Photosynthesis with Single-Rooted Amaranthus Leaves. I. Changes in the Activities of RuBP-1,5-bisphosphate Carboxylase and Phosphoenolpyruvate Carboxylase and the Amounts of Intermediates in Photosynthetic Metabolism in Response to Changes in the Source-Sink Balance |
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Authors: | Sawada, Shinichi Arakawa, Osamu Muraki, Isao Echigo, Hitoshi Miyashita, Miyuki Iwafune, Mito Kasai, Minobu Usuda, Hideaki |
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Affiliation: | 1 Department of Biofunctional Science, Faculty of Agriculture and Life Science, Hirosaki University Hirosaki, 036-8561 Japan 2 Laboratory of Chemistry, Faculty of Medicine, Teikyo University Ohtsuka, Hachioji, Tokyo, 192-0395 Japan |
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Abstract: | C4 model plants composed of single-rooted Amarantus cruentusL. leaves were developed to study source-sink relationships.The photosynthetic activity of CO4 fixation in the rooted leavescultured under the control condition (10-h light/14-h darkness,730 µmol photons m2s1) remained constantand high. When the leaves kept for 8 or 9 d under the controlcondition were exposed to continuous light (CL) for 4 d, theactivity decreased steeply. The starch content increased significantlyduring the first 2 d in CL whereas the sucrose content increasedalmost linearly during the 4 d in CL. The close relationshipbetween the decrease in photosynthetic activity and the increasein sucrose indicates that the activity was subjected to feedbackinhibition when the end product was forced to accumulate inthe leaves. In the treated leaves, the initial activity of ribulose-1,5-bisphoshatecarboxylase decreased significantly. A large increase in ribulose-1,5-bisphosphateand decrease in 3-phosphoglycerate occurred following the deactivationin RuBPcase, which was fully restored by the preincubation withCO2 and Mg2+ and/or P1 and Mg2+. The activity of phosphoenolpyruvatecarboxylase also decreased but not as rapidly as RuBPcase. Thealanine and pyruvate levels decreased markedly but the oxaloacetate,glutamine and asparagine levels significantly increased whilephosphoenolpyruvate and glutamate were kept at almost constantlevels. The malate level first increased slightly and then decreasedsignificantly. The mechanism of coordinated decrease in carboxylationreactions between phtosynthetic carbon reduction cycle and C4metabolism in response to change in the source-sink balanceis discussed. (Received May 24, 1999; Accepted September 1, 1999) |
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