AN ISOELECTRIC POINT FOR PLANT TISSUE 
AND ITS SIGNIFICANCE 1 
William J. Robbins 
(Received for publication December 6, 1922) 
The study of the relation of hydrogen-ion concentration to the growth 
and development of plants or plant parts has, in many cases, indicated that 
there is between the extreme limits of acidity or of alkalinity which are • 
injurious to and interfere with growth and development a third region where 
the reaction affects the plant injuriously. This produces, when the growth 
of the plant or the intensity of the process is plotted against the hydrogen- 
ion concentration of the solution, a curve having a double maximum with a 
minimum located between the two maxima. Webb (25) was apparently 
the first to call attention to this phenomenon. He found that, when the 
spores of Penicillium cyclopium were germinated in buffer mixtures of 
H3PO4 and NaOH containing mannite, two maxima appeared with a 
minimum between when the percentage of spore germination was plotted 
against the pH of the solutions. He found similar curves for Fusarium sp., 
and, under certain temperature conditions, for Aspergillus niger. For 
Botrytis cinerea- and Lenzites saepiaria his curves.showed but one maximum. 
In a more recent paper (26) he has studied the germination of the spores of a 
large number of fungi in various media the hydrogen-ion concentration of 
which was adjusted by means of H 3 P 0 4 and NaOH. In various forms, 
notably Penicillium and Fusarium, and under certain conditions, two 
maxima were found in the curve of spore germination plotted against the 
hydrogen-ion concentration of the medium. In at least one set of condi¬ 
tions, two maxima were found for Aspergillus niger , Penicillium cyclopium , 
Botrytis cinerea , Fusarium sp., Penicillium italicum , Lenzites saepiaria , and 
Puccinia graminis. Only one maximum was found for Colletotrichum 
Gossypii. 
Salter and Mcllvaine (22) found a double-maximum curve for the 
growth of wheat seedlings in solution cultures the reaction of which was 
adjusted with citric acid and NaOH. They concluded that this curve 
resulted from the exhaustion of nitrates due to the development of bacteria 
in the cultures. 
1 Published with the permission of the Director of the Agricultural Experiment Station, 
University of Missouri. The experiments on the absorption of water by potato-tuber 
tissue in buffer mixtures of H3PO4 and NaOH were performed for the writer by Mrs. Edward 
Abbott. The assistance of Mr. Karl Petsch in the experimental work involved is also 
acknowledged. 
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