The damping and reinitiation of the circadian rhythm of CO2 output in Bryophyllum leaves in relation to their malate content
1993
Wilkins, M.B. | Thomas, S.L.
The circadian rhythm of CO2 output in leaves of Bryophyllum fedtschenkoi damps out after 3-4 d in continuous darkness and a CO2-free air stream at 15 degrees C. The rhythm is reinitiated after a single exposure to white light of 2, 4, 6 or 8 h duration, damps out again after a further 3-4 d and can be reinitiated for a second time by a further exposure to light. During the exposure to light there is a burst of CO2 output consistent with the decarboxylation of malate, and the rhythm begins afterwards with an initial high rate of CO2 fixation. Malate gradually accumulates in the leaves in continuous darkness to attain a maximum value (35 mol m-3) at the time when the circadian rhythm disappears, and decreases to a low value (19 mol m-3) after a 4 h exposure to light which reinitiates rhythmicity. These results support the hypothesis that damping of the rhythm of CO2 output in continuous darkness is due to the accumulation of malate in the leaf cells, eventually reaching such a level that its removal from the cytoplasm into the vacuole cannot take place, with the result that PEPc activity, upon which the rhythm of CO2 output depends, remains allosterically inhibited.
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