Characteristics of photosynthetic carbon metabolism of spikelets in rice

In lemmas and paleae of rice, the amount of pyruvate, Pi dikinase (PPDK) protein increased dramatically 6 d after anthesis and this change was consistent with that in the activity of PPDK. Since lemmas and paleae at this stage also showed high activities of the other marker enzymes of C-4 pathway including phosphoenolpyruvate carboxylase (Imaizumi et al. (1990) Plant Cell Physiol 31:835-843), photosynthetic carbon metabolism with lemmas at this stage were characterized. In a C-14 pulse-C-12 chase study by photosynthetic CO2 fixation, about 35% and 25% of C-14 fixed in lemmas were incorporated initially into 3-phosphoglycerate (3-PGA) and C-4 acids, respectively. This suggests that lemmas participate mainly in C-3-type photosynthetic metabolism, but that lemmas may also participate in the metabolism of C-4 acids to some extent. To clarify this possibility, large amounts of C-14-labeled C-4 acids were synthesized in vivo by a light-enhanced dark CO2 fixation (LED) method and the fate of C-14 in C-4 acids in the light was investigated. The percentage distribution of C-14 in C-4 position of malate was about 90% and 83% after 10 s of photosynthetic (CO2)-C-14 fixation and 110 s of LED, respectively. Some of the C-14 incorporated into C-4 acids was transferred into 3-PGA and sugar phosphates. The possibility of direct fixation of CO2 by phosphoenolpyruvate carboxylase and metabolic pathway of CO2 released by decarboxylation of malate produced were discussed.