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     

Biomass production and nitrogen content of C3- and C4- grasses in pure and mixed culture with different nitrogen supply

Summary Two C₃ grasses (Hordeum vulgare L., Avena sativa L.) and two C₄ grasses (Panicum miliaceum L., Panicum crus-galli L.) were cultivated in standard soil in the open air in pure cultures and in various mixed cultures at low and high nitrogen fertilization levels. After three months the dry weight, length and nitrogen content of the aboveground and below-ground parts of the plants and the shoot/root ratios were determined. Hordeum vulgare was the most successful species irrespective of the nitrogen fertilization level, and also exhibited in most cases the highest nitrogen concentrations. Panicum miliaceum, on the other hand, was the species least able to compete. The production of biomass was reduced in cultures growing under nitrogen starvation conditions, this phenomenon being more pronounced with respect to the C₄ than to the C₃ species. The decrease in the production of biomass at low N conditions was most drastic with Panicum crus-galli, the species with the lowest nitrogen content and thus assumed to be best adapted to nitrogen starvation conditions. In cultures growing at low nitrogen fertilization levels the shoot/root ratios of all species.shifted in favour of an increasing root proportion. The extent of this shift, however, differed from species to species.     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

Contribution of root respiration to soil respiration in a C3/C4 mixed grassland

The spatial and temporal variations of soil respiration were studied from May 2004 to June 2005 in a C₃/C₄ mixed grassland of Japan. The linear regression relationship between soil respiration and root biomass was used to determine the contribution of root respiration to soil respiration. The highest soil respiration rate of 11-54 Μmol m^-2 s^-1 was found in August 2004 and the lowest soil respiration rate of 4.99 Μmol m^-2 s^-1 was found in April 2005. Within-site variation was smaller than seasonal change in soil respiration. Root biomass varied from 0.71 kg m^-2 in August 2004 to 102 in May 2005. Within-site variation in root biomass was larger than seasonal variation. Root respiration rate was highest in August 2004 (5.7 Μmol m^-2 s^-1) and lowest in October 2004 (1.7 Μmol m^-2 s^-1). Microbial respiration rate was highest in August 2004 (5.8 Μmol m^-2 s^-1) and lowest in April 2005 (2.59 Μmol m^-2 s^-1). We estimated that the contribution of root respiration to soil respiration ranged from 31% in October to 51% in August of 2004, and from 45% to 49% from April to June 2005.     

Light/dark modulation of phosphoenolpyruvate carboxylase in C3 and C4 species

In this report, the effects of light on the activity and allosteric properties of phosphoenolpyruvate (PEP) carboxylase were examined in newly matured leaves of several C₃ and C₄ species. Illumination of previously darkened leaves increased the enzyme activity 1.1 to 1.3 fold in C₃ species and 1.4 to 2.3 fold in C₄ species, when assayed under suboptimal conditions (pH 7) without allosteric effectors. The sensitivities of PEP carboxylase to the allosteric effectors malate and glucose-6-phosphate were markedly different between C₃ and C₄ species. In the presence of 5 mM malate, the activity of the enzyme extracted from illuminated leaves was 3 to 10 fold higher than that from darkened leaves in C₄ species due to reduced malate inhibition of the enzyme from illuminated leaves, whereas it increased only slightly in C₃ species. The Ki(malate) for the enzyme increased about 3 fold by illumination in C₄ species, but increased only slightly in C₃ species. Also, the addition of the positive effector glucose-6-phosphate provided much greater protection against malate inhibition of the enzyme from C₄ species than C₃ species. Feeding nitrate to excised leaves of nitrogen deficient plants enhanced the degree of light activation of PEP carboxylase in the C₄ species maize, but had little or no effect in the C₃ species wheat. These results suggest that post-translational modification by light affects the activity and allosteric properties of PEP carboxylase to a much greater extend in C₄ than in C₃ species.     

Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C3-and C4-cycle co-function

Carbon-isotope ratios were examined as ¹³C values in several C₃, C₄, and C₃–C₄ Flaveria species, and compared to predicted ¹³C, values generated from theoretical models. The measured ¹³C values were within 4 of those predicted from the models. The models were used to identify factors that contribute to C₃-like ¹³C values in C₃–C₄ species that exhibit considerable C₄-cycle activity. Two of the factors contributing to C₃-like ¹³C values are high CO₂ leakiness from the C₄ pathway and pi/pa values that were higher than C₄ congeners. A marked break occurred in the relationship between the percentage of atmospheric CO₂ assimilated through the C₄ cycle and the ¹³C value. Below 50% C₄-cycle assimialtion there was no significant relationship between the variables, but above 50% the ¹³C values became less negative. These results demonstrate that the level of C₄-cycle expression can increase from, 0 to 50% with little integration of carbon transfer from the C₄ to the C₃ cycle. As expression increaces above 50%, however, increased integration of C₃- and C₄-cycle co-function occurs.Abbreviations and symbols RuBP carboxylase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - PEP carboxylase phosphoenolpyruvate carboxylase (EC 4.1.1.31) - pa atmospheric CO₂ partial pressure - pi intercellular CO₂ partial pressure - isotope ratio - quantum yield for CO₂ uptake     

The occurrence of both C3 and C4 photosynthetic characteristics in a single Zea mays plant

The activities of the carboxylating enzymes ribulose-1,5-biphosphate (RuBP) carboxylase and phosphoenolpyruvate (PEP) carboxylase in leaves of three-week old Zea mays plants grown under phytotron conditions were found to vary according to leaf position. In the lower leaves the activity of PEP carboxylase was lower than that of RuBP carboxylase, while the upper leaves exhibited high levels of PEP carboxylase. Carbon dioxide compensation points and net photosynthetic rates also differed in the lower and upper leaves. Differences in the fine structure of the lowermost and uppermost leaves are shown. The existence of both the C₃ and C₄ photosynthetic pathways in the same plant, in this and other species, is discussed.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose-1,5-biphosphate     

The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

Light-dependent net CO-evolution by C3 and C4 plants

Light-dependent CO-evolution by the green leaves of C₃ and C₄ plants depends on the CO₂/O₂ ratio in the ambient atmosphere. This and other physiological responses suggest that CO-evolution is a byproduct of photorespiration. At CO₂/O₂ ratios up to 10^-3, the ratio of CO evolved: CO₂ fixed in photosynthesis is significantly higher in C₃ than in C₄ plants. This discrepancy disappears when a correction is made for the CO₂-concentrating mechanism in C₄ photosynthesis, by which CO₂-concentration at the site of ribulose-bis-phosphate carboxylase/oxygenase in the bundle sheaths is raised significantly as compared to the ambient atmosphere. Since the oxygenase function of this enzyme is responsible for glycolate synthesis, i.e., the substrate of photorespiration, this result seems to support the conclusion that CO-evolution is a consequence of photorespiration. CO-evolution may turn out to be a useful and rather straightforward indicator for photorespiration in ecophysiological studies.Abbreviations CAM crassulacean acid metabolism - CO net CO-evolution - CO₂ net CO₂-fixation - PEP-C phosphoenolpyruvate carboxylase - RubP-C ribulose-bisphosphate carboxylase/oxygenase Dedicated to Professor André Pirson on the occasion of his 70th birthday     

Le vie fotosintetiche C3, C4 e CAM nel contesto ecologico

Abstract

Ecological aspects of C₃, C₄ and CAM photosynthetic pathways. - Three different photosynthetic CO₂ fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO₂ release. Both C₄ and CAM pathways behave as a CO₂ concentrating mechanism which prevent photorespiration. The CO₂-concentrating mechanism in C₄ plants is based on intracellular symplastic transport of C₄ dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO₂-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C₄ photosynthetic pathway as compared to the C₃ pathway permits higher rates of CO₂ fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO₂.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C₄ plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C₃; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.     

Structural characterization of photosynthetic cells in an amphibious sedge, Eleocharis vivipara, in relation to C3 and C4 metabolism

Eleocharis vivipara Link is a unique amphibious leafless sedge. The terrestrial form has Kranz anatomy and the biochemical traits of C₄ plants while the submerged form develops structural and biochemical traits similar to those of C₃ plants. The structural features of the culms, which are the photosynthetic organs, of the two forms were examined and compared. The culms of the terrestrial form have mesophyll cells and three bundle sheaths which consist of three kinds of cell, namely, the innermost Kranz cells that contain large numbers of organelles, the middle mestome sheath cells that lack chloroplasts, and the outermost parenchyma sheath cells that contain chloroplasts. The culms of the submerged form had a tendency towards reduction in numbers and size of Kranz cells and vascular bundles, as compared to the terrestrial form, and they had spherical mesophyll cells that were tightly packed without intercellular spaces inside the epidermis. The submerged form had a higher ratio of cross-sectional area of mesophyll cells plus parenchyma sheath cells to that of Kranz cells than the terrestrial form. The difference was mainly due to a decrease in the number and the size of the Kranz cells and to a marked increase in the size of the mesophyll cells and the parenchyma sheath cells in the submerged form, as compared to the terrestrial form. The Kranz cells of the terrestrial form had basically the structural characteristics of plants of the NAD-malic enzyme type, with the exception of the intracellular location of organelles. The Kranz cells of the submerged form included only a few organelles, and the percentage of organelles partitioned to the Kranz cells was significantly smaller in the submerged form than in the terrestrial form. In addition, the size of chloroplasts of the Kranz cells was 60–70% of that of the terrestrial form. These structural differences between the two forms may be related to the functional differences in their mechanisms of photosynthesis.Abbreviations KC Kranz cell - MC mesophyll cell - PSC parenchyma sheath cell - NAD-ME NAD-malic enzyme - VB vascular bundle This study was supported by Grants-in-Aid from the Ministry of Agriculture, Forestry and Fisheries of Japan (Integrated Research Program for the Use of Biotechnological Procedures for Plant Breeding) and from the Science and Technology Agency of Japan (Enhancement of Center-of-Excellence, the Special Coordination Funds for Promoting Science and Technology).     

Detection of Pfiesteria spp. by PCR in surface sediments collected from Chesapeake Bay tributaries (Maryland)

In 1997 blooms of Pfiesteria piscicida occurred in association with fish kills and human health problems in tributaries of the Chesapeake Bay (Maryland) and the scientific and media response resulted in large economic losses in seafood sales and tourism. These events prompted the Maryland Department of Natural Resources (MDNR) to begin monitoring for Pfiesteria spp. in water column samples. Real-time PCR assays targeted to the 18S rRNA gene were developed by our laboratories and utilized in conjunction with traditional microscopy and fish kill bioassays for detection of these organisms in estuarine water samples. This monitoring strategy aided in determining temporal and spatial distribution of motile forms of Pfiesteria spp. (i.e. zoospores), but did not assess resting stages of the dinoflagellates’ life cycle. To address this area, a 3-year study was designed using real-time PCR assays for analysis of surface sediment samples collected from several Chesapeake Bay tributaries. These samples were tested with the real-time PCR assays previously developed by our laboratories. The data reported herein suggest a strong positive association between presence of Pfiesteria spp. in the sediment and water column, based on long-term water column monitoring data. P. piscicida is detected more commonly in Maryland's estuarine waters than Pfiesteria shumwayae and sediment ‘cyst beds’ may exist for these organisms.     

Immunocytochemical localization of enzymes involved in the C3 and C4 pathways in the photosynthetic cells of an amphibious sedge, Eleocharis vivipara

Eleocharis vivipara link, an amphibious leafless sedge, develops traits of C₄ photosynthesis and Kranz anatomy in the terrestrial form but develops C₃-like traits with non-Kranz anatomy when submerged. The cellular localization of C₃ and C₄ enzymes in the photosynthetic cells of the two forms was investigated by immunogold labeling and electron microscopy. The terrestrial form has mesophyll cells and three kinds of bundle sheath cell, namely, parenchyma sheath cells, non-chlorophyllous mestome sheath cells, and Kranz cells. Phosphoenol-pyruvate carboxylase (PEPCase) was present in the cytosol of both the mesophyll cells and the parenchyma sheath cells, with higher-density labeling in the latter, but not in the Kranz cells. Pyruvate, Pi dikinase (PPDK) was found at high levels in the chloroplasts of both the mesophyll cells and the parenchyma sheath cells with some-what stronger labeling in the latter. This enzyme was also absent from the Kranz cells. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was found in the chloroplasts of all types of photosynthetic cell, but labeling was significantly less intense in the parenchyma sheath cells than in other types of cell. The submerged form also has three types of photosynthetic cell, as well as non-chlorophyllous mestome sheath cells, but it lacks the traits of Kranz anatomy as a consequence of modification of the cells. Rubisco was densely distributed in the chloroplasts of all the photosynthetic cells. However, PEPCase and PPDK were found in both the mesophyll cells and the parenchyma sheath cells but at lower levels than in the terrestrial form. These data reveal that the terrestrial form has a unique pattern of cellular localization of C₃ and C₄ enzymes, and they suggest that this pattern and the changes in the extent of accumulation of the various enzymes are the main factors responsible for the difference in photosynthetic traits between the two forms.Abbreviations CAM crassulacean acid metabolism - MC meso phyll cell - PSC parenchyma sheath cell - KC Kranz cell - PEP-Case phosphoenolpyruvate carboxylase - PPDK pyruvate, Pi dikinase - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - LS large subunit - RuBP ribulose-1,5-bisphosphate This study was supported by Grants-in-Aid from the Ministry of Agriculture, Forestry and Fisheries of Japan (Integrated Research Program for the Use of Biotechnological Procedures for Plant Breeding) and from the Science and Technology Agency of Japan (Enhancement of Center-of-Excellence, the Special Coordination Funds for Promoting Science and Technology). The author is grateful to Drs M. Matsuoka and S. Muto for providing the antisera and Dr. M. Samejima for his advice at the early stages of this study.     

Incorporation of 14CO2 into C4 acids by leaves of C3-C4 intermediate and C3 species of Moricandia and Panicum at the CO2 compensation concentration

Comparative ¹⁴CO₂ pulse-¹²CO₂ chase studies performed at CO₂ compensation ()-versus air-concentrations of CO₂ demonstrated a four-to eightfold increase in assimilation of ¹⁴CO₂ into the C₄ acids malate and aspartate by leaves of the C₃-C₄ intermediate species Panicum milioides Nees ex Trin., P. decipiens Nees ex Trin., Moricandia arvensis (L.) DC., and M. spinosa Pomel at . Specifically, the distribution of ¹⁴C in malate and aspartate following a 10-s pulse with ¹⁴CO₂ increases from 2% to 17% (P. milioides) and 4% to 16% (M. arvensis) when leaves are illuminated at the CO₂ compensation concentration (20 l CO₂/l, 21% O₂) versus air (340 l CO₂/l, 21% O₂). Chasing recently incorporated ¹⁴C for up to 5 min with ¹²CO₂ failed to show any substantial turnover of label in the C₄ acids or in carbon-4 of malate. The C₄-acid labeling patterns of leaves of the closely related C₃ species, P. laxum Sw. and M. moricandioides (Boiss.) Heywood, were found to be relatively unresponsive to changes in pCO₂ from air to . These data demonstrate that the C₃-C₄ intermediate species of Panicum and Moricandia possess an inherently greater capacity for CO₂ assimilation via phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) at the CO₂ compensation concentration than closely related C₃ species. However, even at , CO₂ fixation by PEP carboxylase is minor compared to that via ribulosebisphosphate carboxylase (EC 4.1.1.39) and the C₃ cycle, and it is, therefore, unlikely to contribute in a major way to the mechanism(s) facilitating reduced photorespiration in the C₃-C₄ intermediate species of Panicum and Moricandia.Abbreviations Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - PEP phosphoenolpyruvate - CO₂ compensation concentration - 3PGA 3-phosphoglycerate - SuP sugar monophosphates - SuP₂ sugar bisphosphates Published as Paper No. 8249, Journal Series, Nebraska Agricultural Research Division     

Biomass production and nitrate metabolism of Atriplex hortensis L. (C3 plant) and Amaranthus retroflexus L. (C4 plant) in cultures at different levels of nitrogen supply

Summary Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C₃ plant) and Amaranthus retroflexus L. (C₄ plant) were maintained under open air conditions in standard soil at low and high nitrogen supply levels.A comparison of shoot dry weight and shoot length in the various series shows that the growth of the aboveground parts of both species was severely reduced under low N conditions. In both pure and mixed cultures the differences resulting from low N vs. high N conditions was less pronounced with Atriplex (C₃ plant) than with Amaranthus (C₄ plant). The root dry weight of the two species was not reduced so much under low N conditions as was the shoot dry weight. The low N plants were found to contain a larger proportion of their biomass in the roots than did the high N plants. In general the root proportion of Atriplex was greater than that of Amaranthus. The contents of organic nitrogen and nitrate and the nitrate reductase activity (NRA) per g dry weight of both species decreased continually throughout the experiments. With the exception of young plants, the low N plants always had tower contents of organic nitrogen and nitrate and nitrate reductase activities than did the high N plants. The highest values of NRA were measured in the leaf laminae. The eaves also exhibited the highest concentrations of organic nitrogen. The highest nitrate concentrations, however, were observed in the shoot axis, and in most cases the lowest nitrate values were found in the laminae. At the end of ne growing season this pattern was found to have been reversed with Atriplex, but not with Amaranthus. Thus Atriplex was able to maintain a higher NRA in the laminae than Amaranthus under low N conditions.The transpiration per leaf area of the C₄ plant Amaranthus during the course of a day was substantially lower than that of the C₃ plant Atriplex. There were no significant differences in transpiration between the low N and high N series of Amaranthus. The low N plants of Atriplex, however, clearly showed in most cases higher transpiration rates than the corresponding high N plants. These different transpiration rates of the high N and the low N Atriplex plants were also reflected in a distinct ¹³C discrimination.The sum of these results points to the conclusion that the C₃ plant Atriplex hortensis can maintain a better internal inorganic nitrogen supply than the C₄ plant Amaranthus retroflexus under low N conditions and an ample water supply, due to the larger root proportion and the more pronounced and flexible transpiration of the C₃ plant.Dedicated to Prof. Dr. Karl Mägdefrau, Deisenhofen, on the ocasion of his 80th birthday     

C4-derived soil organic carbon decomposes faster than its C3 counterpart in mixed C3/C4 soils

The large difference in the degree of discrimination of stable carbon isotopes between C3 and C4 plants is widely exploited in global change and carbon cycle research, often with the assumption that carbon retains the carbon isotopic signature of its photosynthetic pathway during later stages of decomposition in soil and sediments. We applied long-term incubation experiments and natural ¹³C-labelling of C3 and C4-derived soil organic carbon (SOC) collected from across major environmental gradients in Australia to elucidate a significant difference in the rate of decomposition of C3- and C4-derived SOC. We find that the active pool of SOC (ASOC) derived from C4 plants decomposes at over twice the rate of the total pool of ASOC. As a result, the proportion of C4 photosynthesis represented in the heterotrophic CO₂ flux from soil must be over twice the proportional representation of C4-derived biomass in SOC. This observation has significant implications for much carbon cycle research that exploits the carbon isotopic difference in these two photosynthetic pathways.     

Geographical and environmental distribution of C3 and C4 grasses in the Sinai,Negev, and Judean deserts

Summary The relation between photosynthetic pathway and habitat of the grass species recorded in the desert regions of Sinai, Negev, and Judea was investigated. The climatic conditions and micro-environments in the study area vary considerably, and the distribution of the various species is found to conform to specific patterns which reveal the adaptive advantages of the different photosynthetic pathways. There is also a distinct correlation between the phytogeographic origin of the grass species and the photosynthetic pathways that they utilize.The survey shows that the majority of the grass species in the region are of the C₃ type and all except one of these species belong to the Holarctic domain. This is in accordance with the fact that the region forms part of the Mediterranean winter rainfall regime and that C₃ species have an adaptive advantage where minimum temperatures are low during the winter growing season.The occurence of C₄ species increases with decreasing rainfall and they dominate in those districts where temperatures are high throughout the year. These C₄ grasses are of both Holarctic and Palaeotropic origin according to the classification adopted here, but they are essentially all elements of the Saharo-Arabian, Irano-Turanian, Sudanian, or Tropical phytogeographic regions and are not typical of the Mediterranean or Euro-Siberian floras. The plants with multi-regional distributions that occur in Mediterranean communities may well be intrusive.Analysis of the three subtypes of the C₄ species suggests that the malate-forming NADP-me grasses grow where water stress is not a dominating factor, while the aspartateforming NAD-me grasses are more successful under xeric conditions. The PEP-ck species are not abundant and form an intermediate group between the NADP-me and NAD-me subtypes.     

Using growth analysis to interpret competition between a C3 and a C4 annual under ambient and elevated CO2

Summary Detailed growth analysis in conjunction with information on leaf display and nitrogen uptake was used to interpret competition between Abutilon theophrasti, a C₃ annual, and Amaranthus retroflexus, a C₄ annual, under ambient (350 l l^-1) and two levels of elevated (500 and 700 l l^-1) CO₂. Plants were grown both individually and in competition with each other. Competition caused a reduction in growth in both species, but for different reasons. In Abutilon, decreases in leaf area ratio (LAR) were responsible, whereas decreased unit leaf rate (ULR) was involved in the case of Amaranthus. Mean canopy height was lower in Amaranthus than Abutilon which may explain the low ULR of Amaranthus in competition. The decrease in LAR of Abutilon was associated with an increase in root/shoot ratio implying that Abutilon was limited by competition for below ground resources. The root/shoot ratio of Amaranthus actually decreased with competition, and Amaranthus had a much higher rate of nitrogen uptake per unit of root than did Abutilon. These latter results suggest that Amaranthus was better able to compete for below ground resources than Abutilon. Although the growth of both species was reduced by competition, generally speaking, the growth of Amaranthus was reduced to a greater extent than that of Abutilon. Regression analysis suggests that the success of Abutilon in competition was due to its larger starting capital (seed size) which gave it an early advantage over Amaranthus. Elevated CO₂ had a positive effect upon biomass in Amaranthus, and to a lesser extent, Abutilon. These effects were limited to the early part of the experiment in the case of the individually grown plants, however. Only Amaranthus exhibited a significant increase in relative growth rate (RGR). In spite of the transitory effect of CO₂ upon size in individually grown plants, level of CO₂ did effect final biomass of competitively grown plants. Abutilon grown in competition with Amaranthus had a greater final biomass than Amaranthus at ambient CO₂ levels, but this difference disappeared to a large extent at elevated CO₂. The high RGR of Amaranthus at elevated CO₂ levels allowed it to overcome the difference in initial size between the two species.This study was supported by a grant from the US Department of Energy     

Inhibition of oxygen release by anoxia in a C3-plant (Nicotiana tabacum cv. Wisconsin 38). Comparison with C4-plants

Tobacco leaves (Nicotiana tabacum var. Wisconsin 38) submitted to anaerobic conditions behave in a manner similar to that of maize, sugarcane, or sorghum leaves (C₄-plants); more precisely, a lag time in O₂ release is exhibited when the leaves are exposed to light after treatment in the dark under pure nitrogen. Although the conditions for the appearance of this phenomenon in tobacco are somewhat different, the main features are identical to those observed with maize: abolition of the lag time upon immediate exposure to light, release of CO₂ under light (illumination burst of CO₂), photochemical nature of the reactions involved in the abolition of the lag time, activation of oxygen release by far-red light, and the antagonistic effect of red and far-red light on the lag time. The high CO₂ compensation point of tobacco leaves permits the classification of this plant among the C₃ group. A comparison of these experimental results with others from the literature suggests than the distinguishing features between C₃- and C₄-plants are not as sharp as generally thought.