THE INTERACTIVE EFFECTS OF CARBON-DIOXIDE ENRICHMENT AND DAYLENGTH ONGROWTH AND DEVELOPMENT IN PETUNIA-HYBRIDA

Plants were grown at either 350 or 1000 mu l l(-1) CO2 and in one of t hree photoperiod treatments: continuous short days (SD), continuous lo ng days (LD), or short switched to long days at day 41 (SD-LD). All pl ants received 9 h of light at 450 mu mol m(-2) s(-1) and LD plants rec eived an additional 4 h of light at 8 mu mol m(-2) s(-1). Growth of SD plants responded more positively to elevated CO2 than did LD plants, due largely to differences in the effect of CO2 on unit leaf rate. Hig h CO2 increased height and decreased branching under SD conditions, bu t had no effect under LD conditions. Elevated CO2 also increased the n umber of buds and open flowers, the effect for flower number being gre ater in short than in long days. The specific leaf area of plants grow n at 1000 mu l l(-1) CO2 was reduced regardless of daylength. High CO2 also decreased leaf and increased reproductive allocation, the magnit ude of these effects being greater under SD conditions. Bud formation and flower opening was advanced under high CO2 conditions in SD plants but bud formation was delayed and there was no effect on flower openi ng under LD conditions. The effects of CO2 on plants switched from SD to LD conditions were 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 factor c ontrolling response of plants to elevated CO2. (C) 1997 Annals of Bota ny Company.