Responses of carboxylating enzymes, sucrose metabolizing enzymes and plant hormones in a tropical epiphytic CAM orchid to CO2 enrichment


Correspondence: Choy Sin Hew. Fax: +65 7792486; E-mail:


After exposure to a doubled CO2 concentration of 750 µL L−1 for 2 months, average relative growth rate (RGR) of Mokara Yellow increased 25%. The two carboxylating enzymes, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase), responded differently to CO2 enrichment. There was a significant daytime down-regulation in Rubisco activity in the leaves of CO2-enriched plants. However, PEPCase activity in CO2-enriched plants was much higher in the dark period, although it was slightly lower during the daytime than that at ambient CO2. Leaf sucrose–phosphate synthase (SPS) and sucrose synthase (SS) activities in CO2-enriched plants increased markedly, along with a night-time increase in total titratable acidity and malate accumulation. There was a remarkable increase in the levels of indole-3-acetic acid (IAA), gibberellins A1 and A3 (GA1+3), isopentenyladenosine (iPA) and zeatin riboside (ZR) in the expanding leaves of plants grown at elevated CO2. It is suggested that (1) the down-regulation of Rubisco and up-regulation of SPS and SS are two important acclimation processes that are beneficial because it enhanced both photosynthetic capacity at high CO2 and reduced resource investment in excessive Rubisco capacity; (2) the increased levels of plant hormones in CO2-enriched M. Yellow might play an important role in controlling its growth and development.