Age. at flowering differentially affects vegetative and reproductive responses of a determinate annual plant to elevated carbon dioxide

Plant population and community dynamics may be altered by increasing atmospheric CO2 concentrations {[CO2]} through intraspecific variation in the responses of vegetative and reproductive growth. Although these responses may be regulated by age at flowering, little is known about the direct effects of age at flowering on growth responses to elevated [CO2]. In this study, we examined the interactive effects of elevated [CO2] and age at flowering on absolute and relative allocation to vegetative and reproductive growth in the determinate, short-day species Xanthium strumarium L. (common cocklebur). Six cohorts were planted at 5-day intervals in chambers maintained at either 365 or 730 mumol mol(-1) CO2, with an 18-h photoperiod and a non-limiting nutrient supply. All plants were simultaneously induced to flower by switching the photoperiod to 12 h for 2 days, then switching back to an 18-h photoperiod for the remainder of the experiment. All plants were harvested 15 days after the onset of flowering. Total plant biomass increased 11-41% with increasing [CO2] and 45% from the youngest to the oldest cohort. Vegetative growth responses to elevated [CO2] significantly increased with increasing age at flowering, associated with increasing sink relative to source capacity. In contrast, total fruit mass decreased 32% from the youngest to the oldest cohort and was not significantly affected by CO2 supply. Relative biomass allocation to fruit decreased 47% from the youngest to the oldest cohort, reflecting decreased numbers of fruit, and 6-28% with increasing [CO2], reflecting decreased mean mass per mature fruit. Our findings suggest that elevated [CO2] MAY increase vegetative growth in Xanthium without increasing reproductive biomass, and that age at flowering may influence these responses through effects on source:sink balance. Further, changes in the allometric relationship between vegetative and reproductive growth associated with growth in elevated [CO2] suggest that long-term population and community-level responses to elevated [CO2] may differ substantially from predictions based on vegetative responses.