A RESPIRATORY STUDY OF POWDERY MILDEW OF WHEAT

Abstract

OBLIGATE PARASITES comprise a group of organisms highly specialized from a nutritional point of view. As their name implies, they are restricted to a parasitic existence and cannot be grown on artificial media. This has been an impediment to physiological researches on such parasites and has seriously limited the study of their nutrition and metabolism. In view of the fundamental importance of this aspect of obligate parasitism, a study of the respiration of the powdery mildew of wheat Erysiphe graminis D. C. var. tritici and of its host was undertaken. Wheat mildew is an ectoparasite-that is, the only invasion of the host is by haustoria which penetrate the epidermal cells. Thus it is easy to remove all of the mildew, except the haustoria, from infected wheat. This was one reason why Erysiphe graminis was chosen for this study. Trelease and Trelease (1929) studied the effect of the carbohydrate supply of wheat in relation to its infection by mildew. If the carbohydrate supply was cut off by placing plants in the dark, infection by mildew would not occur. Isolated leaves kept in the dark supported the growth of the mildew if their lower ends were placed in solutions of suitable carbohydrates. Development of the mildew under these conditions was independent of the chlorophyll content of the leaves. In the light, growth of the mildew on wheat would occur only if the wheat was supplied with CO2 or with carbohydrates. Nicolas (1990) reported that infection of Prasium majts by Erysiphe lamprocarpa and of Tonilis nodosa by E. communis brought about a decrease in the respiratory rate of the host. Nicolas did not report on the age of the infection nor the change in the respiratory rate during the course of the disease; therefore it is impossible to decide whether the decrease was the primary result of infection or a change occurring in a late phase of the disease. Yarwood (1934) compared the respiration of clover leaflets infected with E. polygonium with that of normal leaflets, and found that the infected leaves had a rate about 50 per cent higher than the normal ones. Since his measurements were of total respiration over a period of several days, they are not directly comparable with those reported in this paper. Pratt (1938) has followed the course of respiration of wheat after inoculation with E. graminis. He found that, although the respiration of normal wheat gradually declines with age, wheat inoculated with mildew shows an increasing respiration which gradually rises to a value about three times that of healthy wheat, maintains this level for about a week, and then drops off to zero as the wheat dies. The increased respiration is not appreciably reduced by killing the mildew with sulfur. In a preliminary report Allen and God-