction rateof turgor in mealy cultivars (Fig. 3), indicating that the smaller the change in turgor after
harvest, the larger the degree of mealinessdeveloping during storage. The reduction
rates of turgor in the mealy cultivars Starking
Delicious and Macoun were nearly identicalto those in cultivars that had barely softened
during storage in our experiment. This possibly means that the strength of adhesion
between neighboring cells determines thefate of a cultivar that has a low reduction
rate of turgor whether the cultivar has severe
mealiness or long shelf life. On the other
hand, there were mealy cultivars with highreduction rates of turgor, suggesting thattissue can also fail by cell rupture at highratios, although the tissue mainly failed bycell-to-cell debonding. The fruit of the mealy
cultivar Braeburn softened with a high reduction rate of turgor (Fig. 2). Harker and Hallett(1992) indicated that 20% of low- and 80% of
high-maturity fruit became mealy after 16
weeks storage at 0 C in ‘Braeburn’ andHarker et al. (2002) also observed that fruitof ‘Braeburn’ failed very easily as a resultof cell rupture although the fruit was firm
(72 N).
Heritability of the softening rate has been
estimated to be high (h2 = 0.93) (Iwanami
et al., 2008). High heritability means thatprogeny closely resembles its parents in aphenotypic value (softening rate). The resemblance can be displayed by parent–progeny
regression. In this study, the relationships
between the progeny cultivars and theirparents regarding the softening rate weresignificant at the 5% level, but there were
outliers (Fig. 4B). The softening rate of‘Jonagold’ was considerably higher than thatof its parents, and the softening rates of ‘Fuji’and ‘Shinano Gold’ were considerably lower
than those of their parents. Iwanami et al.
(2008) indicated that the softening rates of
mealy progeny tend to shift higher than themean values of the softening rates of theirparents. Therefore, the mealy cultivar Jonagold showed a remarkably higher softening
rate than its parents. On the other hand, one ofthe parents of ‘Fuji’ and ‘Shinano Gold’ is amealy cultivar with the result that its parents
also probably showed a remarkably higher
softening rate than the nonmealy progenies‘Fuji’ and ‘Shinano Gold’. Iwanami et al.
(2008) also indicated that the contribution ofa mealy parent to the softening rate of nonmealy progenies cannot be predicted by its
phenotypic value (softening rate). In contrast
to the softening rate, the relationships between progeny cultivars and their parents in
the reduction rate of turgor were highly
significant (P = 0.003), and the regression
coefficient was 1.01, which indicated thatprogeny cultivars closely resembled its parents and the reduction rate of turgor could be
HORTSCIENCE VOL. 43(5) AUGUST 2008

considered to be the character with the higher
heritability (Fig. 4A).
Iwanami et al. (2008) proposed the softening rate as a phenotypic value and estimated the heritability and the breeding valueof softening to produce a new cultivar withgood storage and shelf life potential. They
also indicated that it was necessary to estimate the breeding value in mealy cultivars
because the softening rates (phenotypic values) could be influenced by the occurrenceof mealiness and were inconsistent with thebreeding values in mealy cultivars. That is,mealy cultivars can produce a nonmealy
progeny, but the softening rate of the nonmealy progeny cannot be predicted from thesoftening rates of the mealy parents. To
estimate the breeding values, many progenies
have to be made, and it takes time and labor.
On the contrary, the reduction rate of turgor
does not relate to the occurrence of mealinessand seems to be a better index than thesoftening rate to select a parent for producing
cultivars with good storage potential. The
softening rate of a progeny from a crossbetween parents with a low reduction rateof turgor can be predicted to be low unless theprogeny develops mealiness. A cultivar witha reduction rate of turgor below 0.01 MPad–1,
although the cultivar can be mealy, has thepotential for producing a progeny with goodstorage potential such as ‘Fuji’ which has along storage potential.
Because textural attributes are determinedby the structural, physiological, and biochemical characteristics of cells, many studies have tried to clarify the mechanism ofsoftening at the cellular levels by investigating the changes in the pectin composition ofcell walls and the activity of cell wallmodifying enzymes (Glenn and Poovaiah,
1990; Nara et al., 2001; Pena and Carpita,
2004; Tong et al., 1999; Yoshioka et al.,1995). Recently, Vicente et al. (2007) identified several tomato mutants whose fruit
exhibit exceptional shelf life, remaining firm
and maintaining an edible texture for many
months after reaching a fully ripe stage. Cellwall degradation and cell separation of the
fruit occurred to the same extent and at thesame rate in these mutants, as in normallysoftening fruit. However, cellular turgor
remained high in them, unlike normallysoftening fruit. These results suggest thatwall disassembly is not the sole and primarycause of fruit softening and that turgor is

HORTSCIENCE VOL. 43(5) AUGUST 2008

critically important. In our study, cultivars
with good shelf life had low reduction rates ofturgor, but cell separation could occur independently of the rate of reduction in turgor.
Therefore, further studies are required toelucidate factors that can influence turgor
and determine the relationship between thesefactors and softening. One of the factors may
be water loss from fruit. However, cultivar
differences in weight loss (equivalent towater loss) of fruit during storage did not
relate to those in the softening rate and the
reduction rate of turgor in our study (data notshown).
In conclusion, fruit softened with a reduction in turgor in almost all cultivars for 10 dof storage at 20 C. The softening rates weresignificantly correlated with the reduction
rates of turgor in nonmealy cultivars, andthe reduction rates of turgor were all low in
cultivars that barely softened in storage. The
softening rates of mealy cultivars were all
high, but the reduction rates of turgor varied
widely. Furthermore, there were cultivars
showing low reduction rates of turgor, whichwere almost identical to those of cultivars
that barely softened in storage. Because
the reduction rates of turgor in progeny cultivars were consistent with the mean reduction rates of turgor in their parents, acultivar with a low reduction rate of turgor,
although the cultivar can be mealy, has thepotential to produce a progeny with a low
reduction rate of turgor that has good storage
potential.
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