FURTHER STUDIES ON ISOELECTRIC POINTS FOR PLANT 
TISSUE 1 
By William J. Robbins, Botanist, and Irl T. Scott, Assistant Plant Pathologist , 
Missouri Agricultural Experiment Station 2 
INTRODUCTION 
Previous papers by one of the writers (5, 6 ) 3 have indicated that 
plant tissue—in its absorption of water, in its absorption and reten¬ 
tion of acid and basic dyes, and in its effect upon the reaction of 
buffer mixtures in whicn the tissue is placed—acts much like an 
amphoteric colloid hating an isoelectric point. 
The so-called isoelectric point for potato-tuber tissue in the 
previous experiments seemed to be near P H 6.0, for the mycelium of 
Rhizopus nigricans near P H 5.0, and for Fusarium lycopersici near 
P H 5.5. These points are apparently of importance in the physiology 
of the organisms because a low point in the growth of R . nigricans (6) 
and of F. lycopersici (8) is correlated with the so-called isoelectric 
point when the organisms are grown on media of different hydrogen- 
ion concentrations. This point is also apparently correlated with the 
ability of F. lycopersici to infect tomato plants (8) grown in soils of 
varying hydrogen-ion concentration. 
In this paper the results of further experiments on the analogy 
between plant tissue and an amphoteric colloid with a definite iso¬ 
electric point are presented. As has been pointed out by Michaelis 
(3), an ampholyte placed in weak buffer mixtures changes toward 
greater alkalinity the reaction of those buffer mixtures that are more 
acid than the isoelectric point, and changes toward greater acidity 
the reaction of those buffer mixtures that are more alkaline than the 
isoelectric point. This is because the ampholyte acts as an acid in 
solutions more alkaline than the isoelectric point, and as an alkali in 
solutions that are more acid than the isoelectric point. At the iso¬ 
electric point, the reaction of the solution in which the ampholyte is 
placed remains unchanged. One of the writers has to some extent 
used this method of determining the isoelectric point with potato- 
tuber tissue (5), and the mycelium of R . nigricans ( 6 ). The previous 
experiments, however, were, subsidiary to those on water absorption, 
dye absorption, and growth. The experiments reported here were 
designed to follow out more elaborately with plant tissue this method 
of determining the isoelectric point. 
PROCEDURE 
The general procedure was as follows. The plant tissue was 
added to a small quantity of a dilute buffer mixture, and the changes 
in reaction were measured at intervals until equilibrium was reached. 
If the tissue acts like an amphoteric colloid, with a definite isoelectric 
1 Received for publication Oct. 16, 1924; issued September, 1925. 
2 Published with the permission of the director of the Agricultural Experiment Station, University of 
Missouri. The careful assistance of Roy O. Campbell with part of the laboratory work of this investigation 
is scknowlGd^cd 
3 Reference is made by number (italic) to “Literature cited,” page 398. 
Journal of Agricultural Research, 
Washington, D. C. 
( 385 ) 
Vol. XXXI, No. 4 
Aug. 15, 1925 
Key No. Mo.-8 
