Library

Notes: 
ATP, adenosine triphosphate
Km, Michaelis-Menton coefficient
Ca, concentration of CO2 in the air (μmol mol–1)
NAD, oxidized nicotin adenine dinucleotide
NADH, reduced nicotin adenine dinucleotide
NADP, oxidized nicotin adenine phosphate dinucleotide
NADPH, reduced nicotine adenine phosphate dinucleotide
R, rate of respiration per unit DW [μmol g 
DW–1], Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
Vc,max, maximum in vivo rate of carboxylation at Rubisco (μmol m–2 s–1)

There is abundant evidence that a reduction in mitochondrial respiration of plants occurs when atmospheric CO2 (Ca) is increased. Recent reviews suggest that doubling the present Ca will reduce the respiration rate [per unit dry weight (DW)] by 15 to 18%. The effect has two components: an immediate, reversible effect observed in leaves, stems, and roots of plants as well as soil microbes, and an irreversible effect which occurs as a consequence of growth in elevated Ca and appears to be specific to C3 species. The direct effect has been correlated with inhibition of certain respiratory enzymes, namely cytochrome-c-oxidase and succinate dehydrogenase, and the indirect or acclimation effect may be related to changes in tissue composition. Although no satisfactory mechanisms to explain these effects have been demonstrated, plausible mechanisms have been proposed and await experimental testing. These are carbamylation of proteins and direct inhibition of enzymes of respiration. A reduction of foliar respiration of 15% by doubling present ambient Ca would represent 3 Gt of carbon per annum in the global carbon budget.

Notes: Stands of Scirpus olneyi, a native saltmarsh sedge with C3 photosynthesis, had been exposed to normal ambient and elevated atmospheric CO2 concentrations (Ca) in their native habitat since 1987. The objective of this investigation was to characterize the acclimation of photosynthesis of Scirpus olneyi stems, the photosynthesizing organs of this species, to long-term elevated Ca treatment in relation to the concentrations of Rubisco and non-structural carbohydrates. Measurements were made on intact stems in the Held under existing natural conditions and in the laboratory under controlled conditions on stems excised in the field early in the morning. Plants grown at elevated Ca had a significantly higher (30–59%) net CO2 assimilation rate (A) than those grown at ambient Ca when measurements were performed on excised stems at the respective growth Ca. However, when measurements were made at normal ambient Ca, A was smaller (45–53%) in plants grown at elevated Ca than in those grown at ambient Ca. The reductions in A at normal ambient Ca, carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30–58%) in plants grown at elevated Ca; these plants also contained less soluble protein (39–52%). The Rubisco content was 43 to 58% of soluble protein, and this relationship was not significantly altered by the growth CO2 concentrations. The Rubisco activation state increased slightly, but the in situ carboxylase activity decreased substantially in plants grown at elevated Ca. When measurements were made on intact stems in the field, the elevated Ca treatment caused a greater stimulation of,A (100%) and a smaller reduction in carboxylation efficiency (which was not statistically significant) than when measurements were made on excised stems in the laboratory. The possible reasons for this arc discussed.Plants grown at elevated Ca contained more non-structural carbohydrates (25–53%) than those grown at ambient Ca. Plants grown at elevated Ca appear to have sufficient sink capacity to utilize the additional carbohydrates formed during photosynthesis.Overall, our results are in agreement with the hypothesis that elevated Ca leads to an increased carbohydrate concentration and the ensuing acclimation of the photo-synthetic apparatus in C3 plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated Ca, relative to plants grown at normal ambient Ca. Nevertheless, when compared at their respective growth Ca, Scirpus olneyi plants grown at elevated Ca in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient Ca even after 8 years of growth at elevated Ca.

Notes: Abstract. While a short-term exposure to elevated atmospheric CO2 induces a large increase in photosynthesis in many plants, long-term growth in elevated CO2 often results in a smaller increase due to reduced photosynthetic capacity. In this study, it was shown that, for a wild C3 species growing in its natural environment and exposed to elevated CO2 for four growing seasons, the photosynthetic capacity has actually increased by 31%. An increase in photosynthetic capacity has been observed in other species growing in the field, which suggests that photosynthesis of certain field grown plants will continue to respond to elevated levels of atmospheric CO2

Notes: In this study, the response of N2 fixation to elevated CO2 was measured in Scirpus olneyi, a C3 sedge, and Spartina patens, a C4 grass, using acetylene reduction assay and 15N2 gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO2 significantly (P〈 0·05) caused a 35% increase in nitrogenase activity and 73% increase in 15N incorporated by Scirpus olneyi. In Spartina patens, elevated CO2 (660 ± 1 μmol mol−1) increased nitrogenase activity and 15N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N2 fixation in Scirpus olneyi under elevated CO2 was 611 ± 75 ng 15N fixed plant−1 h−1 compared with 367 ± 46 ng 15N fixed plant−1 h−1 in ambient CO2 plants. In Spartina patens, however, the rate of N2 fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng 15N fixed plant−1 h−1 for elevated and ambient CO2, respectively. Heterotrophic non-symbiotic N2 fixation in plant-free marsh sediment also increased significantly (P〈 0·05) with elevated CO2. The proportional increase in 15N2 fixation correlated with the relative stimulation of photosynthesis, in that N2 fixation was high in the C3 plant in which photosynthesis was also high, and lower in the C4 plant in which photosynthesis was relatively less stimulated by growth in elevated CO2. These results are consistent with the hypothesis that carbon fixation in C3 species, stimulated by rising CO2, is likely to provide additional carbon to endophytic and below-ground microbial processes.

Notes: The effects of elevated atmospheric CO2 concentration on plant-fungi and plant-insect interactions were studied in an emergent marsh in the Chesapeake Bay. Stands of the C3 sedge Scirpus olneyi Grey, and the C4 grass Spartina patens (Ait.) Muhl. have been exposed to elevated atmospheric CO2 concentrations during each growing season since 1987. In August 1991 the severities of fungal infections and insect infestations were quantified. Shoot nitrogen concentration ([N]) and water content (WC) were determined. In elevated concentrations of atmospheric CO2, 32% fewer S. olneyi plants were infested by insects, and there was a 37% reduction in the severity of a pathogenic fungal infection, compared with plants grown in ambient CO2 concentrations. S. olneyi also had reduced [N], which correlated positively with the severities of fungal infections and insect infestations. Conversely, S. patens had increased WC but unchanged [N] in elevated concentrations of atmospheric CO2 and the severity of fungal infection increased. Elevated atmospheric CO2 concentration increased or decreased the severity of fungal infection depending on at least two interacting factors, [N] and WC; but it did not change the number of plants that were infected with fungi. In contrast, the major results for insects were that the number of plants infected with insects decreased, and that the amount of tissue that each insect ate also decreased.

Notes: Native scrub-oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO2] (approx. 350 μmol mol−1) and to high [CO2] (increased by 350 μmol mol−1). Stomatal and photosynthetic acclimation to high [CO2] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (gs) was approximately 40% lower in the high- compared to low-[CO2]-grown plants when measured at their respective growth concentrations. Reciprocal measurements of gs in both high- and low-[CO2]-grown plants showed that there was negative acclimation in the high-[CO2]-grown plants (9–16% reduction in gs when measured at 700 μmol mol−1), but these were small compared to those for net CO2 assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO2] (ci) which showed a reduction in stomatal sensitivity at low ci in the high-[CO2]-grown plants. Stomatal density showed no change in response to growth in high growth [CO2]. Long-term stomatal and photosynthetic acclimation to growth in high [CO2] did not markedly change the 2·5- to 3-fold increase in gas-exchange-derived water use efficiency caused by high [CO2].

Notes: Abstract. There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO2: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO2 increased canopy photosynthesis in all cases. The relative effect of CO2 was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO2 were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO2 treatment to date, monospecific stands of a C3 sedge, Scirpus olneyi (Grey), and a C4 grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO2 concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO2 exchange per unit green biomass (NCEb) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO2 increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.

Notes: Over a large part of the photoperiod, light energy absorbed by upper canopy leaves saturates photosynthesis and exceeds the energetic requirements for light-saturated linear electron flow through photosystem II (JPSII), so that photoinhibition results. From a theoretical consideration of the response of light-saturated photosynthesis to elevated atmospheric CO2 partial pressure (pCO2) it may be predicted that, where light-saturated photosynthesis is Rubisco-limited, an increase in pCO2 will stimulate JPSII. Therefore, the proportion of absorbed quanta dissipated photochemically will increase and the potential for photoinhibition of photosynthesis will decrease. This was tested by measuring modulated chlorophyll a fluorescence from Quercus myrtifolia Willd. growing in the field in open-top chambers, at either current ambient or elevated (ambient + 35 Pa) pCO2 on Merritt Island, Florida, USA. During spring and summer, light-saturated photosynthesis at current ambient pCO2 was Rubisco-limited. Consistent with theoretical prediction, JPSII was increased and photoinhibition decreased by elevated pCO2 in spring. In the summer, when growth had largely ceased, an acclimatory decrease in the maximum Ribulose 1,5 bisphosphate saturated carboxylation capacity (Vc max) removed the stimulation of JPSII seen in the spring, and photoinhibition was increased in elevated pCO2. It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated pCO2 on JPSII and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated pCO2 in a predictable manner.

Notes: Elevated atmospheric carbon dioxide (Ca) usually reduces stomatal conductance, but the effects on plant transpiration in the field are not well understood. Using constant-power sap flow gauges, we measured transpiration from Quercus myrtifolia Willd., the dominant species of the Florida scrub-oak ecosystem, which had been exposed in situ to elevated Ca (350 µmol mol−1 above ambient) in open-top chambers since May 1996. Elevated Ca reduced average transpiration per unit leaf area by 37%, 48% and 49% in March, May and October 2000, respectively. Temporarily reversing the Ca treatments showed that at least part of the reduction in transpiration was an immediate, reversible response to elevated Ca. However, there was also an apparent indirect effect of Ca on transpiration: when transpiration in all plants was measured under common Ca, transpiration in elevated Ca-grown plants was lower than that in plants grown in normal ambient Ca. Results from measurements of stomatal conductance (gs), leaf area index (LAI), canopy light interception and correlation between light and gs indicated that the direct, reversible Ca effect on transpiration was due to changes in gs caused by Ca, and the indirect effect was caused mainly by greater self-shading resulting from enhanced LAI, not from stomatal acclimation. By reducing light penetration through the canopy, the enhanced self-shading at elevated Ca decreased stomatal conductance and transpiration of leaves at the middle and bottom of canopy. This self-shading mechanism is likely to be important in ecosystems where LAI increases in response to elevated Ca.

Notes: Leaf conductance often decreases in response to elevated atmospheric CO2 concentration (Ca) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated Ca during an eight-month period two years after Ca manipulation began. Whole-chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated Ca, despite an increase in leaf area index (LAI). Elevated Ca also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated Ca in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated Ca.