Adaptive respiratory significance of organophosphates (ATP & GTP) in air-breathing fishes

The ATP and GTP contents of erythrocytes of Protopterus annectens, Polypterus senegalus and Clarias lazera were assessed in relation to maturation and hypoxia. The steady state normal concentrations of ATP were: 446, 316 and 475 µM/100 ml and those of GTP were 286, 93 and 85 µM/100 ml in Protopterus, Polypterus and Clarias, respectively. The level of ATP decreased and that of GTP increased with increments in body growth, especially during development of the pulmonary organs. The concentration of GTP also increased progressively with hypoxia (O₂: 7.4 to 3.6 mg/l); the level ATP remained almost unaltered. The ratio of GTP increased markedly with increased hypoxia particularly in the adaptive respiratory range of aquatic oxygen concentrations (6.5–4.8 mg/l) when the fish were forced to depart from branchial respiration and became reliant on air-breathing.     

Development of dependence on aerial respiration in Polypterus senegalus (Cuvier)

The respiratory behaviour and partitioning of O₂ uptake between air and water were investigated in Polypterus genegalus using continuous-flow and two-phase respirometers and lung gas replacement techniques P. senegalus rarely resorts to aerial respiration under normal conditions. Partitioning of O₂ consumption depends on the activity and age of fish and the availability of aquatic oxygen. Immature fish (12–22 g) cannot utilize aerial O₂ but older fish exhibit age-dependent reliance on aerial respiration in hypoxic and hypercarbic waters. Pulmonary respiration accounts for 50% of the total requirement at aquatic O₂ concentrations of about 3.5 mg · l^–1 (or CO₂ of about 5%) and fish rely exclusively on aerial respiration at O₂ concentrations of less than 2.5 mg · l^–1. Branchial respiration is initially stimulated by hypercarbia (CO₂: 0.5–0.8%) but increased hypercarbia (CO₂ – 1%) greatly depresses (by over 90%) brancial respiration and initiates (CO₂: 0.5%) and sustains pulmonary respiration.     

Experimental hybridization between catfishes of the families Clariidae and Pangasiidae in Thailand

Synopsis Hybrids between the clariid speciesClarias macrocephalus andC. batrachus and the pangasiid speciesPangasius sutchi were obtained by hormone injection of brood stock and artificial fertilization. Pure parental crosses as well as all possible hybrid combinations were obtained. Fertility, hatchability, and post-yolk absorption survival was high (66–99%) in all pure parental crosses and in all crosses between the two species ofClarias. In crosses betweenClarias andPangasius fertilization was also very high (68–97%) but hatchability varied from 11 to 23% and post-yolk absorption survival from 0% inPangasius sutchi ×Clarias batrachus to about 50% inPangasius sutchi ×Clarias macrocephalus . The longest-lived hybrids ofPangasius andClarias were those ofPangasius sutchi ×Clarias macrocephalus which survived until the experiment was prematurely terminated due to contaminated food after 4 1/2 months, at which point they had grown to total lengths of 8–14cm. The hybrids comprised four morphotypes, two relativelyPangasius-like and two relativelyClarias-like, but all markedly different from the parental species.     

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.     

Respiration and photosynthesis in cones of Norway spruce (Picea abies (L.) Karst.)

Summary Dark respiration and photosynthetic carbon dioxide refixation in purple and green Picea abies cones were investigated from budbreak to cone maturity. The rate of dark respiration per unit dry weight and CO₂ refixation capacity decreased during cone maturation. At the beginning of the growing season, photosynthetic CO₂ refixation could reduce the amount of CO₂ released by respiration in green and purple cones by 50% and 40%, respectively. The seasonal performance of the components of the cone carbon balance was calculated using information on the seasonal course of respiration, refixation capacity and the light response curves of cone photosynthesis, as well as the actual light and temperature regime in the field. The daily gain of CO₂ refixation reached 28%–34% of respiration in green and 22%–26% in purple cones during the first month of their growth, but decreased later in the season. Over the entire growth period refixation reduced carbon costs of cone production in both cone colour polymorphs by 16%–17%.     

Carbon cost of nitrogenase activity in Frankia–Alnus incana root nodules

The carbon cost of nitrogenase activity was investigated to determine symbiotic efficiency of the actinorhizal root nodule symbiosis between the woody perennial Alnus incana and the soil bacterium Frankia. Respiration (CO₂ production) and nitrogenase activity (H₂ production) by intact nodulated root systems were continuously recorded in short-term assays in an open-flow gas exchange system. The assays were conducted in N₂:O₂, thus under N₂-fixing conditions, in all experiments except for one. This avoided the declines in nitrogenase activity and respiration due to N₂ deprivation that occur in acetylene reduction assays and during extended Ar:O₂ exposures in H₂ assays. Two approaches were used: (i) direct estimation of root and nodule respiration by removing nodules, and (ii) decreasing the partial pressure of O₂ from 21 to 15% to use the strong relationship between respiration and nitrogenase activity to calculate CO₂/H₂. The electron allocation of nitrogenase was determined to be 0.6 and used to convert the results into moles of CO₂ produced per 2e^– transferred by nitrogenase to reduction of N₂. The results ranged from 2.6 to 3.4mol CO₂ produced per 2e^–. Carbon cost expressed as gC produced per gN reduced ranged from 4.5 to 5.8. The result for this actinorhizal tree symbiosis is in the low range of estimates for N₂-fixing actinorhizal symbioses and crop legumes. Methodology and comparisons of root nodule physiology among actinorhizal and legume plants are discussed.     

Growth,photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O2 concentrations and photon flux density

Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO₂ enrichment up to 6000 l CO₂ l^–1. This high CO₂ optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO₂ was less or absent following four days growth in 2% O₂. The Leaf Area Ratio decreased in response to CO₂ enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO₂ enrichment up to 1500 l CO₂ l^–1 during measurement, showing only small increases with further CO₂ enrichment up to 5000 l CO₂ l^–1 at a photon flux density of 210 mol m^–2 s^–1 and small decreases at 2000 mol m^–1 s^–1. The actual rate of photosynthesis of those plants cultivated at high CO₂ levels, however, was less than the air grown plants. The response of photosynthesis to O₂ indicated that the enhancement of growth and photosynthesis by CO₂ enrichment was a result of decreased photorespiration. Plants cultivated in low O₂ produced abnormal morphological features and after a short time showed a reduction in growth.     

The reproductive biology of four African catfishes (Osteichthyes: Clariidae) in Anambra River basin,Nigeria

The reproductive biology of Clarias ebriensis, C. agboyiensis, C. macromystax and C. buthupogon was investigated over a period of 32 months in Anambra River basin, Nigeria. Immature (stage I) C. ebriensis and C. agboyiensis were recruited into the fishery in October, and in September in C. macromystax and C. buthupogon. The occurrence of mature (stage III) fish all the year round indicated the potential of the Clarias species to breed at any time of the year. C. ebriensis bred from April–September while breeding occurred from May–September in the other species. The clariids matured between their first and second year of life. C. ebriensis females matured earlier than males but the reserve was the case in C. agboyiensis, C. macromystax and C. buthupogon. Ovary weight was a better indicator of egg number than either total length or body weight in all species. Communal spawning is reported for the first time in Clarias species and its significance to survival discussed.     

CO2 and O2 gas exchange in outdoor thin-layer high density microalgal cultures

Two thin layer culture units operated as batch cultures with the algaChlorella kessleri were used in gas exchange experiments. The mass transfer coefficient K_g [g m^–2 h^–1 kPa^–1] of O₂ and CO₂ desorption from culture surface decreased with increasing culture temperature. Between 60–70% of supplied CO₂ was used for algal growth. It was estimated that the length of growth surface may be extended to about 50 m, without additional saturation by CO₂. On average 1.35 g CO₂ was consumed by the alga per 1 g of produced O₂. Net CO₂ consumption (R_CO2) and O₂ production (R_O2) were not inhibited by irradiance. R_O2 did not decrease (in some cases it even increased) along the culture surface, despite increased accumulation of O₂. Measurement of pO₂ where the culture leaves the reactor before being pumped back onto the illuminated surface, correlated with O₂ production and CO₂ consumption and may be used to monitor the reactors growth performance.     

Soil O2 and CO2 effects on root respiration of cacti

Through use of a recently developed technique that can measure CO₂ exchange by individual attached roots, the influences of soil O₂ and CO₂ concentrations on root respiration were determined for two species of shallow-rooted cacti that typically occur in porous, well-drained soils. Although soil O₂ concentrations in the rooting zone in the field were indistinguishable from that in the ambient air (21% by volume), the CO₂ concentrations 10 cm below the soil surface averaged 540 μLL⁻¹ for the barrel cactusFerocactus acanthodes under dry conditions and 2400 μLL⁻¹ under wet conditions in a loamy sand. For the widely cultivated platyopuntiaOpuntia ficus-indica in a sandy clay loam, the CO₂ concentration at 10 cm averaged 1080 μLL⁻¹ under dry conditions and 4170 μLL⁻¹ under wet conditions. For both species, the respiration rate in the laboratory was zero at 0% O₂ and increased to its maximum value at 5% O₂ for rain roots (roots induced by watering) and 16% O₂ for established roots. Established roots ofO. ficus-indica were slightly more tolerant of elevated CO₂ than were those ofF. acanthodes, 5000 μLL⁻¹ inhibiting respiration by 35% and 46%, respectively. For both species, root respiration was reduced to zero at 20,000 μLL⁻¹ (2%) CO₂. In contrast to the reversible effects of 0% O₂, inhibition by 2% CO₂ was irreversible and led to the death of cortical cells in established roots in 6 h. Although the restriction of various cacti and other CAM plants to porous soils has generally been attributed to their requirement for high O₂ concentrations, the present results indicate that susceptibility of root respiration to elevated soil CO₂ concentrations may be more important.     

In situ CO2 gas-exchange in fruits of a tropical tree,Durio zibethinus Murray

We examined the in situ CO₂ gas-exchange of fruits of a tropical tree, Durio zibethinus Murray, growing in an experimental field station of the Universiti Pertanian Malaysia. Day and night dark respiration rates were exponentially related to air temperature. The temperature dependent dark respiration rate showed a clockwise loop as time progressed from morning to night, and the rate was higher in the daytime than at night. The gross photosynthetic rate was estimated by summing the rates of daytime dark respiration and net photosynthesis. Photosynthetic CO₂ refixation, which is defined as the ratio of gross photosynthetic rate to dark respiration rate in the daytime, ranged between 15 and 45%. The photosynthetic CO₂ refixation increased rapidly as the temperature increased in the lower range of air temperature T _c (T _c <28.5 °C), while it decreased gradually as the temperature increased in the higher range (T _c 28.5 °C). Light dependence of photosynthetic CO₂ refixation was approximated by a hyperbolic formula, where light saturation was achieved at 100 mol m^–2 s^–1 and the asymptotic CO₂ refixation was determined to be 37.4%. The estimated gross photosynthesis and dark respiration per day were 1.15 and 4.90 g CO₂ fruit^–1, respectively. Thus the CO₂ refixation reduced the respiration loss per day by 23%. The effect of fruit size on night respiration rate satisfied a power function, where the exponent was larger than unity.     

Respiration of individual honeybee larvae in relation to age and ambient temperature

The CO₂ production of individual larvae of Apis mellifera carnica, which were incubated within their cells at a natural air humidity of 60–80%, was determined by an open-flow gas analyzer in relation to larval age and ambient temperature. In larvae incubated at 34 °C the amount of CO₂ produced appeared to fall only moderately from 3.89±1.57 µl mg^–1 h^–1 in 0.5-day-old larvae to 2.98±0.57 µl mg^–1 h^–1 in 3.5-day-old larvae. The decline was steeper up to an age of 5.5 days (0.95±1.15 µl mg^–1 h^–1). Our measurements show that the respiration and energy turnover of larvae younger than about 80 h is considerably lower (up to 35%) than expected from extrapolations of data determined in older larvae. The temperature dependency of CO₂ production was determined in 3.5-day-old larvae, which were incubated at temperatures varying from 18 to 38 °C in steps of 4 °C. The larvae generated 0.48±0.03 µl mg^–1 h^–1 CO₂ at 18 °C, and 3.97±0.50 µl mg^–1 h^–1 CO₂ at 38 °C. The temperature-dependent respiration rate was fitted to a logistic curve. We found that the inflection point of this curve (32.5 °C) is below the normal brood nest temperature (33–36 °C). The average Q₁₀ was 3.13, which is higher than in freshly emerged resting honeybees but similar to adult bees. This strong temperature dependency enables the bees to speed up brood development by achieving high temperatures. On the other hand, the results suggest that the strong temperature dependency forces the bees to maintain thermal homeostasis of the brood nest to avoid delayed brood development during periods of low temperature.Abbreviations m body mass - R rate of development or respiration - T_I inflexion point of a logistic (sigmoid) curve - T_L lethal temperature - T_O temperature of optimum (maximum) developmentCommunicated by G. Heldmaier     

Energy metabolism of Chironomus anthracinus (Diptera: Chironomidae) from the profundal zone of Lake Esrom,Denmark, as a function of body size,temperature and oxygen concentration

The profundal zone of Lake Esrom, Denmark has a dense population of Chironomus anthracinus, which survives 2–4 months of oxygen depletion each summer during stratification. The metabolism of 3rd and 4th instar larvae was examined in regard to variation in biomass and temperature. Respiration at air saturation was described by a curvilinear multiple regression relating oxygen consumption to individual AFDW and temperature. At 10 °C and varying oxygen regimes the O₂ consumption and CO₂ production of 4th instar larvae were almost unaltered from saturation to about 3 mg O₂ l^–1, but decreased steeply below this level. The respiratory quotient increased from 0.82 at saturation to about 3.4 at oxygen concentrations near 0.5 mg O₂ l^–1. This implied a shift from aerobic to partially anaerobic metabolism. At 0.5 mg O₂ l^–1 the total energy production equalled 20% of the rate at saturation of which more than one third was accounted for by anaerobic degradation of glycogen. This corresponded to a daily loss of 12 µg mg AFDW^–1 or approximately 5% of the body reserves. At unchanged metabolic rate the glycogen store would last three weeks, but long term oxygen deficiency causes a further suppression of the energy metabolism in C. anthracinus.     

A technique for distinguishing between bacterial and non-bacterial respiration in decomposing Spartina alterniflora

A number of antibiotics were used to suppress bacterial activity in decomposing Spartina alterniflora. The effectiveness of each treatment was quantified using INT formazan vital staining and epifluorescent microscopy. Bacterial suppression of selected treatments was verified using standard plate count procedures. Chloramphenicol treated samples (exhibiting 87–90% bacterial suppression) were analyzed respirometrically and found to consume only 30% less O₂ than controls. Non-bacterial respiration (probably fungal) accounted for 70% of the respiration.     

Chemolithoautotrophy in the marine,magnetotactic bacterial strains MV-1 and MV-2

Magnetite-producing magnetotactic bacteria collected from the oxic–anoxic transition zone of chemically stratified marine environments characterized by O₂/H₂S inverse double gradients, contained internal S-rich inclusions resembling elemental S globules, suggesting they oxidize reduced S compounds that could support autotrophy. Two strains of marine magnetotactic bacteria, MV-1 and MV-2, isolated from such sites grew in O₂-gradient media with H₂S or thiosulfate (S₂O₃^2–) as electron sources and O₂ as electron acceptor or anaerobically with S₂O₃^2– and N₂O as electron acceptor, with bicarbonate (HCO₃^–)/CO₂ as sole C source. Cells grown with H₂S contained S-rich inclusions. Cells oxidized S₂O₃^2– to sulfate (SO₄^2–). Both strains grew microaerobically with formate. Neither grew microaerobically with tetrathionate (S₄O₆^2–), methanol, or Fe²⁺ as FeS, or siderite (FeCO₃). Growth with S₂O₃^2– and radiolabeled ¹⁴C-HCO₃^– showed that cell C was derived from HCO₃^–/CO₂. Cell-free extracts showed ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) activity. Southern blot analyses indicated the presence of a form II RubisCO (cbbM) but no form I (cbbL) in both strains. cbbM and cbbQ, a putative post-translational activator of RubisCO, were identified in MV-1. MV-1 and MV-2 are thus chemolithoautotrophs that use the Calvin–Benson–Bassham pathway. cbbM was also identified in Magnetospirillum magnetotacticum. Thus, magnetotactic bacteria at the oxic–anoxic transition zone of chemically stratified aquatic environments are important in C cycling and primary productivity.     

Ecophysiological characterization of carnivorous plant roots: Oxygen fluxes,respiration, and water exudation

Various ecophysiological investigations on carnivorous plants in wet soils are presented. Radial oxygen loss from roots of Droseraceae to an anoxic medium was relatively low 0.02 – 0.07 mol(O₂) m^{– 2} s^–1 in the apical zone, while values of about one order of magnitude greater were found in both Sarracenia rubra roots and Genlisea violacea traps. Aerobic respiration rates were in the range of 1.6 – 5.6 mol kg^–1 (f.m.) s^–1 for apical root segments of seven carnivorous plant species and 0.4 – 1.1 mol kg^–1 (f.m.) s^–1 for Genlisea traps. The rate of anaerobic fermentation in roots of two Drosera species was only 5 – 14 % of the aerobic respiration. Neither 0.2 mM NaN₃ nor 0.5 mM KCN influenced respiration rate of roots and traps. In all species, the proportion of cyanide-resistant respiration was high and amounted to 65 – 89 % of the total value. Mean rates of water exudation from excised roots of 12 species ranged between 0.4 – 336 mm 3 kg^–1 (f.m.) s^–1 with the highest values being found in the Droseraceae. Exudation from roots was insensitive to respiration inhibitors. No significant difference was found between exudation rates from roots growing in situ in anoxic soil and those kept in an aerated aquatic medium. Carnivorous plant roots appear to be physiologically very active and well adapted to endure permanent soil anoxia.     

Photosynthesis and respiration of Griffithsia monilis (Rhodophyceae): Effect of light,salinity, and oxygen

The giant-celled alga Griffithsia monilis has a low light compensation point and saturates photosynthesis at 60–90 mol photons m^-2s^-1 (oxygen evolution and CO₂ fixation). Under dark and low light intensities ¹⁴C is preferentially incorporated into amino acids (mainly aspartate and alanine). With increasing light a gradual change was observed and, under light saturation, compounds of the anionic fraction (digeneaside and hexosephosphates) were the most strongly labeled compounds, together with the amino acids glycine and serine. To a large extent (30–40% of the total) ¹⁴C was fixed into EtOH-insoluble products, the hydrolysates of which consisted mainly of glucose and mannose. In the steady state the rates of photosynthesis and respiration decreased with increasing salinity. Changes in the rates after hyperosmotic shocks were less severe in cells adapted to high salinities. Photorespiration exists in Griffithsia: Glycine and serine are the major labeled compounds in O₂-saturated media.     

Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C₃, C₃-C₄, C₄-like, and C₄): the O₂ inhibition of photosynthesis measured above the O₂ partial pressure which gives a maximum rate, and O₂- and light-dependent whole-chain electron flow measured at the CO₂ compensation point (). The optimum level of O₂ for maximum photosynthetic rates under atmospheric levels of CO₂ (34 Pa) was lower in C₃ and C₃-C₄ species (ca. 2 kPa) than in C₄-like and C₄ species (ca. 9 kPa). Increasing O₂ partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO₂. This inhibition was calculated as the O₂ inhibition index (_A, the percentage inhibition of photosynthesis per kPa increase in O₂). From measurements of 18 Flaveria species at atmospheric CO₂, the _A values decreased from C₃ (1.9–2.1) to C₃-C₄ (1.2–1.6), C₄-like (0.6–0.8) and C₄ species (0.3–0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at under atmospheric levels of O₂, and increasing O₂ partial pressure at 300 mol quanta·m^–2·s^–1, there was a similar increase in the rate of O₂ evolution associated with whole-chain electron flow (J_{o 2}, calculated from chlorophyll fluorescence analysis) in the C₃ and C₃-C₄ species compared to a much lower rate in the C₄-like and C₄ species. The results indicate that there is substantial O₂-dependent electron flow in C₃ and C₃-C₄ species, reflecting a high level of photorespiration compared to that in C₄-like and C₄ species. Consistent with these results, there was a significant decrease in from C₃ (6–6.2 Pa) to C₃-C₄ (1.0–3.0 Pa), to C₄-like and C₄ species (0.3–0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C₃ and C₃-C₄ species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower values, primarily by refixing photorespired CO₂. The C₄-like and C₄ Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO₂ pump via the C₄ pathway.Abbreviations and Symbols A CO₂ assimilation rate - CE carboxylation efficiency - C_i intercellular CO₂ partial pressure - I_a absorbed PPFD - J_{o 2} oxygen evolution from PSII - PPFD photosynthetic photon flux density (mol · m^–2· s^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - VPD water-vapor pressure difference between the leaf and atmospheric air - CO₂ compensation point - _{CO 2} quantum yield of CO₂ assimilation - _PSII quantum yield of photosystem II - _A O₂ inhibition index for photosynthesis (percentage inhibition of photosynthesis per kPa increase in O₂) This research was supported by the National Science Foundation Grant IBN 9317756 and Equipment (Grant DMB-8515521 and DOE/USDA/NSF Triagency Plnat Biochemistry Research Training Grant Program.     

Root respiration and its importance for the carbon balance of beech saplings (Fagus sylvatica L.) in a montane beech forest

Root respiration of 10-year-old beech saplings (Fagus sylvatica L.) grown in the understorey (UND) and in a natural gap (GAP) of a mature beech forest in the Solling mountains, FRG, was investigated from April until December, 1990. Respiration rates of fine, medium and coarse roots were measured in situ by a PC-controlled cuvette system. Fine root respiration rates were in the range of 0.5–9.8 nmol CO₂ gDW^–1 s^–1 at both sites, but respiration rates of UND saplings were higher, compared to those of GAP saplings. The dependence of respiratory activity on soil temperature proved to be highly significant (p<0.001) for both plots, following a quasi-Arrhenius type curve. Fine root respiration rates of UND saplings were highly significantly, negatively correlated with the water content of the attached organic material, whereas respiration rates of GAP saplings did not show such a correlation. Further, a significant correlation (p<0.01) between mycorrhizal biomass and respiration rate was detected at the UND site, but not at the GAP site. Medium and coarse root respiration rates were very similar and no significant differences between the two sites were detected. Maximum respiration rates of 3.1 nmol CO₂ gDW^–1 s^–1 were reached in the middle of July. Due to low light intensities in the under storey, daily net CO₂ assimilation rates of UND saplings were much smaller than those of GAP saplings. At both sites, net CO₂ assimilation rates varied more than respiration rates and thus the carbon balance of beech saplings was more affected by the rate of carbon fixation than by the rate of respiratory carbon loss.     

Co-function of C3-and C4-photosynthetic pathways in C3, C4 and C3-C4 intermediate Flaveria species

The potential for C₄ photosynthesis was investigated in five C₃-C₄ intermediate species, one C₃ species, and one C₄ species in the genus Flaveria, using ¹⁴CO₂ pulse-¹²CO₂ chase techniques and quantum-yield measurements. All five intermediate species were capable of incorporating ¹⁴CO₂ into the C₄ acids malate and aspartate, following an 8-s pulse. The proportion of ¹⁴C label in these C₄ products ranged from 50–55% to 20–26% in the C₃-C₄ intermediates F. floridana Johnston and F. linearis Lag. respectively. All of the intermediate species incorporated as much, or more, ¹⁴CO₂ into aspartate as into malate. Generally, about 5–15% of the initial label in these species appeared as other organic acids. There was variation in the capacity for C₄ photosynthesis among the intermediate species based on the apparent rate of conversion of ¹⁴C label from the C₄ cycle to the C₃ cycle. In intermediate species such as F. pubescens Rydb., F. ramosissima Klatt., and F. floridana we observed a substantial decrease in label of C₄-cycle products and an increase in percentage label in C₃-cycle products during chase periods with ¹²CO₂, although the rate of change was slower than in the C₄ species, F. palmeri. In these C₃-C₄ intermediates both sucrose and fumarate were predominant products after a 20-min chase period. In the C₃-C₄ intermediates, F. anomala Robinson and f. linearis we observed no significant decrease in the label of C₄-cycle products during a 3-min chase period and a slow turnover during a 20-min chase, indicating a lower level of functional integration between the C₄ and C₃ cycles in these species, relative to the other intermediates. Although F. cronquistii Powell was previously identified as a C₃ species, 7–18% of the initial label was in malate+aspartate. However, only 40–50% of this label was in the C-4 position, indicating C₄-acid formation as secondary products of photosynthesis in F. cronquistii. In 21% O₂, the absorbed quantum yields for CO₂ uptake (in mol CO₂·[mol quanta]^-1) averaged 0.053 in F. cronquistii (C₃), 0.051 in F. trinervia (Spreng.) Mohr (C₄), 0.052 in F. ramosissima (C₃-C₄), 0.051 in F. anomala (C₃-C₄), 0.050 in F. linearis (C₃-C₄), 0.046 in F. floridana (C₃-C₄), and 0.044 in F. pubescens (C₃-C₄). In 2% O₂ an enhancement of the quantum yield was observed in all of the C₃-C₄ intermediate species, ranging from 21% in F. ramosissima to 43% in F. pubescens. In all intermediates the quantum yields in 2% O₂ were intermediate in value to the C₃ and C₄ species, indicating a co-function of the C₃ and C₄ cycles in CO₂ assimilation. The low quantum-yield values for F. pubescens and F. floridana in 21% O₂ presumably reflect an ineffcient transfer of carbon from the C₄ to the C₃ cycle. The response of the quantum yield to four increasing O₂ concentrations (2–35%) showed lower levels of O₂ inhibition in the C₃-C₄ intermediate F. ramosissima, relative to the C₃ species. This indicates that the co-function of the C₃ and C₄ cycles in this intermediate species leads to an increased CO₂ concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase and a concomitant decrease in the competitive inhibition by O₂.Abbreviations PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate