Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: V. RESPONSE OF PANICUM MAXIMUM, PANICUM MILIOIDES, AND TALL FESCUE (FESTUCA ARUNDINACEA) TO NITROGEN NUTRITION

The response of apparent photosynthesis to N nutrition was studied in the C₃ grass, tall fescue (Festuca arundinacea Schreb.), in the C₄ species Panicum maximum Jacq., and in Panicum milioides Nees ex Trin., a species with characteristics intermediate between C₃ and C₄ photosynthetic types. Plants were grown in culture solution containing 1, 5, 50, and 200 milligrams N per liter. Apparent photosynthesis was measured on the youngest fully expanded leaves at 320 microliters of CO₂ per liter of air and 21% O₂. Leaf conductance was calculated from transpiration measurements, and CO₂ compensation concentrations were also estimated. Several leaf anatomical characteristics were studied on plastic-embedded material. Leaf N content was determined on leaves which were used in photosynthesis measurements.     

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.     

Developmental Changes in Photosynthetic Gas Exchange in the Polyol-Synthesizing Species, Apium graveolens L. (Celery)

Developmental changes in photosynthetic gas exchange were investigated in the mannitol synthesizing plant celery (Apium graveolens L. `Giant Pascal'). Greenhouse-grown plants had unusually high photosynthetic rates for a C₃ plant, but consistent with field productivity data reported elsewhere for this plant. In most respects, celery exhibited typical C₃ photosynthetic characteristics; light saturation occurred at 600 micromoles photons per square meter per second, with a broad temperature optimum, peaking at 26°C. At 2% O₂, photosynthesis was enhanced 15 to 25% compared to rates at 21% O₂. However, celery had low CO₂ compensation points, averaging 7 to 20 microliters per liter throughout the canopy. Conventional mechanisms for concentrating CO₂ were not detectable.     

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.     

Nitrogen distribution and leaf area indices in relation to photosynthetic nitrogen use efficiency in savanna grasses

Leaf photosynthetic characteristics, distribution patterns of nitrogen content per unit leaf area (n_L) and leaf area production per unit n_Lwere measured in natural stands of a C₄ grass (Hyparrhenia rufa) from the seasonal savannas and of a C₄grass (Paspalum fasciculatum) and two C₃grasses (Leersia hexandra and Hymenachne amplexicaulis) from the flooded savannas in central Venezuela. Daily rates of canopy photosynthesis (P_cD) as well as the optimal leaf area production per unit n_Lat which P_cDfor a given total amount of nitrogen in the canopy (i.e., canopy-PNUE) is maximized were also calculated. The C₃and C₄species from the flooded savannas had similar light saturated rates of photosynthesis per unit n_L(i.e. leaf-PNUE) and similar canopy-PNUEs which was in strong contrast with previous studies. Especially H. rufa but also L. hexandra and H. amplexicaulis had leaf- and canopy-PNUEs which were considerably higher than the values calculated for most other species with the same photosynthetic pathway (i.e., C₃or C₄). In contrast to previous studies, differences in the light gradient in the canopy between stands only partially explained differences in N distribution. Measured leaf area indices were greater and the average n_L values were consequently smaller than the calculated optima. There was, however, a very strong linear correlation between the optimal and actual average n_Lindicating that even though the model overestimated average n_L, it did predict the differences in leaf area production per unit nitrogen – the inverse average n_L– very well. This result strongly indicates that leaf area production per unit of leaf nitrogen increases with leaf-PNUE and decreases with the extinction coefficient for light. Grass species from seasonal savannas have extremely high leaf-PNUEs and thus optimally produce large amounts of leaf area per unit n_L. This helps explain how stands of these species may have high leaf area indices and achieve high photosynthetic productivity despite the very low nutrient availability at which they grow.     

Decarboxylation of malate by isolated bundle-sheath cells of certain plants having the C4-dicarboxylic acid cycle of photosynthesis

Summary Bundle-sheath cells isolated by the grinding and filtration procedure of Edwards and Black (1971b) from species of plants having the C₄-dicarboxylic acid pathway of photosynthesis were tested for the decarboxylation of malate from the C₄-carboxyl position. The bundle-sheath cells, which showed high malic enzyme activity in extracts, decarboxylated 4[¹⁴C]malate at rates sufficient to be involved in photosynthesis. The malate decarboxylation is dependent on the addition of magnesium or manganese and NADP⁺. The activity was increased by raising the temperature from 30 to 50°. The evidence supports the idea that malate may be a carboxyl donor to the reductive pentose-phosphate cycle in bundle-sheath cells in certain C₄-dicarboxylic acid pathway plants such as Zea mays L., Sorghum bicolor L., and Digitaria sanguinalis (L.) Scop.Abbreviations C₄ pathway C₄-dicarboxylic acid pathway - RPP pathway reductive pentose phosphate pathway - C₄ plants plants having the C₄ and the RPP pathways - C₃ plants plants having only the RPP pathway - R5P ribose-5-phosphate - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - Tricine N-tris-(hydroxymethyl)methylglycine     

Mechanism for the differential translocation of amiben in plants

The proportion of the total plant radioactivity present in shoots at the end of a 24-hour exposure of the roots to 0.5 milligram per liter ¹⁴C-3-amino-2,5-dichlorobenzoic acid (¹⁴C-amiben) ranged from 1.4 to 74.3% in 13 species. When roots of 10-day-old wheat (Triticum aestivum L. em. Thell., Triumph) and 13-day-old barnyard grass (Echinochloa crusgalli L. Beauv.) plants were treated with 0.5 milligram per liter ¹⁴C-amiben for 12 or 24 hours, barnyard grass shoots contained at least eight times more of the total plant radioactivity than did wheat shoots. In similar experiments with ¹⁴C-2-chloro-4-(ethylamino)-6-(isopropylamine)-s-triazine (¹⁴C-atrazine), there were no differences in translocation between these two species.     

Temperature response of photosynthesis in C3, C4, and CAM plants: temperature acclimation and temperature adaptation

Most plants show considerable capacity to adjust their photosynthetic characteristics to their growth temperatures (temperature acclimation). The most typical case is a shift in the optimum temperature for photosynthesis, which can maximize the photosynthetic rate at the growth temperature. These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures. In this review article, we summarize the basic differences in photosynthetic reactions in C₃, C₄, and CAM plants. We review the current understanding of the temperature responses of C₃, C₄, and CAM photosynthesis, and then discuss the underlying physiological and biochemical mechanisms for temperature acclimation of photosynthesis in each photosynthetic type. Finally, we use the published data to evaluate the extent of photosynthetic temperature acclimation in higher plants, and analyze which plant groups (i.e., photosynthetic types and functional types) have a greater inherent ability for photosynthetic acclimation to temperature than others, since there have been reported interspecific variations in this ability. We found that the inherent ability for temperature acclimation of photosynthesis was different: (1) among C₃, C₄, and CAM species; and (2) among functional types within C₃ plants. C₃ plants generally had a greater ability for temperature acclimation of photosynthesis across a broad temperature range, CAM plants acclimated day and night photosynthetic process differentially to temperature, and C₄ plants was adapted to warm environments. Moreover, within C₃ species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis (i.e., the photosynthetic rate at high-growth temperature divided by that at low-growth temperature was close to 1.0) than deciduous woody plants and annual herbaceous plants, indicating that photosynthetic acclimation would be particularly important in perennial, long-lived species that would experience a rise in growing season temperatures over their lifespan. Interestingly, across growth temperatures, the extent of temperature homeostasis of photosynthesis was maintained irrespective of the extent of the change in the optimum temperature for photosynthesis (T _opt), indicating that some plants achieve greater photosynthesis at the growth temperature by shifting T _opt, whereas others can also achieve greater photosynthesis at the growth temperature by changing the shape of the photosynthesis–temperature curve without shifting T _opt. It is considered that these differences in the inherent stability of temperature acclimation of photosynthesis would be reflected by differences in the limiting steps of photosynthetic rate.     

CO(2) Exchange, Cytogenetics, and Leaf Anatomy of Hybrids between Photosynthetically Distinct Flaveria Species

Hybrids between the C₄-like species, Flaveria brownii, A. M. Powell and the C₃-C₄ intermediate species Flaveria linearis Lag., Flaveria floridana Johnston, and Flaveria oppositifolia (DC.) Rydb. exhibited bivalent chromosome pairing during meiosis and stainability of pollen was high, ranging from 51 to 95%. An F₂ population produced from an F. brownii × F. linearis F₁ hybrid, exhibited bivalent chromosome pairing and high pollen stainability indicating a high degree of fertility in the hybrid. Oxygen inhibition of apparent photosynthesis averaged 6.8% for F. brownii and 22.2% for the C₃-C₄ species (in two experiments), and F₁ hybrids exhibited inhibitions which were intermediate to their parents. Values of carbon dioxide compensation concentration determined at low irradiance were 4.0, 34.0, and 6.5 microliters per liter for F. brownii, F. linearis and their F₁ hybrid, respectively. The mean value at low irradiance for 33 F₁ plants was 6.8 microliters per liter, and individual values ranged only from 3.7 to 11.7 microliters per liter. Anatomical characteristics for the F₁ hybrid leaves were intermediate to those of the parents, and there was considerable variation among F₂ plants derived from F. brownii × F. linearis. In the F₂ population δ¹³C values ranged from −27‰ to −20‰. The expression of more C₄-like characteristics by the F₁ hybrids in this study and their apparent high fertility make them promising specimens for producing segregating populations for use in C₄ inheritance studies.     

Leaf anatomical characters in relation to the C3 and C4 photosynthetic pathway in Cyperus (Cyperaceae)

Leaf anatomical characters of twelve species from the genus Cyperus, a genus known to contain species with both C₃ and C₄ plants, have been investigated. We investigated and established the usefulness of all‐inclusive functional leaf anatomical characters for identifying the photosynthetic pathways of these species. The species investigated were C. articulatus L., C. compressus L., C. difformis L., C. dilatatus Schum. & Thonn., C. distans L., C. esculentus L., C. haspan L., C. imbricatus Retz., C. iria L., C. rotundus L., C. sphacelatus Rottb. and C. tenuiculmis (Boeck.) Hooper, collected from locations in southwestern Nigeria. Standard anatomical procedures for examining epidermal and cross sections of leaves were employed. Our data suggested that a combination of characters, such as the occurrence of Kranz tissue, maximum cell distant count, maximum lateral cell count, interveinal distance, and to some extent leaf and mesophyll thickness, provide a reliable basis for the assessment of the photosynthetic pathways of the investigated species as compared to the isolated characters used previously. The study indicate that C. difformis and C. haspan are C₃ species while the rest follow the C₄ photosynthetic pathway. A salient feature of this study is the identification of C. dilatatus as belonging to the C₄ group.     

Measures of ecological association

Summary The relationships of photosynthetic characteristics to the competitive interactions of a C₃ plant, Chenopodium album, and a C₄ plant, Amaranthis retroflexus, were investigated in different temperature and water supply regimes. Both species had similar photosynthetic rates at 25°C, but at higher temperatures, Amaranthus had substantially greater rates than Chenopodium. Conversely, at lower temperatures, Chenopodium had an advantage. The competitive abilities in mixtures exhibited a close parallel to the photosynthetic performances with Amaranthus having an advantage at high temperatures and Chenopodium an advantage at low temperatures. These competitive outcomes were determined primarily by differences in relative growth rates prior to canopy closure. In the respective, temperature regimes, the species having the highest photosynthetic rate, which resulted an more rapid growth, overtopped and shaded the other species at the time of canopy closure. These results demonstrate that differences in photosynthetic temperature response between C₄ and C₃ plants can be an important determinant in competitive interactions, but at least in this case, the influence is primarily through, events prior to the actual initiation of competition.In contrast to temperature, growth of the plants under limited water supply had no influence on the competitive interactions. Thus, the presence of the C₄ pathway alone was not sufficient to yield a competitive advantage over the C₃ species under water limited conditions.     

The effects of clipping and soil moisture on leaf and root morphology and root respiration in two temperate and two tropical grasses

Numerous studies have explored the effect of environmental conditions on a number of plant physiological and structural traits, such as photosynthetic rate, shoot versus root biomass allocation, and leaf and root morphology. In contrast, there have been a few investigations of how those conditions may influence root respiration, even though this flux can represent a major component of carbon (C) pathway in plants. In this study, we examined the response of mass-specific root respiration (μmol CO₂ g⁻¹ s⁻¹), shoot and root biomass, and leaf photosynthesis to clipping and variable soil moisture in two C₃ (Festuca idahoensis Elmer., Poa pratensis L.) and two C₄ (Andropogon greenwayi Napper, and Sporobolus kentrophyllus K. Schum.) grass species. The C₃ and C₄ grasses were collected in Yellowstone National Park, USA and the Serengeti ecosystem, Africa, respectively, where they evolved under temporally variable soil moisture conditions and were exposed to frequent, often intense grazing. We also measured the influence of clipping and soil moisture on specific leaf area (SLA), a trait associated with moisture conservation, and specific root length (SRL), a trait associated with efficiency per unit mass of soil resource uptake. Clipping did not influence any plant trait, with the exception that it reduced the root to shoot ratio (R:S) and increased SRL in P. pratensis. In contrast to the null effect of clipping on specific root respiration, reduced soil moisture lowered specific root respiration in all four species. In addition, species differed in how leaf and root structural traits responded to lower available soil moisture. P. pratensis and A. greenwayi increased SLA, by 23% and 33%, respectively, and did not alter SRL. Conversely, S. kentrophyllus increased SRL by 42% and did not alter SLA. F. idahoensis responded to lower available soil moisture by increasing both SLA and SRL by 38% and 33%, respectively. These responses were species-specific strategies that did not coincide with photosynthetic pathway (C₃/C₄) or growth form. Thus, mass-specific root respiration responded uniformly among these four grass species to clipping (no effect) and increased soil moisture stress (decline), whereas the responses of other traits (i.e., R:S ratio, SLA, SRL) to the treatments, especially moisture availability, were species-specific. Consequently, the effects of either clipping or variation in soil moisture on the C budget of these four different grasses species were driven primarily by the plasticity of R:S ratios and the structural leaf and root traits of individual species, rather than variation in the response of mass-specific root respiration.     

Molecular phylogenies disprove a hypothesized C4 reversion in Eragrostis walteri (Poaceae)

Background and Aims

The main assemblage of the grass subfamily Chloridoideae is the largest known clade of C₄ plant species, with the notable exception of Eragrostis walteri Pilg., whose leaf anatomy has been described as typical of C₃ plants. Eragrostis walteri is therefore classically hypothesized to represent an exceptional example of evolutionary reversion from C₄ to C₃ photosynthesis. Here this hypothesis is tested by verifying the photosynthetic type of E. walteri and its classification.

Methods

Carbon isotope analyses were used to determine the photosynthetic pathway of several E. walteri accessions, and phylogenetic analyses of plastid rbcL and ndhF and nuclear internal transcribed spacer DNA sequences were used to establish the phylogenetic position of the species.

Results

Carbon isotope analyses confirmed that E. walteri is a C₃ plant. However, phylogenetic analyses demonstrate that this species has been misclassified, showing that E. walteri is positioned outside Chloridoideae in Arundinoideae, a subfamily comprised entirely of C₃ species.

Conclusions

The long-standing hypothesis of C₄ to C₃ reversion in E. walteri is rejected, and the classification of this species needs to be re-evaluated.     

Pathway of Photosynthetic Malate Formation in Vitis vinifera, a C(3) Plant

The time course of intramolecular isotope distribution in phosphoglyceric acid and serine was determined after exposure of grape leaf discs (Vitis vinifera L.) to ¹⁴CO₂ (1000 microliters per liter) for variable metabolic periods, and the labeling patterns were compared with the respective isotope distribution in the C_1-3 fragment of malic acid. The results clearly support the classical concept of a close precursor-product relationship between photosynthetic phosphoglycerate and malic acid. Under the assimilatory conditions used in this study, there was no indication of an immediate carbon transfer from serine to malate as has been suggested for C₃ plants (Kent et al. 1974 Plant Physiol 53: 491-495) because of a coincident labeling of these compounds in Vicia faba. According to our data, previous evidence in favor of this hypothetical pathway is based largely on an unusual ¹⁴C distribution in serine, due to an extreme suppression of photorespiration, as well as on arbitrary comparisons between compounds of divergent kinetic characteristics and consequently different degrees of metabolic label randomization.     

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.     

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.

     

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.     

C(4) Pathway Photosynthesis at Low Temperature in Cold-tolerant Atriplex Species

Two species of Atriplex were grown under low temperature (8 C day/6 C night) and high temperature (28 C day/20 C night) regimes. The photosynthetic capacity of these plants was studied as a function of temperature in a leaf gas exchange cuvette. Both species showed substantial photosynthetic capacity between 4 and 10 C and this was not enhanced by growth at low temperatures but rather, was somewhat greater in plants grown at higher temperature. Photosynthetic capacity of low temperature-grown plants at high temperature was greater in Atriplex confertifolia (Torr. and Frem.) S. Watts., a native of cool deserts, than in Atriplex vesicaria (Hew. ex. Benth.) from warmer desert areas. Leaves of both species were also subjected to ¹⁴CO₂ pulse-chase and steady-state feeding experiments under controlled temperature conditions. These experiments revealed that the kinetics of carbon assimilation through the intermediates of the C₄ pathway is not substantially disrupted at low temperature in either species. There was, however, a substantial interchange of label between aspartate and malate at low temperature which was not evident at high temperature. There was also an increase in the pool sizes of the C₄ acids involved in photosynthesis of A. confertifolia. Speculation as to the explanation of these changes and their possible significance in promoting low temperature C₄ photosynthesis in these plants is presented.     

Trait differences between grass species along a climatic gradient in South and North America

Question: Are trait differences between grasses along a gradient related to climatic variables and/or photosynthetic pathway? Location: Temperate grassland areas of South and North America. Methods: In a common garden experiment, we cultivated C₃ and C₄ grasses from grasslands under different climatic conditions, and we measured a set of 12 plant traits related to size and resource capture and utilization. We described (1) interspecific plant trait differences along a climatic gradient defined by the precipitation and temperature at the location where each species is dominant and (2) the association between those plant trait differences and the photosynthetic pathway of the species. Results: Trait differences between grasses were related to the precipitation at the area where each species is dominant, and to the photosynthetic pathway of the species. Leaf length, leaf width, plant height, leaf area per tiller, specific leaf area, leaf δ¹³C ratio, and nitrogen resorption efficiency increased while leaf dry matter content and nitrogen concentration in senesced leaves decreased as precipitation increased. A proportion of these changes along the gradient was related to the photosynthetic pathway because dominant grass species in cold areas with low precipitation are mainly C₃ and those from warm and wet areas are C₄. Conclusions: A previous worldwide analysis showed that traits of graminoid species measured in situ changed slightly along climatic gradients (< 10% variance explained). In contrast, under a common environment we observed that (1) grass traits changed strongly along a climatic gradient (30‐85% variance explained) and, (2) a proportion of those changes were related to the association between photosynthetic pathway of the species and precipitation.     

Effect of soil drying on growth,biomass allocation and leaf gas exchange of two annual grass species

Influence of short-term water stress on plant growth and leaf gas exchange was studied simultaneously in a growth chamber experiment using two annual grass species differing in photosynthetic pathway type, plant architecture and phenology:Triticum aestivum L. cv. Katya-A-1 (C₃, a drought resistant wheat cultivar of erect growth) andTragus racemosus (L.) All. (C₄, a prostrate weed of warm semiarid areas). At the leaf level, gas exchange rates declined with decreasing soil water potential for both species in such a way that instantaneous photosynthetic water use efficiency (PWUE, mmol CO₂ assimilated per mol H₂O transpired) increased. At adequate water supply, the C₄ grass showed much lower stomatal conductance and higher PWUE than the C₃ species, but this difference disappeared at severe water stress when leaf gas exchange rates were similarly reduced for both species. However, by using soil water more sparingly, the C₄ species was able to assimilate under non-stressful conditions for a longer time than the C₃ wheat did. At the whole-plant level, decreasing water availability substantially reduced the relative growth rate (RGR) ofT. aestivum, while biomass partitioning changed in favour of root growth, so that the plant could exploit the limiting water resource more efficiently. The change in partitioning preceded the overall reduction of RGR and it was associated with increased biomass allocation to roots and less to leaves, as well as with a decrease in specific leaf area. Water saving byT. racemosus sufficiently postponed water stress effects on plant growth occurring only as a moderate reduction in leaf area enlargement. For unstressed vegetative plants, relative growth rate of the C₄ T. racemosus was only slightly higher than that of the C₃ T. aestivum, though it was achieved at a much lower water cost. The lack of difference in RGR was probably due to growth conditions being relatively suboptimal for the C₄ plant and also to a relatively large investment in stem tissues by the C₄ T. racemosus. Only 10% of the plant biomass was allocated to roots in the C₄ species while this was more than 30% for the C₃ wheat cultivar. These results emphasize the importance of water saving and high WUE of C₄ plants in maintaining growth under moderate water stress in comparison with C₃ species.