ALBERT

Notes: Abstract  We compared the CO2- and light-dependence of photosynthesis of four tree species (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 µl l–1 CO2. Naturally-established saplings were fumigated with a free-air CO2 enrichment system. Light-saturated photosynthetic rates were 159–190% greater for Ce. canadensis saplings grown and measured under elevated CO2. This species had the greatest CO2 stimulation of photosynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO2 enrichment and Ca. glabra and L. styraciflua had intermediate responses. Elevated CO2 stimulated light-saturated photosynthesis more than the apparent quantum yield. The maximum rate of carboxylation of ribulose-1,5-bisphosphate carboxylase, estimated from gas-exchange measurements, was not consistently affected by growth in elevated CO2. However, the maximum electron transport rate estimated from gas- exchange measurements and from chlorophyll fluorescence, when averaged across species and dates, was approximately 10% higher for saplings in elevated CO2. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yield and elevated rates of maximum electron transport suggests that saplings growing under elevated CO2 make more efficient use of sunflecks. The stimulation of light-saturated photosynthesis by CO2 did not appear to correlate with shade-tolerance ranking of the individual species. However, the species with the greatest enhancement of photosynthesis, Ce. canadensis and L. styraciflua, also invested the greatest proportion of soluble protein in Rubisco. Environmental and endogenous factors affecting N partitioning may partially explain interspecific variation in the photosynthetic response to elevated CO2.

Notes: Abstract  Atmospheric CO2 partial pressure (pCO2) was as low as 18 Pa during the Pleistocene and is projected to increase from 36 to 70 Pa CO2 before the end of the 21st century. High pCO2 often increases the growth and reproduction of C3 annuals, whereas low pCO2 decreases growth and may reduce or prevent reproduction. Previous predictions regarding the effects of high and low pCO2 on C3 plants have rarely considered the effects of evolution. Knowledge of the potential for evolution of C3 plants in response to CO2 is important for predicting the degree to which plants may sequester atmospheric CO2 in the future, and for understanding how plants may have functioned in response to low pCO2 during the Pleistocene. Therefore, three studies using Arabidopsis thaliana as a model system for C3 annuals were conducted: (1) a selection experiment to measure responses to selection for high seed number (a major component of fitness) at Pleistocene (20 Pa) and future (70 Pa) pCO2 and to determine changes in development rate and biomass production during selection, (2) a growth experiment to determine if the effects of selection on final biomass were evident prior to reproduction, and (3) a reciprocal transplant experiment to test if pCO2 was a selective agent on Arabidopsis. Arabidopsis showed significant positive responses to selection for high seed number at both 20 and 70 Pa CO2 during the selection process. Furthermore, plants selected at 20 Pa CO2 performed better than plants selected at 70 Pa CO2 under low CO2 conditions, indicating that low CO2 acted as a selective agent on these annuals. However, plants selected at 70 Pa CO2 did not have significantly higher seed production than plants selected at 20 Pa CO2 when grown at high pCO2. Nevertheless, there was some evidence that high CO2 may also be a selective agent because changes in development rate and biomass production during selection occurred in opposite directions at low and high pCO2. Plants selected at high pCO2 showed no change or reductions in biomass relative to control plants due to a decrease in the length of the life cycle, as indicated by earlier initiation of flowering and senescence. In contrast, selection at low CO2 resulted in an average 35% increase in biomass production, due to an increase in the length of the life cycle that resulted in a longer period for biomass accumulation before senescence. From the Arabidopsis model system we conclude that some C3 annuals may have produced greater biomass in response to low pCO2 during the Pleistocene relative to what has been predicted from studies exposing a single generation of C3 plants to low pCO2. Furthermore, C3 annuals may exhibit evolutionary responses to high pCO2 in the future that may result in developmental changes, but these are unlikely to increase biomass production. This series of studies shows that CO2 may potentially act as a selective agent on C3 annuals, producing changes in development rate and carbon accumulation that could not have been predicted from single-generation studies.

Notes: Abstract In order study C3 and C4 plant growth in atmospheric CO2 levels ranging from past through predicted future levels, Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) were grown from seed in growth chambers controlled at CO2 partial pressures of 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current) and 70 Pa (predicted future). After 35 days of growth, CO2 had no effect on the relative growth rate, total biomass or partitioning of biomass in the C4 species. However, the C3 species had greater biomass accumulation with increasing CO2 partial pressure. C3 plants grown in 15 Pa CO2 for 35 days had only 8% of the total biomass of plants grown in 35 Pa CO2, C3 plants had lower relative growth rates and lower specific leaf mass than plants grown in higher CO2 partial pressures, and aborted reproduction. C3 plants grown in 70 Pa CO2 had greater root mass and root-to-shoot ratios than plants grown in lower CO2 partial pressures. These findings, support other studies that show C3 plant growth is more responsive to CO2 partial pressure than C4 plant growth. Differences in growth responses to CO2 levels of the Pleistocene through the future suggest that competitive interactions of C3 and C4 annuals have changed through geologic time. This study also provided evidence that C3 annuals may be operating near a minimum CO2 partial pressure for growth and reproduction at 15 Pa CO2.

Notes: 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 CO2 partial pressures increased with increasing CO2 for C3 plants, but not C4 plants. Net photosynthesis of Abutilon at 15 Pa CO2 was 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 CO2 was 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 (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% in Abutilon grown at 15 Pa compared with 35 Pa CO2. Abutilon grown at 15 Pa CO2 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 CO2. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J max and 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 CO2 compared with 35 Pa CO2, suggesting that the rise in CO2 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 CO2 treatments. Differences in photosynthetic response to growth over a wide range of CO2 partial pressures suggest changes in the relative performance of C3 and C4 annuals as atmospheric CO2 has fluctuated over geologic time.