Differences in Rubisco content and its synthesis in leaves at different positions in Eucalyptus globulus seedlings

The dynamics of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) content and turnover during leaf development are not well understood in woody plants. Rubisco synthesis, N influx and the mRNA levels of Rubisco-encoding genes were determined as a function of leaf position in 4.5-month-old Eucalyptus globulus seedlings. Rubisco concentration was slightly higher in the top leaves as leaf expansion progressed and was almost maximal in the uppermost fully expanded leaves. Rubisco concentration remained almost constant in the fully expanded leaves at the top and middle positions and then became slightly low at the lowest positions. Rubisco synthesis was active only in the top leaves. These results suggest that Rubisco turnover rate is low in the middle leaves, leading to the maintenance of Rubisco contents, and that Rubisco degradation primarily occurs in the lowest leaves. Changes in the RBCS and rbcL mRNA levels were roughly parallel with Rubisco synthesis, but N influx was more closely correlated with Rubisco synthesis. These results suggest that N influx rather than the transcript abundance of Rubisco-encoding genes is of primary importance in regulating the rate of Rubisco synthesis. Additionally, expression of RBCS multigene family in E. globulus leaves was discussed.