Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: IV. ANALYSIS OF REDUCED OXYGEN RESPONSE IN PANICUM MILIOIDES AND PANICUM SCHENCKII

Reduced photorespiration has been reported in Panicum milioides on the basis of lower CO₂ compensation concentrations than in C₃ species, lower CO₂ evolution in the light, and less response of apparent photosynthesis to O₂ concentration. The lower response to O₂ in P. milioides could be due to reduced O₂ competition with CO₂ for reaction with ribulose 1,5-bisphosphate, to a reduced loss of CO₂, or to an initial fixation of CO₂ by phosphoenolpyruvate carboxylase. Experiments were carried out with Panicum maximum Jacq., a C₄ species having no apparent photorespiration; tall fescue (Festuca arundinacea Schreb.), a C₃ species; P. milioides Nees ex Trin.; and Panicum schenckii Hack. The latter two species are closely related and have low photorespiration rates. CO₂ exchange was measured at five CO₂ concentrations ranging from 0 to 260 microliters per liter at both 2 and 21% O₂. Mesophyll conductance or carboxylation efficiency was estimated by plotting substomatal CO₂ concentrations against apparent photosynthesis. In the C₄ species P. maximum, mesophyll conductance was 0.96 centimeters per second and was unaffected by O₂ concentration. At 21% O₂ mesophyll conductance of tall fescue was decreased 32% below the value at 2% O₂. Decreases in mesophyll conductance at 21% O₂ for P. milioides and P. schenckii were similar to that for tall fescue. On the other hand, loss of CO₂ in CO₂-free air, estimated by extrapolating the CO₂ response curve to zero CO₂, was increased from 1.8 to 6.5 milligrams per square decimeter per hour in tall fescue as O₂ was raised from 2-21%. Loss of CO₂ was less than 1 milligram per square decimeter per hour for P. milioides and P. schenckii and was unaffected by O₂. The results suggest that the reduced O₂ response in P. milioides and P. schenckii is due to a lower loss of CO₂ in the light rather than less inhibition of carboxylation by O₂, since the decrease in carboxylation efficiency at 21% O₂ was similar for P. milioides, P. schenckii, and tall fescue. The inhibition of apparent photosynthesis by 21% O₂ in these three species at low light intensities was similar at 31 to 36% which also indicates similar O₂ effects on carboxylation. Apparent photosynthesis at high light intensity was inhibited less by 21% O₂ in P. milioides (16.8%) and P. schenckii (23.8%) than in tall fescue (28.4%). This lower inhibition in the Panicum species may have been due to a higher degree of recycling of photorespired CO₂ in these species than in tall fescue.     

Oxygen Stimulation of Apparent Photosynthesis in Flaveria linearis

A plant was found in the C₃-C₄ intermediate species, Flaveria linearis, in which apparent photosynthesis is stimulated by atmospheric O₂ concentrations. A survey of 44 selfed progeny of the plant showed that the O₂ stimulation of apparent photosynthesis was passed on to the progeny. When leaves equilibrated at 210 milliliters per liter O₂ were transferred to 20 milliliters per liter O₂ apparent photosynthesis was initially stimulated, but gradually declined so that at 30 to 40 minutes the rate was only about 80 to 85% of that at 210 milliliters per liter O₂. Switching from 20 to 210 milliliters per liter caused the opposite transition in apparent photosynthesis. All other plants of F. linearis reached steady rates within 5 minutes after switching O₂ that were 20 to 24% lower in 210 than in 20 milliliters per liter O₂. At low intercellular CO₂ concentrations and low irradiances, O₂ inhibition of apparent photosynthesis of the aberrant plant was similar to that in normal plants, but at an irradiance of 2 millimoles quanta per square meter per second and near 300 microliters per liter CO₂ apparent photosynthesis was consistently higher at 210 than at 20 milliliters per liter O₂. In morphology and leaf anatomy, the aberrant plant is like the normal plants in F. linearis. The stimulation of apparent photosynthesis at air levels of O₂ in the aberrant plant is similar to other literature reports on observations with C₃ plants at high CO₂ concentrations, high irradiance and/or low temperatures, and may be related to limitation of photosynthesis by triose phosphate utilization.     

Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways: III. OXYGEN RESPONSE AND ENZYME ACTIVITIES OF SPECIES IN THE LAXA GROUP OF PANICUM

Measurements of CO₂ exchange at varying O₂ concentrations in seven grass species of the Laxa group of Panicum and activities of five photosynthetic enzymes were compared to values obtained for these characters in a cool season C₃ grass, tall fescue (Festuca arundinacea Schreb.) and a C₄ grass, P. maximum Jacq. Plants were divided into three groups on the basis of the inhibition of apparent photosynthesis by 21% O_2. Rates of apparent photosynthesis in P. prionitis Griseb. and P. maximum were virtually unaffected by changes in O₂ concentration. In another group consisting of P. hylaeicum Mez., P. rivulare Trin., P. laxum Sw., and tall fescue apparent photosynthesis was inhibited by 28.2 to 36.0% at 21% O_2. An intermediate inhibition of 20.6 to 23.3% at 21% O₂ was exhibited by P. milioides Nees ex Trin., P. schenckii Hack., and P. decipiens Nees ex Trin. The CO₂ compensation concentration for P. prionitis and P. maximum was low (≤6 microliters per liter CO₂ at 21% O₂) and affected little by O₂, whereas values for P. hylaeicum, P. rivulare, P. laxum, and tall fescue were much greater, and increased almost linearly from 2 to 48% O_2. Values for P. milioides, P. schenckii, and P. decipiens were intermediate to the other two groups. The effect of O₂ on total leaf conductance to CO₂ was similar to the C₃ grasses and the intermediate Panicums. However, estimates of photorespiration in the intermediate species were low and changed little with O₂ in comparison to estimates for the C₃ species which were higher and increased greatly with increased O_2.     

Photosynthesis in Grass Species Differing in Carbon Dioxide Fixation Pathways: II. A Search for Species with Intermediate Gas Exchange and Anatomical Characteristics

Thirty-three grass species were examined in two experiments in an attempt to locate plants with photosynthetic responses to O₂, CO₂ compensation concentrations, and leaf anatomy intermediate to those of C₃ and C₄ species. Species examined included seven from the Laxa group in the Panicum genus, one of which, P. milioides Nees ex Trin., has been reported earlier to have intermediate characteristics. The species with O₂-sensitive photosynthesis typical of C₃ plants showed more than 37% increase in apparent photosynthesis at 2% O₂ compared to 21% O₂ at 25 C and 335 microliters per liter CO₂, whereas in Panicum milioides, P. schenckii Hack., and P. decipiens Nees ex Trin., members of the Laxa group of Panicum, increases ranged from 25 to 30%. The remainder of the species did not respond to O₂. Species with O₂ responses characteristic of C₃ plants exhibited CO₂ compensation concentrations of 44 microliters per liter or higher at 21% O₂ and 25 to 27.5 C and species characterized as O₂-insensitive had values of microliters per liter or less. The CO₂ compensation concentration (Г) values of P. milioides, P. schenckii, and P. decipiens ranged from 10.3 to 23.3 microliters per liter. Other species of the Laxa group of Panicum exhibited O₂ response and Г values of either C₃ (P. laxum Sw., P. hylaeicum Mez., and P. rivulare Trin.) or C₄ (P. prionitis Griseb.) plants. Leaves of species with O₂ response and CO₂ compensation values typical of C₃ plants had poorly developed or nearly empty bundle sheath cells, and much larger distances and mesophyll cell numbers between veins than did the O₂-insensitive ones. Vein spacings in P. milioides, P. schenckii, and P. decipiens ranged from 0.18 to 0.28 millimeter and mesophyll cell number between veins from 5.2 to 7.8. While these vein spacings are closer than those of most C₃ grasses, two O₂-sensitive species of Dactylis had vein spacings similar to these Panicums and veins in Glyceria striata, another O₂-sensitive plant, were separated by only four mesophyll cells and 0.12 millimeter. Bundle sheath cells of the three intermediate Panicums contained greater quantities of organelles than are typical for C₃ grasses.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways : VI. DIFFERENTIAL EFFECTS OF TEMPERATURE AND LIGHT INTENSITY ON PHOTORESPIRATION IN C(3), C(4), AND INTERMEDIATE SPECIES

The effects of temperature and photosynthetically active radiation levels on photorespiration were investigated in Panicum milioides Nees ex Trin. and Panicum schenckii Hack., species known to have low photorespiration rates and other characteristics intermediate between C₃ and C₄ species. Comparisons were made with the C₃ grass species tall fescue (Festuca arundinacea Schreb.). An increase in temperature from 20 to 35 C raised photorespiration from 7.3 to 10.2 milligrams per square decimeter per hour in tall fescue, but the increase in P. schenckii was less than 1 milligram per square decimeter per hour. Increases in temperature caused much less change in CO₂ compensation concentration in P. milioides and P. schenckii than in tall fescue, values of 160 microliters per liter being obtained in tall fescue at 40 C compared to about 40 microliters per liter for P. milioides and P. schenckii. Photorespiration in P. schenckii increased by only about 1 milligram CO₂ per square decimeter per hour as the photosynthetically active radiation level was raised from 100 to 2,000 microEinsteins per square meter per second. Loss of CO₂ into CO₂-free air actually decreased from 2.2 to 1.0 milligrams per square decimeter per hour as the radiation level was raised from 100 to 1,100 microEinsteins per square meter per second but tended to rise again at 2,000 microEinsteins per square meter per second. In contrast, photorespiration in tall fescue tripled with radiation level over the same range, reaching a maximum value of 7.2 milligrams per square decimeter per hour as determined by extrapolation of the CO₂ response curves to zero CO₂. The CO₂ compensation concentration in tall fescue was nearly insensitive to photosynthetically active radiation above 140 microEinsteins per square meter per second but, in P. milioides and P. schenckii, it decreased from values of 69 and 62 microliters per liter, respectively, to values of 21 and 16 as the radiation level was increased from 50 to 1075 microEinsteins per square meter per second. Interpolation of CO₂-response curves showed that an increase in photosynthetically active radiation level from 100 to 2,000 microEinsteins per square meter per second reduced the CO₂ compensation value of P. schenckii from 38 to 19 microliters per liter. Data from these experiments indicate reduced photorespiration or a CO₂-recycling mechanism in P. milioides and P. schenckii which causes apparent photorespiration to be nearly insensitive to temperature in the 20 to 35 C range and to decrease at high radiation intensities.     

Leaf photosynthesis and conductance of selected triticum species at different water potentials

Leaf gas exchange characteristics of a desert annual (Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 (Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from −0.50 to −2.9 megapascals. At an ambient [CO₂] of 330 microliters per liter, T. kotschyi had higher conductance and CO₂ assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO₂ concentration (C_i) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO₂] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C_i and lower water use efficiency. Under water deficits, however, C_i at 330 microliters per liter ambient [CO₂] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C_i response curves had higher A at C_i values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis.     

Inorganic Carbon Diffusion between C(4) Mesophyll and Bundle Sheath Cells: Direct Bundle Sheath CO(2) Assimilation in Intact Leaves in the Presence of an Inhibitor of the C(4) Pathway

Photosynthesis rates of detached Panicum miliaceum leaves were measured, by either CO₂ assimilation or oxygen evolution, over a wide range of CO₂ concentrations before and after supplying the phosphoenolpyruvate (PEP) carboxylase inhibitor, 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (DCDP). At a concentration of CO₂ near ambient, net photosynthesis was completely inhibited by DCDP, but could be largely restored by elevating the CO₂ concentration to about 0.8% (v/v) and above. Inhibition of isolated PEP carboxylase by DCDP was not competitive with respect to HCO₃⁻, indicating that the recovery was not due to reversal of enzyme inhibition. The kinetics of ¹⁴C-incorporation from ¹⁴CO₂ into early labeled products indicated that photosynthesis in DCDP-treated P. miliaceum leaves at 1% (v/v) CO₂ occurs predominantly by direct CO₂ fixation by ribulose 1,5-bisphosphate carboxylase. From the photosynthesis rates of DCDP-treated leaves at elevated CO₂ concentrations, permeability coefficients for CO₂ flux into bundle sheath cells were determined for a range of C₄ species. These values (6-21 micromoles per minute per milligram chlorophyll per millimolar, or 0.0016-0.0056 centimeter per second) were found to be about 100-fold lower than published values for mesophyll cells of C₃ plants. These results support the concept that a CO₂ permeability barrier exists to allow the development of high CO₂ concentrations in bundle sheath cells during C₄ photosynthesis.     

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.     

Environmental Effects on Photorespiration of C(3)-C(4) Species : I. Influence of CO(2) and O(2) during Growth on Photorespiratory Characteristics and Leaf Anatomy

The possibility of altering CO₂ exchange of C₃-C₄ species by growing them under various CO₂ and O₂ concentrations was examined. Growth under CO₂ concentrations of 100, 350, and 750 micromoles per mole had no significant effect on CO₂ exchange characteristics or leaf anatomy of Flaveria pringlei (C₃), Flaveria floridana (C₃-C₄), or Flaveria trinervia (C₄). Carboxylation efficiency and CO₂ compensation concentrations in leaves of F. floridana developed under the different CO₂ concentrations were intermediate to F. pringlei and F. trinervia. When grown for 12 days at an O₂ concentration of 20 millimoles per mole, apparent photosynthesis was strongly inhibited in Panicum milioides (C₃-C₄) and to a lesser degree in Panicum laxum (C₃). In P. milioides, acute starch buildup was observed microscopically in both mesophyll and bundle sheath cells. Even after only 4 days exposure to 20 millimoles per mole O₂, the presence of starch was more pronounced in leaf cross-sections of P. milioides compared to those at 100 and 210 millimoles per mole. Even though this observation suggests that P. milioides has a different response to low O₂ with respect to translocation of photosynthate or sink activity than C₃ species, the concentration of total available carbohydrate increased in shoots of all species by 33% or more when grown at low O₂. This accumulation occurred even though relative growth rates of Festuca arundinacea (C₃) and P. milioides grown for 4 days at 210 millimoles per mole O₂, were inhibited 83 and 37%, respectively, when compared to plants grown at 20 millimoles per mole O₂.     

Photosynthesis, Leaf Anatomy, and Morphology of Progeny from Hybrids between C(3) and C(3)/C(4)Panicum Species

Species in the Laxa group of Panicum have C₃ or C₃/C₄ photosynthesis based on leaf anatomical and CO₂ exchange characteristics. Hybrids were previously made between C₃/C₄ and C₃ species in this group (RH Brown et al. 1985 Plant Physiol 77: 653-658). In this paper, CO₂ exchange, morphological, and leaf anatomical characteristics of F₂ or F₅ progeny from colchicine-induced amphiploids of C₃/C₄ × C₃ hybrids (Panicum milioides Nees ex Trin. [C₃/C₄] × Panicum laxum Mez [C₃] and Panicum spathellosum Doell [C₃/C₄] × Panicum boliviense Hack. [C₃]) were studied.

There were no differences found in morphology or physiology between the amphiploids and the F₁ hybrids from which they were produced. In the segregating progeny, CO₂ compensation concentration and photorespiration values typical of C₃, but not of C₃/C₄ plants, were recovered. Progeny were found from both crosses which possessed O₂ inhibition of apparent photosynthesis typical of the parents, and in the case of the P. milioides × P. laxum cross, leaf anatomy and overall plant morphology typical of the parents were observed in some progeny. The progeny were found to possess recombinations of various traits associated with reduced photorespiration, so that no correlation existed among O₂ inhibition of apparent photosynthesis, CO₂ compensation concentration, and leaf anatomical traits. One plant was especially noteworthy in possessing leaf anatomy typical of C₃/C₄ plants, but with CO₂ exchange characteristics of C₃ plants.

     

C(3)-C(4) Intermediate Species in Alternanthera (Amaranthaceae) : Leaf Anatomy, CO(2) Compensation Point, Net CO(2) Exchange and Activities of Photosynthetic Enzymes

Two naturally occurring species of the genus Alternanthera, namely A. ficoides and A. tenella, were identified as C₃-C₄ intermediates based on leaf anatomy, photosynthetic CO₂ compensation point (Γ), O₂ response of г, light intensity response of г, and the activities of key enzymes of photosynthesis. A. ficoides and A. tenella exhibited a less distinct Kranz-like leaf anatomy with substantial accumulation of starch both in mesophyll and bundle sheath cells. Photosynthetic CO₂ compensation points of these two intermediate species at 29°C were much lower than in C₃ plants and ranged from 18 to 22 microliters per liter. Although A. ficoides and A. tenella exhibited similar intermediacy in г, the apparent photorespiratory component of O₂ inhibition in A. ficoides is lower than in A. tenella. The г progressively decreases from 35 microliters per liter at lowest light intensity to 18 microliters per liter at highest light intensity in A. tenella. It was, however, constant in A. ficoides at 20 to 25 microliters per liter between light intensities measured. The rates of net photosynthesis at 21% O₂ and 29°C by A. ficoides and A. tenella were 25 to 28 milligrams CO₂ per square decimeter per hour which are intermediate between values obtained for Tridax procumbens and A. pungens, C₃ and C₄ species, respectively. The activities of key enzymes of C₄ photosynthesis, phosphoenolpyruvate carboxylase, pyruvate Pi dikinase, NAD malic enzyme, NADP malic enzyme and phosphoenolpyruvate carboxykinase in the two intermediates, A. ficoides and A. tenella are very low or insignificant. Results indicated that the relatively low apparent photorespiratory component in these two species is presumably the basis for the C₃-C₄ intermediate photosynthesis.     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

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.     

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.     

Changes in Levels of Intermediates of the C(4) Cycle and Reductive Pentose Phosphate Pathway under Various Light Intensities in Maize Leaves

The rate of CO₂ assimilation and levels of metabolites of the C₄ cycle and reductive pentose phosphate pathway in attached leaves of maize (Zea mays L.) were measured over a range of light intensity from 0 to 1,900 microEinsteins per square meter per second under a saturated CO₂ concentration of 350 microliters per liter and a limiting CO₂ concentration of 133 microliters per liter. The level of ribulose 1,5-bisphosphate (RuBP) stayed almost constant (around 60 nanomoles per milligram chlorophyll [Chl]) from low to high light intensities under 350 microliters per liter. Levels of 3-phosphoglycerate (PGA) increased from 100 to 650 nanomoles per milligram Chl under 350 microliters per liter CO₂ with increasing light intensity. The calculated RuBP concentration of 6 millimolar (corresponded to 60 nanomoles per milligram Chl) was about two times above the estimated RuBP binding-site concentration on ribulose bisphosphate carboxylase-oxygenase (Rubisco) of ~2.6 millimolar in maize bundle sheath chloroplasts in the light. The ratio of RuBP/PGA increased with decreasing light intensity under 350 microliters per liter CO₂. These results suggest that RuBP carboxylation is under control of light intensity possibly due to a limited supply of CO₂ to Rubisco through the C₄ cycle whose activity is highly dependent on light intensity. Pyruvate level increased with increasing light intensity as long as photosynthesis rate increased. A positive relationship between levels of PGA and those of pyruvate during steady-state photosynthesis under various conditions suggests that an elevated concentration of PGA increases the carbon input into the C₄ cycle through the conversion of PGA to PEP and consequently the level of total intermediates of the C₄ cycle can be raised to mediate higher photosynthesis rate.     

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways : VIII. Ultrastructural Characteristics of Panicum Species in the Laxa Group

Ultrastructural studies of leaves of seven Panicum species in or closely related to the Laxa group and classified as C₃, C₄ or C₃-C₄ intermediate were undertaken to examine features associated with C₃ and C₄ photosynthesis. The C₃ species Panicum rivulare Trin. had few organelles in bundle sheath cell profiles (2 chloroplasts, 1.1 mitochondria, and 0.3 peroxisomes per cell section) compared to an average of 10.6 chloroplasts, 17.7 mitochondria, and 3.2 peroxisomes per bundle sheath cell profile for three C₃-C₄ species, Panicum milioides Nees ex Trin., Panicum decipiens Nees ex Trin. and Panicum schenckii Hack. However, two other C₃ species, Panicum laxum Sw. and Panicum hylaeicum Mez, contained about 0.7, 0.5, and 0.3 as many chloroplasts, mitochondria, and peroxisomes, respectively, as in bundle sheath cell profiles of the C₃-C₄ species. Chloroplasts and mitochondria in bundle sheath cells were larger than those in mesophyll cells for the C₄ species Panicum prionitis Griseb. and the C₃-C₄ species, but in C₃ species the organelles were similar in size or were smaller in the bundle sheath cells. The C₃-C₄ species and P. laxum and P. hylaeicum exhibited an unusually close association of organelles in bundle sheath cells with mitochondria frequently surrounded in profile by chloroplasts. The high concentrations in bundle sheath cells of somewhat larger organelles than in mesophyll cells correlates with the reduced photorespiration of the C₃-C₄ species.     

The Regulation of Photosynthesis in Leaves of Field-Grown Spring Wheat (Triticum aestivum L., cv Albis) at Different Levels of Ozone in Ambient Air

Wheat (Triticum aestivum L. cv Albis) was grown in open-top chambers in the field and fumigated daily with charcoal-filtered air (0.015 microliters per liter O₃), nonfiltered air (0.03 microliters per liter O₃), and air enriched with either 0.07 or 0.10 microliters per liter ozone (seasonal 8 hour/day [9 am-5 pm] mean ozone concentration from June 1 until July 10, 1987). Photosynthetic ¹⁴CO₂ uptake was measured in situ. Net photosynthesis, dark respiration, and CO₂ compensation concentration at 2 and 21% O₂ were measured in the laboratory. Leaf segments were freeze-clamped in situ for the determination of the steady state levels of ribulose 1,5-bisphosphate, 3-phosphoglycerate, triose-phosphate, ATP, ADP, AMP, and activity of ribulose, 1,5-bisphosphate carboxylase/oxygenase. Photosynthesis of flag leaves was highest in filtered air and decreased in response to increasing mean ozone concentration. CO₂ compensation concentration and the ratio of dark respiration to net photosynthesis increased with ozone concentration. The decrease in photosynthesis was associated with a decrease in chlorophyll, soluble protein, ribulose bisphosphate carboxylase/oxygenase activity, ribulose bisphosphate, and adenylates. No decrease was found for triose-phosphate and 3-phosphoglycerate. The ratio of ATP to ADP and of triosephosphate to 3-phosphoglycerate were increased suggesting that photosynthesis was limited by pentose phosphate reductive cycle activity. No limitation occurred due to decreased access of CO₂ to photosynthetic cells since the decrease in stomatal conductance with increasing ozone concentration did not account for the decrease in photosynthesis. Ozonestressed leaves showed an increased degree of activation of ribulose bisphosphate carboxylase/oxygenase and a decreased ratio of ribulose bisphosphate to initial activity of ribulose bisphosphate carboxylase/oxygenase. Nevertheless, it is suggested that photosynthesis in ozone stressed leaves is limited by ribulose bisphosphate carboxylation possibly due to an effect of ozone on the catalysis by ribulose bisphosphate carboxylase/oxygenase.     

Photosynthetic Characteristics of the C(3)-C(4) Intermediate Parthenium hysterophorus

The weedy species Parthenium hysterophorus (Asteraceae) possesses a Kranz-like leaf anatomy. The bundle sheath cells are thick-walled and contain numerous granal chloroplasts, prominent mitochondria, and peroxisomes, all largely arranged in a centripetal position. Both mesophyll and bundle sheath chloroplasts accumulate starch. P. hysterophorus exhibits reduced photorespiration as indicated by a moderately low CO₂ compensation concentration (20-25 microliters per liter at 30°C and 21% O₂) and by a reduced sensitivity of net photosynthesis to 21% O₂. In contrast, the related C₃ species P. incanum and P. argentatum (guayule) lack Kranz anatomy, have higher CO₂ compensation concentrations (about 55 microliters per liter), and show a greater inhibition of photosynthesis by 21% O₂. Furthermore, in P. hysterophorus the CO₂ compensation concentration is relatively less sensitive to changes in O₂ concentrations and shows a biphasic response to changing O₂, with a transition point at about 11% O₂. Based on these results, P. hysterophorus is classified as a C₃-C₄ intermediate. The activities of diagnostic enzymes of C₄ photosynthesis in P. hysterophorus were very low, comparable to those observed in the C₃ species P. incanum (e.g. phosphoenolpyruvate carboxylase activity of 10-29 micromoles per milligram of chlorophyll per hour). Exposures of leaves of each species to ¹⁴CO₂ (for 8 seconds) in the light resulted in 3-phosphoglycerate and sugar phosphates being the predominant initial ¹⁴C products (77-84%), with ≤4% of the ¹⁴C-label in malate plus aspartate. These results indicate that in the C₃-C₄ intermediate P. hysterophorus, the reduction in leaf photorespiration cannot be attributed to C₄ photosynthesis.     

Photosynthesis, Morphology, Leaf Anatomy, and Cytogenetics of Hybrids between C(3) and C(3)/C(4)Panicum Species

The Laxa group of the Panicum genus contains species which have CO₂ exchange and anatomical characteristics intermediate to C₃ and C₄ photosynthetic types (C₃/C₄), and also species characterized as C₃. Hybrids were made between two of the C₃/C₄ species and two C₃ species. Carbon dioxide exchange and morphological, leaf anatomical, and cytogenetic characteristics of F₁ hybrids between Panicum milioides Nees. ex Trin (C₃/C₄) and P. laxum Mez. (C₃), P. spathellosum Doell (C₃/C₄) and P. boliviense Hack. (C₃), and P. spathellosum and P. laxum were studied. There were no consistent differences in apparent photosynthesis, although two of the three hybrids had higher net CO₂ uptake than the C₃ parent. Values of inhibition of apparent photosynthesis by 21% O₂, CO₂ loss in the light, and CO₂ compensation concentration for the hybrids were between those of the parents. All three hybrids showed leaf anatomical traits, especially organelle quantities in the bundle sheath cells, between those of their respective parents. Linear regression of CO₂ compensation concentration on the percentage of mitochondria and chloroplasts in vascular bundle sheaths of the parents and hybrids gave correlation coefficients of −0.94. This suggests that the reduction in CO₂ loss in the C₃/C₄ species, and to a lesser degree in the F₁ hybrids, was due to development of organelles and perhaps a higher proportion of leaf photorespiration in bundle sheaths. The overall morphology of the hybrids was so different from the parents that they could be described as new taxonomic forms. The chromosomes in the hybrids were mainly unpaired or paired as bivalents indicating possible homology between some parental genomes.     

Photorespiration-induced reduction of ribulose bisphosphate carboxylase activation level

Leaf photosynthesis and ribulose bisphosphate carboxylase activation level were inhibited in several mutants of the C₃ crucifer Arabidopsis thaliana which possess lesions in the photorespiratory pathway. This inhibition occurred when leaves were illuminated under a photorespiratory atmosphere (50% O₂, 350 microliters per liter CO₂, balance N₂), but not in nonphotorespiratory conditions (2% O₂, 350 microliters per liter CO₂, balance N₂). Inhibition of carboxylase activation level was observed in strains with deficient glycine decarboxylase, serine transhydroxymethylase, serine-glyoxylate aminotransferase, glutamate synthase, and chloroplast dicarboxylate transport activities, but inhibition did not occur in a glycolate-P phosphatase-deficient strain. Also, the photorespiration inhibitor aminoacetonitrile produced a decline in leaf and protoplast ribulose bisphosphate carboxylase activation level, but was without effect on intact chloroplasts. Fructose bisphosphatase, a light-activated enzyme which is strongly dependent on stromal pH and Mg²⁺ for regulation, was unaffected by conditions which caused inhibition of ribulose bisphosphate carboxylase. Thus, the mechanism of inhibition does not appear to involve changes in stromal Mg²⁺ and pH but rather is associated with metabolite flux through the photorespiratory pathway.