Effects of super-elevated CO2 on the growth and carboxylating enzymes in an epiphytic CAM orchid plantlet

Abstract

Responses of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) to super-elevated CO2 were determined along with dry mass production, chlorophyll, soluble protein and nocturnal malate increases (NMI) for an epiphytic Crassulacean acid metabolism (CAM) orchid plantlet, Mokara Yellow. After 3-month culture period, the total dry mass under super-elevated CO2 was 170% higher than the plantlets grown in 0.03% CO2; young leaf dry mass was 4-fold higher while the root dry mass increased 278 % and 344% under 1% and 5 % CO2 respectively. Higher root: shoot ratio was observed under super-elevated CO2; 0.22 in 0.03 % CO2, 0.32 in 1% CO2 and 0.38 in 5 % CO2. The averaged increase in total young leaf area was 244% and 373 % under 1% and 5 % CO2 respectively. Leaf chlorophyll expressed per unit fresh weight was reduced under 5 % CO2 but it increased 19% and 67% in old and young leaves of 3-month plantlets under 1% CO2. The root chlorophyll content increased 108 % and 154 % under 1% and 5 % CO2 respectively. Soluble protein in young leaves increased 32 % under 1% CO2 and 75 % under 5 % CO2, while the increase in root protein varied from 36 % to 100 %. The activities of Rubisco and PEPCase expressed per unit protein were reduced under super-elevated CO2, particularly in 5 % CO2, the decreases ranged from 12 % to 90 % in Rubisco and 27% to 90% in PEPCase. Nevertheless, the leaf Rubisco:PEPCase ratio increased 110% to 362% under super-elevated CO2. Increased NMI, ranged from 23 % to 182% under super-elevated CO2, contributed to the increased dry matter accumulation in Mokara plantlets. Throughout the 3-month culture period, the CO2-enriched plantlets showed enhanced growth particularly under 1% CO2 in terms of biomass production, chlorophyll, soluble protein and NMI despite a concomitant decrease in the activities of the carboxylating enzymes.