Tuberization of the Colorado Wild Potato As Affected by X-Radiation

TUBERIZATION OF THE COLORADO WILD POTATO AS AFFECTED BY X-RADIATION EDNA LouIsE JOHNSON (WITH ONE FIGURE) Introduction The Colorado wild potato (Solanum jamesii) is the only species among more than 100 different kinds of plants that the writer (6) has irradiated in which a plant organ has made greater growth leading to increased weight as a result of the treatment. The growth and development of the underground parts of 22 species (7) exposed to medium doses showed detrimental effects to adventitious, fibrous, and tap roots. Corms and bulbs of certain species of Iridaceae and Liliaceae, although less sensitive than tubers of the Jerusalem artichoke, agree in showing retarded growth. The wild potato was the only species, among those in which the underground organs espe- cially were studied, for which a dose favoring tuber production was found. JACOBSON (2) reported that by x-radiation the crop of one variety of cultivated potatoes was increased 84 per cent. in weight over the control plants, while with another variety the increase was as much as 200 per cent. He stated that not only was each tuber larger than normal but that there was an increase in the total number as well. SPRAGUE and LENZ (9) from preliminary experiments concluded that strong doses may reduce the number of tubers formed, but that such tubers attain a greater size so that the total production is not lowered. In the writer's preliminary experiments (3) with cultivated potatoes grown in the greenhouse, tubers which were given light doses of x-rays formed more young tubers per hill than did the controls, but the average weight was less than that for the controls. Consequently the average total weight per hill was practically the same for controls and experimental plants. Since only a very limited number of cultivated potatoes could be grown in the greenhouse at one time, further experiments were carried on with the Colorado wild potato which has proved to be useful in ecological experiments (8) since it requires little room and matures quickly. A summary (1) of the studies reporting stimulative action of x-radiation has pointed out that in the great majority of cases where it has oc- curred, light doses have been used. Experiments of two seasons, in which the use of light doses on the wild potato failed to give increased production, have been previously reported (5). Results based on 14,000 tubers obtained from the second year's experiment, indicated that the controls and experimental plants did not vary more in yield than would two groups of untreated plants. 547

Downloaded from on May 12, 2019 - Published by www.plantphysiol.org Copyright © 1937 American Society of Plant Biologists. All rights reserved. 548 PLANT PHYSIOLOGY In the study herein reported, the application of one dose of 1500 r-units to mother tubers was found to favor tuberization. Five different trials involving a large number of potatoes indicated that radiation of the unsprouted tubers resulted in only a slight increase in number of progeny and in average weight per hill, but treatment of sprouted tubers gave marked increase not only in average number and weight per hill but in average weight per tuber. Experimentation UNSPROUTED TUBERS TREATED WITH MEDIUM X-RAY DOSAGE Tubers were sorted so that those which were to serve as controls were of practically the same size and weight as the lot to be treated. Immediately after irradiation with a medium dose, the experimental tubers were planted either in plots in the University greenhouse, or outside in garden soil. In all cases, the controls alternated in both directions with treated tubers. When the crop was harvested about four months after planting, care was taken to sift the soil in order that even the very small tubers should not be lost. TABLE I PRODUCTION PROM UNSPROUTED TUBERS IRRADIATED WITH MEDIUM DOSES

Av. No. DIFFER- DIFFER- DIFFER- No. OF TUBERS ENCE IN Av. WT. ENCE IN Av. WT. ENCE IN HILLS PER HILL TREATED PER HILL TREATED PER TUBER TREATED PLANTS PLANTS PLANTS Tubers grown in greenhouse (Set-up for dose: 51 K.V. 7i ma. 30 cm. 25 min.) % gm. % gn. %

Control ...... 47 16.5 9.5 0.58 Irradiated ... 57 17.0 + 3.0* 11.1 + 16.8 0.65 + 12.1 Tubers grown in garden plot at altitude of 8000 ft. (Set-up for dose: 56 K.V. 5 ma. 30 cm. 33 min.)

Control ...... 50 19.5 1 13.8 0.71 Irradiated ... 51 20.3 + 4.1 16.7 + 21.0 l 0.82 + 15.5 Tubers grown in garden plot at altitude of 5600 ft.) (Dose: 1510 r-units) Control 48 20.3 22.6 1 1.1 Irradiated ... 43 20.8 + 2.5 20.3 -10.2 1 1.0 - 9.1 Totals from controls and 3 groups of unsprouted irradiated tubers Control 145 18.8 15.3 1 0.81 Irradiated ... 151 19.2 l + 2.1 | 15.6 | + 2.0 0.81 1 * In tables I and II of this paper, the plus sign indicates that the treated plants ex- hibited greater growth than the controls; the minus sign that the treated plants showed less growth.

Downloaded from on May 12, 2019 - Published by www.plantphysiol.org Copyright © 1937 American Society of Plant Biologists. All rights reserved. JOHNSON: TUBERIZATION AND X-RADIATION 549 In all cases the average number of tubers per hill, average weight per hill, and average weight per tuber were determined. These are recorded in table I. In one group which was grown in the greenhouse, it was found that the average length of time for sprouts of the irradiated tubers to appear aboveground was 36 days, whereas the average for the controls was 43 days. Dates of sprouting in the other groups were not recorded. In some cases experimental plants blossomed earlier than the check plants. Results from the three groups reported in table I indicate that when unsprouted tubers are treated with medium doses of x-rays there is a slight increase in number of progeny and in average weight per hill. SPROUTED TUBERS TREATED WITH 1500 R-UNITS Tubers in the late spring were planted in soil in 3-inch paper pots. When the shoots had reached a height of from 1 to 2 cm., they were treated with a dose of 1500 r-units and placed in the ground outdoors without dis- turbance of the roots. Results obtained the first year indicated that this treatment was particularly favorable, for the number of tubers per hill was increased 59 per cent., the weight per hill 110 per cent., and the weight per tuber 30 per cent. The experiments, when repeated the following sum- mer, confirmed the results of the previous year; 50 hills each of control and treated sprouted tubers yielded 79 and 113 tubers per hill respectively. The average weight of the progeny from the treated mother tubers was increased 50 per cent. and the experimental tubers averaged heavier than those from check plants (table II). TABLE II PRODUCTION FROM SPROUTED TUBERS IRRADIATED WITH 1500 R-UNITS

Av. No. DFrnpE-I DiFER- DIPPER- No. or TABERS ENCE IN Av. WT. ENCE IN Av. WT. ENCE IN HILLS PER HILL TREATED PER HILL TREATED PER TUBER TREATED PLANTS PLANTS PLANTS Tubers grown in garden plot at altitude of 5600 ft. % gin. gm. % Control...... 30 19.9 17.5 0.88 Irradiated ... 34 31.8 + 59.6 36.8 + 110.3 1.15 + 30.7 Tubers grown in garden plot at altitude of 5600 ft. Control 50 79.2 53.6 0.67 Irradiated... 50 113.4 +± 43.1 80.5 + 50.2 0.71 + 6.0 Totals showing production from controls and 2 groups of sprouted irradiated tubers Control 80 57.0- 40.1 0.70 Irradiated 84 80.4 + 41.0 62.8 + 56.6 0.78 + 11.4 Figure 1 represents the entire crop produced the second year. The small size of the tubers can be appreciated when it is known that 3961 prog-

FIG. 1. Colorado wild potato tubers. Left, 3961 tubers from 50 control mothers; right, 5668 tubers from 50 sprouted mother tubers treated with 1500 r-units. Average weight of tubers is approximately 0.7 grams. eny from 50 control mother tubers are in the jar on the left and the 5668 offspring produced by the 50 irradiated sprouted mother tubers are on the right. The average weight of the tubers is about 0.7 grams. The application of the correct dose of x-rays favors the production of wild potato tubers. Radiated unsprouted tubers produced a crop with a slight increase in number of tubers and also in average weight per hill. Treated sprouted tubers gave striking increases in number and weight per hill and in weight per tuber. The conclusion drawn is that the probable

Downloaded from on May 12, 2019 - Published by www.plantphysiol.org Copyright © 1937 American Society of Plant Biologists. All rights reserved. JOHNSON: TUBERIZATION AND X-RADIATION 551 increased rhizome development resulting in greater tuber production is similar to the increased aerial branching which occurs in some other members of this family (4) when very young plants are treated with medium doses. Summary 1. Radiation of wild potato tubers with light doses of x-rays failed to give increased number of progeny. Treatment of unsprouted tubers with moderate doses resulted in only a slight increase in number of progeny and in average weight per hill. 2. Progeny from sprouted mothers which had been treated with 1500 r-units gave the following percentage of increase over the controls: average number of tubers per hill, 41 per cent.; average weight per hill, 56 per cent.; average weight per tuber, 11 per cent. The explanation advanced for the greatly increased production from treated tubers is that there is a probable increased rhizome development which results in greater tuber production. This is similar to the increased aerial branching which occurs in some other members of Solanaceae when young plants are treated with medium doses. UNIVERSITY OF COLORADO BOULDER, COLORADO LITERATURE CITED 1. DUGGAR, B. M. Biological effects of radiation. McGraw-Hill Book Co., New York. 1936. 2. JACOBSON, M. Die Wirkung der R6ntgenstrahlen auf das Wachstum der Pflanzen. Beilage zum Rigaschen Rundschau. 54: 5. 1923. 3. JOHNSON, EDNA L. Tuberization of potatoes increased by x-rays. Sci- ence n. s. 68: 231. 1928. 4. . Effect of x-irradiation upon growth and reproduction of tomato. Plant Physiol. 6: 685-694. 1931. 5. . On the alleged stimulating action of x-rays upon plants. Amer. Jour. Bot. 18: 603-614. 1931. 6. . Susceptibility of seventy species of flowering plants to x-radiation. Plant Physiol 11: 319-342. 1936. 7. . Development of roots and underground stems as influenced by x-radiation. Univ. Colorado Studies 23: 169-187. 1936. 8. , and BURKE, MARIAN. Tuber production of the Colorado wild potato as influenced by certain environmental factors. (Unpublished.) 9. SPRAGUE, H. B., and LENZ, M. The effect of x-rays on potato tubers for seed. Science n. s. 69: 606. 1929.