Abstract
Summary Responses of ribulose-I,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) to super-elevated CO2 were determined along with dry mass production, chloro phyll, soluble protein and nocturnal malate increases (NMI) for an epiphytic Crassulacean acid metabo lism (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 con tent 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 CO 2 -enriched plantlets showed enhanced growth particularly under 1 % CO2 in terms of biomass pro duction, chlorophyll, soluble protein and NMI despite a concomitant decrease in the activities of the car boxylating enzymes.
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