CRASSULACEAN ACID METABOLISM UNDER NATURAL TROPICAL CONDITIONS

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Summary

The content of various metabolic intermediates has been determined in material from several species of succulent plants known to show a Crassulacean acid metabolism, grown under natural conditions at Kampala, Uganda. Free titratable acidity, malate, citrate and isocitrate, present in considerable amounts, and various keto-acids present in relatively small amounts, have been measured both at dawn and during the later afternoon. In the case of leaf from two species of Bryophyllum natural changes of keto-acid content were followed in detail.

Overnight changes in free titratable acidity could, in all cases, be largely attributed to malate accumulation, but citrate also increased considerably in some species. Diurnal fluctuations in isocitrate were not observed. Concentrations of both oxaloacetate and α-oxo-glutarate increased markedly overnight. In the two Bryophyllum species, pyruvate concentrations were maximal during photodeacidification. The phosphopyruvate content of B. crenatum leaf increased slightly during photodeacidification but was low during dark acidification. No isocitrate lactone was detected in B. calycinum leaf.

Observed oxygen uptake rates would account for the complete oxidation during photodeacidification of only a small proportion of the malate decarboxylation product. Probable pathways involved in the formation and utilization of malate are discussed.

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