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Metabolism and Enzymology

CO₂ and O₂ Exchanges in the CAM Plant Ananas comosus (L.) Merr

Determination of Total and Malate-Decorboxylation-Dependent CO₂-Assimilation Rates; Study of Light O₂-Uptake

Service de Radioagronomie, Département de Biologie, Cen Cadarache, 13108 St. Paul lez Durance, France, Laboratoire de Physiologie et Biochimie, Institut de Recherche sur les Fruits et Agrumes Tropicaux (IRFA-CIRAD), 34032 Montpellier, France

Photosynthesis and light O₂-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO₂ and O₂ gas exchange measurements. The amount of CO₂ which was fixed during a complete day-night cycle was equal to the amount of total net O₂ evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O₂-evolution and of net light atmospheric CO₂-uptake give the rates of malate-decarboxylation-dependent CO₂ assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O₂ evolved/net CO₂ fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO₂ for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO₂ assimilation deduced from net O₂-evolution was 3 times higher than the maximum rate of atmospheric CO₂-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO₂-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO₂-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO₂ fixed during the last hour of the light period was stored as malate. Light O₂-uptake determined with ¹⁸O₂ showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O₂-uptake increased progressively; during the following part of the light period, the rate of O₂-uptake was 3.5 times higher than the maximum rate of CO₂-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO₂-free atmosphere or in continuous illumination, the rate of O₂-uptake was higher than in the control. This supports the hypothesis that the low rate of O₂-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO₂ concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O₂ and CO₂ gas exchange suggest that at the end of the light period the intracellular CO₂ concentration was very low. We propose that the high ratio of O₂-uptake/CO₂-fixation is principally caused by the stimulation of photorespiration during this period.

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				H. T. K. Hong, A. Nose, and S. Agarie Respiratory properties and malate metabolism in Percoll-purified mitochondria isolated from pineapple, Ananas comosus (L.) Merr. cv. smooth cayenne J. Exp. Bot., October 1, 2004; 55(406): 2201 - 2211. [Abstract] [Full Text] [PDF]