38 AMERICAN POTATO JOURN~AL [Vol. 46

R E D U C I N G C H I L L I N G I N J U R Y O F P O T A T O E S BY I N T E R M I T T E N T W A R M I N G

HOWARD W. HRUSCHKA, 1 WILSON' L. SMITH, JR. 2 AND JAMES E. BAKER 1

ABSTRACT

Potato tubers held continuously 19 weeks at 32 F developed chilling- injury symptoms early during storage and the symptoms worsened with time in storage. Early symptoms included browning and surface mold on skinned areas. By 11 weeks nlahogany browning, blackheart, and hollow heart, bluish skin discoloration and sinking of intact skin were visible in daylight. And under ultraviolet light, yellow fluorescence was visible in halos, washes, or spots around or near internal tuber tissues discolored by other chilling-injury symptoms. Some damage at 32 F became visible after subsequent holding of tubers for 1 week at 60 F. Tubers accumulated high amounts of reducing and total sugar at 32 F and respiration, measured at 60 F as CO_~ evolved, was progressively stimulated by time at 32 F. Sugar build-up, respiration-rate increase, and chilling injury seemed to show no cause-and-effect relationship but appeared as separate phenomena each caused by the stress of holding tubers at too low a temperature.

Moving potatoes to 60 F for 1 week after every 3rd week to interrupt storage at 32 practically eliminated all forms of chilling injury except skin browning and surface mold which were greatly reduced. Sugar con- tent and respiration rates in tubers intermittently held at low and high temperatures were higher after periods in low temperature and lower after periods at 60 F, and in general were maintained at levels nmch lower than in potatoes held continuously at 32, 36, or 40 F. In all treatments sugar content was higher than desirable for chipping type potatoes.

Holding potatoes at 60 F for 1 to 4 weeks before placing them at 32 F reduced skin browning and surface mold but had no effect on other forms of chilling injury, sugar accunmlation, or respiration rates in tubers as measured after 15 to 19 weeks continuously at 32 F.

Practically no chiUing-injury symptom except for very little mold growth and browning of skinned areas developed in tubers held in 36 or 40 F.

RESU~IEN

Tub6rculos de papa mantenidos continuamente 19 semanas a 32 F desarrollaron slntomas de dafio por frio temprano durante el almacena- miento y estos slntomas empeoraron con el tiempo en almacenamiento. Los sintomas tempranos incluyeron el empardecimiento y el enmohecimiento superficial en 1as fireas peladas. A1 cabo de 11 semanas se pudieron observar a la luz de[ dia el empardecimiento oseuro, coraz6n negro, coraz6n hueco, decoloraci6n azul de la pie1, y el hundimiento de la pie1 intacta. Y bajo luz ultravioleta era visible una fluorescencia amarilla en halos y manchas alrededor o cerca de los tejidos internos descoloridos por atros sintomas del dafio por frio. Algunos de los dafios producidos

1 Plant Physiologists and -".Plant Pathologist, Agricultural Research Service, U. S. Dept. of Agriculture, Plant Industry Station, Beltsville, Maryland. Accepted for publication June 28, 1968.

1969] tlRUSCtIKA ct al: REDUClNC CItlLLING INJURY 39

por 32 F fueron visibles cuando los tub6rculos fueron en seguida ahnacen- ados por una semana a 60 F. Los tubfirculos acumularon cantidades grandes de azficares reductores y totales a 32 F y la respiracidn, medida a 60 F cuando CO._, se form6, fu~ estimulada progresivamente con el tiempo a 32 F. E1 aumento de azficar, el incremento de la tasa de respiracidn, y el dafio por frio, al parecer, no indicaban uan relacidn de causa y efecto: m/is bien parecian fendmenos separados, todos causados por el hecho de nlantener los tub&culos a temperatura demasiado baja. Moviendo las papas a 60 F pot una semana despu~s de cada tercera semana para interrumpir el almacenamiento a 32 F, practicamente elimin6 ioda clase de dafio por frio con la excepcidn del empardecimiento de la piel y del enmobecimiento superficial que fueron reducidos considerablemente. E1 contenido en azfl- cares y las tasas de respiracidn en tub&culos mantenidos intermitentemente a telnperaturas bajas 3' altas, fueron naris altos despufis de periodos en temperaturas bajas y mils bajos en periodos a 60 F y e n general fueron mantenidos a niveles ross majos que los de las papas ahnacenadas continua- mente a 32, 36. o 40 F. En todos los tratamientis el contenido en azflcar fu~ m~,s alto de lo deseable para papas fritas.

Manteniendo !as papas a 60 F durante 1 a 4 semanas antes de ahnocenarlas a 32 F redujo el empardecimiento de la piel y el enmo- hecimiento superficial pero no tuvo efecto alguno sobre otras formas de dafio por frio, tal COlnO la acunmlacidn de azflcar o las tasas de respiracidn medidas despu~s de 15 a 19 semanas continuas a 32 F.

Practicamente no apareci6 sintoma alguno de dafio por frio, con la excepcidn de un poco de enmohecimiento y empardecimiento de fireas peladas, en tub6rculos lnantenidos a 36 o 40 F.

Chilling injury in economic plants is an ancient probleln (10) which is still with us (18. 22, 31). Conditions producing chilling injury vary and range from 1 minute's cold exposure for one plant to several months' exposure for other plants (14. 15, 18, 26). Theories describe chilling injury in terms of shock reaction (12, 19), changes in cell wall permeability (26, 28) or disturbed metabolisln (20, 21).

Many studies have been made (3, 11, 24) and reviewed (4, 9, 17, 27, 29) on effects of low. high, and changing temperatures on potato (Solarium tuberosum I~.) tubers. However, chilling injury still occurs in potatoes during winter months in cold climates (14).

Chilling injury of grapefruit (6) and of sweetpotatoes (20, 21) was reduced by interrupting cold storage with short periods of warm storage. Therefore, we tested the possibility that interlnittent warming would reduce chilling injury of potatoes. We also investigated the relationship between respiration rate and sugar content, measured at intervals (luring storage, and chilling injury of potatoes.

Preliminary Tests and Observations Preliminary tests and observations with Katahdin tubers during" the

1965-6 storage season indicated that all symptoms in the potato chilling- injury syndrome (15) and yellow-green fluorescence (14) in tuber tissue can be prevented during 20 weeks' storage by breaking time at 32 F

40 AMERICAN POTATO JOURNIAL [Vol. 46

(0 C) into 3-week periods with 1-week periods at 60 F (15.5 C) every 4th week. Tubers held at 32 F continuously for 7, 11, 15, or 19 weeks developed chilling injury symptoms which were progressively worse as time at 32 F increased. Tubers held continuously at 40 F developed no chilling-injury symptoms.

MATERIALS, METHODS AND RESULTS

Three one-ton lots (2-Katahdin and 1-Kennebec) of certified-seed- quality potatoes were harvested at Presque Isle, Maine, October 3, 1966, and trucked for receipt at Beltsville, Maryland, October 12. The tubers were maintained at 40 F (4.4 C) from harvest to start of the tests. On October 19, the tubers were sorted to 2-4 inch size. Sound tubers from each lot were divided at random into 158 samples of 22 tubers each and each sample was placed into a burlap bag. Bagged samples from each of the three varietal lots were distributed into each of two replicate sets of storage rooms at 85-95% relative ]tumidity and 32, 36, 40, and 60 F (0, 2.2, 4.4, 15.5 C).

Potatoes in test bags were held up to 20 weeks in various cycles (Table 1) alternating between low and high temperatures. Some samples were held in five 4-week cycles, 3 weeks of each cycle at 32, 36, or 40 F and the fourth week at 60 F. Additional samples for comparison were held at 32, 36 ,or 40 F for 19 weeks continuously followed by 1 week at 60 F (20-week cycle). Other bags were given 8-week (7 at 32 and 1 at 60 F ) cycles and 12-week (I1 at 32 and 1 at 60 F ) cycles. All but the 4- and 20-week cycling systems had one incomplete cycle near the end of the test. Thus, we observed the effect on potato tubers of different maximum continuous times (3, 7, 11, 15, 19 weeks) at low temperatures. In addition, other test bags were held 1, 2, 3, or 4 weeks at 60 F froni the start of the tests and at 32 F subsequently with some moved to 60 F for the 20th week of the test. Test bags from each of the six repli- cates and representing all treatments were examined at the start of the tests, after 3, 7, 11, 15 and 19 weeks' storage and also after 1 week following each of these six examination times. Data were processed and reported using the analysis of variance and the Duncan Multiple Range Test at 5% level of statistical significance.

External Condi t ions of Tubers

Each 22-tuber sample was removed from its burlap bag and examined in daylight. Samples were rated for general appearance, sprout growth, browning (scald) of skinned areas, surface mold, and decay.

General Appearance: Tubers at 40 F continuously or intermittently (by alternating with 60 F ) had best appearance throughout the tests. Tubers from 36 F were about as good as those from 40 F storage. Tubers from 32 F continuous storage had the poorest appearance.

Appearance of tubers from 32 F was improved considerably by warming at 60 F during part of storage time. Thus, tubers moved to 60 F for 1 week after every 3rd week of 32 F storage or tubers held at 60 F for 1 to 4 weeks before placing at 32 F appeared practically as good as those held at 40 F.

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42 AMERICAN POTATO JOURN'AL [Vol. 46

Sprout Growth: No sprouts developed during 20 weeks in tubers held 19 weeks continuously at 32, 36, or 40 F followed by 1 week at 60 F nor in tubers held 1 to 4 weeks at 60 F and then at 32 F until the final week at 60 F. At the final 20-week examination, only tubers warmed 1 week at 60 F after every ,3 weeks at 32, 36, or 40 F had sprouts; they were less than ~/~", ~ " , and a~ inch long respectively.

Decay: Tubers were practically decay-free until after 15 weeks' storage. At the 20-week examination about 2 and 4% of the tubers pre- viously at 40 F and 36 F, respectively, were decayed. About 15% of the tubers at 32 F continuously for 11 or 19 weeks were decayed. Seven per cent of the tubers in 32 F storage interrupted after 3 or 7 weeks by a week at 60 F were decayed. Less decay developed in tubers warmed 1 to 4 weeks at 60 F before placing at 32 F than in those warmed inter- mittently during storage.

Browning (scald) of Skinned Areas: Browning was found on an average of about 30, 21, and 12% of tubers held continuously at 32, 36, and 40 F respectively (Table 1). Incidence of browning increased steadily during storage.

Warming tubers to 60 F for 1-week periods to interrupt storage at 32, 36, or 40 F after every 3 weeks, reduced average percentage of tubers with skin browning to 21, 18, and 11.5% respectively. This reduction probably resulted from the healing effect of warm temperature.

Holding tubers at 60 F for 1 to 4 weeks before holding at 32 F reduced percentage of tubers with browning to about half that found when the same amount of time at 60 F, in 1-week periods, was spaced throughout storage following initial time of 3, 7, 11, or 19 weeks at 32 F.

Surface Mold: Surface mold was first noted on tubers at the 7-week examination (Table 2). During storage, surface mold was found on an average of about 20, 4, and 0% of tubers held continuously at 32, 36, or 40 F respectively. Warming tubers to 60 F for 1-week periods after every 3 weeks at 32 F or 36 F, reduced incidence of surface mold to 2 and 1% respectively. Holding tubers at 60 F for 1 to 4 weeks before storing at 32 F reduced average percentage of tubers with surface mold to about one-sixth that in tubers given intermittent warm-up periods. The longer the initial warm time was extended, the lower was the sub- sequent percentage of moldy tubers.

Internal Condi t ion of Tubers

After the external examination had been completed, 15 sound (non- decayed) tubers were taken at random from each 22-tuber sample and examined internally in daylight and in ultraviolet light. Tuber tissue was analyzed for sugar content and respiration rates were determined for intact tubers.

Internal Chilling-Injury Symptoms Visible in Daylight: Internal chilling-injury symptoms, visible in daylight, developed in 28, 66, and 94% of the tubers held continuously at 32 F for 11, 15, and 19 weeks respectively (Table 3, Fig. 1 and 2). Per cent affected tubers and severity of symptoms increased during the subsequent 1 week at 60 F following removal from 32 F storage. When potatoes were warmed to 60 F for

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1969] HRUSCHKA et al: REDUCING CHILLING INJURY 45

EFFECT OF COLD STORAGE ON POTATOES 52 ~ P. CONSTANTLY VS. 32 ~ ALTERNATED WITH 60 ~

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WEEKS IN STORAGE

FIG..1--Sugar content, respiration rate and chilling injury in potato tubers held 19 weeks constantly at 32 F or for 20 weeks in five 4-week cycles of 3 weeks at 32 F and the fourth week at 60 F. Chill ing-injury data are based on tubers with internal

symptoms visible in daylight.

46 AMERICAN POTATO JOURN,AL [Uol. 46

FiG. 2.--Internal view of Katahdin (A) and Kennebec (B) potato tubers after 20 weeks' storage showing chilling injury symptoms visible in daylight. (1) Tubers held at 32 F interrupted by 60 F every fourth week; (2) Tubers held at 32 F for 19 weeks and at 60 F the 20th week; (3) Tubers held at 60 F the first week and then 19 weeks at 32 F ; (4) Tubers held at 60 F the first 4 weeks and last week, with middle 15 weeks at 32 F. Both 1 and 4 had total of 15 weeks at 32 F and 5 weeks

at 60 F. Both 2 and 3 had total of 19 weeks at 32 F and 1 week at 60 F.

1969] IIRUSCtIKA et al: REDUCING CHILLING INJURY 47

1 week after every 3 weeks at 32 F 11o internally visible chilling-injury symptoms developed during 19 weeks, and after 20 weeks only 4% of the tubers had very slight sylnptoms. In contrast, chilling-injury symptoms developed in 98 to 100% of the tubers held at 60 F for 1 to 4 weeks before holding 15 or more weeks at 32 F with or without a final week at 60 F. Thus development of internal chilling injury symptoms depends oll the length of continuous time at 32 F and not on the total time at 32 F and 60 F. Internal chilling-injury was found in less than 1% of the tubers at 36 or 40 F continuously or intermittently with 60 F, and sylnptoms were very slight.

The early stages of chilling injury, as seen in daylight, were some- what different in Kennebec than in Katahdin tubers (Fig. 2). In Ken- nebec tubers, injury appeared as scattered areas of black discolored tissue and looked like blackheart. Hollow-heart cavities later developed within these blackened areas. In Katahdins, discoloration was first greyish and later dark-red to brown, generally more diffuse, and typical of mahogany browning.

Advanced stages of chilling injury, as seen in daylight, were silnilar in both varieties, injured tissue was dark brown to black and often con- tained cavities. Symptoms in the chilling-injury syndrome were indis- tinguishable from mahogany browning, blackheart, and hollow heart (15) (Fig. 2). \Vhel~ dark tissue and cavities were near the tuber surface, the intact skin appeared bluish and developed sunken areas (Fig. 3). About equal percentages of Kennebecs and Katahdins developed chilling- injury symptoms.

t~Iue-HTh#e Fhlorescelzce: In long-wave ultraviolet light the cut sur- faces of all halved potato tubers fluoresced to emit some blue-white visible light (Fig. 4). This fluorescence seemed to be the type found by others in tubers with freezing injury (25) or infections diseases (5, 16, 23) and in apparently healthy tubers (8). A colorless compound which fluoresces

Fro. 3.--Katahdin tubers with severe chilling injury. Internal chilling injury symp- toms near tuber surface (left) cause sinking and dark bluish appearance of intact

skin (right) .

48 AMERICAN POTATO JOURN,AL [Vol. 46

FIG. 4.--Blue-white fluorescence in cut potato tubers exposed to long-wave ultra- violet light after 1 week at 60 F at start of tests. Area of fluorescence ranged from 100%, upper left, to trace, lower right. Dark areas are not fluorescing. No chilling-

injury symptoms were found in these tubers.

blue-white has been extracted from virus-infected potato tubers and in very small amounts from healthy tubers and identified as scopoletin (2). However, other compounds such as chlorogenic acid also may be involved in potato tubers which fluoresce blue-white.

The area fluorescing in each tuber ranged from a trace to 100% and averaged 31% in Kennebec tubers, and 66% and 84% for the two lots of Katahdin tubers at the start of the tests. Gomphrena readings in- dicaed virus X in 85, 18, and 50% of the above three respective lots. Thus, the blue-white fluorescence seems unrelated to virus X. The area of blue-white fluorescence apparently was influenced much less by storage treatments than by varietal lots. However the area of blue-white fluor- escence could not be estimated accurately in many samples with necrosis from chilling injury.

Yellow fluorescence: In long-wave ultraviolet light, portions of the cut surfaces of halved tubers fluoresced, to emit yellow to yellow-green visible light (Fig. 5). Before storage a few tubers fluoresced in very small yellowish spots resembling more extensive fluorescence seen later. During storage under all conditions tested, yellow fluorescence was found in about 6% of the tubers while blue-white fluorescence was found in 100% of the tubers (Fig. 4). Whether or not the yellow fluorescence we observed is the unnamed derivative of scopoletin observed by Andreae and Andreae (1) has not been determined.

1969] HRUSCHKA et al: REDUCING CHILLING INJURY 49

FIG. 5.--Internal view of tubers, in daylight (upper left and right tubers) and long- wave ultraviolet light (middle, and lower left, and right tubers), previously held 19 weeks at 32 F and 1 week at 60 F. Necrotic areas of chilled tubers are brown to black in daylight and in ultraviolet light. Transition zones between black-brown necrotic and healthy tissue are brilliant yellow halos, spots and washes in ultra- violet light, Gray shaded areas in three tubers in ultraviolet light are either not

/tuorescing or fluorescing blue-white.

Yellow fluorescence was found at the 7-week and subsequent examina- tions in tubers from 32 F continuous and 32 F alternated with 60 F storage in most but not all t ime-temperature-examinat ion combinations. Yellow fluorescence was found in an average of 12% of tubers at 32 F continuously for 19 weeks and 1 week at 60 F, and in tubers at 32 F with interruptions at 60 F after every 7 or 11 weeks. In contrast yellow fluorescence was found in only 0.5% of tubers when cold storage was interrupted by 1 week at 60 F after every 3 weeks at 32 F. Generally percentage of tubers with yellow fluorescence increased during the 1 week at 60 F following removal f rom 32 F. Holding tubers at 60 F for 1 to 4 weeks before placing at 32 F for 15 to 19 weeks did not reduce per- centage of tubers with yellow fluorescence.

Yellow fluorescence generally occurred as streaks and halos in or around tissues showing other chill ing-injury symptoms. It was also af- fected by storage tenlperatures in much the same way as chilling injury. Thus we feel that the yellow fluorescence seen is another symptom in the chill ing-injury syndrome (15) . No tubers held at 36 F and practically no tubers held at 40 F developed yellow-fluorescing tissue.

Sugar Content: Sugar content was determined for tubers held con- tinuously at 32 or 40 F and for tubers held at 32 or 40 F intermittently warmed after every 3rd week with 1 week at 60 F. Analyses of tubers f rom 40 F were made only at examinations during the last third of the storage season. In each de~:ermination tissue from six tubers was ground

50 AMERICAN POTATO JOURNAL [Vol. 46

to a slurry with a meat grinder and thoroughly mixed. Twenty-gram samples were placed in Kohlrausch flasks, and boiling 95~/o ethanol was added to give a final concentration of 8070 ethanol. A portion of the extract was clarified with neutral lead acetate and excess lead was removed with dibasic sodium phosphate. Each clarified sample was brought to volume and reducing sugars were estimated colorimetrically by the method of Ting (30). Inversion of non-reducing sugar for total sugar estimation was done with hydrochloric acid.

Tubers held continuously at 32 F accumulated and maintained high amounts of reducing (4.227o, maximum) and total (7.43%, maximum) sugar throughout the storage period (Fig. 1). In potatoes alternately held 3 weeks at 32 F and 1 week at 60 F during 20 weeks' storage, sugar content was higher following time at 32 F and lower following time at 60 F. These changes with temperature were most striking in total sugar content and followed a pattern quite similar to changes in respiration rate.

In comparable determinations, per cent sugar paralleled, but was consistently lower ()~ to ~ as great) , in potatoes from 40 F than in those from 32 F storage. In potatoes from both 32 and 40 F storage, tubers, held at 60 F for 1 week after every 3rd week accunmlated much less sugar than comparable tubers held continuously at 32 or 40 F. The lowest reducing sugar content measured during storage was 0.60% in tubers at 40 F alternated with 60 F ; this is two to three times the maxinmm reducing sugar content found in tubers from which potato chips of acceptable color and quality can be made. Varietal lots differed very little in sugar content.

Respiration Rate: Respiration rates were estimated as milligrams of carbon dioxide evolved per kilogram of fresh whole potatoes per hour at 60 F (rag CO, , /Kg /h r at 60 F) . At start of tests or after removal from 32, 36, 40, or 60 F, tubers were held an additional day at 60 F before start of carbon dioxide collection. The respiration measurements thus represented the respiratory burst rate at 60 F following removal from low temperature and the more or less stabilized respiration rate after a 1-week warm-up at 60 F.

Respiration measurement methods used were adaptations based on Brown and Escombe, and of Har t (7. 13). Within each of three replicate 1-gallon wide-mouthed jars, carbon dioxide produced during 48 hours at 60 F by five whole tubers (1000 grams) was absorbed in 50 ml of 20~'e potassium hydroxide in a 250-ml beaker. Each jar was vented with a ~ - i nch diameter tube and in spot checks, oxygen content of jar atmos- phere at the end of the respiration period was 17c/c , or over. At the start of the tests, tubers produced 10 mg CO. , /Kg/hr at 60 F and 8 mg C O 2 / K g / h r at 60 F after 1 week at 60 F. After 19 weeks continuous storage at 32, 36. or 40 F respiration rates, as measured by CO2 pro- duction, were 47, 24, and 11 mg C O 2 / K g / h r at 60 F respectively. Holding tubers at 60 F for 1 week after removal from 32, 36. or 40 significantly reduced respiration rates to 29, 13, and 7 mg CO,_,/Kg/hr at 60 F respectively.

Respiration rates, for tubers that were moved back and forth from 32. 36, or 40 to 60 F were high following time at low temperature and

1969] HRUSCHKA et al: REDUCING CHILLING INJURY 51

low following time at 60 F. This increase and decrease was successively acconaplished one to five times by moving potatoes back and forth from low and high telnperature storage over a 20-week period.

Moving potatoes from 32, 36, or 40 to 60 F for 1 week after every 3 weeks slowed up increase or reduced respiration rates of tubers to rates, after 19 weeks, of 18, 11, and 6 mg CO2/Kg/hr at 60 F respectively and after 20 weeks (the last week at 60 F) of 14, 8, and 6 nag CO2/Kg/hr at 60 F respectively. Moving potatoes from 32 to 60 F for 1 week after every 7 weeks reduced respiration rate less (to 21 nag), and moving potatoes from 32 to 60 F for 1 week after 11 weeks reduced respiration rate least (to 26 mg).

Tubers, war,ned 4 weeks at 60 F before and 1 week after placing at 32 F for 15 weeks, had about the sanle respiration rate after the 20 weeks' storage as tubers, held 19 weeks at 32 F and a final week at 60 F.

DISCUSSION

By naanipulating storage temperatures, we were able to slow down or speed up physiological activity and produce or prevent physiological damage in potato tubers. Thus we produced chilling injury, sugar accumu- lation, and increased respiration burst (measured at 60 F) by long con- tinuous storage at 32 F, and we prevented chilling injury and de- creased sugar content and respiration rate (measured at 60 F) by warming the tubers intermittently. Changes in sugar content and respiration rate were reversed repeatedly in five consecutive cycles of 3 weeks at cold temperature and 1 week at warm temperature. This is not meant as a reconamendation for storing potatoes at 32 F. However, if storage-laouse temperature has been low for some time, potatoes could be warmed to greatly reduce or avoid damage. Intermittent warming may also benefit tubers held at temperatures higher than 32 by preventing excessive sugar build-up. Build up of sugars and increase of respiration and chilling injury seem to be separate phenomena, each caused by holding the tubers at too low a temperature (3, 11, 24).

Yellow fluorescence found after the start of these tests was restricted almost entirely to tubers from 32 F storage. Most vellow fluorescence appeared as a chilling-injury symptoln associated wit~l internal areas of tubers discolored by other chilling injury symptoms. Yellow fluorescence was prevented in the same way that other chilling-injury symptoms were prevented. Identification of the fluorescing compounds is in progress.

ACKNOWLEDGMENTS

Jack Wilson, Presque Isle, Maine, procured potatoes for these tests. Robert V. Akeley identified the potato varieties. Avery Rich, Jasper R. Chase and Muriel J. O'Brien tested the tubers for virus infection using Gomphrena indicator plants. George A. Brown prepared graphs and with H. Carl Vanght made titrations for respiration measurements. Margaret Blomquist, Vaughn Crouch, Stephen L. Bowen and Mary Jane Kriemel- meyer assisted with inspections. E. James Koch advised on experimental design and analysis.

52 AMERICAN POTATO JOURNAL [Vo] . 46

LITERATURE CITED

1. Andrere, S. R. and W. A. Andre~e. 1949. ,The metabolism of scopoletin by healthy and virus infected potato tubers. Can. J. Res., Sect. C, Bot. Sci., 27(2) : 15-22.

2. Adnre0e, W . A . 1948. The isolation of a blue fluorescent compound, scopoletin, from green mountain potato tubers, infected with leaf rolll virus. Can. J. Res., Sect. C, Bot. Sci., 26(1) : 31-34.

3. Appleman, C. O. 1911. ~Physiological behavior of enzymes and carbohydrate transformations in after-ripening of the potato tuber. Bot. Gaz. 52: 306-315.

4. Arreguin-Lozano, B. and J. Bonner. 1949. Experiments on sucrose formation by potato tubers as influenced by temperature. Plant Physiol. 24 (4) : 720-738.

5. Best, R . J . 1936. Studies on a fluorescent substance present in plants. 1. Pro- duction of the substance as a result of virus infection and some application of the phenomenon. Australian J. Exptl. Biol. Med. Sci. 14: 199-213.

6. Brooks, C. and L. P. McColloch. 1936. Some storage diseases of grapefruit. J. Agr. Res. 52: (5) 319-351.

7. Brown, t-I. T. and F. Escombe. 1905. ~On a new method for the determination ~f atmospheric carbon dioxide, based on the rate of absorption by a free surface of a solution of caustic alkali. Proc. Roy. Soc. London (B) 76: 112-117.

8. Burton, W. G. 1956. Some observations on the growth substances in ether extracts of the potato tuber. Footnote 1 p. 584, Physiologia Plantarum 9: 567-587.

9. Burton, W. G. 1966. The potato. 2nd ed. H. Veenman und Zonen. Wagenin- gen, Netherlands, 382 p.

10. Cooley, J. S. 1951. Origin of the sweet potato and primitive storage practices. The Scientific Monthly 77 (5) : 325-331.

l l . ,Craft, C. C. 1963. Respiration of potatoes as influenced by previous storage temperatures. Amer. Potato J. 40 (9) : 289-298.

12. Ezell, B. D. and M. S. Wilcox. 1967. Frost injury systemic in sweet potatoes. Agric. and Food Chem. 15 (4) : 729-735.

13. Hart , P. 1887. On the estimation of the relative amounts of caustic and car- bonate of soda in commercial soda. J. Soc. Chem. Ind. London: 347 j.

14. Hilborn, M. T. and R, Bonde. 1942. A new form of low-temperature injury of potatoes. Amer. Potato J. 19 : 24-29.

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