Shifts in photosynthetic carbon labelling pattern by etiolated rice (Oryza sativa L.) seedlings during greening

Abstract

The carbon assimilation pattern in light by dark-grown rice (Oryza sativa) seedlings was studied during greening. After exposure of etiolated seedlings to light their capacity to synthesize the C₄-acids, malate and aspartate, increased for 12 h. The labelling of 3-phosphoglycerate, sugar phosphates, sucrose and insolubles, did not increase until after 12 h. Thereafter a continuous steep increase in synthesis of Calvin cycle intermediates, but not of C₄-acids, recurred. The levels of carboxylating enzymes (phosphoenolpyruvate and ribulose diphosphate carboxylases) in the seedlings correlated positively with the appearance of label in C₄ acids and in Calvin cycle compounds. We suggest that a β-carboxylation mechanism was activated immediately on illumination but was persistent for only 12 h. After an 8-h lag period following illumination, the Calvin cycle began to operate and continued during further growth of seedlings in light.