The effect of elevated CO2 on the vegetative and generative growth of Phalaenopsis

Phalaenopsis is a crassulacean acid metabolism (CAM) plant which absorbs and binds CO2 as malate during the night. During daytime the stomata close and the CO2 stored in the vacuole is released and used for photosynthesis. Because the CO2 taken up by CAM plants was assumed to be unaffected by the CO2 concentration in the air, additional CO2 for increased growth was generally not supplied in Phalaenopsis. However, a literature study indicated that elevated CO2 might have a positive effect in Phalaenopsis. To assess the effects of elevated CO2 concentrations in Phalaenopsis, two experiments were performed with elevated CO2. In the first experiment, 8 Phalaenopsis clones were induced to flower at 400 and 1000 ppm CO2 in the greenhouse at day/night temperatures of 19.2/18°C. Two or more spikes were produced by 82% of the plants grown at 1000 ppm CO2, and in 70% of the plants grown at 400 ppm CO2. The mean number of spikes per plant was 9% higher and their dry weight was 12% higher. A second experiment was then designed to see if the effect of elevated CO2 was limited to the flowering phase, or was effective during the vegetative phase as well. In this experiment, four Phalaenopsis clones were grown in climate controlled growth chambers at CO2 concentrations of 400 or 1000 ppm. During the vegetative growth phase (30 weeks), the leaf area and root dry weight of plants grown at elevated CO2 appeared to be higher than at low CO2. At the end of the vegetative growth phase, plants from both CO2 treatments were split into two groups: one group at low CO2 and the other at elevated CO2 for the generative growth phase, resulting in 4 treatment groups. Day/night temperatures were lowered to 19/18°C for 6 weeks to induce spiking followed by 12 weeks with day/night temperatures of 21/19°C during the growth and development of the spikes. Plants grown at elevated CO2 during the generative growth phase yielded 15 and 10% more plants with multiple spikes and an improved flower stem quality, when grown at low and high CO2 in the vegetative phase respectively. Elevated CO2 during the vegetative growth phase had no significant effect on the number of spikes, but the trend indicates an increase in spike quality