CO2 Fixation in Opuntia Roots

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Abstract
Nonautotrophic CO2 metabolism in O. echinocarpa roots was studied with techniques of manometry and radiometry. The roots were grown in a 1/4 strength nutrient solution for several days. Roots assimilated significant quantities of C14O2 and appeared to show a crassulacean-type acid metabolism with respect to quality and quantity. Most of the C14 activity was associated with the distal portion of the elongating root indicating correlation with metabolic activity. The C14O2 assimilation was comparable to a crassulacean leaf succulent, but 3 times greater than that found for stem tissue of the same Opuntia species. The rates of O2 and CO2 exchange and estimated CO2 fixation were 180, 123, and 57 [mu]l/g/hour. A respiratory quotient of 0.66 was found. Equilibration of the predominant product, malic acid, with isocitric, succinic, and fumaric acids was not evident. The latter observation was interpreted as metabolic isolation of the fixation products rather than poor citric acid cycle activity. A rapid turnover of the fixed C14O2 was measured by following decar-boxylation kinetics and by product analysis after a postincubation period. The first order rate constant for the steady state release was 4.4 x 10-3 min-1 with a half-time of 157.5 min. Amino acids decayed at a more rapid rate than organic acids.