Poster: Photosynthesis

Abs # 323: Photosynthetic responses of poplar and soybean to free air enrichment of CO₂

Presenter:	Bernacchi, Carl J, bernacch@life.uiuc.edu
Authors	Bernacchi, Carl J (A) (B)  Morgan, Patrick B (B)   Ort, Donald R (A) (B)  Long, Stephen P (B)
Affiliations:	(A): USDA Agricultural Research Service (B): University of Illinois, Urbana Champaign

As atmospheric concentrations of CO₂ ([CO₂]) rise, metabolic control of light saturated photosynthesis (A_sat) may shift from ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) toward the processes determining the rate of RuBP regeneration. Optimal acclimation to growth at an elevated [CO₂] ([CO₂]_e) is expected to decrease the carboxylation capacity (V_c,max) and increase the capacity for RuBP regeneration (J_max). Numerous efforts have been made to determine how photosynthesis may respond to growth at [CO₂]_e, but very few have studied plants that develop from planting to harvest under open-air [CO₂]_e. Here we use two such experiments to test the hypothesis that growth in [CO₂]_e will decrease the V_c,max to J_max ratio. This research was conducted using the Soybean Free Air Concentration Enrichment (FACE) facility (two soybean cultivars) and the Poplar FACE facility (three poplar species). FACE allows open air CO₂ enrichment of whole canopies without alterations in microclimate. Soybean and poplar grown in [CO₂]_e had higher rates of A_sat over the duration of the experiments. A decrease in V_c,max and in the ratio of V_c,max/J_max was evident for soybean grown in [CO₂]_e compared with the control plants. This suggests a decrease in photosynthetic potential, with soybean shifting resources away from Rubisco content and/or activation, consistent with optimal resource partitioning away from processes that are no-longer limiting. Similarly, two of the poplar species showed down-regulation of V_c,max with growth in [CO₂]_e, however, there was no clear shift in the V_c,max/J_max ratios. For poplar and soybean, two widely contrasting species, down-regulation of photosynthetic potential to growth in [CO₂]_e was apparent, although not sufficient to prevent an overall increase in A_sat.