The Interactive Effects of Carbon Dioxide Enrichment and Daylength on Growth and Development inPetunia hybrida

Plants were grown at either 350 or 1000 µl l^-1CO₂and inone of three photoperiod treatments: continuous short days (SD),continuous long days (LD), or short switched to long days atday 41 (SD–LD). All plants received 9 h of light at 450µmol m^-2s^-1and LD plants received an additional 4 h oflight at 8 µmol m^-2s^-1. Growth of SD plants respondedmore positively to elevated CO₂than did LD plants, due largelyto differences in the effect of CO₂on unit leaf rate. High CO₂increasedheight and decreased branching under SD conditions, but hadno effect under LD conditions. Elevated CO₂also increased thenumber of buds and open flowers, the effect for flower numberbeing greater in short than in long days. The specific leafarea of plants grown at 1000 µl l^-1CO₂was reduced regardlessof daylength. High CO₂also decreased leaf and increased reproductiveallocation, the magnitude of these effects being greater underSD conditions. Bud formation and flower opening was advancedunder high CO₂conditions in SD plants but bud formation wasdelayed and there was no effect on flower opening under LD conditions.The effects of CO₂on plants switched from SD to LD conditionswere largely intermediate between the two continuous treatments,but for some parameters, more closely resembled one or the other.The results illustrate that daylength is an important factorcontrolling response of plants to elevated CO₂. Petunia hybridaHort. ex Vilm; carbon dioxide; photoperiod; functional growth analysis; daylength; global change; development; phenology