EFFECTS OF LOW AND ELEVATED CO2 ON C-3 AND C-4 ANNUALS .2. PHOTOSYNTHESIS AND LEAF BIOCHEMISTRY

Abutilon theophrasti (C-3) and Amaranthus retroflexus (C-4), were grow n 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 well-water ed conditions to evaluate their photosynthetic response to historic an d future levels of CO2. Net photosynthesis at growth CO2 partial press ures increased with increasing CO2, for C-3 plants, but not C-4 plants . Net photosynthesis of Abutilon at 15 Pa CO2 was 70% less than that o f plants grown at 35 Pa CO2, due to greater stomatal and biochemical l imitations at 15 Pa CO2. Relative stomatal limitation (RSL) of Abutilo n at 15 Pa CO2 was nearly 3 times greater than at 35 Pa CO2. A photosy nthesis model was used to estimate ribulose-1,5-bisphosphate carboxyla se (rubisco) activity (Vc(max)), electron transport mediated RuBP rege neration capacity (J(max)), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO2 (A-C-i) curves. All three component processes decreased by approximately 25% i n Abutilon grown at 15 Pa compared with 35 Pa CO2. Abutilon grown at 1 5 Pa CO2 had significant reductions in total rubisco activity (25%), r ubisco content (30%), activation state (29%), chlorophyll content (39% ), N content (32%), and starch content (68%) compared with plants grow n at 35 Pa CO2. Greater allocation to rubisco relative to light reacti on components and concomitant decreases in J(max) and PiRC suggest co- regulation of biochemical processes occurred in Abutilon grown at 15 P a CO2. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO2 compared with 35 Pa CO2, su ggesting that the rise in CO2 since the beginning of the industrial ag e has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between s tomatal and biochemical factors such that net photosynthesis was simil ar in all CO2 treatments. Differences in photosynthetic response to gr owth over a wide range of CO2 partial pressures suggest chang es in th e relative performance of C-3 and C-4 annuals as atmospheric CO2 has f luctuated over geologic time.