Rhythms in respiratory metabolism ofLemna gibbaG3under continuous illumination1

摘要

Changes in chemical constituents and respiratory metabolism of a long-day duck-weed,Lemna gibbaG3exposed to continuous illumination after short-day cultivation were investigated. The dry weight to fresh weight ratio was constant during the first 72 hr of continuous illumination. pH of the crude extract was constant at 6.6, but pH of the culture medium was raised with theLemnagrowth. Titratable acidity decreased after about 44 hr, whereas malic acid content increased in 18 hr. There were no significant changes in total reducing sugar and pentose. Total protein content and lipid showed rhythmical changes with cycles of 48 hr.O2-Uptake gave a damped oscillation with cycles of 24 hr. It was low in the first half day and high in the second half.14CO2-Output from glucose-l-14C showed a similar damped oscillation.14CO2-Output from glucose-2-14Cor glucose-6-14C was almost constant. The C6/C2ratio, then, showed damped oscillation in the reverse way to O2- uptake between 0.3–0.5, and the C8/C2ratio was constant at 0.9. Accordingly, the diurnal rhythm of O2-uptake was thought to be brought about by variation in activity of the pentose-phosphate pathway. Reproduction of glucose-6-phosphate by the pentose-phosphate pathway was presumably limited in amount.Glyceraldehyde-3-phosphate dehydrogenase activity varied diurnally. The activities of NADP-linked and NAD-linked enzymes increased and decreased, respectively, in the first half day. Variations in these enzymatic activities are discussed in correlation with rhythmical changes in O2-uptake and in the C6/C1ratio. Acid phosphatase activity also followed a diurnal variation. No activities of alcohol and formic dehydrogenases were found. The activities of NADP glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, pyruvic kinase and NADP isocitric dehydrogenase were high, but showed no rhythmical variatio