Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris

Among plants grown under enriched atmospheric CO2, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to root s than among ambient-grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase alloca tion to shoot. We tested these RSB predictions, and whether responses to CO 2 enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south-eastern USA. Plants were grown under near-a mbient (400 mu mol mol(-1)) and enriched (700 mu mol mol(-1)) levels of atm ospheric CO2. Defoliation consisted of the weekly removal of 25% of each ne w fully expanded, but not previously defoliated, leaf from either rosette o r bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in the se plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO2-enriched plan ts conformed to the predictions of RSB, with higher root:shoot ratio than a mbient-grown plants (P < 0.02), whereas older, slower growing plants did no t show a CO2 effect on root:shoot ratio. Defoliation resulted in smaller pl ants, among which both root and shoot biomass were reduced, irrespective of CO2 treatment (P < 0.03). However, H. subaxillaris plants were able to com pensate for leaf area removal through flexible shoot allocation to more lea ves vs. stem (P < 0.01). Increased carbon availability through CO2 enrichme nt did not enhance the response to defoliation, apparently because of compl ete growth compensation for defoliation, even under ambient conditions. CO2 -enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other h and, earlier and more abundant yield of flower biomass was an important con sequence of growth under CO2 enrichment.