CHANGES IN THE METABOLISM OF WHEAT LEAVES INDUCED BY INFECTION WITH POWDERY MILDEW

THE POWDERY mildews infecting wheat are parasites whose nutritional requirements are extremely exacting. These requirements are satisfactorily met in wheat leaves which are adequately supplied with carbohydrate (Trelease and Trelease, 1929), but not in leaves whose carbohydrate supply is deficient. It is, therefore, probable that the development of mildew is closely related to the carbohydrate metabolism of its host, and that a knowledge of this metabolism may throw some light on the nature of the host contributions of importance in the development of the parasite. A study has, therefore, been made of some aspects of the carbohydrate metabolism of wheat leaves as they are influenced by mildew and as they in turn influence the development of mildew. Respiration, respiratory quotients, photosynthesis and chlorophyll content, and glucose, sucrose, and starcl have been measured at frequent intervals from the time of infection until the death of the leaves. The peculiar changes leading to the formation of green islands have also been followed, and are discussed in this paper. MATERIALS AND METHODS.-Growth of the plants. -All of the wheat plants used in these experiments were grown in full strength Hoagland's solution in two-quart jars. The early series were grown in thv greenhouse, the later series in a control chamber.2 In the latter the temperature was kept at 1942OC.., the humidity at 70?10 per cent, and the plants were illuminated for sixteen hours daily with a circle of eleven 30-watt daylight fluorescent lamps supplemented with seven red lamps. These gave an intensity of approximately 150 foot candles. For series F a somewhat higher intensity was obtained using six additional daylight lamps. In table 1 is presented a summary of the culture conditions and the kind of seed used in the different series. The strain of Erysiphe graminis tritici used in this study is highly pathogenic for Axminster and, according to Mains' classification (1933), is, therefore, strain number two. Inoculations were made by painting spores on one side of the leaf with a dry camel's hair brush. In this way the density of inoculation could be roughly controlled, though never with great accuracy. With heavy inoculations differences in the actual number of colonies per cm.2 be1 Received for publication December 16, 1941. Contribution from the Department of Botany, University of California, Berkeley, California. I wish to express my thanks to Prof. A. R. Davis for his generous support of this research project, and to Dr. L. C. Marshall for advice and help in obtaining uniform light intensity. 2 A description of this control chamber will be published at a later date. came less significant, as shown by the even tenor of changes in heavily inoculated leaves. Handling of experimental material.-Manometric experiments were always begun between 8.30 and 9.30 a.m. and required a total of four to five hours to complete. The first foliage leaf, which has been dealt with exclusively in these experiments, was removed from the seedling, one to two centimeters of the base and tip were discarded, and the rest of the leaf was cut into two or three pieces in order to fit the experimental vessels. The error involved due to loss of water while weighing was minimized by standardizing the weighing procedure, but may still have been great enough to account for some of the variability experienced throughout these investi'gations. In series A, and A the chlorophyll content was determined on the same piece of leaf whose respiration and photosynthesis were measured. In series D and E one of the two or three pieces of each leaf was cut longitudinally and the two halves used respectivelv for measurements of photosynthesis and chlorophyll content. The measurements of respiration were made, using one of these halves together with the remainder of the leaf. Usually the remainder of the leaf was removed before measuring photosynthesis, in order that the rate of gas exchange should be suitable. Determination of chlorophyll content.-The chlorophyll content was determined by a method similar to that of Emerson (1929). After boiling for two minutes in distilled water, weighed samples were extracted until white in commercial methyl alcohol. The per cent transmission of the resulting solutions was measured at 670 my with a Bausch and Lomb spectrophotometer. The density, D, in arbitrary units, was obtained from the transmission, T, from the relation D=log I/T. The values of D were converted to moles and expressed as the chlorophyll content of one milligram of fresh leaf tissue, using a conversion factor obtained by simultaneously determining the transmission and the Mg content of a methyl alcohol extract of two kilograms of seedling wheat leaves. The extractwas purified of inorganic Mg by transferring to toluene and washing repeatedly with distilled water. Determinations of this factor were made on two separate samples of wheat and yielded values which differed from each other by only 5 per cent. The two were therefore averaged, giving a conversion factor of 23XIO-9, which is the moles of chlorophyll per ml. which would give an absorption of 1.0. Manometric technique.-All measurements of gas exchange were made with a Fenn microvolumeter at 220C. The experimental conditions were similar to those described previously (Allen and Goddard,