tration of ACC increases in the locule tissue of mature-green fruit coincident with gel formation and prior to the appear ance of internal color. It is possible that ...
Proc. Fla. State Hort. Soc. 98: 184-187. 1985.
LOCATION OF DEVELOPMENT OF ETHYLENE BIOSYNTHESIS DURING TOMATO FRUIT MATURATION Jeffrey K. Brecht
University of Florida, IFAS, Vegetable Crops Department Gainesville, FL 32611 Additional index words. Lycopersicon esculentum, 1-aminocyclopropane-1-carboxylic acid, ripening. Abstract. Proper maturity at harvest is a major determinant of post harvest ripening rate and the ultimate quality of to matoes picked prior to the color break. Maturity in tomato (Lycopersicon esculentum Mill.) is associated with seed de velopment and development of a jelly-like consistency in the locular contents. Initiation of autocatalytic ethylene produc tion is thought to signal the initiation of ripening in tomato and other climacteric fruits. The location of initiation of ethylene production in tomato fruit was investigated by separating fruit of different maturity stages into their compo nent tissues and measuring the levels of the ethylene precur sor l-aminocyclopropane-1-carboxylic acid (ACC). The concen tration of ACC increases in the locule tissue of mature-green fruit coincident with gel formation and prior to the appear ance of internal color. It is possible that some factor produced in the locular tissue at the time of gel formation promotes further locular tissue breakdown and subsequently ripening of the whole fruit.
Approximately 85% of the Florida tomato crop is har vested and marketed in the green stage, i.e. as 'maturegreens' (6). "Mature" is defined in the U.S. Standards for grades of tomatoes as ". . . the stage of development which will insure a proper completion of the ripening process, and that the contents of two or more seed cavities have developed a jelly-like consistency and the seeds are well developed" (15). Unfortunately, determination of these in ternal maturity indices is not practical during the harvest ing and handling operations due to their destructive na ture and satisfactory nondestructive indices are not cur rently available. Thus, a wide range of maturities may be included in commercial shipments, resulting in
nonuniform ripening and lowered quality. As indicated in the definition given above, proper maturity at harvest is associated with the ability of the to
mato fruit to complete normal ripening following harvest. Ripening in tomatoes, as in other climacteric type fruits, is signaled by a rapid increase in production of ethylene (11) followed by a number of other ripening-associated changes including increased respiration, synthesis of lycopene and other carotenoid pigments, autolysis of cell wall pectins, Florida Agricultural Experiment Stations Journal Series No. 6958. The author wishes to thank G. B. Odell and D. J. Cantliffe; D. J. Huber; and R. G. Gardner for supplying tomatoes used in this study.
184
and changes in various acid and sugar components (7). The following sequence has been established for the biosynthesis of ethylene: Methionine ■* SAM (S-adenosylmethionine) ■* ACC - C2H4 (2). Although it has been shown that both the conversion of SAM to ACC and ACC to ethylene are low in preripe fruit (1,8), in all cases studied, ACC formation was the limiting step in the pathway (17). Thus, an increase in ACC concentration may be taken as an indication of the initiation of ethylene production and ripening. The locular tissue of tomatoes is derived from the placenta of the fruit, overgrowing the seeds and filling the locules (12,13). Cell division in the locules ceases early on in fruit development and subsequent growth is by expan sion (5). The cell walls become progressively thinner and more fragile, eventually rupturing to produce the typical jelly-like consistency found in mature fruits (4). The re lationship between this sequence of events and the initia tion of fruit ripening is unclear. It is well known however that color development begins internally and precedes the external appearance of color by one or more days. The objective of the present study was to determine the location of the initiation of ethylene production in to mato fruit by measuring the levels of ACC in the various tissues at different stages of maturity. The information thus obtained may be useful in increasing our understand ing of the relationship between maturation and ripening of tomato fruit and lead to further studies eventually re sulting in the development of a reliable method of identify ing mature-green fruit. Materials and Methods
'Flora-Dade' and 'Sunny' tomatoes were obtained from plots at the Horticultural Unit in Gainesville. 'Mountain Pride' tomatoes were from the Mountain Horticultural Crops Research Station in North Carolina. Harvested green fruit of different maturities were treated with 1000 ppm NaOCl, rinsed with deionized water, dried, weighed, and each fruit placed in a 480 ml glass jar at 24± 1°C. After holding overnight, jars were sealed for 1 to 2 hours; ethylene concentration in the headspace was determined by gas chromatography and the rate of ethylene produc tion calculated. After measurement of the initial rates of ethylene production, some of the fruits were cut open and rated for maturity class while others were left in the jars for additional measurements of ethylene production and examined at various times during storage. The maturity of the green tomatoes was rated according to the subjective classifications of Kader and Morris (9) as shown in Table 1. According to this scheme, typical mature-green fruit will reach the 'breaker' stage in 5 or fewer days after harvest when held at 20°, while immature green fruit will not begin Proc. Fla. State Hort. Soc. 98: 1985.
Table 1. Maturity classes of green tomatoes Maturity
Ml
M2
Immature-green
No jelly-like material in locules; seeds are cut by a sharp knife.
Partially
Jelly-like material formed in at least one, but not all locules; seeds are well developed. Jelly-like matrix in all locules; seeds are not cut by a sharp knife. Typical mature-green with some internal red color.
mature-green
Typical
M3
mature-green
M4
Internal appearance
Class
Score
Advanced mature-green
'From Kader and Morris (9).
to ripen for 10 or more days. Following examination of fruits for maturity class, a 1 cm thick transverse slice of tissue was cut from the center portion of each fruit and frozen at -20°. The frozen slices were separated into the various tissues (13) and ACC assayed (10). Initial tests de termined that very little ACC was present in seeds or skin at any stage. In subsequent tests the seeds and skin were discarded and ACC levels determined in the outer pericarp wall, radial pericarp walls, placental and core (columella) tissue, and locular gel.
dered to coincide with color development since, as shown here, the first obvious increase in ethylene production by whole fruit corresponds to or slightly precedes the breaker stage or appearance of red color. However, when the JeveJ of ACC in the various tissues was examined (Fig. 2), it was apparent that a significant increase in ACC occurs mainly in the locular gel tissue at the M2 or M3 stage, followed by increased ACC levels in the placental tissue and, later, in the septa and outer pericarp. The locular gel accounts for only about 20% of the total weight of tomato fruit. Thus the small increase in ethylene production by whole fruit at the M3 stage might represent a significant increase in ethylene production in the gel tissue at this stage. The increase in ACC in the locular gel is temporally related to the cytological changes occurring in that tissue and pre cedes the appearance of red color. Since external color development is usually first observed at the blossom end, the possibility cannot be discounted that an initial increase in ACC may have occurred in the pericarp which was not detected in samples taken from the central portion of the fruit. However, the fact that internal red color was often observed in the locular gel of fruits with green skin and pericarp, while the reverse was never observed, and ACC concentration rose in the gel prior to the appearance of red color tend to support the idea that ripening normally begins in the locules.
Results and Discussion As shown in Fig. 1, there was a small increase in ethylene production by the tomato fruit at the M3 stage and much larger increases at the M4 and breaker stages. The initiation of ripening in tomato fruit is often consi-
12.0 4
i
6.0
2.50
Gel
5.Of
g
4.0
|
3.0
c
2.0
1.0
M1
M2
M3
M4
Br
Maturity/Ripeness Stage Fig. 1. Ethylene production by whole tomato fruit at different matur ity and ripeness stages (Data ± S.E., N = 9 to 12 fruit).
Proc. Fla. State Hort. Soc. 98: 1985.
Pericarp
M1
M2
M3
M4
Maturity/Ripeness
Br
Stage
Fig. 2. Levels of ACC in the outer pericarp wall (•), radial pericarp walls (■, septa), placenta (^), and locular gel (♦) of tomato fruit at differ ent maturity and ripeness stages (Data ± S.E., n = 3 to 5 fruit).
185
In a study of the development of tomato placental tis sue, Cocking and Gregory (4) described how the cells of the locules become progressively larger and thinner walled until, at maturity, they rupture, releasing their contents. Several possibilities exist relating this sequence of events to ripening in the whole fruit: (a) increased ethylene produc tion by locule cells during their expansion and separation, similar to that which occurs in abscission zones (16), may promote further gel formation and trigger ripening in ad jacent tissues; (b) cell wall fragments released at this stage may act as elicitors of ethylene production in adjacent cells (Brecht and Huber, unpublished); or (c) ACC released from the vacuoles of ruptured locule cells may stimulate ethylene production in intact cells. Confirmation of the involvement of any one or more of these possibilities in tomato fruit ripening awaits further investigation. Parthenocarpic or puffy fruit which do not contain locular gel
presumably ripen normally, however. Thus gel formation is apparently not a prerequisite for ripening to occur. The cells of the locule tissue, when it does occur, may be more sensitive to ethylene or some other factor than the sur rounding pericarp cells, and thus initiate ACC production prior to the other parts of the fruit. Abundant research has shown that if fully mature green tomatoes are held at proper temperatures and al lowed to ripen fully they will develop acceptable quality. Immature fruit, however, require more time to ripen and are of lower quality. Susceptibility to physical injury, water loss, decay, and chilling injury is greater in immature to mato fruit. It is apparent then that an important aspect of studies of the sort described here is the potential for an improved understanding of tomato fruit maturation which may lead to better selection procedures and better quality tomatoes. When the data of Fig. 1 relating tomato fruit maturity and ripeness stages to ethylene production rate is redrawn so that maturity stages MI and M2 are combined and ethylene production is plotted on a log scale, a straight line relation is apparent (Fig. 3). Using this relationship,
5.00
.920 / / y=0 .529x-1.860 r= 0
1.00 0.50
0.10
0.05
I
/
/
M1/M2
7
Literature Cited 1. Adams, D. O. 1979. Methionine metabolism in apple tissue in relation to ethylene biosynthesis. PhD Thesis, Univ. of Calif., Davis. 2. Adams, D. O. and S. F. Yang. 1979. 1-aminocycolpropane-l-carboxylic acid: a metabolic intermediate in the conversion of methionine to ethylene. Proc. Natl. Acad. Sci. 76:170-174. 3. Calfresh Tomato Marketer. 1985. Look-alike tomatoes don't sound alike. Calfresh Tomato Marketer 2(5):2. 4. Cocking, E. C. and D. W. Gregory. 1963. Organised protoplasmic units of the plant cell. I. Their occurrence, origin and structure. J. Exp. Bot. 14:504-511.
5. Davies, J. W. and E. C. Cocking. 1965. Changes in carbohydrates, proteins and nucleic acids during cellular development in tomato fruit locule tissue. Planta 67:242-253. 6. Florida Tomato Committee. 1985. Annual Report, 1984-1985. 7. Hobson, G. E. and J. N. Davies. 1971. The tomato. In: A. C. Hulme, (ed.). The biochemistry of fruits and their products. Vol. 2. Academic Press, New York. 8. Hoffman, N. E. and S. F. Yang. 1980. Changes in 1-aminocyclop-
ropane-1-carboxylic acid content in ripening fruits in relation to their ethylene production rates. J. Amer. Soc. Hort. Sci. 105:492-495. 9. Kader, A. A. and L. L. Morris. 1976. Correlating subjective and ob jective measurements of maturation and ripeness of tomatoes. Proc. 2nd Tomato Quality Workshop, p. 57-62. Univ. Calif., Davis. 10. Lizada, M. C. C. and S. F. Yang. 1979. A simple and sensitive assay for 1-aminocyclopropane-l-carboxylic acid. Anal. Biochem. 100:140145.
11. McGlasson, W. B., N. L. Wade, and I. Adato. 1978. Phytohormones
M3
M4
Br
Maturity/Ripeness Stage Fig. 3. Ethylene production by tomato fruit at different maturity and ripeness stages showing the linear relationship between log ethylene pro
duction rate and developmental stage (Redrawn from Fig. I).
186
the rate of ethylene production could be used as a physiological index of proper harvest maturity. Su, et al. (14) reached the same conclusion in their study of tomato fruit maturation and ripening and an examination of their data indicates an almost identical relationship to that shown here. Ethylene production could not be used to distinquish MI from M2 fruits, however. While measurement of ethylene production rates to de termine tomato maturity may see application in research settings, measurement of changes in the physical proper ties of the fruit tissue related to the changes in the locule may be more likely to be employed in the commercial field. Various techniques for sorting produce in the field or packing house based on measurement of internal quality are currently in use or have been demonstrated, including measurement of X-ray or light transmittance for elimina tion of freeze-damaged citrus fruits, detection of hollow heart in potato and water core in apple, measurement of flesh chlorophyll in peaches, and selection of lettuce heads for mechanical harvest. Recently, measurement of sound transmission or resonance frequencies has been proposed for determining tomato maturity (3). The results of this study demonstrate the relationship between gel formation in the locule tissue of tomato fruit and the initiation of ethylene biosynthesis in the locule tis sue. While gel formation in the locules has previously been considered as part of the maturation process separate from ripening, this work suggests that this process might be more accurately thought of as an early, and perhaps im portant event in the ripening period. An investigation of the possible roles of cell wall fragments, ACC, and ethylene from the locule tissue in the initiation of tomato fruit ri pening is currently underway.
and fruit ripening. In: D. S. Letham, P. B. Goodwin, and T. J. V. Higgins (eds.). Phytohormones and related compounds—a com prehensive treatise. Vol. II. Elsevier/North Holland Biomedical Press, Amsterdam. 12. Smith, O. 1935. Pollination and life-history studies of the tomato (Lycopersicon esculentum Mill.). Cornell Univ. Agr. Exp. Sta. Mem. 184: 3-16.
Proc. Fla. State Hort. Soc. 98: 1985.
13. Spurr, A. R. 1976. Structure and development of the tomato fruit.
Proc. 2nd Tomato Quality Workshop, p. 4-7. Univ. Calif., Davis. 14. Su, L. Y., T. McKeon, D. Grierson, M. Cantwell and S. F. Yang. 1984. Development of 1-aminocyclopropane-l-carboxylic acid synthase and polygalacturonase activities during the maturation and ripening of tomato fruit. HortScience 19:576-578. 15. U.S. Dept. Agr. 1976. United States standards for grades of fresh
market tomatoes. Agr. Mktg. Serv., U.S. Dept. Agr., Washington, D.C.
16. Wright, M. and D. J. Osborne. 1974. Abscission in Phaseolus vulgaris. The positional differention and ethylene-induced expansion growth of specialized cells. Planta 120:163-170. 17. Yang, S. F. and N. E. Hoffman. 1984. Ethylene biosynthesis and its regulation in higher plants. Ann. Rev. Plant Physiol. 35:155-189.
Proc. Fla. Slate Hort. Soc. 98: 187-189. 1985.
IMPACT OF CURING, FILM-WRAPPING AND CHLORINATION ON THE STORABILITY OF BONIATO SWEET POTATOES Kathleen Delate and Jeffrey K. Brecht
sumption and distribution to major U.S. cities could be
Institute of Food and Agricultural Science University of Florida Gainesville, FL 32611
increased and an additional $10 to $12 million generated for Florida producers. This figure has been determined based on average price per bag, yield per acre, and total
Mary Lamberts
acreage data. Current harvesting and handling procedures employed for boniatos have precluded long-term storage due to mechanical injury and lack of curing, resulting in insuffi cient periderm formation, wound healing and subsequent decay (1). Curing is not a current practice and harvested roots may be stored for only 3 to 4 days before shipment. After 7 days, a reduction in quality can be noted (6). Cur rent information for temperate-grown sweet potatoes indi cates that with proper curing and temperature control (to avoid chilling injury), a potential shelf life of 6 months is
Dade County Cooperative Extension Service 18710 SW 288th Street Homestead, FL 33030
Additional index words. Ipomoea batatas Lam., postharvest storage.
Abstract. This paper presents the results of the first year of study on various postharvest treatments for boniato sweet potatoes (Ipomoea batatas Lam.). Numerous postharvest de cays, including Rhizopus sp. and Diplodia sp., have been implicated in precluding long-term storage of boniatos. This problem is particularly important during the best marketing window in the winter months. This study demonstrated the beneficial impact of curing boniatos at high temperature (30°C) and high humidity (95%-100% RH) in preventing stor
age rots during the winter harvests. Chlorination of simulated packinghouse spray water was found to be ineffective in decay prevention in initial studies. Polyethylene shrink-wrap films extended storage life (up to 2 months), but the high humidity in the wrapping caused excessive sprouting which may have a detrimental effect on salability.
It is estimated that boniato, or Cuban sweet potatoes (Ipomoea batatas), are grown on approximately 5,500 acres in the Dade County, Florida area (5). Yields of these whitefleshed sweet potatoes range from 200 to 250 50-lb. bags per acre; price per bag ranges from $6.00 during the heavy summer production to $17.00 during the best marketing window of the winter months (3). A severe freeze in Janu ary, 1985 essentially eliminated production from February to April in the Homestead area. The possibility of compe tition from Caribbean exporters of tropical root crops, due to the Caribbean Basin Initiative, suggests that improve ment of the postharvest quality of domestic boniatos will assume increased importance for domestic growers.
The principal causes of postharvest deterioration in boniatos are water loss and decay, mainly caused by Rhizopus sp., but also by Diplodia and Fusarium sp., (Brecht and Sherman, unpublished). If shelf life could be extended by just 50% beyond the current 1 to 3 weeks, local conFlorida Agricultural Experiment Stations Journal Series No. 6959.
Proc. Fla. State Hort. Soc. 98: 1985.
possible (7). The objective of this study was to evaluate some postharvest techniques for extending the shelf life and reduc ing the decay problems of boniatos based on practical appication to local packinghouse conditions. Wash water chlori nation, curing, and film-wrapping were tested for their effect on storability with two cultivars of boniatos harvested during the late fall and winter of 1984-85.
Materials and Methods
Boniatos are planted year round in South Florida, with harvests normally occuring from 4 to 6 months or more after planting, depending on the season and market condi tions. Two cultivars of boniato sweet potatoes currently in commercial production in Dade County were selected for the study. Ticadita', a dark, purple-red skinned cultivar, is grown predominately in the fall and winter and is reputed to have the best shipping quality and cold-hardiness. 'Campeon' has a lighter red skin with a buff appearance and is usually grown in summer and fall. Three harvests of Ticadita' were made: 22 Nov., 26 Dec, and 27 Jan., ap proximately 4.5, 5.5, and 6.5 months from planting. 'Campeon' was harvested twice, on 26 Dec. and 27 Jan., 4.5 and 5.5 months after planting.
The sweet potato roots were harvested with a mechan ical potato digger, washed, sorted, and packed in 50-lb. burlap bags by a commercial grower-packer in the Home stead area. After transport to Gainesville, roots were sorted for uniform size, and divided by random sample for treat ment. A complete factorial arrangement of treatments con sisted of 2 chlorination levels, 2 curing methods and 2 wrapping procedures with 2 replications of 5 boniatos per 187
