| Experiments were conducted in controlled growth chambers to examine the responses of ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco), sucrose phosphate synthase (SPS) and sucrose synthase (SS), to long term CO2 enrichment in the tropical epiphytic orchid hybrid Oncidium Goldiana. Furthermore, subsequent changes in activities of these enzymes were monitored following transfer to ambient air. After 2 months of growth, relative growth rate (RGR) in plants grown at elevated CO2 doubled compared to those grown at ambient CO2. Concomitantly, there was a decline in Rubisco activity, Rubisco activation state and Rubisco protein content in CO2-enriched plants. In contrast, there was a significant up-regulation of SPS and SS activities. Both leaf sucrose and starch contents were significantly higher in plants grown at elevated CO2 than that at ambient CO2, while leaf total nitrogen content was markedly lower in CO2-enriched plants. Following transfer to ambient CO2, leaf SPS and SS activities, sucrose and starch content in CO2-enriched plants declined sharply within 10 days to the same level of control plants without CO2 enrichment. Contrary to our expectation, after 20 days of growth at ambient air, Rubisco activity in CO2-enriched plants remained much lower than that of control plants, although there was some increase of leaf Rubisco activity. This is primarily due to the consistently low levels of Rubisco protein content in the leaves. Leaf SPS activity was closely associated with leaf sucrose accumulation and SS activity was closely associated with starch accumulation. The activities of SPS and SS in leaf extracts were also closely associated with leaf assimilation rates. It is suggested that (1) the up-regulation of leaf SPS and SS activities might be an acclimation response to optimize the utilization and export of organic-carbon with the increased rate of inorganic-carbon fixation; (2) SPS and SS might play an important role in carbon partitioning between sucrose and starch in O. Goldiana under elevated CO2 conditions. |