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Postharvest water relationships and tissue browning of rambutan fruit
SCIENTIA HORTlCULTuM Scientia Horticulturae 66 ( 1996) 201-208
Postharvest water relationships and tissue browning of rambutan fruit M. Landrigan a**, S.C. Morris b, D. Eamus ‘, W.B. McGlasson d a Northern Territory Department of Primary Industry and Fisheries, P.O. Box 79, Berrimah N.T., 0828, Australia b Commonwealth Scientific and Industrial Research Organisation, Division of Horticulture, North Ryde, NSW, 2113, Australia ’ Northern Territory University, Casuarina, NT, 0810. Australia ’ University of Western Sydney, Hawkbury. Richmond, NSW, 2756, Australia
Accepted 1 April 1996
Abstract The water status of excised spintems or spintem plus pericarp and attached endocarp of rambutan fruits was measured three times during storage at 20°C. Changes in fruit colour were also recorded. The development of browning was preceded by water loss and concomitant declines in water potential of spintems and skin. As water potential decreased cell turgor also declined. There was a strong negative correlation between water potential and browning such that as browning score increased, water potential declined. Similarly relative water content showed a
negative correlation with browning. Water was lost from intact rambutan fruits via spintems and replaced by water from the skin. It is proposed that due to excessive water loss and the development of plasmolysis, a loss of membrane browning processes to proceed.
permeability
occurred and this then allowed the
Keywords: Nephelium lappaceum; Relative water content; Water potential; Water stress
1. Introduction Rambutan is a tree fruit crop cultivated throughout the humid tropics (Van Welzen and Verheij, 1991). The most attractive and distinctive features of rambutan fruit are its bright red and yellow colours, but most particularly its soft fleshy spines (spinterns),
’ Corresponding author: Tel. 61 89 892360; Fax 61 89 892049. 03044238/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. P/I SO304-4238(96)00915-6
202
M. Lundrigan et al./Scientia
Horticulturae 64 (1996) 201-208
typically l-3 cm in length. However rambutan fruit rapidly lose their attractive appearance after harvest because of the development of browning of the skin and spinterns (Watson, 1988). Such changes have a major negative impact upon the sale value of the fruit at market. The development of browning is preceded by water loss from spintems and skin (Pantastico et al., 1975; Landrigan et al., 1994). However, even when water loss is greatly reduced by storage in a high humidity (95% RI-I) environment, browning still develops, albeit more slowly. Furthermore, it has been shown that water lost from the spintem is replaced, at least partially, by water from the skin. This water would seem to travel via the highly developed vascular system linking the spintem and skin (Landrigan et al., 1994). Previous studies of changes in water status of fruit have concentrated on measurements on total fresh weight (Pantastico et al., 1975). Whilst such measures are of value, the use of more precise measurements of tissue water status, such as water potential, and the comparison of spintems with pericarp, will facilitate both tissue comparisons and comparisons with fruit of other species. Furthermore, it will allow consideration of fruit water status in the context of the water relations of the rest of the plant. This research was undertaken to describe the relationships, if any, between water loss, visual browning and water potential of the three separate components that make up the pericarp i.e. the spinterns, the skin and the fibrous layer (endocarp) closest to the pulp. Both water potential and osmotic potential were determined, thereby allowing calculation of turgor pressure, by difference. Relative water content was also determined as an independent assessment of water status.
2. Materials and methods In the first year of study 30 fruits of Nephelium luppaceum (cv. Jitlee) were harvested at the fully red mature stage from a single tree to reduce variability between fruits. Fruit were immediately stored at 20°C and 60% RH in the dark. Measurements of percentage weight loss, subjective scores of skin and spintem browning (Scale l-5: 1 = no browning; 5 = fully browned) and objective colour scores of browning, were made on ten fruits on the day of harvest and then at 4 and 8 days after harvest. Objective colour scores were made using a Minolta Chromameter (model CR-300, Minolta Camera Co., Ltd., Osaka, Japan). A white reference standard was used to calibrate the instrument. The values L* , a * and b * were recorded and the colour term, total colour difference was calculated from these values (Hung, 1990). Four of these fruits were also used for measurements of water potential (see Section 2.1). In the second year, fruits (cv. Jitlee) were harvested at the fully red mature stage and stored at 20°C and 60% RI-I. Measurements of whole fruit weight, weights of excised spintems (excised with a razor blade) and skin (minus spinterns), weights of flesh and seed, subjective scores of skin and spintem browning and objective colour scores of browning using a Minolta Chromameter were taken on ten fruits on the day of harvest, and 1, 2, 4, and 7 days after harvest. Water potential, osmotic potential and relative water content (RWC) were also measured on six of these fruits (see below).
M. L.undrigan et al./ Scientia Horticulturae 66 (1996) 201-208
203
2.1. Water potential Water potential was measured on whole spinterns or discs (10 mm diameter) cut from the skin plus spinterns and fibrous endocarp. These samples were sealed in a C-30 sample chamber coupled to a Wescor HR-33T thermocouple psychrometer (Wescor Inc., UT, USA). The sample chambers were placed in a water bath in a temperature controlled room set at 20°C. Water potential was measured using the psychrometric mode following a minimum equilibration of 2 h. Each chamber was calibrated using potassium chloride solutions of known molality (Wiebe, 1971). Apoplastic dilution from the cut surface of the tissue was not taken into account. 2.2. Osmotic potential Osmotic potential was measured by the psychrometric method in the C-30 chambers. After measurement of the water potential the tissue samples were frozen in liquid N,. Upon thawing osmotic potential was determined. 2.3. Relative water content Relative water content was determined by adapting the method of Turner (1981). Whole spintems or discs (IO mm diameter), cut as above, were weighed, floated in covered petri dishes at 20°C for 4 h to rehydrate before reweighing and then oven dried and dry weight measured. 2.4. Statistical analyses Data from all experiments were subjected to analyses of variance and where appropriate the means compared using Waller-Duncan’s Bayesian k-ratio LSD at k = 100 (Steel and Torrie, 1980). The level of k = 100 used here is approximately equal to the P = 0.05 level in conventional LSD. Simple correlations between objective colour values, subjective browning scores and weight loss measurements were generated by linear regression analyses.
3. Results Weight loss by fruit was substantial (23%) after the first 4 days of storage and increased to 35% (P < 0.01) by Day 8 (Fig. l(a)). Differences were seen in rates of browning between rambutan skin and spinterns. Spintem browning significantly increased (P < 0.01) with duration of storage and was consistently higher than skin browning. Changes in skin browning were not apparent until Day 8 and browning was not as extensive in skin as in spintems (Fig. l(a)). The a * values, which give a measure of the redness of the fruit, only showed a significant (P < 0.01) decline after 8 days storage (Fig. l(a)). The water potential of all component parts of the pericarp decreased significantly
204
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0123456789 Fig. 1. Changes in weight loss, skin and spintem browning (1 = no browning; 5 = fully brown), a* values and water potential, at 20°C and 60% RH, with postharvest storage. In (a) each point is the mean of ten fruit, in (b) each point is the mean of four replicates. ‘Ihe vertical bar represents the difference among the means using the I&SD ratio at k = 100.
(P < 0.01) with time (Fig. l(b)). Decreases in water potential of the skin occurred before browning was observed. It was not until Day 8 that there was a significant difference in water potential (P < 0.01) between the component parts of the pericarp. The spintem exhibited the lowest water potential of - 6.0 MPa (limit of the instrument) and the skin the highest of - 4.0 MPa. All parameters were tested for cross correlations. Changes in water potential of the skin and weight loss were most strongly correlated with the occurrence of browning in both skin and spintems (Table 1). In contrast to previous work there was no correlation with the a* values and water potential. Weight loss was highly correlated with water potential, but weight loss was more strongly correlated with spintem browning than skin browning. Spintem browning was also highly correlated with all changes in water potential. Although skin browning was highly correlated with water potential of the skin
M. Landrigan et al. / Scientiu Horticulturae Table 1 Correlation
matrix for water potential variables,
browning
205
66 (1996) 201-208
and colour scores and weight loss during storage at
20°C and 60% RH
JlSkin a JIspin b JIEndo ’ t/JSkin d
JlSkin L
*spin
l.occl 0.956 * * * 0.996 * * * -0.933 ’* -0.917 * 0.075 -0.910 **
l.OQO 0.942 ’ * -0.831 * * -0.945 ** - 0.050 -0.971 * * *
l
Ye c WT s (o/o)
b
*.f
JIEndo ’
Skin ’
Spin ’
a
1.OOo -0.934 ** -0.908 ** 0.142 -0.895 **
1.ooo 0.712 * - 0.306 0.710 *
1.000 0.198 0.989 * * *
1.000 0.205
wTg(%)
1.000
a Water potential of skin plus spintems; b Water potential of spintems; ’ Water potential of endocarp; browning score; ’ Spintem browning score; f Objective colour score and g Weight loss. ’ P = 0.05, ’ ’ P = 0.01, * * * P = 0.001.
d Skin
and endocarp it was less well correlated with water potential of the spintems. As water potential declined due to water loss during storage, skin browning increased significantly.
60
012345678
612345676
012345678
012345678
Days
Days
after harvest
after harvest
Fig. 2. (a) Changes in weight loss during storage of the whole rambutan fruit, skin minus the spintems, flesh and seed. (b) Changes during skin and spintem browning (1 = no browning; 5 = fully brown) during storage. All measurements were on fruit stored at 20°C and 60% RH, and each point is the mean of ten fruit. The vertical bar represents the difference among the means using the kLSD ratio at k = 100.
206
M. Landrigan et al./ Scientia Horticulturae 46 (1996) 201-208
In the second year, a more detailed account of water loss was obtained for the component parts of rambutan fruit. Fig. 2(a) shows a substantial weight loss (approximately 30%) for the whole fruit, the spintems and skin in the first 48 h. In contrast there was no weight loss of the seed for the first 4 days of storage, and total weight loss was less than 10% after 7 days. Weight loss from the flesh was approximately 20% after 7 days of storage. The spintems consistently lost more weight than the skin or the whole fruit for the first 4 days. By Day 7 both spintems and skin had lost approximately 50% of their weight (Fig. 2(a)). When this weight loss was examined on a percentage weight loss of the entire fruit, most water was lost from the skin, followed by the spintems and then the flesh. The spintems browned at a faster rate than the skin, reaching a commercially unacceptable score (> 2.5) by Day 7 (Fig. 2(b)). Due to high variation between fruit, differences in colour values were only significant (P < 0.01) for the L* values or brightness (Fig. 2(c)), confirming a darkening of fruit with time. Relative water content and water potential decreased with time (Fig. 2(d)). For both determinations, the spintem had a lower relative water content and water potential than the skin. The water potential values ranged from -0.2 MPa for fresh tissue to -3.0 MPa for the brown spintem at Day 7. RWC for fresh tissue was 93% and declined to approximately 60% for the spintems after 7 days. Water potential of spintems declined more rapidly up to Day 2, however, thereafter the rate of decline for spintems and the skin was the same. RWC was plotted against water potential (Fig. 3). Since the slopes of the regression for the skin plus spintems and the spintems alone were shown by comparison of regressions to be not significantly different, a single regression was fitted. The slope of relationship between the water potential and RWC shows an approximate relationship with bulk apoplastic modulus (E,) of the tissue (Tyree and Jarvis, 1982; Kim and Lee-Stadelmann, 1984). This relationship assumes that the osmotic potential of the
.
30
40
% Water potential=(.397)RWC-45.420 (r’.0.455,p<0.01,n=48)
50
60 RWC
70
80
90
100
110
(%)
Fig. 3. Relationship between relative water content (RWC) and water potential were obtained during 1 week of storage at 20°C and 60% RH.
of rambutan
pericarp.
Data
M. Landrigan et ul./ Scientia Horticulturue
66 (1996) 201-208
207
tissue is zero and that changes in RWC are a measure of changes in the relative volume of the symplast. Whilst the former assumption is likely to be incorrect, the latter assumption is likely to be valid. Errors can also be introduced in the determination of RWC due to unknown and variable degrees of rehydration of the apoplast (Eamus and Narayan, 1990). However, in a comparative study of spintems and skin of rambutan fruit, such errors and assumptions may be common to both tissue types. If this is correct, no clearly discernible difference in E, (as revealed by comparing the slopes of the two tissue types, Fig. 3) could be determined. This is in contrast to a range of studies of osmoregulation of leaves in response to dehydration stress, where changes in E, had been found (Morgan, 1984; Eamus and Narayan, 1990). It would also appear that the relationship between RWC and water potential is tighter at higher RWC and water potentials. This may be because at low RWC (< 80%) and low water potentials ( < - 2 MPa), measurement may not be as accurate.
4. Discussion The object of this work was to provide detailed measurements of changes in water status through time for the component parts of rambutan fruit, and to relate these changes to changes in colour of the fruit. Observations of fruit of a range of species have shown that water stress affects fruit, but few measurements of water potential of the component tissues have been made. Ben-Yehoshua et al. (1983) in their study on bell peppers and lemons found a significant negative correlation between weight loss and water potential. Furthermore they found an increase in amino acid and electrolyte leakage with weight loss, suggesting a disruption to membrane integrity. A loss of membrane integrity may contribute to the development of browning symptoms by allowing the enzymes of browning to react with substrates. If such a mechanism occurs, there should be a significant negative correlation between browning and water loss. Water is lost from rambutan fruit via the spintems and replaced by water from the skin as a result of the vascular system linking skin and spintem (Landrigan et al., 1994). Similarly the fruit of lychee also show preferential desiccation (Underhill and Simons, 1993). After 48 h at 25°C and 60% RH, whilst the whole lychee fruit had lost 10% in weight, the pericarp had lost 55% and no loss occurred from the aril (Underhill and Simons, 1993). These authors referred to this as ‘selective dehydration’, with little movement of water between aril and per&up. Rambutan fruit appear to behave in a similar manner. The phenomenon of water being drawn from the peel rather than the pulp has also been reported for oranges (Ben-Yehoshua, 1987). Water potential varies from near zero in unstressed plants to approximately -3.5 MPa in wilting rambutan leaves (Diczbalis and Eamus, 1993); this corresponded to a RWC of approximately 85%. Values of RWC and water potential for ‘browned’ rambutan spintem and skin are of this order. The RWC values for the fruits are lower than those found in the leaves. This is probably due to the presence of the seed in fruit which have a very low water content compared to leaves, endocarp and skin. As the water potential decreases so does cell turgor. With excessive water loss and the potential
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development of plasmolysis, a breakdown of membrane integrity would permit oxidation of phenols and lead to the eventual death of some cells. This manifests as the browning of the spintem followed by the skin. The fact that desiccation of the skin equals that of the desiccation of the spintem by Day 7 may be attributed to plant cell walls drying with declining permeability of gas diffusion (Ben-Yehoshua, 1987). Data is this study illustrate that water loss precedes the occurrence of browning but over time water loss is related to browning. Water stress affects rambutan tissue in the same manner as does senescence. More definitive studies must be undertaken to separate the effects of water stress and senescence.
Acknowledgements We wish to thank T.K. Lim for advice and Yan Diczbalis and Ian Knuckey for reading of the manuscript and discussion. Financial support from the Rural Industries Research and Development Corporation is gratefully acknowledged.
References Ben-Yehoshua, S., 1987. Transpiration, water stress, and gas exchange. In: J. Weichmann (Editor), Postharvest Physiology of Vegetables. Marcel Dekker, New York, pp. 113-170. Ben-Yehoshua, S., Shapiro, B., Chen Z.E. and Lurie, S., 1983. Mode of action of plastic film in extending life of lemon and bell pepper fruits by alleviation of water stress. Plant Physiol., 73: 87-93. Diczbalis, Y. and Eamus, D., 1993. Effect of irrigation schedules on plant and soil water status of container grown rambutans. In: DPIF Technical Ammal Report, Horticulture Branch, No. 207, pp. 11 l- 123. Eamus, D. and Narayan, A., 1990. A pressure-volume analysis of Solanum melongena leaves. J. Exp. Bot., 41: 661-668. Hung, Y., 1990. Effect of curvature and surface area on calorimeter readings-a model study. J. Food Quality, 13: 259-269. Kim, J.O. and Lee-Stadelmamr, O.K., 1984. Water relations and cell wall elastic quantities in Phaseolus vulgar& leaves. J. Exp. Bot., 35: 841-858. Landrigan, M., San&, V., Morris, SC. and McGlasson, W.B., 1994. Structural aspects of rambutan fruits and their relation to postharvest browning. J. Hort. Sci., 69: 571-579. Morgan, J.M., 1984., Osmoregulation and water stress in higher plants. Ann. Rev. Plant Physiol., 35: 299-3 19. Pantastico, E.B., Pantastico, J.B. and Cosico, V.B., 1975. Some forms and functions of the fruit and vegetable epidermis. Philippine J. Biol., 4: 175-197. Steel, R.G.D. and Torrie, J.H., 1980. Multiple comparisons. hi: Principles and Procedures of Statistics. McGraw-Hill, New York, pp. 172- 191. Turner, NC., 1981. Techniques and experimental approaches for the measurement of plant water status. Plant Soil, 58: 339-366. Tyrce, M.T. and Jarvis, P.G., 1982. Water in tissue and cells. In: O.L. Lang, P.S. Nobel, C.B. Osmond and H. Ziegler (Editors), Encyclopedia of Plant Physiology, l2B. Springer-Verlag, Berlin, pp. 35-77. Van Welzen, P.C. and Verheij, E.W.M. (1991). Nephelium lappaceum L. In: E.W.M. Verheij and R.E. Coronel (Editors), Plant Resources of South-East Asia. No. 2. Edible Fruits and Nuts. Pudoc, Wageningen, pp. 235-240. Underhill, S.J.R. and Simon& D.H., 1993. Lychee (Lirchi chinensis SOM.) pericarp desiccation and the importance of postharvest micro-cracking. Sci. Hortic., 54: 287-294. Watson, B.J., 1988. Rambutan cultivars in north Queensland. Qld. Agric. J., Jar-Feb.,: 37-41. Wiebe, H.H., 1971. Measurement of plant and soil water status. UT Ag. Exp. Sm. Bull., 484: l-71.
Postharvest water relationships and tissue browning of rambutan fruit M. Landrigan a**, S.C. Morris b, D. Eamus ‘, W.B. McGlasson d a Northern Territory Department of Primary Industry and Fisheries, P.O. Box 79, Berrimah N.T., 0828, Australia b Commonwealth Scientific and Industrial Research Organisation, Division of Horticulture, North Ryde, NSW, 2113, Australia ’ Northern Territory University, Casuarina, NT, 0810. Australia ’ University of Western Sydney, Hawkbury. Richmond, NSW, 2756, Australia
Accepted 1 April 1996
Abstract The water status of excised spintems or spintem plus pericarp and attached endocarp of rambutan fruits was measured three times during storage at 20°C. Changes in fruit colour were also recorded. The development of browning was preceded by water loss and concomitant declines in water potential of spintems and skin. As water potential decreased cell turgor also declined. There was a strong negative correlation between water potential and browning such that as browning score increased, water potential declined. Similarly relative water content showed a
negative correlation with browning. Water was lost from intact rambutan fruits via spintems and replaced by water from the skin. It is proposed that due to excessive water loss and the development of plasmolysis, a loss of membrane browning processes to proceed.
permeability
occurred and this then allowed the
Keywords: Nephelium lappaceum; Relative water content; Water potential; Water stress
1. Introduction Rambutan is a tree fruit crop cultivated throughout the humid tropics (Van Welzen and Verheij, 1991). The most attractive and distinctive features of rambutan fruit are its bright red and yellow colours, but most particularly its soft fleshy spines (spinterns),
’ Corresponding author: Tel. 61 89 892360; Fax 61 89 892049. 03044238/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. P/I SO304-4238(96)00915-6
202
M. Lundrigan et al./Scientia
Horticulturae 64 (1996) 201-208
typically l-3 cm in length. However rambutan fruit rapidly lose their attractive appearance after harvest because of the development of browning of the skin and spinterns (Watson, 1988). Such changes have a major negative impact upon the sale value of the fruit at market. The development of browning is preceded by water loss from spintems and skin (Pantastico et al., 1975; Landrigan et al., 1994). However, even when water loss is greatly reduced by storage in a high humidity (95% RI-I) environment, browning still develops, albeit more slowly. Furthermore, it has been shown that water lost from the spintem is replaced, at least partially, by water from the skin. This water would seem to travel via the highly developed vascular system linking the spintem and skin (Landrigan et al., 1994). Previous studies of changes in water status of fruit have concentrated on measurements on total fresh weight (Pantastico et al., 1975). Whilst such measures are of value, the use of more precise measurements of tissue water status, such as water potential, and the comparison of spintems with pericarp, will facilitate both tissue comparisons and comparisons with fruit of other species. Furthermore, it will allow consideration of fruit water status in the context of the water relations of the rest of the plant. This research was undertaken to describe the relationships, if any, between water loss, visual browning and water potential of the three separate components that make up the pericarp i.e. the spinterns, the skin and the fibrous layer (endocarp) closest to the pulp. Both water potential and osmotic potential were determined, thereby allowing calculation of turgor pressure, by difference. Relative water content was also determined as an independent assessment of water status.
2. Materials and methods In the first year of study 30 fruits of Nephelium luppaceum (cv. Jitlee) were harvested at the fully red mature stage from a single tree to reduce variability between fruits. Fruit were immediately stored at 20°C and 60% RH in the dark. Measurements of percentage weight loss, subjective scores of skin and spintem browning (Scale l-5: 1 = no browning; 5 = fully browned) and objective colour scores of browning, were made on ten fruits on the day of harvest and then at 4 and 8 days after harvest. Objective colour scores were made using a Minolta Chromameter (model CR-300, Minolta Camera Co., Ltd., Osaka, Japan). A white reference standard was used to calibrate the instrument. The values L* , a * and b * were recorded and the colour term, total colour difference was calculated from these values (Hung, 1990). Four of these fruits were also used for measurements of water potential (see Section 2.1). In the second year, fruits (cv. Jitlee) were harvested at the fully red mature stage and stored at 20°C and 60% RI-I. Measurements of whole fruit weight, weights of excised spintems (excised with a razor blade) and skin (minus spinterns), weights of flesh and seed, subjective scores of skin and spintem browning and objective colour scores of browning using a Minolta Chromameter were taken on ten fruits on the day of harvest, and 1, 2, 4, and 7 days after harvest. Water potential, osmotic potential and relative water content (RWC) were also measured on six of these fruits (see below).
M. L.undrigan et al./ Scientia Horticulturae 66 (1996) 201-208
203
2.1. Water potential Water potential was measured on whole spinterns or discs (10 mm diameter) cut from the skin plus spinterns and fibrous endocarp. These samples were sealed in a C-30 sample chamber coupled to a Wescor HR-33T thermocouple psychrometer (Wescor Inc., UT, USA). The sample chambers were placed in a water bath in a temperature controlled room set at 20°C. Water potential was measured using the psychrometric mode following a minimum equilibration of 2 h. Each chamber was calibrated using potassium chloride solutions of known molality (Wiebe, 1971). Apoplastic dilution from the cut surface of the tissue was not taken into account. 2.2. Osmotic potential Osmotic potential was measured by the psychrometric method in the C-30 chambers. After measurement of the water potential the tissue samples were frozen in liquid N,. Upon thawing osmotic potential was determined. 2.3. Relative water content Relative water content was determined by adapting the method of Turner (1981). Whole spintems or discs (IO mm diameter), cut as above, were weighed, floated in covered petri dishes at 20°C for 4 h to rehydrate before reweighing and then oven dried and dry weight measured. 2.4. Statistical analyses Data from all experiments were subjected to analyses of variance and where appropriate the means compared using Waller-Duncan’s Bayesian k-ratio LSD at k = 100 (Steel and Torrie, 1980). The level of k = 100 used here is approximately equal to the P = 0.05 level in conventional LSD. Simple correlations between objective colour values, subjective browning scores and weight loss measurements were generated by linear regression analyses.
3. Results Weight loss by fruit was substantial (23%) after the first 4 days of storage and increased to 35% (P < 0.01) by Day 8 (Fig. l(a)). Differences were seen in rates of browning between rambutan skin and spinterns. Spintem browning significantly increased (P < 0.01) with duration of storage and was consistently higher than skin browning. Changes in skin browning were not apparent until Day 8 and browning was not as extensive in skin as in spintems (Fig. l(a)). The a * values, which give a measure of the redness of the fruit, only showed a significant (P < 0.01) decline after 8 days storage (Fig. l(a)). The water potential of all component parts of the pericarp decreased significantly
204
M. Landrigan et al. / Scientia Horticulturae 66 (1996) 201-208 -5
-4 m z 3. - 32
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1
0
::
/
/
KS0
:
P’
/
1 / /’
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d/
1
I
I
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-1
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0123456789
-2
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-3 -
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0123456789 Fig. 1. Changes in weight loss, skin and spintem browning (1 = no browning; 5 = fully brown), a* values and water potential, at 20°C and 60% RH, with postharvest storage. In (a) each point is the mean of ten fruit, in (b) each point is the mean of four replicates. ‘Ihe vertical bar represents the difference among the means using the I&SD ratio at k = 100.
(P < 0.01) with time (Fig. l(b)). Decreases in water potential of the skin occurred before browning was observed. It was not until Day 8 that there was a significant difference in water potential (P < 0.01) between the component parts of the pericarp. The spintem exhibited the lowest water potential of - 6.0 MPa (limit of the instrument) and the skin the highest of - 4.0 MPa. All parameters were tested for cross correlations. Changes in water potential of the skin and weight loss were most strongly correlated with the occurrence of browning in both skin and spintems (Table 1). In contrast to previous work there was no correlation with the a* values and water potential. Weight loss was highly correlated with water potential, but weight loss was more strongly correlated with spintem browning than skin browning. Spintem browning was also highly correlated with all changes in water potential. Although skin browning was highly correlated with water potential of the skin
M. Landrigan et al. / Scientiu Horticulturae Table 1 Correlation
matrix for water potential variables,
browning
205
66 (1996) 201-208
and colour scores and weight loss during storage at
20°C and 60% RH
JlSkin a JIspin b JIEndo ’ t/JSkin d
JlSkin L
*spin
l.occl 0.956 * * * 0.996 * * * -0.933 ’* -0.917 * 0.075 -0.910 **
l.OQO 0.942 ’ * -0.831 * * -0.945 ** - 0.050 -0.971 * * *
l
Ye c WT s (o/o)
b
*.f
JIEndo ’
Skin ’
Spin ’
a
1.OOo -0.934 ** -0.908 ** 0.142 -0.895 **
1.ooo 0.712 * - 0.306 0.710 *
1.000 0.198 0.989 * * *
1.000 0.205
wTg(%)
1.000
a Water potential of skin plus spintems; b Water potential of spintems; ’ Water potential of endocarp; browning score; ’ Spintem browning score; f Objective colour score and g Weight loss. ’ P = 0.05, ’ ’ P = 0.01, * * * P = 0.001.
d Skin
and endocarp it was less well correlated with water potential of the spintems. As water potential declined due to water loss during storage, skin browning increased significantly.
60
012345678
612345676
012345678
012345678
Days
Days
after harvest
after harvest
Fig. 2. (a) Changes in weight loss during storage of the whole rambutan fruit, skin minus the spintems, flesh and seed. (b) Changes during skin and spintem browning (1 = no browning; 5 = fully brown) during storage. All measurements were on fruit stored at 20°C and 60% RH, and each point is the mean of ten fruit. The vertical bar represents the difference among the means using the kLSD ratio at k = 100.
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In the second year, a more detailed account of water loss was obtained for the component parts of rambutan fruit. Fig. 2(a) shows a substantial weight loss (approximately 30%) for the whole fruit, the spintems and skin in the first 48 h. In contrast there was no weight loss of the seed for the first 4 days of storage, and total weight loss was less than 10% after 7 days. Weight loss from the flesh was approximately 20% after 7 days of storage. The spintems consistently lost more weight than the skin or the whole fruit for the first 4 days. By Day 7 both spintems and skin had lost approximately 50% of their weight (Fig. 2(a)). When this weight loss was examined on a percentage weight loss of the entire fruit, most water was lost from the skin, followed by the spintems and then the flesh. The spintems browned at a faster rate than the skin, reaching a commercially unacceptable score (> 2.5) by Day 7 (Fig. 2(b)). Due to high variation between fruit, differences in colour values were only significant (P < 0.01) for the L* values or brightness (Fig. 2(c)), confirming a darkening of fruit with time. Relative water content and water potential decreased with time (Fig. 2(d)). For both determinations, the spintem had a lower relative water content and water potential than the skin. The water potential values ranged from -0.2 MPa for fresh tissue to -3.0 MPa for the brown spintem at Day 7. RWC for fresh tissue was 93% and declined to approximately 60% for the spintems after 7 days. Water potential of spintems declined more rapidly up to Day 2, however, thereafter the rate of decline for spintems and the skin was the same. RWC was plotted against water potential (Fig. 3). Since the slopes of the regression for the skin plus spintems and the spintems alone were shown by comparison of regressions to be not significantly different, a single regression was fitted. The slope of relationship between the water potential and RWC shows an approximate relationship with bulk apoplastic modulus (E,) of the tissue (Tyree and Jarvis, 1982; Kim and Lee-Stadelmann, 1984). This relationship assumes that the osmotic potential of the
.
30
40
% Water potential=(.397)RWC-45.420 (r’.0.455,p<0.01,n=48)
50
60 RWC
70
80
90
100
110
(%)
Fig. 3. Relationship between relative water content (RWC) and water potential were obtained during 1 week of storage at 20°C and 60% RH.
of rambutan
pericarp.
Data
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66 (1996) 201-208
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tissue is zero and that changes in RWC are a measure of changes in the relative volume of the symplast. Whilst the former assumption is likely to be incorrect, the latter assumption is likely to be valid. Errors can also be introduced in the determination of RWC due to unknown and variable degrees of rehydration of the apoplast (Eamus and Narayan, 1990). However, in a comparative study of spintems and skin of rambutan fruit, such errors and assumptions may be common to both tissue types. If this is correct, no clearly discernible difference in E, (as revealed by comparing the slopes of the two tissue types, Fig. 3) could be determined. This is in contrast to a range of studies of osmoregulation of leaves in response to dehydration stress, where changes in E, had been found (Morgan, 1984; Eamus and Narayan, 1990). It would also appear that the relationship between RWC and water potential is tighter at higher RWC and water potentials. This may be because at low RWC (< 80%) and low water potentials ( < - 2 MPa), measurement may not be as accurate.
4. Discussion The object of this work was to provide detailed measurements of changes in water status through time for the component parts of rambutan fruit, and to relate these changes to changes in colour of the fruit. Observations of fruit of a range of species have shown that water stress affects fruit, but few measurements of water potential of the component tissues have been made. Ben-Yehoshua et al. (1983) in their study on bell peppers and lemons found a significant negative correlation between weight loss and water potential. Furthermore they found an increase in amino acid and electrolyte leakage with weight loss, suggesting a disruption to membrane integrity. A loss of membrane integrity may contribute to the development of browning symptoms by allowing the enzymes of browning to react with substrates. If such a mechanism occurs, there should be a significant negative correlation between browning and water loss. Water is lost from rambutan fruit via the spintems and replaced by water from the skin as a result of the vascular system linking skin and spintem (Landrigan et al., 1994). Similarly the fruit of lychee also show preferential desiccation (Underhill and Simons, 1993). After 48 h at 25°C and 60% RH, whilst the whole lychee fruit had lost 10% in weight, the pericarp had lost 55% and no loss occurred from the aril (Underhill and Simons, 1993). These authors referred to this as ‘selective dehydration’, with little movement of water between aril and per&up. Rambutan fruit appear to behave in a similar manner. The phenomenon of water being drawn from the peel rather than the pulp has also been reported for oranges (Ben-Yehoshua, 1987). Water potential varies from near zero in unstressed plants to approximately -3.5 MPa in wilting rambutan leaves (Diczbalis and Eamus, 1993); this corresponded to a RWC of approximately 85%. Values of RWC and water potential for ‘browned’ rambutan spintem and skin are of this order. The RWC values for the fruits are lower than those found in the leaves. This is probably due to the presence of the seed in fruit which have a very low water content compared to leaves, endocarp and skin. As the water potential decreases so does cell turgor. With excessive water loss and the potential
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development of plasmolysis, a breakdown of membrane integrity would permit oxidation of phenols and lead to the eventual death of some cells. This manifests as the browning of the spintem followed by the skin. The fact that desiccation of the skin equals that of the desiccation of the spintem by Day 7 may be attributed to plant cell walls drying with declining permeability of gas diffusion (Ben-Yehoshua, 1987). Data is this study illustrate that water loss precedes the occurrence of browning but over time water loss is related to browning. Water stress affects rambutan tissue in the same manner as does senescence. More definitive studies must be undertaken to separate the effects of water stress and senescence.
Acknowledgements We wish to thank T.K. Lim for advice and Yan Diczbalis and Ian Knuckey for reading of the manuscript and discussion. Financial support from the Rural Industries Research and Development Corporation is gratefully acknowledged.
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