Photosynthesis in Plants of Four Tropical Species Growing under Elevated CO2

We studied the responses of leaf gas exchange and growth to an increase in atmospheric CO₂ concentration in four tropical deciduous species differing in carbon fixation metabolism: Alternanthera crucis, C3-C4; Ipomoea carnea, C3; Jatropha gossypifolia, C3; and Talinum triangulare, inducible-CAM. In the first stage, plants were grown in one open-top chamber at a CO₂ concentration of 560±40 mol mol^-1 (EC), one ambient CO₂ concentration chamber (AC), and one unenclosed plot (U). In the second stage, plants were grown in five EC chambers (CO₂ concentration = 680±30 mol mol^-1), five AC chambers, and five unenclosed plots. During the first weeks under EC in the first stage, plants of all the species had a very marked increase in their maximal net photosynthetic rates (P _max) of 3.5 times on average; this stimulatory effect was maintained for 11-15 weeks, rates dampening afterward to values still higher than controls for 37 weeks. After a suspension of CO₂ enrichment for 6 weeks, an increase in P _max of EC plants over the controls was found in plants of all the species until week 82 of the experiment. Stomatal conductance (g) showed no response to EC. Carboxylation efficiency decreased in all the species under EC     

冬小麦光合作用对开放式空气CO2 浓度增高(FACE)的非气孔适应

在同样CO2浓度下测定时,开放式空气CO2浓度增高(FACE,580 μmol CO2 /mol)条件下生长的冬小麦叶片的净光合速率、气孔导度和羧化效率都显著低于普通空气(380 μmol CO2 /mol)中生长的对照叶片.与此相一致,FACE叶片的可溶性蛋白、二磷酸核酮糖羧化酶/加氧酶(Rubisco)和Rubisco活化酶含量也都显著低于对照叶片.这些结果表明,在根系生长不受限制的田间条件下,冬小麦叶片的光合作用对高浓度CO2产生了适应现象,其主要原因可能是碳同化的关键酶Rubisco等含量的降低.     

Interactive Effects of Elevated CO2 and Growth Temperature on the Tolerance of Photosynthesis to Acute Heat Stress in C3 and C4 Species

Determining effects of elevated CO2 on the tolerance of photosynthesis to acute heat-stress (heat wave) is necessary for predicting plant responses to global warming, as photosynthesis is thermolabile and acute heat-stress and atmospheric CO2 will increase in the future. Few studies have examined this, and past results are variable, which may be due to methodological variation. To address this, we grew two C3 and two C4 species at current or elevated CO2 and three different growth temperatures (GT). We assessed photosynthetic thermotolerance in both unacclimated (basal tolerance) and preheat-stressed (preHS = acclimated) plants. In C3 species, basal thermotolerance of net photosynthesis (Pn) was increased In high CO2, but in C4 species, Pn thermotlerance was decreased by high CO2 (except Zea maya at low GT); CO2 effects in preHS plants were mostly small or absent, though high CO2 was detrimental in one C3 and one C4 species at warmer GT. Though high CO2 generally decreased stomatal conductance, decreases in Pn during heat stress were mostly due to non-stomatal effects. Photosystem II (PSII) efficiency was often decreased by high CO2 during heat stress, especially at high GT; CO2 effects on post-PSll electron transport were variable. Thus, high CO2 often affected photosynthetic theromotolerance, and the effects varied with photosynthetic pathway, growth temperature, and acclimation state. Most importantly, in heat-stressed plants at normal or warmer growth temperatures, high CO2 may often decrease, or not benefit as expected, tolerance of photosynthesis to acute heat stress. Therefore, interactive effects of elevated CO2 and warmer growth temperatures on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.     

植物对大气CO2浓度升高的光合适应机理

光合作用对大气中CO2浓度升高适应的可能原因主要表现在以下几个方面:由于CO2浓度升高,碳水化合物过量积累,光合电子传递链中质体醌与过氧化氢(H2O2)的氧化还原信号对光合作用发生反馈抑制;核酮糖1,5-二磷酸羧化/加氧酶(Rubisco)的含量及其活性下降;气孔状态发生变化.此外,植物体内C/N平衡、生长调节物质和己糖激酶对光合基因表达水平的调控等多个方面会对光合适应产生影响.     

植物对大气CO2 浓度升高的光合适应机理

光合作用对大气中CO2浓度升高适应的可能原因主要表现在以下几个方面: 由于CO2浓度升高,碳水化合物过量积累, 光合电子传递链中质体醌与过氧化氢(H2O2)的氧化还原信号对光合作用发生反馈抑制; 核酮糖1,5-二磷酸羧化/加氧酶(Rubisco)的含量及其活性下降; 气孔状态发生变化。此外, 植物体内C/N平衡、生长调节物质和己糖激酶对光合基因表达水平的调控等多个方面会对光合适应产生影响。     

植物光合作用对CO2浓度增高的适应机制

长期在高浓度CO2环境下生长的植物往往会发生光合适应或下调,即在相同CO2浓度下的光合速率明显低于普通空气中生长的对照。虽然关于这种现象已经有许多研究报告和综述文章,但是它的机理还不很清楚。本文结合作者所在研究组的工作,概述了关于植物光合适应机理研究的新进展,提出除了呼吸作用增强和光合产物超常积累的可能作用以外,二磷酸核酮糖（RuBP）羧化限制和RuBP再生限制可能是导致植物光合适应的主要因素。     

Elevated CO2 Effects during Leaf Ontogeny (A New Perspective on Acclimation)

For many plants growth in elevated CO2 leads to reduced rates of photosynthesis. To examine the role that leaf ontogeny plays in the acclimation response, we monitored photosynthesis and some related parameters at short intervals throughout the ontogenetic development of tobacco (Nicotiana tabacum L.) leaves under ambient (350 [mu]L L-1)- and high (950 [mu]L L-1)-CO2 conditions. The pattern of photosynthetic rate over time was similar between the two treatments and consistent with the expected pattern for a typical dicot leaf. However, the photosynthesis pattern in high-CO2-grown tobacco was shifted temporally to an earlier maximum and subsequent senescent decline. Ribulose-1,5-biphosphate carboxylase/oxygenase activity appeared to be the main factor regulating photosynthetic rates in both treatments. Therefore, we propose a new model for interpreting the acclimation response. Lowered photosynthetic rates observed during acclimation appear to be the result of a shift in the timing of the normal photosynthetic stages of leaf ontogeny to an earlier onset of the natural decline in photosynthetic rates associated with senescence.     

Acclimation of Respiratory O2 Uptake in Green Tissues of Field-Grown Native Species after Long-Term Exposure to Elevated Atmospheric CO2

C3 and C4 plants were grown in open-top chambers in the field at two CO2 concentrations, normal ambient (ambient) and normal ambient + 340 [mu]LL-1 (elevated). Dark oxygen uptake was measured in leaves and stems using a liquid-phase Clark-type oxygen electrode. High CO2 treatment decreased dark oxygen uptake in stems of Scirpus olneyi (C3) and leaves of Lindera benzoin (C3) expressed on either a dry weight or area basis. Respiration of Spartina patens (C4) leaves was unaffected by CO2 treatment. Leaf dry weight per unit area was unchanged by CO2, but respiration per unit of carbon or per unit of nitrogen was decreased in the C3 species grown at high CO2. The component of respiration in stems of S. olneyi and leaves of L. benzoin primarily affected by long-term exposure to the elevated CO2 treatment was the activity of the cytochrome pathway. Elevated CO2 had no effect on activity and capacity of the alternative pathway in S. olneyi. The cytochrome c oxidase activity, assayed in a cell-free extract, was strongly decreased by growth at high CO2 in stems of S. olneyi but it was unaffected in S. patens leaves. The activity of cytochrome c oxidase and complex III extracted from mature leaves of L. benzoin was also decreased after one growing season of plant exposure to elevated CO2 concentration. These results show that in some C3 species respiration will be reduced when plants are grown in elevated atmospheric CO2. The possible physiological causes and implications of these effects are discussed.     

The Photosynthetic Response to Elevated CO2 in High Altitude Potato Species (Solanum curtilobum)

Plants of Solanum curtilobum (from high altitude) and Solanum tuberosum (from low altitude) were grown in open-top chambers in a greenhouse at either ambient (AC, 360 µmol mol^–1) or ca. twice ambient (EC, 720 µmol mol^–1) CO₂ concentrations for 30 d. CO₂ treatments started at the reproductive stage of the plants. There were similar patterns in the physiological response to CO₂ enrichment in the two species. Stomatal conductance was reduced by 59 % in S. tuberosum and by 55 % in S. curtilobum, but such a reduction did not limit the net photosynthetic rate (P _N), which was increased by approximately 56 % in S. curtilobum and 53 % in S. tuberosum. The transpiration rate was reduced by 16 % in both potato species while instantaneous transpiration efficiency increased by 80 % in S. tuberosum and 90 % in S. curtilobum. Plants grown under EC showed 36 and 66 % increment in total dry biomass, whereas yields (dry mass of tubers) were increased by 40 and 85 % in S. tuberosum and S. curtilobum, respectively. EC promoted productivity by increasing P _N. Thus S. tuberosum, cultivated around the world at low altitudes, and S. curtilobum, endemic of the highland Andes, respond positively to EC during the tuberisation stage.     

Photosynthetic Acclimation to Elevated CO2 in Relation to Leaf Saccharide Constituents in Wheat and Sunflower

Wheat (T. durum cvs. HD 4502 and B 449, T. aestivum cvs. Kalyansona and Kundan) and sunflower (Helianthus annuus L. cv. Morden) were grown under atmospheric (360±10 cm³ m^–3, AC) and elevated CO₂ (650±50 cm³ m^–3, EC) concentration in open top chambers for entire period of growth and development till maturity. Leaf net photosynthetic rate (P _N) of EC-grown plants of wheat measured at EC was significantly decreased in comparison with AC-plants of wheat measured at EC. Sunflower, however, showed no significant depression in P _N in EC-plants. There was a decrease in ribulose-1,5-bisphosphate carboxylase (RuBPC) activity, its activation state and amount in EC-plants of wheat, whereas no significant decrease was observed in sunflower. The above different acclimation to EC in wheat and sunflower was related with saccharide constituents accumulated in the leaves. Under EC, sunflower accumulated in the leaves more starch, whereas wheat accumulated more sugars.     

C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration)

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% O2 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on O2 (approximately 10%). This O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O2] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light ([phi]CO2). The dual effect of O2 is masked if measurements are made under only 2 versus 21% O2. The inhibition of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with decreasing Ci increases in a very similar manner, characteristically of O2 inhibition due to photorespiration. There is a sharp increase in O2 inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in [phi]CO2 under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying Ci, the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C, and atmospheric levels of CO2 (340 [mu]bar), where there is little inhibition of photosynthesis in maize by O2, the estimated level of CO2 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 [mu]bar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 [mu]bar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.     

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.     

Photosynthesis and Plant Growth at Elevated Levels of CO2

In this review, we discuss the effects of elevated CO₂ levelson photosynthesis in relation to the whole plant growth in terrestrialhigher C₃ plants. Short-term CO₂ enrichment stimulates the rateof photosynthesis. Plant mass is also enhanced by CO₂ enrichment.However, the effects of long-term CO₂ enrichment on photosynthesisare variable. Generally, the prolonged exposure to CO₂ enrichmentreduces the initial stimulation of photosynthesis in many species,and frequently suppresses photosynthesis. These responses areattributed to secondary responses related to either excess carbohydrateaccumulation or decreased N content rather than direct responsesto CO₂. Accumulation of carbohydrates in leaves may lead tothe repression of photosynthetic gene expression and excessstarch seems to hinder CO₂ diffusion. Therefore, the specieswhich have the sink organs for carbohydrate accumulation donot show the suppression of photosynthesis. The suppressionof photosynthesis by CO₂ enrichment is always associated withdecreases in leaf N and Rubisco contents. These decreases arenot due to dilution of N caused by a relative increase in theplant mass but are the result of a decrease in N allocationto leaves at the level of the whole plant, and the decreasein Rubisco content is not selective. Leaf senescence and plantdevelopment are also accelerated by CO₂ enrichment. However,they are independent of each other in some species. Thus, variousresponses to CO₂ observed at the level of a single leaf resultfrom manifold responses at the level of the whole plant grownunder conditions of CO₂ enrichment. (Received July 8, 1999; Accepted August 12, 1999)     

不同氮营养水平下草莓叶片光合作用对高CO2浓度的适应

研究了不同氮素水平（12mmol／L,4mmol／L,0、4mmol／L）下生长的‘丰香’草莓在富C02（700μL/L）和大气CO（390μL/L）下的光合作用。结果表明,高氮（12mmol／L）下,在富CO2环境中生长的‘丰香’草莓叶片未出现光合作用下调,富CO2下草莓叶片的净光合速率、最大羧化速率（Vc.max）、最大电子传递速率（Jmax）、碳同化的电子传递速率（Jc）和光化学猝灭系数（qp）等均显著提高;而在中氮（4mmol／L）、低氮（0．4mmol／L）下,富CO2下生长的草莓叶片的上述参数均出现不同程度的下降。富CO2下,无论氮素水平如何,草莓叶片的光呼吸电子传递速率（Jo）均降低高氮草莓叶片的非光化学猝灭系数（qN或NPQ）降低,光抑制降低,而低氮则相反。上述结果说明,氮素供应不足时草莓叶片在富CO2下光合作用出现下调,因此生产上进行CO2施肥时应适度增加氮素的供应。     

Photosynthesis of F(1) Hybrids between C(4) and C(3)-C(4) Species of Flaveria

Photosynthetic characteristics were studied in several F₁ hybrids between C₄ and C₃-C₄ species of Flaveria. Stable carbon isotope ratios, O₂ inhibition of apparent photosynthesis, and phosphoenolpyruvate carboxylase activities in the hybrids were similar to the means for the parents. Values of CO₂ compensation concentrations were nearer to those of the C₄ parent and apparent photosynthesis was below that of both parents, being only 60 and 74% of that of the lowest (C₃-C₄) parent in two experiments. Reductions of CO₂ compensation concentration and O₂ inhibition of apparent photosynthesis as well as increases in carbon isotope ratios and phosphoenolpyruvate carboxylase activities compared to values in C₃-C₄ species suggest transfer of a limited degree of C₄ photosynthesis to the F₁ hybrids. However, the lower apparent photosynthesis of the hybrids suggests that transfer of C₄ characteristics to non-C₄ species is detrimental unless characteristics associated with C₄ photosynthesis are fully developed. There was a highly significant negative correlation (r = −0.90) between CO₂ compensation concentration and the logarithm of phosphoenolpyruvate carboxylase activity in the parents and hybrids, suggesting involvement of this enzyme in controlling the CO₂ compensation concentration. Although bundle-sheath cells were more developed in leaves of hybrids than in C₃-C₄ parents, they appeared to contain lower quantities of organelles than those of the C₄ parent. Reduced quantities of organelles in bundle-sheath cells could indicate incomplete compartmentation of partial pathways of the C₄ cycle in the hybrids. This may mean that the reduction of CO₂ compensation and O₂ inhibition of apparent photosynthesis relative to the C₃-C₄ parents is less dependent on fully developed Kranz anatomy than is increased apparent photosynthesis.     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Photosynthesis in Flaveria brownii, a C(4)-Like Species: Leaf Anatomy, Characteristics of CO(2) Exchange, Compartmentation of Photosynthetic Enzymes, and Metabolism of CO(2)

Light microscopic examination of leaf cross-sections showed that Flaveria brownii A. M. Powell exhibits Kranz anatomy, in which distinct, chloroplast-containing bundle sheath cells are surrounded by two types of mesophyll cells. Smaller mesophyll cells containing many chloroplasts are arranged around the bundle sheath cells. Larger, spongy mesophyll cells, having fewer chloroplasts, are located between the smaller mesophyll cells and the epidermis. F. brownii has very low CO₂ compensation points at different O₂ levels, which is typical of C₄ plants, yet it does show about 4% inhibition of net photosynthesis by 21% O₂ at 30°C. Protoplasts of the three photosynthetic leaf cell types were isolated according to relative differences in their buoyant densities. On a chlorophyll basis, the activities of phosphoenolpyruvate carboxylase and pyruvate, Pi dikinase (carboxylation phase of C₄ pathway) were highest in the larger mesophyll protoplasts, intermediate in the smaller mesophyll protoplasts, and lowest, but still present, in the bundle sheath protoplasts. In contrast, activities of ribulose 1,5-bisphosphate carboxylase, other C₃ cycle enzymes, and NADP-malic enzyme showed a reverse gradation, although there were significant activities of these enzymes in mesophyll cells. As indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the banding pattern of certain polypeptides of the total soluble proteins from the three cell types also supported the distribution pattern obtained by activity assays of these enzymes. Analysis of initial ¹⁴C products in whole leaves and extrapolation of pulse-labeling curves to zero time indicated that about 80% of the CO₂ is fixed into C₄ acids (malate and aspartate), whereas about 20% of the CO₂ directly enters the C₃ cycle. This is consistent with the high activity of enzymes for CO₂ fixation by the C₄ pathway and the substantial activity of enzymes of the C₃ cycle in the mesophyll cells. Therefore, F. brownii appears to have some capacity for C₃ photosynthesis in the mesophyll cells and should be considered a C₄-like species.     

Relation of CO(2) Compensation Concentration to Apparent Photosynthesis in Maize

Significant differences in CO₂ compensation concentration measured in the field among varieties of the species Zea mays L. are reported for the first time. CO₂ compensation concentrations were significantly (P≤ 0.01) and negatively correlated with apparent photosynthesis at 300 μl CO₂/liter air. The Michaelis constant (as defined) for a leaf was significantly (P≤ 0.01) and positively correlated with apparent photosynthesis among varieties. While the first correlation is similar to behavior of CO₂ compensation among species of different photosynthetic efficiency, the latter correlation is the converse of the behavior of Km among species.     

Degree of C(4) Photosynthesis in C(4) and C(3)-C(4)Flaveria Species and Their Hybrids : I. CO(2) Assimilation and Metabolism and Activities of Phosphoenolpyruvate Carboxylase and NADP-Malic Enzyme

The degree of C₄ photosynthesis was assessed in four hybrids among C₄, C₄-like, and C₃-C₄ species in the genus Flaveria using ¹⁴C labeling, CO₂ exchange, ¹³C discrimination, and C₄ enzyme activities. The hybrids incorporated from 57 to 88% of the ¹⁴C assimilated in a 10-s exposure into C₄ acids compared with 26% for the C₃-C₄ species Flaveria linearis, 91% for the C₄ species Flaveria trinervia, and 87% for the C₄-like Flaveria brownii. Those plants with high percentages of ¹⁴C initially fixed into C₄ acids also metabolized the C₄ acids quickly, and the percentage of ¹⁴C in 3-phosphoglyceric acid plus sugar phosphates increased for at least a 30-s exposure to ¹²CO₂. This indicated a high degree of coordination between the carbon accumulation and reduction phases of the C₄ and C₃ cycles. Synthesis and metabolism of C₄ acids by the species and their hybrids were highly and linearly correlated with discrimination against ¹³C. The relationship of ¹³C discrimination or ¹⁴C metabolism to O₂ inhibition of photosynthesis was curvilinear, changing more rapidly at C₄-like values of ¹⁴C metabolism and ¹³C discrimination. Incorporation of initial ¹⁴C into C₄ acids showed a biphasic increase with increased activities of phosphoenolpyruvate carboxylase and NADP-malic enzyme (steep at low activities), but turnover of C₄ acids was linearly related to NADP-malic enzyme activity. Several other traits were closely related to the in vitro activity of NADP-malic enzyme but not phosphoenolpyruvate carboxylase. The data indicate that the hybrids have variable degrees of C₄ photosynthesis but that the carbon accumulation and reduction portions of the C₄ and C₃ cycles are well coordinated.