Effects of low and elevated CO on C and C annuals : II. Photosynthesis and leaf biochemistry.

Abutilon theophrasti (C) and Amaranthus retroflexus (C), were grown from seed at four partial pressures of CO: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO. Net photosynthesis at growth CO partial pressures increased with increasing CO for C plants, but not C plants. Net photosynthesis of Abutilon at 15 Pa CO was 70% less than that of plants grown at 35 Pa CO, due to greater stomatal and biochemical limitations at 15 Pa CO. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO was nearly 3 times greater than at 35 Pa CO. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vc), electron transport mediated RuBP regeneration capacity (J ), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO (A-C ) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO. Abutilon grown at 15 Pa CO had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO compared with 35 Pa CO, suggesting that the rise in CO since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO treatments. Differences in photosynthetic response to growth over a wide range of CO partial pressures suggest changes in the relative performance of C and C annuals as atmospheric CO has fluctuated over geologic time.