ENGINEERING RESEARCH INSTITUTE THE UNIVERSITY OF MICHIGAN ANN ARBOR Terminal Report May 1, 1955 to December 31, 1956 GAMMA-RAY SPROUT INHIBITION OF POTATOES C. H. Burns J. V. Neh.emias ~ '.. F.~ G. Gustafson R. A. Martens W. J. Hooker A. Pendill D. Isleib F. Heiligman Project 2386 QUARTERMASTER ACTIVITIES DEPARTMENT OF THE ARMY CONTRACT NO. DA-19-129-qm-349 PROJECT NO. 7-84-01-002 March 1957

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TABLE OF CONTENTS Page LIST OF TABLES iv LIST OF FIGURES v SUMMARY 2 I. TECHNICAL OBJECTIVES 2 II. SPRING IRRADIATION AND SUBSEQUENT STORAGE OF IDAHO-GROWN RUSSET BURBANK VARIETY (SEED) POTATOES 4 A. Irradiation 4 B. Sprouting 6 Co Weight Loss 7 Do Rot and Other Losses During Storage 11 IIIo FALL IRRADIATION AND SUBSEQUENT STORAGE OF MICHIGAN-GROWN SEBAGO AND RUSSET RURAL VARIETIES OF POTATOES 11 A. Weight Loss 11 B. Storage of Ring-Rot-Infected Potatoes Following Gamma Irradiation 16 Co Wound Healing, Suberization, and Periderm Formation 20 D, Studies on Potato "Hormones" 26 IV. OTHER STUDIES 29 V. ANALYSIS OF RESULTS 30 Ao Correlation of Weight-Loss Data on Idaho-Grown Russet Burbank Variety Seed Potatoes 30 B. Discussion of Studies on Hormone Activity and on Periderm Formation 39 VI. SUMMARY AND CONCLUSIONS 39 REFERENCES 41 iii

LIST OF TABLES No Page Io Temperature and Humidities near Stored Potatoes in Storage Rooms in Food Service Building 5 IIo Percent Loss to Rot and Severe Shriveling, and Percent Usable at Final Sampling Date (11-1-55) for Idaho Potatoes 13 IIIo Hormone Activity of Pontiac Variety Potatoes 28 IVo Carbohydrate Analyses on Russet Rural Potatoes 29 V. Values of M: Weight Loss Independent of Radiation Dosage for Idaho Tubers 31 iv

LIST OF FIGURES Noo Page 1. Potatoes in cardboard containers in position to be irradiated. 5 2. Irradiated and control Idaho seed potatoes stored at "60"~F, at maximum sprout growth. 6 3o Nonirradiated and 10,000-rep irradiated Idaho seed potatoes (1954 crop) stored at various temperatures until September 27, 1955. 8 40 The graphical relationship between the total percent weight loss, W, the biological weight loss, B, and the weight loss independent of radiation, M (potatoes stored 140 days at 50F and 90% RHoH) 9 5o Weight-loss data as a function of radiation dose, length of storage, and relative humidity. 10 6. Plot of "B" - Rate of weight loss dependent on radiation dosage. 12 7. Accumulative percentage weight loss vs radiation dose for Sebago variety potatoes stored at 45~F since December 7, 1955o 14 8o Accumulative percentage weight loss vs radiation dose for Russet Rural variety potatoes stored at 45~F since December 7, 1955. 15 9. Comparison of typical data for three varieties of potatoes (percentage weight loss vs radiation dose). 17 10o Ring-rot incidence in irradiated field-infected Sebago potatoes stored at 20~Co 18 11o Storage-rot incidence in irradiated field-infected Sebago potatoes stored at 20~Co 19 12. Photomicrographs of Sebago potato-tuber sections six hours after peeling, showing suberization of control only. 22 13. Photomicrographs of Sebago potato-tuber sections 196 hours after peeling, showing suberization of control and treated tubers. 24 14. "B" rate of weight loss dependent on radiation dosage. 33 15. Weight loss vs reciprocal of the absolute temperature (~K) of storage. 36 v

CONTRACT RESEARCH PROGRESS REPORT QUARTERMASTER FOOD AND CONTAINER INSTITUTE FOR THE ARMED FORCES, CHICAGO Research and Development Division Office of the Quartermaster General Fission Products Laboratory Project Noo 7-84-01-002 The University of Michigan Contract No. DA 19-129-qm-349 Engineering Research Institute File No. S-527 Ann Arbor, Michigan Report No. 10 (Terminal) Period: 1 May 1955 to 31 December 1956 Initiation Date: 20 April 1955 Official Investigator: L. Eo Brownell, Supervisor, Fission Products Laboratory, Professor of Chemical and Nuclear Engineering, The University of Michigan Collaborators: C. Ho Burns, Biochemist, Fission Products Laboratory Fo Go Gustafson, Professor of Botany, The University of Michigan Wo Jo Hooker, Associate Professor of Plant Pathology, Michigan State University Do Isleib, Assistant Professor of Farm Crops, Michigan State University Jo Vo Nehemias, Research Associate, Fission Products Laboratory Ro A. Martens, Research Assistant, Fission Products Laboratory A Pendill, Analytical Chemist F. Heiligman, Quartermaster Food and Container Institute Title of Contract: Gamma-Ray Sprout Inhibition of Potatoes THIS INFORMATION IS NOT TO BE REPRINTED OR PUBLISHED WITHOUT WRITTEN PERMISSION FROM HEADQUARTERS, QM R AND D COMMAND, NATICK, MASSACHUSETTS, 1

SUMMARY Earlier limited studies at The University of Michiganl and elsewhere2 indicated that low-dosage gamma irradiation of potatoes was useful in preventing sprouting and spoilage of potatoes under storage without the development of undesirable changes. Because of sprouting followed by rapid deterioration, it usually is not possible to keep northern-grown potatoes under storage for more than eight or nine months of the year. It is believed that desirable types of potatoes can, by irradiation, be made available the year around. This treatment might be particularly useful in increasing the storage life of any type of potato shipped overseas for the armed services. However, little was known about the limitations of the process of irradiation and about the optimum conditions for storage of irradiated potatoes. During the past year a study has been conducted with support from the Quartermaster Corps of the U.So Army to explore the storage properties of irradiated potatoes. The gamma irradiations have been performed at the Fission Products Laboratory, Engineering Research Institute, The University of Michigan, and the research studies have been conducted jointly at The University of Michigan and at Michigan State University. Io TECHNICAL OBJECTIVES Low-dosage gamma irradiation of potatoes has been found to be very successful in preventing sprouting and spoilage of potatoes under storage without the development of undesirable changes. Northern-grown potatoes are available only eight or nine months of the year. Because of sprouting followed by rapid deterioration, it usually is not possible to keep potatoes under storage for longer periods. It is believed that desirable types of potatoes can, by irradiation, be made available the year around. This treatment might be particularly useful in increasing the storage life of any type of potato shipped overseas for the armed services More specifically, the general technical objective is described below: A. A study will be made on the effect of low dosages of gamma radiation (approximately 5,000 to 25,000 rep) on at least one white-skinned and one russet-variety potato with the object of determining the dosage needed to inhibit sprouting when stored at 35~, 4, 0, 60, 0, 0~, 60~, and 80~F with 85% relative humidityo 2

B. An investigation will be made, using doses of gamma radiation as high as 200,000 rep on the same types of potatoes as studied in (A) above, to determine the effect of overdose o C. A study will be made of the effect of three different relative humidities and at two storage temperatures during storage on a white-skinned and a russet-variety potato. D. An evaluation will be made at no less than four scheduled intervals during the storage of the irradiated potatoes that have been stored. The said evaluation shall include: 1. total starch, sucrose, and reducing-sugar content, 2, sprouting and its inhibition, 3. general appearance and texture, 40 interior fleshy region of peeled and sliced potatoes for decay, black heart, blackening, and other manifestations of enzyme and/or microbial action, and 50 loss in weight, to be determined and subdivided into combined respiration and transpiration loss and loss due to sprouts Eo As time allows, a limited study will be made on the effects of wound healing, with special emphasis on formation of cork cambium, cellular organization, and structure. F, A quantitative respiration study will be conducted on at least a white-skinned variety and a selected russet variety of potato. Go The effect of gamma radiation on the activity of specific enzymes involved in potato respiration will be investigated. This will be aimed at understanding the inhibition of enzyme activity as reflected by changes in starch content, total and reducing-sugar content, and color change, allowing for extended storage life of the potato. H. A study will be made of the growth hormone and inhibitors in and around the eyes of irradiated and control potatoes to determine whether or not gamma-ray-induced inhibition of potato sprouting is caused by an increase in the quantity of sprout inhibitorso I. A study will be conducted to determine the incidence of common storage rot in irradiated potatoes. This will include inoculation and storage studies utilizing common potato-rotting bacteria and fungi. J. Samples of potatoes described under (A) will be made available for acceptance testing by personnel of QM F and CI. 5

Respiration is one of the fundamental processes of all living organisms. Therefore it was included as one of the processes to be investigated in the study of the influence of gamma irradiation on the sprout inhibition of potatoes. II. SPRING IRRADIATION AND SUBSEQUENT STORAGE OF IDAHO-GROWN RUSSET BURBANK VARIETY (SEED) POTATOES A. IRRADIATION Because of the late date (May, 1955) of activation of this research program relative to the potato season, uniform potatoes of good quality and known history which were suitable for experimental studies were difficult to obtain when the study began Twenty bags, 100 lb each, of Idaho certified seed potatoes of the Russet Burbank variety and with known history were obtained The potatoes were given a radiation dose of 10,000 rep on or about the first of June, 1955, and were put in storage with control lots of the same potatoes at various temperatures nominally controlled at 350, 40~, 500, 550 and 65~F and at room temperature (75-80~F) at The University of Michigan and Michigan State University. Additional lots of the same potatoes were irradiated with doses of 5,000, 15,000, 50,000, 100,000, and 200,000 rep and all were stored at 45~Fo These potatoes were weighed every two weeks and checked for sprout development. The irradiations were performed in quart cardboard containers in the radiation cave of the Fission Products Laboratory. The use of these small containers permitted more precise evaluation of the radiation dosage delivered, although somewhat more handling time was required to complete the task. Figure 1 shows a typical arrangement of these cartons, full of potatoes, in place around the outside of the source positiono After the irradiation has been half completed, all cartons are rotated 180 degrees, and the upper and lower rows are interchangedo In the background, other irradiation experiments (animal food for feeding studies) may be seen in progress. Samples of potatoes from each of the different dosage lots were put in storage at Michigan State University at 50F (50~ to 550F) and three controlled humidities, The constant-humidity cabinets were originally to be held at 70, 80, and 90% relative humidity However, the equilibrium established after placing the tubers in the cabinets resulted in humidities which varied somewhat and when measured were found to be 66, 74, and 90%. The average value was estimated to be 60, 75, and 90% in one measurement and 55, 75, and 95% in another measurement I

Fig. 1i Potatoes in cardboard containers in position to be irradiated. The bulk of the potatoes 'was stored in the Food Service Building at The University of Michigan and it was found that the actual average temperature of the storage rooms at the location of the potatoes varied considerably and differed from the temperature at the thermostat and the temperature at which the room was reported to be maintained. Table I lists the temperatures and humidities that were observed at the location of potato storage in the various rooms usedo TABLE I TEMPERATURE AND HUMIDITIES NEAR STORED POTATOES IN STORAGE ROOMS IN FOOD SERVICE BUILDING Rated Measured Measured Average of Average Temperature High Low Measured Relative of Room, Temperature, Temperature, Temperatures, Humidity, OF OF OF F 35 39 31 35 87 40 45 40 42 82 45 42 40 41 95 50 53 46 50 85 55 54 49 52 95 60 64 62 63 53 RT 90 75 80 * *Varies as do outside humidity and temperature. 5

During storage the potatoes were examined, weighed, and the sprouts measured monthly. The sprouts on the control tubers were of maximum size at the end of August. Figure 2 shows irradiated potatoes (on left) and sprouted controls stored at "60"~F. Fig. 2. Irradiated (left) and control (right) Idaho seed potatoes stored at "60"'F, at maximum sprout growth (August 29, 1955). B. SPROUTING A slow but steady sprout growth was noted first on the control potatoes stored at "45" and "t40" degrees. The sprouts at "45" and "40" degrees at the end of October were from two to six inches long. Control potatoes stored at "35" degrees at this time had tiny sprouts from three to five mm long. All the sprouts which formed on the control potatoes stored at "room temperature" withered and died by October 26 as a result of lack of nutrient and moisture. None of the irradiated potatoes stored at The University of Michigan or at Michigan State University sprouted, except those which received only 5000 rep. These potatoes were stored at "45" degrees and have developed some very short sprouts (3-5 mm long), much shorter than those of the control potatoes which were stored at the same temperature. Samples of the potatoes which were irradiated to the extent of 10,000 rep were stored at all the temperatures listed and none stored in Michigan developed sprouts. However, some of the 10,000-rep irradiated potatoes which were 10,000-rep irradiated potatoes which were shipped to QMC in Chicago did show some limited sprouting. 6

Figure 3 is a composite of the control (left) and the 10,000-rep irradiated (right) potatoes stored at "35,"," 0" "45," "50, "155," and "60" degrees and at room temperature (RT)o These photographs were taken on September 27, 1955, when the sprouts of the control potatoes stored at "50"~F and above had reached their peak growth and had started to die. The very tiny sprouts in the eyes that were present in the potatoes at irradiation died and disappeared in the irradiated potatoes stored at "55"~F, but remained dormant in the irradiated potatoes stored at "40"~F and higher. C. WEIGHT LOSS Weight loss of Idaho-grown potatoes was determined as a function of radiation dosage, storage temperature, storage humidity, and storage time. Increases in the storage temperature of irradiated potatoes resulted in increases in weight loss with time. Increases in the storage temperature of the control potatoes resulted in a rate of weight loss considerably higher than that of the irradiated potatoes, probably because of the higher metabolic rate and greater surface for transpiration presented by the sprouts. Potatoes receiving increasing doses of radiation, but held at one storage temperature, showed a marked decrease of weight loss with increase in radiation dose up to 15,000 rep. Radiation doses of 50,000 rep and higher resulted in the same weight loss for the same storage conditions. It appears that 15,000 rep is essentially as effective as higher doses in checking weight loss. In storage tests at different humidities, the potatoes lost less weight as the humidity of the air in which they were stored increased, It was also observed that irradiated potatoes lost less weight than nonirradiated potatoes when samples of each were stored at the same relative humidity. Figure 4 is a plot of weight loss vs radiation dosage for potatoes stored for 140 days at 90% relative humidity and 50~Fo It shows the general trend observed with these potatoes of the effect of increasing radiation dosage on the reduction of weight loss and an approach to an equilibrium value at about 20,000-rep dosage. Increases of radiation dosage above this level have little additional effect on the inhibition of weight loss. Figure 5 shows additional data plotted on semilogarithmic graph paper and further illustrates the phenomenon and demonstrates that the equilibrium value of "weight loss independent of radiation dose" varies with storage time and relative humidity. The potatoes lose weight for a number of reasons, but for simplicity the losses may be divided into two groups, (M) "physical" and (B) "biological." Regardless of whether the potato tubers are living or dead, they will lose weight when stored in an atmosphere not saturated with water vapor. This is true because the skin of the potato tuber behaves as a membrane through which water, and also oxygen, carbon dioxide, etc., may diffuseo The "physical" weight loss is largely the result of the evaporation of water, which de7

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30 25 20 8 H J B W =B+M 5 h\\l\\R\\\\ \\\\ \\I\\\\\n\\\ 0 10 20 30 40 50 60 70 80 90 100 DOSE KILOREP Fig. 4. The graphical relationship between the total percent weight loss, W, the biological weight loss, B, and the weight loss independent of radiation, M (potatoes stored 140 days at 50'F and 90% R.H.).

I00 x\ e W \ ~- 60%.R.H:~ ---— a ad^j a" ~1 V~ 190% R.H r V ~ 4b- Dow am GM d_ 60% R.H ' ~ "A"'i-,_.-. —__,_ ______ 5_____ ^^^^1~~~~-"75% R.H 77 DAYS W X — - A~.... 90%R.H I — ~..o60__~/~R.H __R.H.Qf~ S 90 o R H:38 DAYS 10 9.0 R. 0 20 40 60 80 I 00 DOSE KILOREP Fig, 5. Weight-loss data as a function of radiation dose, length of storage, and relative humidity. lO

pends in part on the diffusivity of water through the skin, the partial pressure of water vapor in the storage air, and the velocity of air circulation around the stored tubers. These and other variables which influence the mass transfer coefficients will have some effect on this "physical" or "radiationindependent weight loss." If this value of "radiation-independent weight loss," M, is subtracted from the curve for "total weight loss," W, the remaining "biological" weight loss, B, shown in Fig. 4, is radiation-dependent. For the case of Idaho-grown potatoes used in the studies, the radiation-dependent component of weight loss demonstrates a logarithmic reduction with radiation dosage, as illustrated in Fig. 6. This phenomenon tends to support a hypothesis of weight loss proportional to the number of living cells, if the cells are inactivated by a random, statistical process. Extensive data on a wide variety of unicellular organisms from many laboratories indicate similar exponential radiation inactivation phenomena. Do ROT AND OTHER LOSSES DURING STORAGE Table II presents the percent of loss attributed to rot and to severe shriveling and lists the percent usable for the various doses used for the Idaho potatoes as of November, 1955. The lots stored at various conditions and which received from 10 to 200 kilorep were considered to be 84.9 to 9865% usable, whereas none of the controls were usable on November 1. In November, 1955, no sprouts were found on any tubers receiving treatments of 10,000 rep or more, but the total losses of the controls and low-dosage-rate samples resulted in a discontinuation of the study of these potatoes and the initiation of a new study. IIIo FALL IRRADIATION AND SUBSEQUENT STORAGE OF MICHIGAN-GROWN SEBAGO AND RUSSET RURAL VARIETIES OF POTATOES A. WEIGHT LOSS The procedure described for the Idaho-grown potatoes was repeated in November and December for Michigan-grown Sebago and Russet Rural varieties of potatoes of known history. The weight-loss data for Sebago variety potatoes are plotted in Fig, 7 and clearly indicate that, over the range studied, weight loss in the case is an increasing function of dosage. Comparing these data with those for Russet Burbank vairety Idaho seed potatoes shown in earlier figures indicates that a marked difference exists in the responses of the two lots of potatoes exposed to radiation treatment. Similar data for Russet Rurals which are plotted in Fig. 8 indicate a response intermediate between the other two varieties and show no gross dependence of weight loss 11

4.0 2.0 ~ 1.0 0.8 0.6 0.4 ~~~ 4 060.2 0.1 o \\-.08_ ( 1.06 ~~.04 o.02 -P 'H Cd.01.....008.004-.002. ooi... I_ I I I 1~X 0 5 10 15 20 25 30 35 40 45 50 DOSE IN KILOREP Fig. 6. Plot of "B" - Rate of weight loss dependent on radiation dosage. 12

TABLE II PERCENT LOSS TO ROT AND SEVERE SHRIVELING, AND PERCENT USABLE' AT FINAL SAMPLING DATE (11-1-55) FOR IDAHO POTATOES Relative Dosage, Severe Humidity, kilorRot Usable kilorep Shriveling percent 0 1.1 98.9 0 5 2.1 68.2 29.7 10 5.0 8.4 86.6 60,15 6.8 5.9 87.5 (avg) 50 6.4 8.7 84.9 100 4.6 9.3 86.1 200 7.4 0 92.6 O 0 100 0 5 0.9 54.5 44.6 10 1.9 11.0 87.1 (avg) 15 2.4 29.2 68.4 (av) o50 2.2 11.1 86.7 100 3.2 8.2 88.6 200 4.6 6.7 88.7 0 0 100 0 5 2.5 46.1 41.4 o10 15 0 98.5 9o 9 a) 15 0 5.5 94.5 (ag50 3 4 1.4 95.2 100 2.3 7.1 90.6 200 5.2 0 94,8 13

23%..< ~April 16 20% 20%~~~, I,-April 2 0 o March 19 I< /. I — w 1% - J 0 50 100 150 200 RADIATION DOSE (KILOREP) Fig. 7. Accumulative percentage weight loss vs radiation dose for Sebago variety potatoes stored at 45~F~ since December 7, 1955.

0c _______________^" April 16 I) - 0 o O -~_ 0~ April 2 I I- U^,A _ n,__ ____~ March 19 z U a -- March I w > -----— L Feb 16 5.cI( -J 8 E 0 50 100 150 200 RADIATION DOSE (KILOREP) Fig. 8. Accumulative percentage weight loss vs radiation dose for Russet Rural variety potatoes stored at 45~F since December 7, 1955.

on radiation dosage. In addition to differences in variety and source it should be pointed out that the Idaho Russet Burbank variety seed potatoes were irradiated in May after storage for several months, whereas the Sebago and Russet Rural varieties were irradiated after harvest in the early winter of 1955. These differences among the three varieties with regard to weight loss during storagbe as influenced by radiation dosage are exemplified by typical data in Fig. 9. More detailed evaluation of these differences as functions of storage time, temperature, and humidity could be undertaken from the data. It is clear, however, that three qualitatively distinct gross radiation responses are present in the three varieties. In all three cases, anomalous behavior is exhibited in the dose region less than 20,000 rep. More detailed data in this region might prove enlightening. The hypothesis of radiation destruction of living organisms, assumed to explain the apparent reduction of metabolism with radiation dose in the Idaho seed potatoes, is clearly not a satisfactory explanation for the phenomena observed in the other two varieties. If a second hypothetical process of radiation-induced weight loss is proposed, a satisfactory model for the three responses can be constructed. Physiological differences among the three varieties must then be considered to explain the fact that one process is predominant in the Idaho seed potatoes, the other in Sebagos, and neither in Russet Rurals. The second hypothesized process might be simply a mechanical damage to intercellular structure, facilitating moisture removal. Given a variety particularly sensitive to such destruction, such a process could conceivably overbalance the reduced metabolism due to radiation damage and produce a gross increase in weight loss with dose. B. STORAGE OF RING-ROT-INFECTED POTATOES FOLLOWING GAMMA IRRADIATION Field-infected Sebago potatoes were obtained on December 5, 1955, from a grower near Howard City, Michigan, and were gamma irradiated in the Fission Products Laboratory on December 12-13. The potatoes were stored at 10C and 200C at Michigan State University and were first exani.lned on December 22, ten days after treatment, for incidence of ring rot and other storage rots. The results of this and subsequent inspections on incidence of ring rot for tubers stored at 20~C are plotted in Fig. 10. No trend was observed in incidence of ring rot in turbers stored at 1iC; therefore, these data were not plotted. In the inspection, tubers suspected of being rotted were cut and those showing typical ring-rot symptoms were considered to have ring rot. Stain diagnosis was not made. Other rotted tubers lacking ring-rot symptoms were placed in a second grouping classed as "storage rot" as plotted in Fig. 11. 16

20 Sebogos 2 0 I- I d a h o S e e (IV ~~~)~ 0 CJ ^ 50 100 150 200 (3.15 --- (percentage weight loss vs radiation dose). Russets 10 Idaho Seed 50 100 150 200 RADIATION DOSE (KILOREP) Fig. 9. Comparison of typical data for three varieties of potatoes (percentage weight loss vs radiation dose).

13.2 12.0 11.0 10.0 9.0 8.0 Ol w 7.0 129 Days 0 0 6.0 ar: 5.0 4.0 80 Days 2.0 30 Days 10 Days 100 200 300 350 RADIATION DOSE (KILOREP) Fig. 10. Ring-rot incidence in irradiated field-infected Sebago potatoes stored at 20~C. 18

50.0 / / 40.0.. 80 days / o 30.0 z D / / R 129 days/// 0 p 20.0 94days / ~~~~~~~/ ^ >45 days 10.0 I 0 O 3~ days~ 0 100 200 300 350 RADIATION DOSE (KILOREP) Fig. 11. Storage-rot incidence in irradiated field-infected Sebago potatoes stored at 20~C. 19

It should be pointed out that some of the potatoes infected with ring rot in this lot had been discarded during harvesting operations and in grading operations before shipment. Thus, the amount of ring rot shown in Fig. 10 does not reflect the total amount of infection in the lot. There was no late blight in the shipment of tubers, which made possible rather accurate diagnosis of the tuber injury0 Tuber injury as a result of irradiation was not evident following ten days in storage. By the end of 30 days, the majority of tubers receiving 500,000 rep were rotted and a few receiving 200,000 rep had broken down. Positive diagnosis of ring rot in tubers receiving the 500,000-rep treatment could not be made due to inability to identify ring rot following severe radiation injury Radiation injury resembled severe freezing injury in many respects, Affected tubers were often somewhat cheesy in consistency, later breaking down into a soft rot, Affected tubers often had a fermented odor, and the general appearance was more suggestive of storage rots of the sweet potato than that of the Irish potato, The alcoholic-type fermentation observed in the tubers receiving the higher dose of gamma radiation may have some connection with the sucrose analyses reported later Many tubers held at 20~C were rather badly wilted after 80 days in storage. Nonirradiated controls were so badly sprouted at the 129-day inspection that all tubers were cut for ring-rot evaluation. At 1~C storage there was little wilting after 133 days and tubers had not begun to sprout. At 20~C storage (Fig. 10) there was some evidence that ring rot was developing somewhat more slowly in the irradiated tubers than in the untreated tubers The increase in incidence of ring rot at the 129-day period is due in part to the cutting of all the tubers remaining in the sample because of sprouting. C, WOUND HEALING, SUBERIZATION, AND PERIDERM FORMATION Sebago, Russet Rural, and Katahdin varieties of potatoes were irradiated and then wounded to study wound healing, suberization, and periderm formation. n an initial study, sube ation and peridem formation following wounding of nonirradiated Sebago tubers were observed. These tubers were peeled uniformly by hand, packed loosely in moist sphagnum moss, and held at 26~C for the two-day period during which observations were made. A one-halfinch cork borer was used to obtain tissue samples which were taken from the median region of the tubers. The cylindrical samples were then trimmed to rectangular shape, fixed in FAA, dehydrated in an ethyl alcohol series, changed to chloroform, infiltrated with and embedded in ordinary paraffin, and cut into sections 10 to 20 microns thick on a rotary microtome. The sections were 20

strained with saffranine and fast green. This procedure was standard for all material prepared for this study. The preliminary study of untreated Sebago tubers yielded results similar to those described by Artischwager3 in his classical study of potato histology Six hours after wounding there is evidence of suberization of the surface layer of cells. This layer increases in depth and in density of suberin deposit until after 48 hours three to four cells from the wounded surface show distinct suberization. Periderm formation is not evident after 36 hours, but after 48 hours distinct periderm involving two to four cells is apparent. Some of these cells appear not to have completed division, but are binucleate with traces of developing wall appearing along a median plane of the parent cello In the second series of tests, Sebago tubers which were irradiated at 0-, 15-, and 200-kilorep dosages and stored for 50 days were then peeled and sampled as described for the initial study, but the number of sampling intervals was increased to include 0, 6, 12, 24, 48, 96, and 192 hours after wounding At six hours after wounding only the nonirradiated tubers showed any suberization. Traces of suberization were apparent in treated sections after 36 hours, and at 48 hours there was little to distinguish the suberization in treated sections from that in control sections. Periderm formation in controls was not evident at 48 hours, but periderm was well developed after 96 hours. There was no evidence of periderm formation in either of the two treated series even after 196 hours, and it is assumed that cell division was completely inhibited in the treated potatoes, precluding the possibility of periderm formation. Beginning at the 48-hour stage and continuing through 96 and 192 hours, the depth of the layer of suberized cells and the intensity of suberization were essentially the same in both treated and control potatoes. Figures 12 and 13 illustrate the response of tubers at each treatment level and after 6 and 192 hours, respectively, subsequent to wounding. In the third series, tubers of Sebago, Katahdin, and Russet Rural varieties were irradiated at 0, 2500, 5000, 10,000, and 20,000-rep dosages and were wounded (peeled) eight days after treatment. These tubers were sampled after 36, 72, and 192 hours. Suberization occurred in all tubers with little difference evident among the various treatments and varieties. Irradiation at any level prevented formation of any distinct periderm, although in at least one sample of Katahdin treated at 2500 rep some periderm-like cells could be distinguished. In a fourth series, 29 days elapsed between irradiation and wounding. In other respects this experiment was similar to the preceding test, In Sebago tubers, no distinct periderm could be distinguished in tubers receiving any level of irradiation At 10,000 rep there were scattered periderm-like groups of 21

Control Fig. 12. Photomicrographs of Sebago potato-tuber sections six hours after peeling, showing suberization of control only. 80X 22

15,000 -rep dosage 200,000 -rep dosage Fig 12 Concluded 23

^. Control:::-: ~":.-::-:::.:;...:::..........'": "::. i00. —~ ' 'm^: 0t0yj00- -.t>-..i A __;;ij0 lll~-.^,,' ~~ ^_.B-t'llllll^. - i ""Hi 0: B ': t0000:::.,0..,;:: -izv... " "-",;.......... ':.tX;i-E,.,B,: '.. _ ' 'l'.'a....... ' -..:', "::.: '": '".l~-"....'......: ' -:. ~ '-:. Fig. 13. Photomicrographs of Sebago potatotuber sections 196 hours after peeling, showing suberization of control and treated tubers. Extensive periderm formation has occurred in controls., none in treated tubers. 80X ''::' 0':;0:4::,, 2i0s,2 k::,,2 i~~~~~i::;:::,.:::j0:;;;tigg: iL,:::: i;; 0:: -00;;4: i -'BB::: i i:.'.'0~:;.iI~i:.::~l::::~:::::::: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~::::::::: S:'::::.:::::f:::;: i: -::i::: i;:9:.:::'':-iiiiii~iiiiiiiiiiii:!t ff: E i:gE iC:i.E: ai:ji:liii:::~~~~~~~ ~ ~ ~ ~:y::::y::: 4:i:Ei:i fi iE::L: F iW::ii~l'::s:~~ir Il~~ii~~:::i-:;F i jiii:~~~:iii::::~~~~ ~ ~ ~:::::::i:: -.. ~ ~::: f:: 0 0lii;lij:'l:,:i::iiiii:iii~iii igE -.-..,,:::::::-::D: 0 0 f 'Rt:::t Fig.: 13.__::::iiII Photomicrographs of Sebago potato-lil tuerscton 96hor ate eeinshw ing suberization of control and trea~~~~~~~~~~~~~~~~~~~~~~~~~~~ted tubers.'i'~l;lii~~iii Extensive periderm formation has occurred in~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i.-:ir:::::::::':''''::::''" ~iif~::i''jji: control, nonein treaed tubrs. 80 24~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~iij'iiii

i:sr.; ii;.F iS i i;.:::::cv..........................................................................;............:;::::.:...:..:..::..vX.::::;. -:::;i::; g;E!ii:X:f;;II..,.:B'~ii —;~:iii j.i:Sii ii:- -: —. ---:::::::::;:::-tiE0Si'-I:jS'g.''':.W.,..:i.. )EEi~i,.''ti~ lf i:: i Si ~ i.:'l'.'';.:~ -.:.. i-: i::::j.;: -gi ~:- i-_: fS:ii- i~: i: 8g:: i- i r j >:. i -igE g K gy: 0 0g 0;0fA:j; g00000::: t -.'i:;: 0...........:,> c.00'fSStE'g......... ~~~;|0 00g.:>~..:: fS0. tr:; 00:j>. -:ii-i: t':..., N i:-i '; i:i-::;::::;l:; ~ ~-::L:"ii~:i:ii:, giiii-;..................e>2l t:::0|';i4: "::-_iiii: g~;';:iSSf-: tC:::: f::::i'S::E:A i::::f jl:05ri'T:gi::::t i:s iiiii:dfie i >E iri!iE:.~.:'''g'S'l: ':'''j~ji: V i:::|55::'::::::E,}}::)i::9::tW. fj.F:::f:2i~~~~~~~~~~~~~~~~~~ifidien~~~~i jER..,@y BiEX TlES.,)k <E.;,.i.E,.2. i, i ikE:E~~~~~~~~~~::::::i~:li2EjjE -::k,,\liSE.E-:-:- s'- "::':::::g::~:._:~i i~::i::: 'i e, ii, i,: gifgi g Ei ggggg,, i~~~~~~~~~~ae >"B t0:'tt t:'B'::4:: t:':: 't; eg; ' Bl:;:;:jy;4. <,,.. z } 6. i e i e, 000.:l ig 50i~~~~~~~i00;00: lt ':l fj 'BB:': l:::Xl j0000 tlig>:i::it::ii i::::::i B ~~~~~~~~~~~~dosage::::::?:l~~~~~~~~~~~i'i::j::;|yy: ijii~iiiiiiii_ 0;jl::: 0:j 9; j |;4 l };:;: i| )l:l0 0: )l0 |B. 9\:}X:; | l: ~::i:;: i::i l::::::r::::::::>::::::: Impliciitily8 —:~ I~iiil,,,ig. 13."" Concluded.'' 25~~~~~~~~~i'~

cells, and other treated tubers showed occasional pairs of periderm-like cells. Distinct periderm was evident in controls after 72 hours. Suberization was delayed but not prevented by the irradiation treatments. Katahdin tubers showed evidence of slightly more frequent peridermlike divisions, but again there was no obvious continuous periderm such as appeared in control tubers, Suberization was delayed slightly less than in Sebago tubers o Russet Rural potatoes showed fewer of the scattered periderm-like areas than either Sebago or Katahdino Those which were observed were chiefly in tubers irradiated at 2500 rep, but not at higher dosages However, suberization was not very distinctly slowed in Russet Rural, in contrast to the other two varieties. D. STUDIES ON POTATO "HORMONES" The observation that plants tend to grow toward the source of light is very old. That this phototropic action is related to definite. chemical compounds was first clearly indicated in the classical investigations of BoysenJensen4 on the coleoptile of Avena. Naturally occurring chemical compounds possessing the property of being able to regulate plant growth were given the name "hormone." Any compound, either synthetic or naturally occurring, that has this property is termed auxin. A large number of scientific articles have appeared relating to auxins 5 In general, if there is no auxin present, there will be no growth, but a supra-optimal concentration of auxin will also prevent growth. Only minimal quantities are required for plant growth, and the application of an excess will retard or stop growth. By the use of chemicals it is possible to prolong artificially the rest period of potato tubers. On the other hand, the normal resting period can also be shortened by using ethylene chlorohydrin. The belief prevails that the content of auxin not only varies in quantity during storage, but that the concentration also varies in the different parts of the potato, Prior to sprouting of the potatoes in the spring an increase in auxin has been observed in the fleshy part of the potato, while later the auxin, or its precursor, increases in the potato peelo The change of the precursors into auxin is generally assumed to be enzymatic in origin. One of the chief precursors may be presumed to be the amino acid tryptophane, because it has been shown that the amount of biosynthesis of auxin in a medium depends on the content of tryptophane in the medium. One of the objectives of this research project was to increase the storage life of potatoes as a result of gamma irradiation. Gamma irradiation has been shown to slow down or halt the sprouting of the potato. Therefore, a study of the concentration of the growth-regulating substances which control 26

sprouting was considered advisableo This study involved a comparison of concentrations of phytohormones in irradiated and nonirradiated potatoes under varying storage conditions. In this research study an extract of the "eyes" and a small amount of surrounding tissue of potatoes was made with freshly distilled ether which is completely free of hormone-destroying hydrogen peroxide. The various growthregulating substances were then separated by paper chromatography. The sample consisted of the eyes of several potatoes, including a small portion of the flesh around each eye. The eyes at the apical end were not used because they seemed quite variable About 10 grams of this material were collected and covered with peroxide-free ether. The ether and eye tissue were kept in the refrigerator (about 40~F) for a period of 20 hours The ether was then decanted and evaporated to drynesso The residue was taken up in about 0ol ml of alcohol, and this solution subjected to paper chromatography. The auxins separated by chromatography were assayed by the Avena (oat) technique. In this procedure 72-hour-old plants which had been grown in the dark at a relative humidity of 85% and at a temperature of 25 -260C were usedo Plants of this age and grown under these conditions are very reactive. This assay procedure was originally developed by Went,6 but it has been variously modified by other investigators. A preliminary set of experiments was conducted to test the Avena assay with varying amounts of whole extract (not chromatographed) of the eyes of nonirradiated tubers, using Sebago variety potatoes. The results showed that such extracts had pronounced growth-hormone activity. A second set of experiments was conducted with Sebago variety potatoes to determine if the extract could be resolved by paper chromatography into separate components, and to determine the activity of eacho In the early studies two components were separated, one which moved slowly, by paper chromatography, and one which moved fast. The latter was found to consist of essentially all the growth-promoting activity of the original mixture (positive bending of the oat coleoptile to an approximate angle of 25~)o The former, however, appeared to possess slight growthhormone-inhibiting activity, as shown by a negative bending of the oat coleoptileo The extent of the negative bending was considerably less than 25~, indicating that the hormone inhibition, if any, was considerably weaker than the positive growth-stimulating activityo Furthermore, the subsequent assays showed no evidence at all of the hormone-inhibitor activity in the eyes of nonirradiated as well as irradiated potatoes This would be consistent with other studies which have shown the amount of growth-hormone inhibitor to decrease with cessation of dormancy Another set of experiments was conducted to determine the effect of irradiation of the potatoes on the hormone activity of the eye tissue directly after irradiation. Sebagos given 5, 15, 25, and 50 kilorep were used in the 27

first assay. It was found that all doses of radiation caused a decrease in growth-hormone activity. As mentioned above, there was no hormone-inhibition action in either irradiated or nonirradiated samples. Most of the activity appeared to come from one "spot" on the paper chromatogram, although this retion of the chromatogram may contain several components only partially resolved, It was not possible to distinguish differences in the extent by which different doses of radiation caused a decrease in hormone activity with respect to controls. In a set of experiments with Russet Rurals given 5-, 1o5, and 23-kilorep dosages of radiation, similar results were obtained. Since samples of Sebagos and Russets have not yet been assayed in the same experiment, it is not possible to compare the two varieties. It was also noted that throughout the course of these experiments, the amount of growth-hormone activity in the control (nonirradiated) Sebagos, which were used each time an experiment was conducted, had been slowly but steadily decreasing. Whereas the approximate average angle of bending was 25~ in the first experiments, it decreased to 180 in experiments conducted two and a half months later. When the hormone and inhibitor concentrations of the control and irradiated Sebago tubers were compared, it was found that the observed differences were of such a small magnitude that they could be the result of errors in technic and not due to any significant difference between irradiated and control tubers. This indicates that the inhibition of sprouting as a result of irradiation is not associated with the hormones and inhibitors found in or just around the eyes in Sebago variety stored potatoes. In late March a shipment of freshly harvested Pontiac potatoes was received from Homestead, Florida. These potatoes were obtained for the purpose of investigating the hormone and inhibitor concentrations in freshly harvested tubers o. Three experiments were conducted with the Pontiac tubers. The first test was made for the purpose of determining what amounts to use in subsequent experiments. In the other two tests, dosages of 25 and 75 kilorep were given a week before the analyses. Table III gives the results for the two experiments. TABLE III HORMONE ACTIVITY OF PONTIAC VARIETY POTATOES Total Hormone Total Inhibitor Activity in Activity in Degrees Degrees Curvature Curvature of Oat of Oat Seedlings Seedlings April 13 Control 10.0 1.9 25-kilorep dosage 17o 6 0 April 20 Control 9o2 22o8 75-kilorep dosage 13.5 16o2 28

These two experiments would tend to show that irradiation increases the hormone concentrations. In the April 20 experiment there is for the first time evidence of a high concentration of inhibitor. This was found both in the control and the treated tubers, so there seems to be no error. In both experiments the treated tubers had less inhibitor. IVo OTHER STUDIES Extensive data were obtained on the analysis of irradiated and control tubers with respect to percent reducing sugar, percent sucrose, percent starch, and ascorbic acid content. Reducing-sugar values were obtained on samples of potatoes receiving altogether 15 different treatments The results showed that increasing the storage temperature of potatoes irradiated with 10,000-rep gamma radiation resulted in decreasing the reducing-sugar content from a value of 2.7% for potatoes stored at 35~F for six months to 0.35% for potatoes stored at approximately 80~Fo Potatoes receiving doses of radiation ranging from 0 to 200,000 rep, but stored at the same temperature for five months, showed no consistent change in reducing-sugar content. Table IV gives values of reducing sugar, sucrose, and starch for control and 15,000-rep irradiated Russet Rural variety tubers stored at various temperatures for eight weeks TABLE IV CARBOHYDRATE ANALYSES ON RUSSET RURAL POTATOES Time Control Irradiated (15 Krep) in Temperature % Red'g {{o Red'g Weeks Sugar Sucrose 9 Starch Total Sugar % Sucrose \ Starch Total 0 All temps. 0.43 0.19 0.43 0.30 8 35 1.71 0.47 13.1 15.3 1.25 2.71 13.4 17.4 8 45 0.50 0.28 20.7 21.5 0.40 0.81 21.4 22.6 8 55 0.27 0.31 20.4 21.0 0.43 0.45 20.4 21.3 8 65 0.37 0.25 20.9 21.5 0.55 0.20 20.9 21.7 8 room 0.36 0.46 22.5 23.3 0.37 0.49 17.1 18.0 29

Carbohydrate analyses for both irradiated and control tubers during the late storage period have shown that the percent reducing sugar, sucrose, and starch remained fairly constant with respect to time. A sharp reduction in reduced ascorbic acid content at radiation doses of 10,000 rep was observed with an additional but more gradual reduction at higher doses. Respiration studies were made both with whole potato tubers and with potato-tissue slices and a Warburg apparatus. Respiration studies on potatotissue slices show no significant effect of gamma-radiation doses of 5,000, 10,000, and 15,000 rep. Doses of 50,000, 100,000, and 200,000 rep resulted in higher respiration rates for the slices of irradiated tubers as compared to the controls. This has continued with time of storage. Respiration studies on whole tubers gave results similar to the tests on slices for doses of 50,000, 100,000, and 200,000 repo Low doses of 5,000, 15,000, and 25,000 rep show (as compared to controls) a drop of about -30% in respiration rate during the first day after irradiation, followed by a sharp increase the second day to about plus 60%, followed by a gradual decline to about -5% at the 1ith week of storage. V. ANALYSIS OF RESULTS Ao CORRELATION OF WEIGHT-LOSS DATA ON IDAHO-GROWN RUSSET BURBANK VARIETY SEED POTATOES Based on the method of analysis described and illustrated in Fig. 6, other data were analyzed, using the assumption that the weight loss at 50 kilorep was in equilibrium range and that the weight loss at the 50-kilorep dose was equal to "M," the radiation-independent weight loss. It was desirable to express M in terms of a rate so as to compare data collected at different storage intervals, Therefore, the values of accumulative weight loss independent of dosage were divided by the storage time to give M in terms of weight loss per day. This is considered permissible because observations indicated that weight loss varied nearly directly with time if storage conditions remained constant. Table V gives the average values of M so determined as a function of humidity. The values of M in Table V show a dependence on relative humidity during storage, as might be expectedo As anticipated, the rate of weight loss 30

TABLE V VALUES OF M: WEIGHT LOSS INDEPENDENT OF RADIATION DOSAGE FOR IDAHO TUBERS Relative Humidity, "M" Weight Loss/Day Independent of Dose, percent lb/day per 100 lb of tubers 60.0761 75 o0640 90.0531 decreases as the humidity increases. To correlate these data, the mass transfer relationships for "drying" were considered. Water was removed in any "drying" process involves the phenomenon known as mass transfer. This phenomenon, when limited to drying, consists of the transfer of mass of substance "x" from point 1 to point 2 as a result of a driving force. This driving force is often expressed as the difference in partial pressure of substance x between points 1 and 2. In the case of weight loss in potatoes by "drying," the substance x is water and the driving force is the difference between the partial pressure of water at the existing humidity and the vapor pressure of water at the existing temperature. The water must pass through the skins of the potatoes; therefore, the greater the area of the potatoes, the greater will be the rate of weight loss. It is generally true that the mass transfer depends on the area of the mass transfer surface. The water vapor must pass through a film of stagnant air that surrounds each tuber. The mass transfer characteristic by this film is termed the mass transfer coefficient, kg, and is comparable to heat transfer coefficients in heat transfer relationships. The relationships discussed above can be summarized by the following equation: W1 = k'a (p, - P2), (1) where Wi = rate of weight loss by "drying," lb/day per 100 lb of tubers, kg = mass transfer coefficient, a = area of tubers, sq in. per 100 lb, pi = vapor pressure of water at storage temperature, mm Hg, and P2 = partial pressure of water in bulk of air surrounding tubers, mm Hg. 31

The mass transfer coefficient may be correlated in terms of air velocity, diffusivity of water, temperature, and other properties of the system that affect the resistance of the stagnant-air film to mass transfer. However, as these are seldom major variables for conditions used in potato storage, k' will be considered to be a constant. Potatoes are of fairly uniform size and shape; therefore, the area per 100 lb "a" may be considered constant. It is more convenient to express partial-pressure difference of water vapor in terms of relative humidity. Combining these constants, Equation 1 may be rewritten as W = k (100 - R.H.), (2) where k = a constant and R.Ho = percent relative humidity. Analysis of the data of Table V indicated that k has a value of 0.0008. Using this value of k in Equation 2 did not account for all the weight loss independent of radiation dosage. There was a constant rate of weight loss, W2, which is independent of both radiation dosage and humidity of storage and which is equal to 00045 lb per day per 100 lb of potatoes or "M" = W1 + W2 = o.0008 (100 - R.Ho) + 0.045 = 0.1o25 - 0.008 RH., (3) where M = total rate of weight loss independent of radiation dosage, lb/dayper 100 lb of tubers, and W2 = weight loss independent of both dosage and humidity of storage = 0.045 lb/day per 100 lb of tubers. It is not surprising that some of the weight loss independent of radiation dosage is also independent of humidity of storage, as some of this weight loss occurs by methods other than drying. Certain chemical reactions involved in respiration and which result in weight loss may be independent of radiation dosage and, of course, would be independent of humidity of storage. To calculate values of "B," the value of M (the rate of weight loss at 50 kilorep) was subtracted from the total rate of weight loss for data obtained with storage at 55~F and various radiation dosages and various humidities. The results of the calculations are plotted in Fig. 14, which shows considerable scattering of the data, as might be expected in such measurements. The greatest deviation occurs for 38 days, the shortest storage period analyzed. The percentage error is higher for this short period when the total weight loss was small. 32

1.0 C) 13:: C1 ) 0.1.. 0 aI 0.01 I I Co) 0 0.001 0 a: z 0 a. z w 0 15 0 15 20 5 30 35 40 45 5000001 Fig. 14. "B" rate of weight loss dependent on radiation dosage.

Figures 6 and 14 show that (except for the scattering of the data in the latter figure) the logarithm of percent weight loss for a given storage period or the logarithm of the weight loss per day per 100 lb of tubers is a linear function of the radiation dosage for the Idaho potatoes used. The equation of this line is ~nB = nc + mD or, taking antilogs B = ceD, (4) where B = rate of weight loss dependent on radiation dosage, lb/day per 100 lb of tubers, e = Naperian base, = 203026, m = slope of line in Fig. 14 = 0o15, D = radiation dose, kilorep, and c = antilog of intercept of line in Fig. 14 = 0o130 To solve for c and m, two points are selected on the curve at B = 0.0001 and B = Oo01 2.3026 (-4) = n(48) + inc (5) 2,3026 (-1) = n(2)+ nc o (6) Subtracting Equation 6 from Equation 5, m 23026(3) 0o15 46 c = intercept Fig. 14 - 0.13 ~. 'nB = - 0o15 D + In.0o13 B 013 -0.15D ( B = 0o13 e (7) The semilogarithmic relationship given by Equation 7 indicates that the weight loss "B" dependent on radiation may be proportional to the number of living cells, if the cells are inactivated by a random statistical process. This phenomenon has been termed the "target theory" and has been given appreciable consideration with regard to the effects of radiation on the killing of microorganisms o Equations 3 and 5 may be combined to give the total weight loss for irradiated Idaho seed potatoes stored at 55~F: 34

W55 = (.125 -.0008 R.H. + 0.13e ' )t, (8) where W55 = total weight loss at 55~F, lb/100 lb, R.H. = relative humidity, %, D = radiation dose, kilorep, and t = time, days. Total weight loss as a function of the temperature of storage was measured with tubers stored in different constant-temperature rooms in the Food Service Building at The University of Michigan. Weight loss was found to increase with temperature of storage, as might be expected from a consideration of the effect of temperature on chemical reactionso The Arrhenius equation for reaction rate as a function of temperature and other variables may be stated as follows: k = AeE/RT (9) where kl = specific reaction velocity constant, A = frequency factor for reaction, E = energy of activation, T = absolute temperature, and R = universal gas constant (8.3 x 107 ergs per degree per mole). If storage humidities are high, the weight loss by "drying" will be proportionately small and an appreciable portion of the weight loss will result from respiration losses. Thus, based on the concept that a major portion of the weight loss results from the chemical reactions involved in respiration, Equation 9 may be applied. As all the variables in Equation 9 may be considered constant except temperature, the data for the weight loss were plotted vs the reciprocal of the absolute temperature as shown in Fig. 15. Equation 9 is an exponential relation and, considering the form of the variables, the logarithm of the rate of weight loss (rate of reaction) should be plotted vs the reciprocal of the absolute temperature. The logarithm of the total percent weight loss (W) for 134 days was plotted against the reciprocal of Absolute Temperature (1/T) as shown in Fig. 15. Straight-line curves were obtained for the data on the control and the irradiated tubers, Equations for the straight lines are given as follows: CONTROL Tubers Equation of the straight line n Wo = - + CO, (10) T 55

100 -_ __ _ Control U) o Radiated J \(10,000 rep) I 3: w 1 0 z w N 3.33 3.35 3.40 3.45 3.50 3.55 3.60 3.65 I/TX 103 Fig. 15. Weight loss vs reciprocal of the absolute temperature (OK) of storage.

where Ln = Naperian logarithm, Wo = percentage weight loss for control tubers, mo = slope of line for control tubers, and Co = intercept on y-axis. In order to determine mo, two points are selected on the line for the control tubers: nn 50 = m x 3.44 x 10-3 + C (11),n 8 5 = mO x 3 6 x 10- + C0 O (12) Subtracting Equation 12 from Equation 11, 3.91202 - 2.14007 = - 0.16 x 10- x mo or 1.77195 0 - 0.16 x 10-3 Mo = - 11.075 x 103 -11.075 x 103 Wo = Aoe T (13) Co where Ao = eC. IRRADIATED Tubers mR.n WR C, (14) where WR = weight loss for irradiated tubers, mR = slope of line for irradiated tubers, and CR = intercept on y-axis. To determine mR, two points are selected on the line for irradiated tubers: Ln 2703 = mR x 3.35 x 10 + CR (15) 2n 4.35 = mR x 3.60 x 103 + CR. (16) Subtracting Equation 16 from Equation 15, 3,30689 - 1e47018 = 0,25 mR x l0.e 37

. mR = - 7.3468 x 103 -7.35 x 10 WR = AR e T, (17) CR where AR = eR. Rewriting Equation 17, -7.35 x 10" WR =AR e To evaluate AR, the value of WR at 55~F may be calculated and equated to W55 obtained by Equation 8 or: -7.35 x 103 WR at 550F = AR e 288 (18) W55F = [0.125 - 0.0008 R.H. + 0.13 e1 ] It. (19) Equating W55oF = WR at 55~F or -7~35 x 10~3 AR = 0125 - 0.0008 RoHo + 0.13 e-015D te x 10 (20). General Equation WRTOK = [0.125 - 0.0008 R.H. + 0.13 e ~.15D] te-7 x 103(< - (21) or WRTOK = ebt [0.125 - 0.0008 R.H. + O.13e. 5D], (22) where WRTOK = percentage weight loss at T~K, t = storage time after irradiation, days, T = absolute temperature, ~K, D = radiation dose, kilorep, RoH. = relative humidity, %, e = Naperian base, 203026, and b = - 7035 x 103 [1/288 - 1/T]. The data collected on Idaho seed potatoes from the 1954 crop which were treated with gamma radiation and stored at various temperatures and humidities were summarized by means of Equation 22. However, Equation 22 is applicable only 38

to the potatoes of the variety, size, source, and age used in the experiment and for storage for periods of from 0 to 4 months after irradiation. B, DISCUSSION OF STUDIES ON HORMONE ACTIVITY AND ON PERIDERM FORMATION The results listed in Table III indicate that sprout inhibition as a result of irradiation is not caused by a decrease in activity of the growth hormones or an increase in the amount of growth inhibitor. In fact these results indicate the exact reverse-an increase in growth hormone and a decrease in inhibitor. Additional data will be required to establish this point. However, these results support some earlier studies made with irradiated pearl onions and irradiated seed black walnuts.1 In both of these cases low doses of radiation from 10,000 to 50,000 rep actually stimulated the sprouting of the irradiated onions and walnuts prior to the sprouting of the controls. But these sprouts were short lived and died after reaching a length of about 1/2 inch, whereas the controls subsequently sprouted and the sprouts continued to grow. These limited observations support the idea that growth hormones may be increased and/or growth-hormone inhibitors decreased as a result of gamma irradiation. The studies on periderm formation (see Fig. 13) indicate that cell division is inhibited by gamma irradiation. This also checks with general observations of the effect of low doses of gamma radiation on microorganisms. At the time of irradiation there is little observable effect on the microorganisms, and some time must elapse before the effects of irradiation become apparent. Most cells continue to respire normally after irradiation and to show other biological characteristics such as motility where it occurs normally. However, the irradiation appears to interfere with the process of cell division. Some individual cells may grow to giant size before dividing and when cell division does occur, the daughter cells do not redivide. The hypothesis is proposed that sprout inhibition in irradiated potatoes is caused by a mechanism induced by irradiation which interferes with cell division. The production of growth hormones may be stimulated as a response to the damage to the cells caused by irradiation, VI. SUMMARY AID CONCLUSIONS The optimum effect of gamma irradiation in increasing the storage life of potatoes was obtained with Idaho-grown Russet Burbank variety potatoes irradiated in late spring. Irradiation in the spring appears to be preferable to irradiation in the fall or early winter. Russet Burbank and Russet Rural varieties of potatoes appear to have better storage properties after irradiation than does the Sebago (a white skin) variety~. 39

The optimum radiation dose appears to be about 15,000 rep. The optimum storage conditions appear to be about 450F and about 85%-90% relative humidity. In the studies on Russet Burbank variety tubers, weight loss for a given storage decreased with increased dosage up to 15,000 to 25,000 rep, whereas the reverse effect was observed with the Sebago variety tubers. Length of storage prior to irradiation as well as variety may have caused these differences In studies in suberization and periderm formation, all levels of gamma radiation tested (15,000 to 200,000 rep) were found to inhibit periderm formation completely and to delay but not to inhibit suberizationo Hormone studies on Sebago variety irradiated and control potatoes stored for some time showed such low hormone content that no effect of irradiation could be detected. However, limited data on freshly harvested Pontiac variety (Florida grown) tubers indicate that gamma radiation increases the hormone concentration and decreases inhibitor concentration. Studies on field-infected Sebago variety potatoes indicate that gamma radiation decreases incidence of ring rot on potatoes stored at 200C but has no significant effect on the same tubers stored at 1~C. However, other unclassified forms of rot increased at both storage temperatures with increases in radiation dosage Respiration studies on potato-tissue slices show no significant effect of gamma-radiation doses of 5,000, 10,000, and 15,000 rep. Doses of 50,000, 100,000, and 200,000 rep resulted in higher respiration rates for the slices of irradiated tubers as compared to the controls. This has continued with time of storage. Respiration studies on whole tubers gave results similar to the tests on slices for doses of 50,000, 100,000, and 200,000 repo Low doses of 5,000, 15,000, and 25,000 rep show (as compared to controls) a drop of about -30% in respiration rate during the first day after irradiation, followed by a sharp increase the second day to about plus 60%, followed by a gradual decline to about -5% at the 13th week of storageo Carbohydrate analyses for reducing sugar, sucrose, and starch show little effect of low radiation dose or length of storage on these analyses. Higher radiation dosages and lower storage temperature increase the sucrose concentration Studies on enzyme systems have shown a sharp reduction in reduced ascorbic acid content at radiation doses of 10,000 rep and an additional but more gradual reduction at higher doses. hO

REFERENCES 1. Brownell, L. E., et alo, "Utilization of the Gross Fission Products," Progress Report 7, Univ. of Mich. Eng. Res. Inst. Project M943, November, 1954. 2. Sparrow, A, Ho, and Christensen, E., "Improved Storage Quality of Potato Tubers after Exposure to Cobalt-60 Gammas," Nucleonics, 12:16-17 (1954). 3. Artschwager, E,, Jo Agr. Res., 35:995 (1927). 4. Boysen-Jensen, P., "Uber die Leitung des phototropischen Reizes in Avenakeimpflanzen," Ber. Deut. Bot. Geso, 28:118-120 (1910) 5. Leopold, Ao Co Auxins and Plant Growth. Berkeley: Univ. of Calif. Press, 1955o 6. Went, F. W., and Thimann, Ko V. Phytohormones. New York: The Macmillan Co,, 1937. 41

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