Effects of super-elevated CO 2 on the growth and carboxylating enzymes in an epiphytic CAM orchid plantlet

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.