98 Journal of Agricultural Research voi. xv. No. > 



CHANGE OF HYDROGEN-ION CONCENTRATION OF A PLANT SAP ON 



FREEZING 



INDICATION Olf A CHANGE OF ACIDITY AS SHOWN BY COLOR CHANGES 



One of the most striking of the changes which occur as a result of freez- 

 ing is a change of color in plants which have colored leaves. Haas {15) has 

 shown that the plant pigments act as an indicator of the cell-sap acidity. 

 The change of reaction is best observed in such plants as coleus, which 

 have the pigment in solution in the cell sap and not masked by chloro- 

 phyll. Using such a natural indicator is advantageous because it intro- 

 duces no external factors. Besides, the range of hydrogen-ion concen- 

 tration covered by the change of the pigment from red to blue is the same 

 as that naturally occurring in the plant. Everyone has observed that 

 various conditions affect the color of such pigments. 



The color change on freezing is due to a change in the acidity of the 

 plant juice. The juice of the variety of coleus used here reacts slightly 

 acid to the pigmept, as shown by the bright-red color. If coleus leaves 

 are frozen, the reaction remains the same for a considerable time. If, 

 now, the leaf is rapidly thawed out by being dipped into warm water, the 

 red pigment at once changes to a decided blue color. This change can 

 be observed almost in an instant, before oxidase activity can produce 

 secondary changes. The rapidity of the change indicates a removal of 

 the excess hydrogen-ion in much, the same manner as by neutralization. 



Similar changes of reaction can be produced in the white of egg or in 

 cabbage juice on freezing. If methyl red is added to cabbage juice and 

 the juice allowed to freeze, an increase in the acidity is indicated by the 

 increasing red color of the indicator. Here the change is more or less 

 masked by the chlorophyll. A similar change of acidity can be observed 

 if the white of egg is frozen. It is necessary to use an indicator covering 

 the proper range of acidity, in this case phenolphthalein, since white of 

 egg is slightly alkaline. The white of egg is colored red by the phenol- 

 phthalein; but when frozen solid, the red color disappears, owing to an 

 increase in the hydrogen-ion concentration. This latter case is to serve 

 merely as a demonstration, since we are not especially concerned with 

 such materials. The change of reaction here may be due to the high 

 eutectic point of sodium bicarbonate and its separation in the solid phase 

 on freezing. Certainly such changes in acidity as here shown have some 

 influence on the precipitation of proteins and upon frost injury. It is 

 well known from the work of numerous authors that the acidity of the 

 medium is a factor of great importance in determining the state of hydro- 

 philous colloids such as the proteins. 



