THE COURSE OF ACIDITY CHANGES DURING THE 
GROWTH PERIOD OF WHEAT WITH SPECIAL 
REFERENCE TO STEM-RUST RESISTANCE 1 
By Annie May Hurd 2 
Physiologist , Office of Cereal Investigations , Bureau of Plant Industry , United States 
Department of Agriculture 
INTRODUCTION 
Acidity measurements of juice expressed from wheat at frequent inter¬ 
vals from the early-seedling to the late-flowering stage have shown the oc¬ 
currence of an interesting sequence of changes during the development of 
the plant. As the investigation was initiated for the study of the problem 
of disease resistance, the data were obtained on a number of varieties 
representing extremes of resistance and susceptibility to stem rust 
(Puccinia graminis tritici Erikss. and Henn.). The results therefore 
not only give evidence from a new standpoint of changing metabolic 
processes during the life of the wheat plant, but also permit conclusions on 
the question of a relation between acidity and resistance to stem rust. 
PROCEDURE 
The plants were grown in the greenhouse, although occasional checks 
were made with field-grown plants. The six varieties studied were sown, 
cut, and handled in pairs, each pair consisting of one relatively resistant 
and one relatively susceptible wheat. These pairs were the same in each 
of the three series, namely, Kota and Preston, Pentad (D-5) and Mar¬ 
quis, Khapli emmer and Little Club, respectively. 
The same methods were followed for expressing and handling the juice 
and making the acidity determinations as were described in a preceding 
paper (4 ). 3 Both hydrogen-ion and titratable-acid concentrations were 
determined electrometrically. Immediately following the determination 
of the hydrogen-ion concentration of a sample, which in each case con¬ 
sisted of 10 cc. of undiluted juice, the electrometric titration was made 
on the same sample by adding N/20 sodium hydroxid solution, 1 cc. at a 
time, by means of a burette, the tip of which was inserted through the 
cork of the electrode vessel. The P H values calculated from the successive 
potential differences so obtained were plotted against the volumes of 
sodium hydroxid required to produce them. From the resulting curves, 
the volume of alkali required to bring the reaction to P H 8.3, the turning 
point of phenolphthalein, was determined and taken to represent the 
titratable acidity of the sample. Uniform procedure in the handling 
of each sample, together with the proper reversal of the sequence of 
determinations, reduced the chance of error to a minimum and insured 
the validity of comparisons. 
1 Received for publication Feb. 7.1924. 
J The writer is indebted to Dr. H. B. Humphrey, Dr. C. R. Ball, and Dr. H. Hasselbring for their 
helpful criticisms of the manuscript. 
a Reference is made by number (italic) to " Literature cited,” p. 7 35 . 
(72s) 
Journal of Agricultural Research, 
Washington, D. C. 
Vol. XXVII, No. 10 
Mar. 8 , 1924 
Key No. G-379 
