Relationships between CO(2) Exchange Rates and Activities of Pyruvate,Pi Dikinase and Ribulose Bisphosphate Carboxylase, Chlorophyll Concentration, and Cell Volume in Maize Leaves

The relationships between CO₂-exchange rate (CER), DNA and chlorophyll (Chl) concentrations, pyruvate,Pi dikinase (PPDK) and ribulose bisphosphate carboxylase (RuBPCase) activities in ten maize (Zea mays L.) genotypes were investigated. The in vivo degrees of activation of PPDK and RuBPCase were estimated to make meaningful comparisons with CER. In leaves at a photosynthetic photon flux density (PPFD) of 720 micromoles per square meter per second, in vivo PPDK degree of activation was 80% of that of PPDK fully activated in vitro, whereas RuBPCase could not be further activated in vitro, suggesting that RuBPCase was fully activated in vivo. CER varied about 50% among the genotypes tested. Significant genetic differences were observed for the average weight of a cell (estimated by gram fresh weight per milligram DNA), but this character was not correlated with CER expressed on a fresh weight basis. CER was correlated with Chl concentration, and with estimates of the in vivo degree of activation of PPDK and RuBPCase. We concluded that in maize, CER is controlled by the metabolic components of photosynthesis rather than by membrane resistances to CO₂. If the latter factor were controlling CER, then smaller cells with higher amounts of exposed cell surface area per unit cell volume would have lower resistance to CO₂ diffusion, and therefore higher CER. When data were expressed on a DNA basis (proportional to a per cell basis), results indicated that larger cells (i.e. those with higher fresh weight per milligram DNA) have a higher content of Chl, and higher PPDK and RuBPCase activities, resulting in higher CER than in smaller cells.     

Short-Term Effects of CO(2) on Gas Exchange of Leaves of Bigtooth Aspen (Populus grandidentata) in the Field

The short term effects of increased levels of CO₂ on gas exchange of leaves of bigtooth aspen (Populus grandidentata Michx.) were studied at the University of Michigan Biological Station, Pellston, MI. Leaf gas exchange was measured in situ in the upper half of the canopy, 12 to 14 meters above ground. In 1900 microliters per liter CO₂, maximum CO₂ exchange rate (CER) in saturating light was increased by 151% relative to CER in 320 microliters per liter CO₂. The temperature optimum for CER shifted from 25°C in 320 microliters per liter CO₂ to 37°C in 1900 microliters per liter CO₂. In saturating light, increasing CO₂ level over the range 60 to 1900 microliters per liter increased CER, decreased stomatal conductance, and increased leaf water use efficiency. The initial slope of the CO₂ response curve of CER was not significantly different at 20 and 30°C leaf temperatures, although the slope did decline significantly during leaf senescence. In 1900 microliters per liter CO₂, CER increased with increasing light. The light saturation point and maximum CER were higher in 30°C than in 20°C, although there was little effect of temperature in low light. The experimental results are consistent with patterns seen in laboratory studies of other C₃ species and define the parameters required by some models of aspen CER in the field.     

On the Relationships of Net CO(2) Assimilation and Leaf Expansion to Vegetative Growth in Lycospersicum esculentum, var Jubilee

Relationships between net plant CO₂ exchange rate (CER) and canopy development were examined in `jubilee' tomato over the initial 4 weeks of vegetative growth. A comparison was made between two plant groups that were alternatively exposed to 200 or 800 microeinsteins per square meter per second midday irradiation to establish a differential in net CER. Plants exposed to higher irradiation demonstrated a 2- to 4-fold greater net photosynthetic rate per leaf area and 100% average higher net CO₂ assimilation rate/plant· day. However, leaf-stem growth differed by <50% suggesting a poor relationship to CER. Leaf area growth rate (LAGR) of individual leaves appeared closely related to CER during initial leaf expansion but a greater function of order of emergence in successive leaf growth. LAGR on a per plant basis increased linearly with leaf dry weight but appeared more limited by factors determining maximum leaf enlargement and rate of new leaf development. Net CO₂ assimilation/leaf area and leaf starch consistently declined with time while net CO₂ assimilation plant/day approached a constant rate following 2 to 3 weeks growth. Composite results suggested a simple relationship for sucessive growth where accumulated leaf carbohydrate in excess of 200 milligrams/plant·day could be expected to be partitioned to other plant segments.     

The influence of carbon dioxide-depletion on growth and sinking rate of two planktonic diatoms in culture

Growth in relation to CO₂-depletion and CO₂-enrichment was investigated for the freshwater diatoms Asterionella formosa and Fragilaria crotonensis in batch cultures. Algal concentration and pH were measured during growth cycles, and inorganic carbon quantities determined by potentiometric Gran titrations and from pH-alkalinity relationships. After the primary growth with CO₂-depletion and pH increase, successive CO₂-enrichments induced further such cycles and produced a final three- to fivefold increase in algal biomass over that of unenriched controls. The extent of CO₂-depletion, and pH rise, was greater in later cycles, indicative of some cellular adaptation. Values of pH reached 9·7 for Asterionella and 9·9 for Fragilaria. The lowest residual quantities of free CO₂ were 0·1 and 0·03 μmol 1^-1 for Asterionella and Fragilaria respectively, which were less than 0·05% of the corresponding residual quantities of total CO₂. The primary limitation of CO₂-uptake and growth was probably related to the concentration of free CO₂, given the relative excess of other major nutrients (N, P, Si) in he media used. Limited of CO₂-uptake could be restored without CO₂ additions if the CO₂ present was redistributed between its several forms (increasing free CO₂) by the addition of strong acid, although growth was still restricted.

Limitation of CO₂-uptake, either by CO₂-depletion or the addition of an inhibitor of photo-synthesis (DCMU), increased the sinking rate of Asterionella cells from 0·3 to 1 m day^-1. The possible ecological implications of CO₂-pH-growth and CO₂-pH-buoyancy relationships are discussed, which may contribute to the frequent paucity of diatoms during summer in manv productive lakes.     

The Glycine-Glomus-Rhizobium Symbiosis : VII. Photosynthetic Nutrient-Use Efficiency in Nodulated, Mycorrhizal Soybeans

Four consecutive trifoliate leaves of 56-day-old symbiotic or nonsymbiotic soybean plants were evaluated individually for CO₂ exchange rates (CER), leaf area and dry weight, and leaf N, P, and starch concentrations. Plants had been inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae and Rhizobium japonicum, with either of the endophytes alone, or with neither at time of planting. Plants lacking one or both endophytes received N and/or P fertilizers to produce plants of equal total leaf dry weight in all four treatments. Photosynthetic P-use efficiency (CER per unit leaf P) was higher in the leaves of VAM plants than in P-fertilized plants regardless of the N source (N₂ fixation or combined N). Photosynthetic N-use efficiency was also higher in VAM than in non-VAM plants, but it was affected by the N source, with higher CER in the nodulated plants. The greatest differences in CER, starch accumulation and leaf area were found between the nonsymbiotic plants and those with both endophytes. Statistical evaluations of leaf parameters for treatment or nutrient concentration (N and P) effects between the tri-partite and the nonsymbiotic treatments showed significant changes in concentration of P, but not N, with decreasing leaf age. Both endophytes apparently enhance CO₂ fixation at N and/or P concentrations lower than those of the nonsymbiotic plants. The effects of the endophytes on CO₂ fixation were additive.     

Growth and growth estimation of Saccharomyces cerevisiae in solid-state ethanol fermentation

The anaerobic growth of a respiration-deficient mutant of Saccharomyces cerevisiae on solid medium was estimated by the CO₂ evolution rate (CER). The cell growth and ethanol production were calculated by a growth-model associated with CER. The estimated cell growth agreed with the observed data. The calculation and the observed CER suggested that the maximum ethanol production and maximum cell groth are restricted by the initial moisture content of the solid medium.     

Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature

Peanut (Arachis hypogaea L. cv. Florunner) was grown from seed sowing to plant maturity under two daytime CO₂ concentrations ([CO₂]) of 360 μmol mol⁻¹ (ambient) and 720 μmol mol⁻¹ (elevated) and at two temperatures of 1.5 and 6.0 °C above ambient temperature. The objectives were to characterize peanut leaf photosynthesis responses to long-term elevated growth [CO₂] and temperature, and to assess whether elevated [CO₂] regulated peanut leaf photosynthetic capacity, in terms of activity and protein content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), Rubisco photosynthetic efficiency, and carbohydrate metabolism. At both growth temperatures, leaves of plants grown under elevated [CO₂] had higher midday photosynthetic CO₂ exchange rate (CER), lower transpiration and stomatal conductance and higher water-use efficiency, compared to those of plants grown at ambient [CO₂]. Both activity and protein content of Rubisco, expressed on a leaf area basis, were reduced at elevated growth [CO₂]. Declines in Rubisco under elevated growth [CO₂] were 27–30% for initial activity, 5–12% for total activity, and 9–20% for protein content. Although Rubisco protein content and activity were down-regulated by elevated [CO₂], Rubisco photosynthetic efficiency, the ratio of midday light-saturated CER to Rubisco initial or total activity, of the elevated-[CO₂] plants was 1.3- to 1.9-fold greater than that of the ambient-[CO₂] plants at both growth temperatures. Leaf soluble sugars and starch of plants grown at elevated [CO₂] were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient [CO₂]. Under elevated [CO₂], leaf soluble sugars and starch, however, were not affected by high growth temperature. In contrast, high temperature reduced leaf soluble sugars and starch of the ambient-[CO₂] plants. Activity of sucrose-P synthase, but not adenosine 5′-diphosphoglucose pyrophosphorylase, was up-regulated under elevated growth [CO₂]. Thus, in the absence of other environmental stresses, peanut leaf photosynthesis would perform well under rising atmospheric [CO₂] and temperature as predicted for this century.     

Ploidy effects on anatomy and gas exchange of tall fescue leaves

A growth chamber study was designed to interpret differences in CO₂ exchange rate (CER) and leaf diffusive resistance among 4X, 6X, 8X, and 10X ploidy levels of tall fescue (Festuca arundinacea, Schreb). Mesophyll cell size, stomatal density, number of major and minor veins, and bundle cap size of leaf blades were evaluated. Diffusive resistance tended to decrease and CER to increase with increasing ploidy level. Mean stomatal density decreased from 43.6 per square millimeter to 30.6 per square millimeter as ploidy level increased from 4X to 8X. The 10X ploidy level exhibited the highest stomatal density, 47.2 per square millimeter. Major veins decreased from a mean of 14.2 to 10.2, and minor veins increased from a mean of 4.2 to 6.6, per leaf blade as ploidy increased from 4X to 10X. Total number of veins decreased significantly from a mean of 18.4 to 15.7 as ploidy increased from 4X to 8X.     

Correlations among leaf CO2-exchange rates,areas and enzyme activities among soybean cultivars

Soybean (Glycine max (L.) Merr.) genotypes varying in area per nodal unit (usually a trifoliolate) and maturity class were grown in plots at the University of Illinois experimental farm. Leaf CO₂-exchange rates per unit area (CER) were measured under sunlight on intact plants. In addition to previously reported correlations with specific leaf weight and chlorophyll, CER was positively correlated with ribulose bisphosphate carboxylase (RuBPcase) activity, specific activity, and soluble protein, and was negatively correlated with area per leaf unit. The CER: chlorophyll correlation was destroyed by high CER values in 2 chlorophyll-deficient lines. CER values for 27 of the 35 lines tested fell within the range of those for isolines of cultivar Clark varying in leaf characteristics. The CER values were highest for fully expanded leaves during rapid pod fill. These results suggested that photoperiod (maturity) genes and genes for leaf area growth interact with genes controlling photosynthetic CO₂-exchange to produce the major differences in CER values among soybean genotypes.     

Gradients of Intercellular CO(2) Levels Across the Leaf Mesophyll

Most current photosynthesis models, and interpretations of many wholeleaf CO₂ gas exchange measurements, are based on the often unstated assumption that the partial pressure of CO₂ is nearly uniform throughout the airspaces of the leaf mesophyll. Here we present measurements of CO₂ gradients across amphistomatous leaves allowed to assimilate CO₂ through only one surface, thus simulating hypostomatous leaves. We studied five species: Eucalyptus pauciflora Sieb. ex Spreng., Brassica chinensis L., Gossypium hirsutum L., Phaseolus vulgaris L., and Spinacia oleracea L. For Eucalyptus, maximum CO₂ pressure differences across the leaf mesophyll were 73 and 160 microbar when the pressures outside the lower leaf surface were 310 and 590 microbar, respectively. Using an approximate theoretical calculation, we infer that if the CO₂ had been supplied equally at both surfaces then the respective mean intercellular CO₂ pressures would have been roughly 12 and 27 microbar less than the pressures in the substomatal cavities in these cases. For ambient CO₂ pressures near 320 microbar, the average and minimum pressure differences across the mesophyll were 45 and 13 microbar. The corresponding mean intercellular CO₂ pressures would then be roughly 8 and 2 microbar less than those in the substomatal cavities. Pressure differences were generally smaller for the four agricultural species than for Eucalyptus, but they were nevertheless larger than previously reported values.     

Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery

Greenhouse-grown pigeonpea (Cajanus cajan, [L.] Millsp.; cultivar UW-10) and cowpea (Vigna unguiculata, [L.] Walp.; cultivar California No. 5) were well-watered (control) or subjected to low water potential by withholding water to compare their modes of adaptation to water-limited conditions. Leaf CO₂ exchange rate (CER), leaf diffusive conductance to CO₂ (g_l), and CO₂ concentration in the leaf intercellular air space (C_i) were determined at various CO₂ concentrations and photon flux densities (PFD) of photosynthetically active radiation (400 to 700 nanometer). In cowpea, g_l declined to less than 15% of controls and total water potential (ψ_w) at midafternoon declined to −0.8 megapascal after 5 days of withholding water, whereas g_l in pigeonpea was about 40% of controls even though midafternoon ψ_w was −1.9 megapascal. After 8 days of withholding water, ψ_w at midafternoon declined to −0.9 and −2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (ψ_s) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO₂ concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO₂ concentration in bulk air of 360 microliters per liter, the CER of cowpea had fully recovered to control levels 3 days after rewatering; however, at 970 microliters per liter the PFD-saturated CERs of both species were substantially lower than in controls, indicating residual impairment. In stressed plants of both species the CER responses to C_i from 250 to 600 microliters per liter indicated that a substantial nonstomatal inhibition of CER had occurred. Although the sensitivity of g_l to water limitation in cowpea suggested a dehydration avoidance response, parallel measurements of CER at various C_i and PFD indicated that photosynthetic activity of cowpea mesophyll was substantially inhibited by the water-limited treatment.     

Effect of changes in shoot carbon-exchange rate on soybean root nodule activity

The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean (Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N₂ fixation. Shoot CER, root + nodule respiration, and apparent N₂ fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N₂ fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N₂ fixation was measured within 4 hours, and decreasing CO₂ concentration from 320 to 90 microliters CO₂ per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO₂ concentration around the shoot from 320 to 1,000 microliters CO₂ per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO₂-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO₂ enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N₂ fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N₂-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known.     

Drought Stress and Elevated CO(2) Effects on Soybean Ribulose Bisphosphate Carboxylase Activity and Canopy Photosynthetic Rates

Soybean (Glycine max [L.] cv Bragg) was grown at 330 or 660 microliters CO₂ per liter in outdoor, controlled-environment chambers. When the plants were 50 days old, drought stress was imposed by gradually reducing irrigation each evening so that plants wilted earlier each succeeding day. On the ninth day, as the pots ran out of water CO₂ exchange rate (CER) decreased rapidly to near zero for the remainder of the day. Both CO₂-enrichment and drought stress reduced the total (HCO₃⁻/Mg²⁺-activated) extractable ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity, as expressed on a chlorophyll basis. In addition, drought stress when canopy CER values and leaf water potentials were lowest, reduced the initial (nonactivated) RuBPCase activity by 50% compared to the corresponding unstressed treatments. This suggests that moderate to severe drought stress reduces the in vivo activation state of RuBPCase, as well as lowers the total activity. It is hypothesized that stromal acidification under drought stress causes the lowered initial RuBPCase activities. The K_m(CO₂) values of activated RuBPCase from stressed and unstressed plants were similar; 15.0 and 12.6 micromolar, respectively. RuBP levels were 10 to 30% lower in drought stressed as compared to unstressed treatments. However, RuBP levels increased from near zero at night to around 150 to 200 nanomoles per milligram chlorophyll during the day, even as water potentials and canopy CERs decreased. This suggests that the rapid decline in canopy CER cannot be attributed to drought stress induced limitations in the RuBP regeneration capability. Thus, in soybean leaves, a nonstomatal limitation of leaf photosynthesis under drought stress conditions appears due, in part, to a reduction of the in vivo activity of RuBPCase. Because initial RuBPCase activities were not reduced as much as canopy CER values, this enzymic effect does not explain entirely the response of soybean photosynthesis to drought stress.     

Evidence for Light-Stimulated Fatty Acid Synthesis in Soybean Fruit

In leaves, the light reactions of photosynthesis support fatty acid synthesis but disagreement exists as to whether this occurs in green oilseeds. To address this question, simultaneous measurements of the rates of CO₂ and O₂ exchange (CER and OER, respectively) were made in soybean (Glycine max L.) fruits. The imbalance between CER and OER was used to estimate the diverted reductant utilization rate (DRUR) in the equation: DRUR = 4 × (OER + CER). This yielded a quantitative measure of the rate of synthesis of biomass that is more reduced per unit carbon than glucose (in photosynthesizing tissues) or than the substrates of metabolism (in respiring tissues). The DRUR increased by about 2.2-fold when fruits were illuminated due to a greater increase in OER than decrease in CER. This characteristic was shown to be a property of the seed (not the pod wall), to be present in fruits at all developmental stages, and to reach a maximal response at relatively low light. When seeds were provided with ¹³CO₂, light reduced ¹²CO₂ production but had little effect on ¹³CO₂ fixation. When they were provided with ¹⁸O₂, light stimulated ¹⁶O₂ production but had no effect on ¹⁸O₂ uptake. Together, these findings indicate that light stimulates fatty acid synthesis in photosynthetic oilseeds, probably by providing both ATP and carbon skeletons.     

Effects of Water Stress on Photosynthesis and Carbon Partitioning in Soybean (Glycine max [L.] Merr.) Plants Grown in the Field at Different CO(2) Levels

The effects of water stress and CO₂ enrichment on photosynthesis, assimilate export, and sucrose-P synthase activity were examined in field grown soybean plants. In general, leaves of plants grown in CO₂-enriched atmospheres (300 microliters per liter above unenriched control, which was 349 ± 12 microliters per liter between 0500 and 1900 hours EST over the entire season) had higher carbon exchange rates (CER) compared to plants grown at ambient CO₂, but similar rates of export and similar activities of sucrose-P synthase. On most sample dates, essentially all of the extra carbon fixed as a result of CO₂ enrichment was partitioned into starch. CO₂-enriched plants had lower transpiration rates and therefore had a higher water use efficiency (milligrams CO₂ fixed per gram H₂O transpired) per unit leaf area compared to nonenriched plants. Water stress reduced CER in nonenriched plants to a greater extent than in CO₂-enriched plants. As CER declined, stomatal resistance increased, but this was not the primary cause of the decrease in assimilation because internal CO₂ concentration remained relatively constant. Export of assimilates was less affected by water stress than was CER. When CERs were low as a result of the imposed stress, export was supported by mobilization of reserves (mainly starch). Export rate and leaf sucrose concentration were related in a curvilinear manner. When sucrose concentration was above about 12 milligrams per square decimeter, obtained with nonstressed plants at high CO₂, there was no significant increase in export rate. Assimilate export rate was also correlated positively with SPS activity and the quantitative relationship varied with CER. Thus, export rate was a function of both CER and carbon partitioning.     

Leaf photosynthesis and carbohydrates of CO₂-enriched maize and grain sorghum exposed to a short period of soil water deficit during vegetative development

Among C₄ species, sorghum is known to be more drought tolerant than maize. The objective was to evaluate differences in leaf gas exchanges, carbohydrates, and two enzyme activities of these nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ subtype monocots in response to water deficit and CO₂ concentration ([CO₂]). Maize and sorghum were grown in pots in sunlit environmental-controlled chambers. Treatments included well watered (WW) and water stressed (WS) (water withheld at 26 days) and daytime [CO₂] of 360 (ambient) and 720 (elevated) μmol mol⁻¹. Midday gas exchange rates, concentrations of nonstructural carbohydrates, and activities of sucrose-phosphate synthase (SPS) and adenosine 5′-diphosphoglucose pyrophosphorylase (ADGP) were determined for fully expanded leaf sections. There was no difference in leaf CO₂ exchange rates (CER) between ambient and elevated [CO₂] control plants for both maize and sorghum. After withholding water, leaf CER declined to zero after 8 days in maize and 10 days for sorghum. Sorghum had lower stomatal conductance and transpiration rates than maize, which resulted in a longer period of CER under drought. Nonstructural carbohydrates of both control maize and sorghum were hardly affected by elevated [CO₂]. Under drought, however, increases in soluble sugars and decreases in starch were generally observed for maize and sorghum at both [CO₂] levels. For stressed maize and sorghum, decreases in starch occurred earlier and were greater at ambient [CO₂] than at elevated [CO₂]. For maize, drought did not meaningfully affect SPS activity. However, a decline in SPS activity was observed for drought-stressed sorghum under both [CO₂] treatments. There was an increase in ADGP activity in maize under drought for both [CO₂] treatments. Such a response in ADGP to drought, however, did not occur for sorghum. The generally more rapid response of maize than sorghum to drought might be related to the more rapid growth of leaf area of maize.     

An alpine treeline in a carbon dioxide-rich world: synthesis of a nine-year free-air carbon dioxide enrichment study

We evaluated the impacts of elevated CO₂ in a treeline ecosystem in the Swiss Alps in a 9-year free-air CO₂ enrichment (FACE) study. We present new data and synthesize plant and soil results from the entire experimental period. Light-saturated photosynthesis (A _max) of ca. 35-year-old Larix decidua and Pinus uncinata was stimulated by elevated CO₂ throughout the experiment. Slight down-regulation of photosynthesis in Pinus was consistent with starch accumulation in needle tissue. Above-ground growth responses differed between tree species, with a 33 % mean annual stimulation in Larix but no response in Pinus. Species-specific CO₂ responses also occurred for abundant dwarf shrub species in the understorey, where Vaccinium myrtillus showed a sustained shoot growth enhancement (+11 %) that was not apparent for Vaccinium gaultherioides or Empetrum hermaphroditum. Below ground, CO₂ enrichment did not stimulate fine root or mycorrhizal mycelium growth, but increased CO₂ effluxes from the soil (+24 %) indicated that enhanced C assimilation was partially offset by greater respiratory losses. The dissolved organic C (DOC) concentration in soil solutions was consistently higher under elevated CO₂ (+14 %), suggesting accelerated soil organic matter turnover. CO₂ enrichment hardly affected the C–N balance in plants and soil, with unaltered soil total or mineral N concentrations and little impact on plant leaf N concentration or the stable N isotope ratio. Sustained differences in plant species growth responses suggest future shifts in species composition with atmospheric change. Consistently increased C fixation, soil respiration and DOC production over 9 years of CO₂ enrichment provide clear evidence for accelerated C cycling with no apparent consequences on the N cycle in this treeline ecosystem.     

Effects of Atmospheric CO(2) Enrichment on Photosynthesis, Respiration, and Growth of Sour Orange Trees

Numerous net photosynthetic and dark respiratory measurements were made over a period of 4 years on leaves of 24 sour orange (Citrus aurantium) trees; 8 of them growing in ambient air at a mean CO₂ concentration of 400 microliters per liter, and 16 growing in air enriched with CO₂ to concentrations approaching 1000 microliters per liter. Over this CO₂ concentration range, net photosynthesis increased linearly with CO₂ by more than 200%, whereas dark respiration decreased linearly to only 20% of its initial value. These results, together with those of a comprehensive fine-root biomass determination and two independent aboveground trunk and branch volume inventories, suggest that a doubling of the air's current mean CO₂ concentration of 360 microliters per liter would enhance the growth of the trees by a factor of 3.8.     

Variation in photosynthetic components among photosynthetically diverse cotton genotypes

Photosynthesis is an important component of upland cotton (Gossypium hirsutum L.) yield, but little has been done to increase the photosynthetic performance within the cotton germplasm pool. Part of this dilemma is due to the multi-component aspect of this process and also to lack of information on genetic variation among such components. The objectives of this research were to identify genetic variability in photosynthetic components for six cotton genotypes previously shown to differ in leaf CO₂-exchange rates (CER) and to determine if an afternoon decline in photosynthesis altered genotypic differences in CER. CO₂-exchange rates were measured at several internal CO₂ levels (C_i) to generate CER vs. C_i curves for each genotype and thereby isolate some of the components of photosynthesis. Ribulose 1,5 bisphosphate carboxylase-oxygenase (Rubisco), hydroxypyruvate reductase, malate dehydrogenase, and catalase activities were assayed on leaves used to generate the CER vs. C_i curves. Ambient CER and chlorophyll (Chl) fluorescence measurements were taken before and after solar noon to test for an afternoon decline in photosynthesis. Dixie King, a low ambient CER genotype, exhibited a greater CO₂ compensation point, lower carboxylation efficiency, and reduced Photosystem II (PS II) activity than the other genotypes. The carboxylation efficiency of DES 119 was 13% greater than STV 508 and 29% greater than Dixie King, but not different from the other genotypes in 1994. Pee Dee 3 had greater maximum assimilation rate (A) than all other genotypes except STV 213 in 1993. Although no significant genotype by time of day interaction was detected, CER and Chl fluorescence variable to maximum ratio (Fv/Fm) were reduced 8% and 39%, respectively, in the afternoon as compared to the morning. This study demonstrates genetic variations in many of the components of photosynthesis. However, the narrow range of variation in such components for superior photosynthesizing genotypes explains why difficulties are encountered when breeding for increased photosynthesis.