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

After exposure to a doubled CO2 concentration of 750 muL 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 A(1) and A(3) (GA(1+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.