title: Effects of low and elevated CO2on C3and C4annuals : II. Photosynthesis and leaf biochemistry.
abstract: Abutilon theophrasti (C3) and Amaranthus retroflexus (C4), were grown from seed at four partial pressures of CO2: 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 CO2. Net photosynthesis at growth CO2partial pressures increased with increasing CO2for C3plants, but not C4plants. Net photosynthesis of Abutilon at 15 Pa CO2was 70% less than that of plants grown at 35 Pa CO2, due to greater stomatal and biochemical limitations at 15 Pa CO2. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO2was nearly 3 times greater than at 35 Pa CO2. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vcmax), electron transport mediated RuBP regeneration capacity (Jmax), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO2(A-Ci) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO2. Abutilon grown at 15 Pa CO2had 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 CO2. Greater allocation to rubisco relative to light reaction components and concomitant decreases in Jmaxand PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO2. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO2compared with 35 Pa CO2, suggesting that the rise in CO2since 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 CO2treatments. Differences in photosynthetic response to growth over a wide range of CO2partial pressures suggest changes in the relative performance of C3and C4annuals as atmospheric CO2has fluctuated over geologic time.
authors.affiliation: Department of Botany, Duke University, 27708-0340, Durham, NC, USA.
authors.forename: D T
authors.initials: DT
authors.lastname: Tissue
authors.affiliation: Department of Botany, Duke University, 27708-0340, Durham, NC, USA.
authors.forename: K L
authors.initials: KL
authors.lastname: Griffin
authors.affiliation: Department of Botany, Duke University, 27708-0340, Durham, NC, USA.
authors.forename: R B
authors.initials: RB
authors.lastname: Thomas
authors.affiliation: Department of Botany, Duke University, 27708-0340, Durham, NC, USA.
authors.forename: B R
authors.initials: BR
authors.lastname: Strain
keywords: Abutilon theophrasti; Amaranthus retroflexus; Low CO2; Photosynthesis; Rubisco