RESISTANCE TO VIRUS DISEASES OF SOME F, CLONES DESCENDED FROM THE SPECIES HYBRIDIZATION SOLANUM DEMISSUM x S. TUBEROSUM Onni Pohjakallio and Laura Karhuvaara Department of Plant Pathology, University of Helsinki Received January 11, 1960 The species hybridization Solanum demissum Lindl. x S. tuberosum L. (Rosa- folia) carried out: n 1947 (7) gave rise to a potato plant forming tubers with a purple skin. Most of the progeny of this plant continued to form purple tubers, but some, as the result of bud mutations, formed tubers with a white skin. Bud mutations also lead to changes of other kinds in the properties of plants (3). One of the clones forming purple tubers appeared to be earlier in its development than the others. Some other symptoms suggested that the peculiar character of this clone was solely due to a virus infection (3, 10). Later, the early maturing character also emerged in many of the ether Fj clones forming purple tubers, while all the potato plants forming white tubers remained true to their type of late maturation. Consequently, it looked as though as a result of bud mutations, progenies resistant to a virus had been raised from susceptible potato plants. Material All the potato plants from the species hybridization Solanum demissum x S. tuberosum (Rasafolia) examined in the following are the vegetative progeny of one potato seedling grown in the summer of 1948. The skin of the tubers of this potato plant was purple; the whole of its appearance bore evidence of its hybrid origin. It and its vegetative progeny were frost-hardy (7, 11), resistant to potato blight (7, 8), and formed tubers later than the S. tuberosum varieties (10). In 1949, this potato clone seemed to retain its original properties, but in 1950 some of the plants formed white tubers (3). The haulms of these colour mutants were very luxuriant at the end of the growing season (10), thus resembling the bolter type (cf. 5, 13). Only the tubers which were attached to the potato plant https://www.c-info.fi/en/info/?token=fTRNUOBDgqTAniOh.quQwBkymHI86XNvtIl9oFQ.aOlQTcdAvFrXLD47QP5ZMVzytSSZ0ApKb5xGhmlUTmEsm56C1bDGclK4bHXmKk6XsxPHSBjIJG2qfFltOTptGn6uv6XI5Hh1DW22eMkVc_PwSUXBAArYE4QJI44EvhqvAOfyEVmoGIR6RYNFnu2Wx_taI41CxIfhjTJvOskAdNwuQe146qwi_ZjfhpRBX4ceHGXA 74 were reserved for setting. The tuber yield of each plant was put into a separate paper bag, in which it was preserved over the winter. In the field experiments the tubers were planted at distances of 1.2 metres. Consequently, the tubers of the different potato plants were not mixed with each other. The potato plants forming tubers with a white skin, which were evidently bud mutants of the original potato plant (cf. 3), also formed white tubers in all subsequent vegetative generations. Since the year 1950, however, none of the potato plants of the original type has given rise to further white tubers. On the other hand, many of the plants forming purple tubers changed with regard to the rate of their development becoming earlier. Because this phenomenon was assumed to be due to some virus disease, infection experiments and serological investigations were performed. For the serological investigations Prof. Dr. van Slogtern (Labora- torium voor Bloembollenonderzoek, Lisse, Holland) kindly sent normal sera as well as antisera against the potato viruses X, S, and Y. Attention was also paid to the symptoms of the disease. In addition, the length of the flowering time and the productivity of the potato clones were investigated. Results The potato plants forming tubers with a purple skin. As the result of bud mutations different kinds of clones were found among the Fx material investigated, the plants of which developed purple tubers (10). The differen- ces between the properties of these clones, however, were relatively slight, and therefore in what follows this material has been treated as a unit. A characteristic of all these clones was their tendency to be converted into early maturing types. To solve the question of whether this phenomenon also arose from a bud mutation or was due to a virus infection, core graftings were made. A piece of healthy tuber was removed with a cork borer, and into the hole was inserted a piece taken from a tuber of a plant of the early maturing type by means of a slightly larger cork borer. Graftings were carried out late in the winters of 1956 and 1959 and in autumn 1957. In the following summers grafted as well as ungrafted tubers were planted in field experiments. In 1956 and 1957 the field experiments were placed near the other potato stands. Consequently spontaneous virus infections were possible. In 1959, the potato plants were set in specially, isolated plots located at distances of ca. 50 metres from any other potato stands. The setting tubers for these latter experiments had also been cultivated in special isolation (in 1958). In spite of these precautions, nearly a fourth of the plants had spontaneously acquired the character of early maturity in 1959. However, in all the 3 field experi- ments the corresponding transformation appeared in larger numbers among the grafted than among the ungrafted plants (Table 1); the result, which appeared to be statistically significant, did not depend on whether the piece inserted at grafting carried a bud or not. Consequently, the phenomenon in ques- tion seemed to be an infectious disease. This opinion was sup- ported by the facts that in field experiments in which the original and earlymaturing 75 Table 1. The results of core grafting infections Distance from virus- Clone No. of Percentage diseased ear (colour Piece tubers of plants potato plants of tuber) inserted grafted infected m. 1956 purple 15 20 10 —»— *— carrying a bud 20 65 10 — »— — »— no bud 20 65 10 1957 — »- 33 36 10 —carrying a bud 22 59 10 no bud 13 62 10 1959 - 50') 24 50 —»— —no bud 49 *) 100 50 1956 white 10 0 10 —»— * carrying a bud 20 0 10 *— —no bud 20 0 10 1957 —*- - 21 0 10 * *— carrying a bud 20 0 10 — »— no bud 21 0 10 1) For setting, the tubers were cultivated in specially isolated plots in 1958, when symptoms of early maturity were observed in one of the 50 plants; this plant was eliminated on August 4. types of potato plants were cultivated at distances of only 1.2 metres from each other, transformation into an earlier maturing type of plant was more frequent than when the plam;s of the original type were cultivated in special isolated plots. In 1958, only one o; the 50 potato plants of the special plot was transformed into an early maturing type, while the corresponding percentage in the mixed plot with both types was 58. In 1959, when the weather was exceptionally warm and rainless, the corresponding numbers calculated in percentages were 24 and 91. The disease reported above was first observed in 1952. In 1952 and 1953 only a few diseased plants were found. In the years 1954 to 1959 the rates of new infec- tions were as follows: 21 % in 1954, 49 % in 1955, 51 % in 1956, 57 % in 1957, 58 % in 1958, and 91 % in 1959. Because the disease appeared to be infectious, and without a single exception transmittable in the tubers to the following vegeta- tive generations of the potato plant (7 generations investigated) the disease seemed to be caused by some virus. Serological investigations showed that the viruses X and S were not present in the diseased potato plants. By contrast an agglutination test with antiserum against virus Y gave a positive result. It is true that in some cases the normal serum used also reacted positively with the sap of that plants.Thus the result cannot be 76 Table 2. The earliness of flowering of the Fi potato plants forming tubers with a purple skin Date of onset Date of end of Date of Date of of flowering flowering Year setting lifting Diseased Healthy Diseased Healthy plants *) plants plants plants 1954 29/5 1/10 8/7 7/7 21/7 6/9 1955 7/6 26/9 13/7 13/7 1/8 20/8 1956 5/6 26/9 19/7 18/7 1/8 27/8 1957 29/5 20/9 9/7 10/7 28/7 20/9 1958 5/6 18/9 18/7 19/7 1/8 19/8 1959 15/5 19/9 5/7 6/7 14/7 12/8 i) Many plants did not flower at all. The haulms died early; the haulms of 93 % of the diseased plants died before the beginning of September, when the haulms of all the healthy plants were still alive. Table 3. The effect of the date of lifting on the tuber yield of Ft potato plants forming tubers with a purple skin Yields of 6 potato plants (g) Condition of the 1954 1955 1956 plants . 31/7 6/10 1/8 15/9 1/8 10/9 Healthy 79 2487 12 1163 7 517 Virus diseased 273 1585 21 590 43 815 Table 4. The relation between the number of vegetative generations during which the potato plants had had virus disease and the tuber yield (lifted late in autumn). Results in 1956 Results in 1957 Results in 1958 Results in 1959 Year of No. of Tubers Year of No. of Tubers Year of No. of Tubers Year of No. of Tubers infect- plants g/plant infect- plants g/plant infect- plants g/plant infect- plants g/plant ion ion ion ion 1952 3 87 1952 3 145 1952 3 119 1952 3 59 1953 3 83 1953 2 256 1953 3 92 1953 3 46 1954 12 170 1954 3 202 1954 3 257 1954 3 86 1955 6 57 1955 7 75 1955 2 66 1955 3 11 1956 5 124 1956 6 257 1956 3 106 1956 5 39 Healthy 28 330 1957 13 536 1957 5 75 1957 9 30 plants Healthy 10 526 1958 16 120 1958 5 55 plants Healthy 11 218 1959 31 247 plants Healthy 3 258 plants 77 considered quite conclusive. However, in addition to the results of the agglutination test the symptoms occurring in the diseased plants (leaflets a little wrinkled, and small, dark brown necrotic streaks on the under side of the leaflet) also pointed to the presence of vims Y. Consequently, in the following the disease has been considered to be the potato rugose. The earliness of flowering of the potato plants was not affected by the virus disease (Table 2). By contrast, the length of the flowering time of the diseased potato plants was decreased relative to that of the healthy plants; besides, the haulms of the diseased plants turned yellow and died relatively early. When lifted early the tuber yield of the diseased potato plants appeared to be heavier than that of the healthy p ants (Table 3). But at the time of late lifting the largest amount of tubers was yielded by the healthy potato plants. The effect of the virus disease on the tuber yield did not always appear in the same summer in which the potato plants had contracted the virus infection (Table 4). This, perhaps, arose from the fact that the virus hastened the rate of development of the potato plants, and in that manner partly compensated for the loss caused by the disease. In ;hese experiments lifting was carried out late in autumn (cf. Table 2). Consequently, the tuber yields of the systemically diseased plants were generally much lower than those of the healthy plants. On the other hand the tuber yield appeared to be independent of the number of vegetative generations during which the potato plants had had the virus disease. The variations in the yields of the plants infected in the different years depend, in part at least, on the fact that the potato material included different clones the properties of which varied somewhat or account of bud mutations. In 1959, the material investigated included 21 potato clones forming purple tubers. All these clones appear- ed to be susceptible to the virus infection. The potato plants forming tubers with a white skin. From the potato plants forming white tubers descended from the bud mutation, 4 clones were chosen in 1950. One of these was cultivated for a few years only. Later, in 1954 and 1955, however, 2 new clones were separated from the original clones. Consequently, altogether 5 clones forming white tubers were examined: Table 5. Th« yields (g/plant) of the bud mutant clones forming tubers with a white skin, 1951 to 1959 Year W 1 W 2 W 3 W 4 W 5 1951 841 233 1221 1952 932 53 953 1953 1773 556 1880 1954 682 541 2283 1955 975 222 1750 616 1956 775 100 787 968 47 1957 1189 56 1036 770 55 1958 1115 260 1545 1225 13 1959 1388 1054 1751 845 332 78 W 1. Flowers dark blue. W 2. Flowers dark blue; maturing later than clone W 1. W 3. Flowers light blue. W 4. Flowers light blue; maturing later than clone W 3. W 5. Flowers light blue; maturing still later than clone 4. Generally, the heaviest yields were obtained from the clones developing earliest. Especially in years when the temperature was relatively low (1956 to 1958) and the length of the growing season decreased, the slow developing clones yielded very little (Table 5). In the experiments in which the potato plants were set at distances of 1.2 metres from each other, the numbers of plants investigated in different years were as follows: Year 1952 1953 1954 1955 1956 1957 1958 1959 79 47 36 37 30 24 24 20No. of plants In none of these plants were signs of the virus disease visible to the naked eye. Besides, the results of the core grafting experi- ments showed that these clones were resistant to the virus (Y) which caused the early maturity in the clones forming purple tubers (Table 1). There was no evidence of virus Y or X by means of the serological tests either. The antiserum of virus S on the contrary produced agglutination in the sap of some of the plants forming white tubers. Corresponding results were obtained with the sap of Solanum demissum. However, at no time during the whole period from year 1945 (cf. 12), when the S. demissum material was obtained for our experiments, were visible symptoms of a virus disease discerned in this species. Discussion According to the investigations reported above, the rate of development of the potato plant was hastened considerably by infection with virus Y. It is true that the earliness of onset of flowering has not been determined by the virus infection, but the flowering manifested itself less abundantly and the flowering time was shorter in the diseased than in the healthy plants (Table 2). The virus disease hastened the yellowing and withering of the haulms. The onset of tuber formation also manifested itself earlier in the diseased than in the healthy plants. After the withering of the haulms, however, the tuber yield could no longer increase. Conse- quently, when not lifted until late autumn, the largest amount of tubers was yielded by the healthy plants (Tables 3 and 4). In the same way, exposure of the potato plants to a short photoperiod does not affect the onset of flowering, but decreases the number of the flowers, as well as the length of the flowering time, hastens the onset of tuber formation, the yellowing and withering of the haulms, and at the same time the increase in the tuber yield ceases relatively soon (cf. 9, 10). Con- sequently, a high degree of similarity has been found between the development of healthy potato plants exposed to short day, on the one hand, and of the virus- 79 diseased Fx clones forming purple tubers, on the other hand. Lihnell (4), too, has found that the potato rugose, especially, but also potato crinkle, lead to early maturation of the haulms of the potato plant. According to Kennedy (1) potato leaf roll stimulates the onset of new tubers after dry periods. In our experiments the virus did not bring about further degeneration of the potato plant in successive vegetative generations, but permanently altered its nature once and fcr all (Table 4). The clones with white tubers arising from bud mutations appeared to be resistant to the virus Y investigated (Table 1). In these clones, a tendency to change to still later maturing types was observed (Table 5; 10). Thus the bud mutations have produced clones with properties even more closely resembling those of the mother plant ( Solanum demissum), which forms white tubers, develops tubers extremely late in ttie longday conditions of Finland (12), and has appeared tobe resistant to potato rugose in our experiments. The results of the reciprocal crosses indicate that the cytoplasms of S. demissum and of S. tuberosum are of different nature (7). These results invite the thought that the S. tuberosum genes, when in contact with 5. demissum plasma, become labile and readily mutable (3; cf. also 2,6), when their a:tion declines and the properties of plants descended from this species hybridization readily change in the direction of those of the mother plant. Conclusions The following are the main conclusions to be drawn from the results of experi- ments in which the resistance of Fx clones descended from the species hybridization Solanum demissum x S. tuberosum (Rosafolia) was investigated: The clones forming purple tubers appeared to be very susceptible to the potato rugose. Bud mutants descended from the former, forming white tubers, appeared to be wholly resistant to this virus. The virus accelerated the onset of tuber formation and the decline of the haulms of the susceptible Fj plants. The degeneration of the virus-diseased plants did not progress from one vegeta- tive generation to the next; instead, the virus altered the rate of development of the potato plant to a new level. REFERENCES (1) Kennedy, J. I. 1955. Some physiological aspects of virus degeneration of potatoes. Proceeding in the Ann. appi. biol. 43: 149—151. (2) Koopmans, A. 1951.Cytogenetic studies on Solanum tuberosum L. and some of its relatives. Genet. 25: 193-337. (3) Laurila, Kaiho 1957. Solanum tuberosum L. ja S. demissum Lindl. -lajien välisten risteytysten Fi-polvessa ilmenneestä kasvullisesta muuntelusta (Über vegetative Mutation in der Generat on Fi bei Kreuzungen zwischen den Arten Solanum tuberosum L. und S. demissum Lindl.). Maatal. tiet. aikak. 29: 56 67. 80 (4) Lihnell, D. 1943. De viktigaste potatisviroserna; symptom, spriding och betydelse. Handlingar till lantbruksveckan 1943: 303—309. (5) Manner, Rolf 1952. Erfarenheter rörande spontant uppträdande förändring av bestäende natur i Early Puritan (Experiences from spontaneous changes in Early Puritan). Medd. Gull- äkers växtförädh anst. 1952: 240—247. (6) Michaelis, P. 1935. Erhöhte Wachstumsintensität und Pilzresistenz durch Plasmavererbung, sowie über die Bedeutung des Plasmas bei Kreuzungsschwierigkeiten. Züchter 7: 74 77. (7) Pohjakallio, O. 1951. Potatisens resistensfrägor. Nord, jordbr.forskn, 1951: 486 492. (8) —»— 1954. Eräitä lisäselvityksiä vuoden 1953 perunarutto- [Phytophthora infeslans (Mont.) de Bary] epidemian luonteesta (Some features of the Phytophthora infestans epidemic in the summer of 1953). Maatah tiet. aikak. 26: 142—147. (9) —»— & Salonen, A. 1947. Der Einfluss der Tageslänge auf Entwicklung und Energiehaushalt einiger Kulturpflanzen. Acta agr. fenn. 67, 1. (10) —*— Salonen, A. & Antila, S. 1957. Analysis of Earliness in the Potato. Acta agr. scand. 7; 361-388. (11) —*— Vaartaja, O. & Antila, S. 1955. Frost resistance of potato tubers. Acta agr. fenn. 83: 42-62. (12) Viiri lä, F. 1949. Päivän pituuden vaikutuksesta meksikolaisen luonnonvaraisen perunan, Solanum demissum Lindh, biologiaan (On the effect of day length on the biology of Solanum de- missum Lindh). Archivum soc. zooh bot. fenn. »Vanamo* 4: 1: 60 72. (13) Steineck, O. 1955. Die photoperiodische Reaktion von »Schosser» -Stauden der Sorte Erstling. Z. Pflanzenz. 35:137-148. SELOSTUS: LAJIRISTEYKSESTÄ SOLANUM DEMISSUM x S. TUBEROSUM POLVEUTUVIEN ERÄIDEN Fi-KLOONIEN VIRUSTAUDINKESTÄVYYDESTÄ Onni Pohjakallio ja Laura Karhuvaara Yliopiston kasvipatologian laitos, Helsinki V. 1947 suoritetusta lajiristeytyksestä Solanum demissum Lindl. x S. tuberosum L. (Ruusulehti) saatiin siementaimi, joka syksyllä 1948 muodosti punakuorisia mukuloita. Mukuloista kasvatetut jälkeläiskasvit muodostivat myös v. 1949 punakuorisia mukuloita, mutta silmumutaatiosta johtuen osa kasveista v. 1950 muodosti mukuloita, joiden kuori oli valkea. Jatketuissa tutkimuksissa ilmeni, että punamukulaiset kloonit olivat viiruviroosinarkoja, valkomukulaiset sen sijaan virustaudinkestä- viä; silmumutaatio oli siis aiheuttanut myös virustaudinkestävyyden. Virustauti joudutti perunan mukulanmuodostumisen alkamista, josta johtuen sairas peruna antoi aikaisin korjattuna suuremman mukulasadon kuin terve. Virus ei vaikuttanut perunan kukinnan alkamisaikaan, mutta lyhensi tuntuvasti kukinta-aikaa ja vähensi kukkien määrää. Sitä paitsi virus joudutti varsiston tuleentumista ja kuolemista, josta johtuen myös mukulasadon suurentuminen tyreh- tyi verraten aikaisin. Tästä johtuen terve peruna antoi paljon suuremman mukulasadon silloin kun sadon korjuu tapahtui vasta myöhään syksyllä. Virustaudin vaikutus perunan mukulanmuodostukseen ilmeni yleensä osittain jo samana kesänä kuin peruna sai virustartunnan. Kaikki sairaan kasvin mukuloista kasvatetut kasvit osoittautuivat virustautisiksi. Viruksen vaikutus mukulasadon kehitykseen ilmeni jo saastuntaa seuraavana vuonna lopullisena. Sitä seuraavina vuosina ei virus enää enempää surkastuttanut perunaa, joten virus vain muutti perunan kehityksen toisenlaiseksi.