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 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.