TITRATION CURVES OF ETIOLATED AND OF GREEN WHEAT SEEDLINGS REPRODUCED WITH BUFFER MIXTURES

Abstract

Both the alkali- and acid-titration curves of the juice of etiolated and of green wheat seedlings have been approximately reproduced with chemical mixtures containing asparagin, phosphates, sodium malate, glucose, and, in the case of green seedlings, leucin. Asparagin appears to be the substance in the juice of the etiolated seedlings which is responsible for the characteristic point of inflection of the titration curve near pH 8.9. Phosphates seem to be the principal buffers between pH 6.0 and 7.5. The equilibrium between the primary and secondary phosphates may be intimately associated with the maintenance of the normal reaction of the tissues. Over the range of the acid-titration of the juice, i.e., below pH 6.0, the buffering seems largely due to the presence of an organic-acid constituent. A solution of sodium malate alone, or a combination of tartrate and citrate, with or without malate, gives acid-titration values similar to those of the juice. The zone of decreasing buffer capacity below pH 3.5 and pH 4.0, respectively, in the oxalate and succinate curves, giving rise to characteristic inflections which do not occur in the titration curves of wheat juice, is believed to preclude the presence of these salts in wheat juice in sufficient amount to affect the titration values. Glucose supplied the requisite buffer capacity of the etiolated-seedling juice above pH 9.5. In the case of green seedlings, both leucin and glucose were used in reproducing the titration curve over this region because the requisite amount of glucose alone was so much greater than that found in the juice. The glucose was considered to be representative of all the soluble carbohydrates of the juice which react with alkali above pH 9.5. The most pronounced changes in the buffer system of the wheat plant during the period of seedling development seem, from the evidence presented in this paper, to be due to a reduction in asparagin content and an increase in an organic-acid constituent as assimilation processes become established in the young plant.