Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid plantlet exposed to super-elevated CO2

Abstract

The effects on growth in super-elevated (1%) CO2 in terms of photosynthetic capability and carbohydrate production were studied in an epiphytic CAM (Crassulacean acid metabolism) orchid plantlet, Mokara Yellow (Arachnis hookeriana x Ascocenda Madame Kenny). The growth of the plantlets was greatly enhanced after growing for 3 months at 1% CO2 compared with the control plantlets (0.035% CO2). CO2 enrichment produced more than a 2-fold increase in dry matter production. The enhanced root growth at 1% CO2 led to a higher root:shoot ratio. Plantlets grown at super-elevated CO2 had higher F(v)/F(m) values, a higher photochemical quenching (q(p)) and a relatively lower non-photochemical quenching (q(N)). CO2 at 1% appeared to enhance the utilization of captured light energy in the orchid plantlets. CO2 enrichment also increased contents of soluble sugars (glucose and sucrose) and starch in the orchid plantlets. The extra starch formed under 1% CO2 did not cause a disruption of the chloroplasts. Chlorophyll content was higher and a clear granal stacking was evident in young leaves and roots of plantlets grown at 1% CO2. An extensive thylakoid system was observed in the young leaf chloroplasts of the CO2-enriched plantlets indicating an improved development of the photosynthetic apparatus when compared to that of the control plantlets. The increased photosynthetic capacity and enhanced growth of the epiphytic roots under CO2 enrichment would facilitate the generation of more photoassimilates and acquisition of essential resources, thereby increasing the survival rate of orchid plantlets under stressful field conditions.