STUDIES IN THE PHYSIOLOGY OF THE VIRUS DISEASES OF THE POTATO

SUMMARY. The present investigation was concerned with the formation of carbohydrates in healthy and crinkle infected potato plants, and the nature of the sugar or sugars of transport. A series of observations were also made to determine whether there were any differences in the carbohydrate metabolism when a latent virus (paracrinkle) was present in a variety. The varieties used were Arran Victory for crinkle and President for paracrinkle. The disease was transmitted by stem‐grafting to our virus‐free units with Irish Chieftain for crinkle and King Edward for paracrinkle. All plants were grown in insect‐proof greenhouses. A series of experimental runs were then carried out at different times in the season. Samples of laminae and petioles were taken at hourly intervals over nineteen hours for crinkle and at two‐hourly intervals over twenty‐four hours for paracrinkle. It was found that in the early stages of the disease there was no statistically significant difference between carbohydrate formation in healthy and crinkle plants. The chief differences that were detected were (1) that the percentages of all three carbohydrates that were estimated (hexose, sucrose and starch) were higher in the diseased material than in the healthy, and (2) thatthere was a time lag of approximately two hours between the maximum and minimum points in the crinkle and healthy laminae; the former preceding the latter. Sucrose was found to be the sugar of transport in both cases. In the later part of the season, significant differences were discovered in the two cases. Sucrose was found to show a marked accumulation in the diseased laminae, and it was ascertained that this sucrose was formed by the hydrolysis of starch, whereas in the healthy laminae the formation of sucrose was brought about by synthesis from hexose, which in turn was formed from hydrolysis of starch. A certain amount of the sucrose was also discovered to be produced directly from starch. It was also ascertained that sucrose was still the sugar of transport, but evidence was obtained to show that its passage down the diseased petioles was not so readily effected as in the healthy. The low yields from crinkle infected potatoes is discussed in the light of these results. The presence of a latent virus in a variety was shown to produce no significant difference in carbohydrate formation, either at the beginning or the close of the growing season. This investigation was carried out at the Station of the Scottish Society for Research in Plant Breeding, Corstorphine, Edinburgh, under a scheme financed by the Empire Marketing Board. We would like to take this opportunity of thanking the Director, Mr William Robb, F.R.S.E., and Dr A. Nelson, F.R.S.E., for their helpful criticism in a number of different directions.