(With Plate i and 4 figures in the text)In the main body of the present paper we shall report and discuss the results of physiological experiments performed during the period 1947-9 to strengthen the evidence that COj-fixation is a metabolic event invariably associated with and related to dark acidification in plants showing Crassulacean acid metabolism. We shall view these results against a general background of modern biochemical knowledge, but we shall not give more than incidental consideration to details of partial reactions.We have confirmed (see § II) all the results obtained in 1946 which were reported for Kalanchoe (see p. 2) and Bryophyllum, briefly by and more fully by Thomas & Beevers (1949). Further (see § III), we have extended our investigations to certain other genera in the list Bennet-Clark (1933a) gave of plants showing diurnal variation in acidity. For most of the species clear evidence was obtained of CO2 absorption in the dark from atmospheres enriched in carbon dioxide. Fluctuations in values of apparent respiratory quotients in such atmospheres, and in ordinary air, can be related to stages in Crassulacean acid metabolism in the dark (Thomas, 1949;Wolf, 1949). Accordingly, in most of our experiments we have measured oxygen uptake as well as CO2 output (positive or negative) and the accumulation or disappearance of titratable acid. In general, during dark acidification either in C02-enriched atmospheres or in air, fluctuations in R.Q.'s have appeared to relate to the extent of CO2-fixation and acid accumulation over the period of measurement.To obtain further evidence that acid accumulation and CO2-fixation march together, experiments were performed at different temperatures ( § IV) and in atmospheres containing different concentrations of carbon dioxide ( § V). The results obtained were in accord with the view ( § II, (i)) that the enzyme bringing about CO^-fixation is an integral part of the mechanism which occasions the conversion of carbohydrate into malic acid.Throughout we assume that over the periods of our experiments, changes in titratable acidity relate predominantly to the production and consumption of malic acid (see p. 6), summarized by overall equations (ii) and (iii) respectively. From equation (ii) we have calculated the malic acid equivalent of the amount of CO2 fixed, i.e. the malic acid production according to our hypothesis. In § VI we discuss possible reasons why, in our experiments, malic acid accumulation, as measured by increase in titratable acidity, has usually been less than the calculated value of malic acid production. New Phytol. 53, i '
upon Tyne, England Receiv-ed March 8. 1967. Sumiitmary. Young seedlings of buckwheat (Fagopyrum escidentium) respire in air with an RQ of unity. Analysis of respiratory substrates coupled with a study of the utilization of acetate-14C and glucose-_4C suggest that both the Embden-Meyerhof-Parnas, tricarboxylic acid and pentose phosphate sequences participate in the total respiratory catabolism.In anoxia CO, dropped to one third of the aerobic rate and ethanol accumulated to only about one half the rate of CO, output on a molar basis. Smaller amounts of lactate, succinate and free amino acids (particularly alanine and y-aminobutyric acid) accumulated, carboxylic acids decreased and there were initial increases in pyruvate and a-ketoglutarate. The observed changes are consistent with residual tricarboxylic acid and pentose phosphate cycle activity in anoxia and may account for the excess CO2 production over ethanol accumulation. CO, ethanol and lactate production did not account for all of the carbohydrate consumed in anoxia.Relative rates of carbon loss w-ere measured in air and in atmospheres containing 3.5 %, 2.1 %, 1.3 % and 0.6 % oxygen. The extinction point of anaerobic metabolism was 1.5 %.On return to air from anoxia the CO2 output increased and the RQ rose from 0.8 to 1.0 over the first 2-hour period. Ethanol. lactate and succinate were colsullmed and other constituents returned to their previous aerobic level. Some of these changes suggest a rather slow resumption of tricarboxylic acid cycle activity on return to air.Carbon loss as CO, in air was greater than the carbon loss as CO2 at the extinction point. Carbon loss in anoxia as CO, ethanol and lactate was similar to carbon loss at the extinction point. Assessed in this orthodox manner buckwheat seedlings shoNv no Pasteur effect but the complex nature of the changes in levels of metabolic substrates and intermediates do not allow firm conclusions to be draw^n on the effects of oxygen on the rates of glycolysis and other respiratory processes.In earlier work on the respiratory iiietabolism of germinating buckwheat seedlings Leach (t1) measured CO., outputs in air (N) and in pure nitrogen (I) and hence determined I/N quotients for the moment of transfer from aerobic to anaerobic conditions. The values were all close to 0.33 and Leach inferred that at the mnoment of transition the complete oxidation of carbohydrate to CO, and w ater was replaced by fermentation to yield CO.. and Later, Ranson ( 18) measured the rates of both ethanol accumulation and CO., output in young buckwheat seedlings on first-transfer to anoxia. Values obtained for the I/N quotients were again near 0.33 but the ethanol/CO, quotients were between 0.5 and 0.6. Thus, for these buckwheat seedlings the rate of carbon loss in CO, evolved and ethanol accumulated in anoxia was less than the carbon loss as CO. in air.Thomas (22) argued that a truer assesstnent of the effect of oxygen in conserving respiratorv substrate in plant tissues would be made by a comparison of the anaerobic car...
Many aspects of the growth and development of plants and of their metabolism are known to be influenced by the carbon dioxide concentration in the surrounding atmosphere (see, for example Crocker, 1948; Harrison, 1953). Notwithstanding
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