Short-Term and Long-Term Responses of Crassulacean Acid Metabolism Plants to Elevated CO(2)

For the leaf succulent Agave deserti and the stem succulent Ferocactus acanthodes, increasing the ambient CO₂ level from 350 microliters per liter to 650 microliters per liter immediately increased daytime net CO₂ uptake about 30% while leaving nighttime net CO₂ uptake of these Crassulacean acid metabolism (CAM) plants approximately unchanged. A similar enhancement of about 30% was found in dry weight gain over 1 year when the plants were grown at 650 microliters CO₂ per liter compared with 350 microliters per liter. Based on these results plus those at 500 microliters per liter, net CO₂ uptake over 24-hour periods and dry weight productivity of these two CAM succulents is predicted to increase an average of about 1% for each 10 microliters per liter rise in ambient CO₂ level up to 650 microliters per liter.     

Acclimation to High CO(2) in Bean : Carbonic Anhydrase and Ribulose Bisphosphate Carboxylase

Young bean plants (Phaseolus vulgaris L. cv Seafarer) grew faster in air enriched with CO₂ (1200 microliters per liter) than in ambient CO₂ (330 microliters per liter). However, by 7 days when increases in overall growth (dry weight, leaf area) were visible, there was a significant decline (about 25%) in the leaf mineral content (N, P, K, Ca, Mg) and a drop in the activity of two enzymes of carbon fixation, carbonic anhydrase and ribulose 1,5-bisphosphate (RuBP) carboxylase under high CO₂. Although the activity of neither enzyme was altered in young, expanding leaves during the acclimation period, in mature leaves the activity of carbonic anhydrase was reduced 95% compared with a decline of 50% in ambient CO₂. The drop in RuBP carboxylase was less extreme with 40% of the initial activity retained in the high CO₂ compared with 50% in the ambient atmosphere. While CO₂ enrichment might alter the flow of carbon into the glycolate pathway by modifying the activities of carbonic anhydrase or RuBP carboxylase, there is no early change in the ability of photosynthetic tissue to oxidize glycolate to CO₂.     

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.     

Growth Kinetics, Carbohydrate, and Leaf Phosphate Content of Clover (Trifolium subterraneum L.) after Transfer to a High CO(2) Atmosphere or to High Light and Ambient Air

Intact air-grown (photosynthetic photon flux density, 400 microeinsteins per square meter per second) clover plants (Trifolium subterraneum L.) were transfered to high CO₂ (4000 microliters CO₂ per liter; photosynthetic photon flux density, 400 microeinsteins per square meter per second) or to high light (340 microliters CO₂ per liter; photosynthetic photon flux density, 800 microeinsteins per square meter per second) to similarly stimulate photosynthetic net CO₂ uptake. The daily increment of net CO₂ uptake declined transiently in high CO₂, but not in high light, below the values in air/standard light. After about 3 days in high CO₂, the daily increment of net CO₂ uptake increased but did not reach the high light values. Nightly CO₂ release increased immediately in high light, whereas there was a 3-day lag phase in high CO₂. During this time, starch accumulated to a high level, and leaf deterioration was observed only in high CO₂. After 12 days, starch was two- to threefold higher in high CO₂ than in high light, whereas sucrose was similar. Leaf carbohydrates were determined during the first and fourth day in high CO₂. Starch increased rapidly throughout the day. Early in the day, sucrose was low and similar in high CO₂ and ambient air (same light). Later, sucrose increased considerably in high CO₂. The findings that (a) much more photosynthetic carbon was partitioned into the leaf starch pool in high CO₂ than in high light, although net CO₂ uptake was similar, and that (b) rapid starch formation occurred in high CO₂ even when leaf sucrose was only slightly elevated suggest that low sink capacity was not the main constraint in high CO₂. It is proposed that carbon partitioning between starch (chloroplast) and sucrose (cytosol) was perturbed by high CO₂ because of the lack of photorespiration. Total phosphate pools were determined in leaves. Concentrations based on fresh weight of orthophosphate, soluble esterified phosphate, and total phosphate markedly declined during 13 days of exposure of the plants to high CO₂ but changed little in high light/ambient air. During this time, the ratio of orthophosphate to soluble esterified phosphate decreased considerably in high CO₂ and increased slightly in high light/ambient air. It appears that phosphate uptake and growth were similarly stimulated by high light, whereas the coordination was weak in high CO₂.     

Effects of CO(2) Concentration on Rubisco Activity, Amount, and Photosynthesis in Soybean Leaves

Growth at an elevated CO₂ concentration resulted in an enhanced capacity for soybean (Glycine max L. Merr. cv Bragg) leaflet photosynthesis. Plants were grown from seed in outdoor controlled-environment chambers under natural solar irradiance. Photosynthetic rates, measured during the seed filling stage, were up to 150% greater with leaflets grown at 660 compared to 330 microliters of CO₂ per liter when measured across a range of intercellular CO₂ concentrations and irradiance. Soybean plants grown at elevated CO₂ concentrations had heavier pod weights per plant, 44% heavier with 660 compared to 330 microliters of CO₂ per liter grown plants, and also greater specific leaf weights. Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activity showed no response (mean activity of 96 micromoles of CO₂ per square meter per second expressed on a leaflet area basis) to short-term (~1 hour) exposures to a range of CO₂ concentrations (110-880 microliters per liter), nor was a response of activity (mean activity of 1.01 micromoles of CO₂ per minute per milligram of protein) to growth CO₂ concentration (160-990 microliters per liter) observed. The amount of rubisco protein was constant, as growth CO₂ concentration was varied, and averaged 55% of the total leaflet soluble protein. Although CO₂ is required for activation of rubisco, results indicated that within the range of CO₂ concentrations used (110-990 microliters per liter), rubisco activity in soybean leaflets, in the light, was not regulated by CO₂.     

Acclimation of Two Tomato Species to High Atmospheric CO(2): II. Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase and Phosphoenolpyruvate Carboxylase

Lycopersicon esculentum Mill. cv Vedettos and Lycopersicon chmielewskii Rick, LA 1028, were exposed to two CO₂ concentrations (330 or 900 microliters per liter) for 10 weeks. The elevated CO₂ concentrations increased the initial ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity of both species for the first 5 weeks of treatment but the difference did not persist during the last 5 weeks. The activity of Mg²⁺-CO₂-activated Rubisco was higher in 900 microliters per liter for the first 2 weeks but declined sharply thereafter. After 10 weeks, leaves grown at 330 microliters per liter CO₂ had about twice the Rubisco activity compared with those grown at 900 microliters per liter CO₂. The two species showed the same trend to Rubisco declines under high CO₂ concentrations. The percent activation of Rubisco was always higher under high CO₂. The phosphoenolpyruvate carboxylase (PEPCase) activity measured in tomato leaves averaged 7.9% of the total Rubisco. PEPCase showed a similar trend with time as the initial Rubisco but with no significant difference between nonenriched and CO₂-enriched plants. Long-term exposure of tomato plants to high CO₂ was previously shown to induce a decline of photosynthetic efficiency. Based on the current study and on previous results, we propose that the decline of activated Rubisco is the main cause of the acclimation of tomato plants to high CO₂ concentrations.     

Reversibility of Photosynthetic Inhibition in Cotton after Long-Term Exposure to Elevated CO(2) Concentrations

Cotton (Gossypium hirsutum L. cv Stoneville 213) was grown at 350 and 1000 microliters per liter CO₂. The plants grown at elevated CO₂ concentrations contained large starch pools and showed initial symptoms of visible physical damage. Photosynthetic rates were lower than expected based on instantaneous exposure to high CO₂.

A group of plants grown at 1000 microliters per liter CO₂ was switched to 350 microliters per liter CO₂. Starch pools and photosynthetic rates were monitored in the switched plants and in the two unswitched control groups. Photosynthetic rates per unit leaf area recovered to the level of the 350 microliters per liter CO₂ grown control group within four to five days. To assess only nonstomatal limitations to photosynthesis, a measure of photosynthetic efficiencies was calculated (moles CO₂ fixed per square meter per second per mole intercellular CO₂). Photosynthetic efficiency also recovered to the levels of the 350 microliters per liter CO₂ grown controls within three to four days.

Recovery was correlated to a rapid depletion of the starch pool, indicating that the inhibition of photosynthesis is primarily a result of feedback inhibition. However, complete recovery may involve the repair of damage to the chloroplasts caused by excessive starch accumulation. The rapid and complete reversal of photosynthetic inhibition suggests that the appearance of large, strong sinks at certain developmental stages could result in reduction of the large starch accumulations and that photosynthetic rates could recover to near the theoretical capacity during periods of high photosynthate demand.

     

Prechilling of Xanthium strumarium L. Reduces Net Photosynthesis and, Independently, Stomatal Conductance, While Sensitizing the Stomata to CO(2)

Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO₂ exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO₂ concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO₂ concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO₂ concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO₂ concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO₂; they responded to changes in CO₂ concentration in the range from 100 to 1000 microliters per liter.     

Acclimation to High CO(2) in Monoecious Cucumbers : I. Vegetative and Reproductive Growth

CO₂ concentrations of 1000 compared to 350 microliters per liter in controlled environment chambers did not increase total fruit weight or number in a monoecious cucumber (Cucumis sativus L. cv Chipper) nor did it increase biomass, leaf area, or relative growth rates beyond the first 16 days after seeding. Average fruit weight was slightly, but not significantly greater in the 1000 microliters per liter CO₂ treatment because fruit numbers were changed more than total weight. Plants grown at 1000 and 350 microliters per liter CO₂ were similar in distribution of dry matter and leaf area between mainstem, axillary, and subaxillary branches. Early flower production was greater in 1000 microliters per liter plants. Subsequent flower numbers were either lower in enriched plants or similar in the two treatments, except for the harvest at fruiting when enriched plants produced many more male flowers than the 350 microliters per liter treatments.     

Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2

Few studies have investigated the effects of elevated CO₂ on the physiology of symbiotic N₂-fixing trees. Tree species grown in low N soils at elevated CO₂ generally show a decline in photosynthetic capacity over time relative to ambient CO₂ controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO₂ on net CO₂ assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO₂, A was always greater in elevated compared to ambient CO₂ plants. Late season A vs. internal leaf p(CO₂) response curves indicated no negative adjustment of photosynthesis in elevated CO₂ plants. Rather, elevated CO₂ plants had 16% greater maximum rate of CO₂ fixation by Rubisco. Leaf dark respiration was greater at elevated CO₂ on an area basis, but unaffected by CO₂ on a mass or N basis. In elevated CO₂ plants, leaf N content (μg N cm^?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO₂ plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO₂ plants. Our results suggest that N₂-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO₂ on N-poor soils.     

Effects of High Atmospheric CO(2) and Sink Size on Rates of Photosynthesis of a Soybean Cultivar

The effect of sink strength on photosynthetic rates under conditions of long-term exposure to high CO₂ has been investigated in soybean. Soybean plants (Merr. cv. Fiskeby V) were grown in growth chambers containing 350 microliters CO₂ per liter air until pod set. At that time, plants were trimmed to three trifoliolate leaves and either 21 pods (high sink treatment) or 6 pods (low sink treatment). Trimmed plants were either left in 350 microliters CO₂ per liter of air or placed in 1000 microliters CO₂ per liter of air (high CO₂ treatment) until pod maturity. Whole plant net photosynthetic rates of all plants were measured twice weekly, both at 350 microliters CO₂ per liter of air and 1000 microliters CO₂ per liter of air. Plants were also harvested at this time for dry weight measurements. Photosynthetic rates of high sink plants at both measurement CO₂ concentrations were consistently higher than those of low sink plants, and those of plants given the 350 microliter CO₂ per liter of air treatment were higher at both measurement CO₂ concentrations than those of plants given the 1000 microliters CO₂ per liter of air treatment. When plants were measured under treatment CO₂ levels, however, rates were higher in 1,000 microliter plants than 350 microliter CO₂ plants. Dry weights of all plant parts were higher in the 1,000 microliters CO₂ per liter air treatment than in the 350 microliters CO₂ per liter air treatment, and were higher in the low sink than in the high sink treatments.     

Physiological investigations of a tobacco mutant with o(2)-resistant photosynthesis and enhanced catalase activity

Experiments are described further indicating that O₂-resistant photosynthesis observed in a tobacco (Nicotiana tabacum) mutant with enhanced catalase activity is associated with decreased photorespiration under conditions of high photorespiration relative to net photosynthesis. The effects on net photosynthesis of (a) increasing O₂ concentrations from 1% to 42% at low CO₂ (250 microliters CO₂ per liter), and (b) of increasing O₂ concentrations from 21% to 42% at high CO₂ (500 microliters CO₂ per liter) were investigated in M₆ progeny of mutant and wild-type leaf discs. The mutant displayed a progressive increase in net photosynthesis relative to wild type with increasing O₂ and the faster rate at 42% O₂ was completely reversed on returning to 21% O₂. The photosynthetic rate by the mutant was similar to wild type in 21% and 42% O₂ at 500 microliters CO₂ per liter, and a faster rate by the mutant was restored on returning to 250 microliters CO₂ per liter. The results are consistent with a lowered release of photorespiratory CO₂ by the mutant because greater catalase activity inhibits the chemical decarboxylation of α-keto acids by peroxisomal H₂O₂. Higher catalase activity was observed in the tip and middle regions of expanding leaves than in the basal area. On successive selfing of mutant plants with enhanced catalase activity, the percent of plants with this phenotype increased from 60% in M₄ progeny to 85% in M₆ progeny. An increase was also observed in the percent of plants with especially high catalase activity (averaging 1.54 times wild type) on successive selfings suggesting that homozygosity for enhanced catalase activity was being approached.     

Acclimation of Two Tomato Species to High Atmospheric CO(2): I. Sugar and Starch Concentrations

Lycopersicon esculentum Mill. cv Vedettos and Lycopersicon chmielewskii Rick, LA 1028, were exposed to two CO₂ concentrations (330 or 900 microliters per liter) for 10 weeks. Tomato plants grown at 900 microliters per liter contained more starch and more sugars than the control. However, we found no significant accumulation of starch and sugars in the young leaves of L. esculentum exposed to high CO₂. Carbon exchange rates were significantly higher in CO₂-enriched plants for the first few weeks of treatment but thereafter decreased as tomato plants acclimated to high atmospheric CO₂. This indicates that the long-term decline of photosynthetic efficiency of leaf 5 cannot be attributed to an accumulation of sugar and/or starch. The average concentration of starch in leaves 5 and 9 was always higher in L. esculentum than in L. chmielewskii (151.7% higher). A higher proportion of photosynthates was directed into starch for L. esculentum than for L. chmielewskii. However, these characteristics did not improve the long-term photosynthetic efficiency of L. chmielewskii grown at high CO₂ when compared with L. esculentum. The chloroplasts of tomato plants exposed to the higher CO₂ concentration exhibited a marked accumulation of starch. The results reported here suggest that starch and/or sugar accumulation under high CO₂ cannot entirely explain the loss of photosynthetic efficiency of high CO₂-grown plants.     

Downward Regulation of Photosynthesis and Growth at High CO(2) Levels : No Evidence for Either Phenomenon in Three-Year Study of Sour Orange Trees

Numerous photosynthesis and growth measurements of sour orange (Citrus aurantium L.) trees maintained in ambient air and air enriched with an extra 300 microliters per liter of CO₂ have revealed the CO₂-enriched trees to have consistently sequestered approximately 2.8 times more carbon than the control trees over a period of three full years. Under field conditions in the natural environment, plants may not experience the downward regulation of photosynthetic capacity typically observed in long-term CO₂ enrichment experiments with plants growing in pots.     

Seasonal response of photosynthesis to elevated CO2 in loblolly pine (Pinus taeda L.) over two growing seasons

Trees growing in natural systems undergo seasonal changes in environmental factors that generate seasonal differences in net photosynthetic rates. To examine how seasonal changes in the environment affect the response of net photosynthetic rates to elevated CO₂, we grew Pinus taeda L. seedlings for three growing seasons in open-top chambers continuously maintained at either ambient or ambient + 30 Pa CO₂. Seedlings were grown in the ground, under natural conditions of light, temperature nd nutrient and water availability. Photosynthetic capacity was measured bimonthly using net photosynthetic rate vs. intercellular CO₂ partial pressure (A-C_i) curves. Maximum Rubisco activity (Vc_max) and ribulose 1,5-bisphosphate regeneration capacity mediated by electron transport (J_max) and phosphate regeneration (PiRC) were calculated from A-C_i curves using a biochemically based model. Rubisco activity, activation state and content, and leaf carbohydrate, chlorophyll and nitrogen concentrations were measured concurrently with photosynthesis measurements. This paper presents results from the second and third years of treatment. Mean leaf nitrogen concentrations ranged from 13.7 to 23.8 mg g^?1, indicating that seedlings were not nitrogen deficient. Relative to ambient CO₂ seedlings, elevated CO₂ increased light-saturated net photosynthetic rates 60–110% during the summer, but < 30% during the winter. A relatively strong correlation between leaf temperature and the relative response of net photosynthetic rates to elevated CO₂ suggests a strong effect of leaf temperature. During the third growing season, elevated CO₂ reduced Rubisco activity 30% relative to ambient CO₂ seedlings, nearly completely balancing Rubisco and RuBP-regeneration regulation of photosynthesis. However, reductions in Rubisco activity did not eliminate the seasonal pattern in the relative response of net photosynthetic rates to elevated CO₂. These results indicate that seasonal differences in the relative response of net photosynthetic rates to elevated CO₂ are likely to occur in natural systems.     

Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol⁻¹) [CO₂] and ambient or elevated (+4 g N m⁻² year⁻¹) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (−57 and −37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].     

Effects of elevated CO2 on growth,photosynthesis and respiration of sweet chestnut (Castanea sativa Mill.)

Two year old sweet chestnut seedlings (Castanea sativa Mill) were grown in pots at ambient (350 µmol·mol^–1) and double (700 µmol·mol^–1) atmospheric CO₂ concentration in constantly ventilated greenhouses during entire growing seasons. CO₂ enrichment caused either no significant change or a decrease in shoot response, depending on yearly weather conditions. Similarly, leaf area was either reduced or unchanged under elevated CO₂. However, when grown under controlled conditions in a growth chamber, leaf area was enlarged with elevated CO₂.The CO₂ exchanges of whole plants were measured during the growing season. In elevated CO₂, net photosynthetic rate was maximum in May and then decreased, reaching the level of the control at the end of the season. End of night dark respiration of enriched plants was significantly lower than that of control plants; this difference decreased with time and became negligible in the fall. The original CO₂ level acted instantaneously on the respiration rate: a double concentration in CO₂ decreased the respiration of control plants and a reduced concentration enhanced the respiration of enriched plants. The carbon balance of a chestnut seedling may then be modified in elevated CO₂ by increased carbon inputs and decreased carbon outputs.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations

We constructed a carbon budget for young birch trees grown in ambient and elevated CO₂ concentrations over their fourth year of growth. The annual total of net leaf photosynthesis was 110% more in elevated CO₂ than in ambient CO₂. However, the trees in elevated CO₂ grew only 59% more biomass than the trees in ambient CO₂ over the year. Modelling studies showed that larger loss of carbon from fine-root production and growth of the root-associated mycorrhiza by the trees in elevated CO₂ probably accounted for all the remaining difference in net photosynthesis between the two treatments. Our modelling also showed that the fraction of net photosynthate consumed by respiration of nonleaf tissue was similar in the two CO₂ treatments, and was 26% and 24% for trees in ambient and elevated CO₂, respectively. Trees in elevated CO₂ had 43% more leaves, and produced 110% more net photosynthate than trees in ambient CO₂, even though the maximum rate of carboxylation per unit leaf nitrogen decreased by 21%. Sensitivity studies showed that down-regulation reduced the annual net photosynthetic production of the trees in elevated CO₂ by only 6%. Direct effects of higher CO₂ on photosynthesis and greater leaf area of the trees in elevated CO₂ increased the net photosynthesis of the trees by 68% and 60%, respectively; and together accounted for most of the difference in net photosynthesis between the two treatments.     

Effects of Atmospheric CO(2) Enrichment on the Growth and Mineral Nutrition of Quercus alba Seedlings in Nutrient-Poor Soil

One-year-old dormant white oak (Quercus alba L.) seedlings were planted in a nutrient-deficient forest soil and grown for 40 weeks in growth chambers at ambient (362 microliters per liter) or elevated (690 microliters per liter) levels of CO₂. Although all of the seedlings became severely N deficient, CO₂ enrichment enhanced growth by 85%, with the greatest enhancement in root systems. The growth enhancement did not increase the total water use per plant, so water-use efficiency was significantly greater in elevated CO₂. Total uptake of N, S, and B was not affected by CO₂, therefore, tissue concentrations of these nutrients were significantly lower in elevated CO₂. An increase in nutrient-use efficiency with respect to N was apparent in that a greater proportion of the limited N pool in the CO₂-enriched plants was in fine roots and leaves. The uptake of other nutrients increased with CO₂ concentration, and P and K uptake increased in proportion to growth. Increased uptake of P by plants in elevated CO₂ may have been a result of greater proliferation of fine roots and associated mycorrhizae and rhizosphere bacteria stimulating P mineralization. The results demonstrate that a growth response to CO₂ enrichment is possible in nutrient-limited systems, and that the mechanisms of response may include either increased nutrient supply or decreased physiological demand.