Reproductive response to elevated CO2 : the roles of vegetative carbon storage, nitrogen and seed traits

This study focused on the reproductive response to elevated CO2 of plants possessing below-ground storage. I tested the hypotheses that under elevated CO2: (1) Plants with greater non-foliar storage capacity win show more reproductive response and (2) The altered foliar physiology of carbon (C) and nitrogen (N) use will cause increases in seed number and quality. Carbon dioxide treatments of High (650 muL L-1) and Ambient (360 muL L-1) were used in a controlled environment, simulated growing season, and in a natural pasture community. Hypothesis 1 was tested experimentally using four Raphanus varieties that differed in hypocotyl and leaf sizes. N fertilization and harvest times were used to obtain a range of root:shoot ratios. Enhancements in vegetative leaf area rather than the hypocotyl predicted reproductive responsiveness to CO 2. However, after three years of CO2 exposure in the pasture, hypocotyl-storing Taraxacum officinale responded strongly in vegetative biomass which correlated with inflorescence size and number. Fitness was enhanced four-fold, while the leaf-storing Plantago major produced more ramets and had only a two-fold fitness increase. Hypothesis 2 was tested by examining the C and N physiology underlying the vegetative organs and seeds of the pasture plants. Under elevated CO2, photosynthesis increased two-fold and senescence was delayed. Total plant C:N ratio did not differ, suggesting N acquisition increased. N similarly limited seed number in all cases suggesting an unchanged Physiology of N use in reproduction. While morphology constrained total biomass response, provisioning to seeds increased as shown by higher seed mass and number and decreased variability in number and mass. In all cases, leaf mass increase under high CO2 corresponded with fitness increase. Phenology constrained response to CO 2 as there was no plasticity in flowering day in Raphanus and Plantago, while there were flowering delays but greater seed maturation rate in Ta