HYDROGEN-ION CONCENTRATION AND VARIETAE RE¬ 
SISTANCE OP WHEAT TO STEMRUST AND OTHER 
DISEASES 1 
By Annie May Hurd 2 
Assistant Physiologist, Office of Cereal Investigations , Bureau of Plant Industry, United 
States Department of Agriculture 
INTRODUCTION 
The historical development of the subject of varietal resistance to 
disease among plants has been traced in considerable detail by a number 
of writers (4, 9, 21, 29, 31)? so that a brief summary of the many theories 
proposed in the literature will suffice to give the background for the pres¬ 
ent study. Among the morphological characters which have been con¬ 
sidered responsible for resistance to some parasite are size of stomata, 
waxiness or hairiness of leaves or stems, toughness or thickness of cell 
walls or cuticle, and structure and extent of root system. Among the 
physiological characters are the ability to form cork around the invading 
organism; such rapid growth of the host that the parasite is unable to 
keep up with the growing tip; the presence of phagocytes or antitoxins 
in the host (conferring immunity in a manner analogous to that of 
animals); the activity of enzyms; the concentration of the cell sap; the 
lack of any substance stimulating the chemotropic responses of the 
organism necessary for the latter’s successful entry and progress in the 
susceptible variety; the lack or subminimal amount of some nutrient 
necessary to the growth of the parasite in the host tissue; the presence 
in lethal quantity of some substance toxic to the invading organism. In 
support of these various views comparative determinations of many plant 
constituents, among them silica, manganese, cellulose, starch, sugar, 
tannin, protein, volatile oils, and acids, have been made. 
Of late years there has been considerable speculation on the probability 
that the concentration of free hydrogen-ions in the cell sap of the host is 
the determining factor in resistance. So far, no definite relation has 
been established between this character and degree of resistance to 
parasites. Hawkins and Harvey ( 14 ), working on potato varieties 
resistant and susceptible to Pythium debaryanum , Weiss and Harvey {33) 
on Chrysophlyctis endobiotica in potatoes, and Spriesterbach 4 on wheat 
rust, all report that hydrogen-ion concentration bears no relation to 
ability to resist disease. The present investigation was undertaken for 
the purpose of furnishing additional data on the hydrogen-ion concentra¬ 
tion of the expressed juice of a number of wheat varieties, some resistant 
and others susceptible to stemrust (Puccinia graminis tritici Pers.). 
1 Accepted for publication May 13, 1932. 
* Grateful acknowledgement is here made to Dr. H. B. Humphrey and Dr. H. Hasselbring for their 
helpful suggestions and criticisms. To the office of Physiological and Fermentation Investigations I am 
indebted for the loan of equipment. 
8 Reference is made by number (italic) to “ Literature cited,” 384-386. 
8 Spriesterbach, D. O. biochemical study of resistance to disease in plants. Unpublished 
report referred to by Gortner (10, p. 40). 
Journal of Agricultural Research, 
Washington, D. C. 
adh 
Vol. XXIII, No. s 
Feb. 3,1923 
Key No. G-277 
(373) 
