Carbon dioxide and flowering in Pharbitis nil Choisy

The effects of photoperiod on floral and vegetative development of Pharbitis nil were modified by atmospheric CO2 concentrations maintained during plant growth. Short day (SD) photoperiods caused rapid flowering in Pharbitis plants growing in 0.03 or 0.1% CO2, while plants in long day (LD) conditions remained vegetative. At 1 or 5% CO2, however, flower buds were developed under both the SD and LD photoperiods. Flowering was earliest in the plants exposed to SD at low CO2 concentrations which formed floral buds at stem node 3 or 4. At high CO2 concentrations, floral buds did not form until stem node 6 or 7. Both high CO2 concentrations and LD photoperiods tended to enhance stem elongation and leaf formation. The occurrence of flowering under normally noninductive LD photoperiods at 1 or 5% CO2 is readily explained in terms of higher photosynthetic rates. Plants grown at 0.03 or 1% CO2 in either photoperiod tended to approach maximum photosynthesis between 0.1% and 0.5% CO2. In addition, relative growth rates were not significantly increased by growth at 1 or 5% CO2. Possible alternative mechanisms for the interactive effects of CO2 and photoperiod are discussed.