EVIDENCES FOR THE LOSS OF RESISTANCE OF POTATO TO MOSAIC VIRUSES UNDER THE INFLUENCE OF SOIL TEMPERATURE IN RELATION TO DEGENERATION OF SEED TUBERS

P TIEN S.H. CHANC(Institute of Microbiology, Academia Sinica)C. K. LIN(Peking Instimte of Agriculture)

On the basis of previous notion of the extremely high rate of degeneration of spring sown early potatoes coincident with general expression of severe mosaic symptom in areas south of the Great Wall and the experience of successful seed-tuber production in Damin, Hobei under annual double crops system employing ridge planting and ditch irrigation methods, it has been anticipated that the degeneration of potatoes may not be due to the increase of the percentage of virus infection, but to the loss of resistance of potatoes to latent infection under the influence of soil temperature as conditioned by diurnal temperature difference. Experiments herein reported give evidences for the above hypothesis. The mosaic viruses involved in our chief early potato variety, Irish cobbler have been identified.as x and y. Test of a large number of tubers from non-degeneration area during the past 5 years indicated thier general presence as judged from the symptoms produced on several test plants, including Gomphrene globosa, Petunia hybrida, Physalis floridana and Nicotiana tabacum. A very high percentage of the tubers also gave positive precipitin test against x antiserum. An isolated y inoculum which caused typical vien-banding and crinkling of virus-free seedlings led to no expression of sympton on plants grown from tubers naturally produced in non-degeneration area, a phenomenon indicating the interference of a latent y infection in the later. Planting of tubers, from nondegeneration area in insect-proof seedbeds under natural conditions in Peking gave superficially good new seed tubers which almost invariably produced plants showing severe mosaic symptom in the following crop season. Ecological conditions affected the concentration of x virus in the tubers of the current year's crop. Thus, twelve non-degenerated halved tubers planted at 1,500 M. and 50 M. above sea level yielded tubers with x virus concentration of a ratio of 1:3.5, based upon the average number of local lesions on Gomphrena globosa leaves inoculated with the tuber extracts. Comparable experiments with degenerated tubers gave a ratio of 1:1.5. With non-degenerated halved tubers planted in the spring and in the early Autumn, both at 50 M. above sea level, the corresponging ratio was 1.8:1. There was a consistent reduction of x virus concentration during 10—20 days treatment under 32—37℃. of both resting and sprouting tubers, but the concentration again steadily raised to original level after the tuders returned to normal storage and sprouting temperature of 4℃. and 20℃. respectively. Seed tubers which had undergone high temperature treatment gave rise to plants with more severe mosaic symptom and lower tuber yield. However, the effect was far less striking with virus-containing non-degenerated tubers than with degenerated tubers. Since 1956, a special soil-conditioned bed was used for experimentation. The bed was built in a screened insect proof house with removable glass roof so that the plant tops were exposed to natural air temperature and humidity conditions. With naturally infected but non-degenerated tubers separately planted under soil tenperature regulated at 25℃. and 15℃., the ratio of x virus concentration in the leaves of the current year's plants was 1.4:1, in the tubers 3.6:1, tuber yield per plant averaged 143.5 g. and .355.8 g. respectively. The harvested tubers planted the next year under the same field conditions produced respectively 94.4% and 19.4% mosaic plants, with a ratio of x virus concentration in the leaves of 1.3:1, respective average tuber yield per plant 140.7 g. and 302.2 g. With tubers of the progeny of a virus-free seedling of the same variety, the tuber yield of the current, year's crop was lower under 25℃. than under 15℃. soil temperature. When the harvested tubers were grown the next year under natural field conditions, the former gave rise to weak plants with low tuber yield, but without any mosaic symptom and negative to virus test. Virusfree plants inoculated with x virus, y virus and in combination showed characteristic symptoms in the current season. The symptoms were intensified by high soil temperature (25℃.) with corresponding reduction of yield. The soil temperature effect became more striking in the following crop, especially those inoculated with combined x and y viruses. Short period of high temperature treatment of whole potted virusfree potato plants before inoculation increased the rate of accumulation of the inoculated x-virus and the degree of the mosaic symptom it caused, but the same treatment did not affect the reaction of tobacco plants to the same virus. From the available data, it appears that the resistance of potatoes grown under favorable environmental conditions may manifest itself in the limitation of the multiplication of virus in the tubers, in the prevention of establishment of more virulent virus strains or in the inhibition of symptom expression of the plants. However, it is not yet clear as to what part each of anti-multiplicational, anti-variational and anti-injurious types of resistance to x and y viruses plays.