Metabolism of Slices of the Tomato Stem

Abstract

The rate of respiration of tomato stem slices varied considerably, the highest values (Qo₂, 2–3) being obtained for plants in a good nutritional state, and the lowest (Qo₂, 1) in starved plants. The respiratory quotient of 1.0 remained constant. Glucose fermentation was found to follow both glycolytic and alcohol fermentation pathways, the ratio of ethyl alcohol: lactic acid being 6.6:1. Fermentation seems to take place according to the Embden-Meyerhof scheme, as shown by the presence of some of these enzymes operative in this scheme and by inhibition experiments. In the presence of oxygen there was no formation of alcohol or lactic acid. Pyruvate added to tomato stem slices was metabolized by direct oxidation to acetic acid and by dismutation to lactic and acetic acids and CO₂ The metabolism of acetic acid was demonstrated by its condensation with oxaloacetic acid to form citrate, this being the second time that synthesis of citric acid by this mechanism has been found in plants. The presence of aconitase, of isocitric dehydrogenase, of succinic dehydrogenase, and of malic dehydrogenase, as well as the inhibition of respiration by malonic acid, favour the hypothesis that oxidation of carbohydrate in tomato stem slices proceeds via the citric acid cycle. The possibility of an auxiliary route, the malic acid oxidation pathway, also was demonstrated. Tomato stem tissue anaerobically split malic acid into glycolic acid. The further oxidation of glycolic, glyoxylic, and formic acid was demonstrated. In experiments with C¹⁴-labelled acetate and butyrate a dilution of the C¹⁴-labelled acids was found after incubation indicating new formation of these acids and of active participation of fatty acid metabolism in the metabolic activities of the tissue. With the exception of alanine, added amino acids produced a definite increase in O₂ uptake without extra formation of ammonia. Experimental demonstration of the possibility of electron transport from substrate to molecular oxygen in respiration via polyphenol oridase was provided by the attainment in a tomato tissue homogenate of a coupled oxidation-reduction between α-ketoglutarate and catechol with DPN and tyrosinase as the catalysts. The presence of cytochrome oxidase was also demonstrated. Thus both systems possibly may take part in the respiration of tomato stem slices.