Glycolate metabolism in algal chloroplasts: inhibition by salicylhydroxamic acid (SHAM)

Unicellular green algae such as Chlamydomonas and Dunaliella excrete small amounts of glycolate during active photosynthesis. This phenomenon has been explained by the fact that these algae do not have leaf-type peroxisomes and glycolate oxidase; instead, they have a limited capacity to metabolise glycolate in their mitochondria by a membrane-associated glycolate dehydrogenase. Salicylhydroxamic acid (SHAM), an inhibitor of alternative oxidase in plant and algal mitochondria, stimulates glycolate excretion by the algae or their isolated chloroplasts 5-fold. In the presence of SHAM, cells of Chlamydomonas or Dunaliella grown with high-CO₂ (5% CO₂ in air, v/v) or adapted with air levels of CO₂ excreted glycolate at a rate of about 14 µmol glycolate mg⁻¹ Chl h⁻¹. Aminooxyacetate (AOA), an inhibitor of aminotransferases, also increases glycolate excretion by the algal cells or chloroplasts but at a lower rate (about 50%) than SHAM. The algal, light dependent, SHAM-sensitive glycolate oxidizing system in the chloroplasts appears to be the primary site for glycolate oxidation, and it is different and more active then the minor mitochondrial glycolate dehydrogenase.     

Microbodies of the alga Chara

Chara fragilis possesses microbodies with a remarkably large size of up to 2 µm in diameter . Many of the organelles contain huge nucleoids of amorphous material or paracrystalline inclusions. After isolation of the organelles by gradient centrifugation the specific density of the microbodies was determined to be 1.25 g cm⁻³. Catalase, glycolate oxidase and hydroxypyruvate reductase as well as enzymes of the fatty acid β -oxidation pathway were demonstrated to be constituents of the microbodies in Chara indicating that they are similar to those in green leaves. The data obtained are in agreement with the view that the Charophyceae and especially the algae in the subgroup of Charales are very closely related to the land plants.     

Glycolate metabolism in cyanobacteria

A comparative analysis of glycolate excretion in 11 cyanobacteria showed that 8 strains, although grown and assayed in air, excreted glycolate. The largest quantities were excreted by the filamentous strains Plectonema boryanum 73110 and Anabaena cylindrica (Lemm). The carbon lost by excretion was at most 9% of the net fixed carbon in air for heterocystous cyanobacteria but increased (up to 60%) in some strains under a high pO₂ (0.03 kPa CO₂ in pure O₂). A. cylindrica excreted glycolate at a maximum level of 2 and 10 μmol (mg chl a )⁻¹ h⁻¹ in air and at high pO₂, respectively. The excretion continued for several hours. Increases in light intensity and pO₂ and a shift in pH from 7 to 9 increased the amount of glycolate excreted. A. cylindrica also showed the most O₂-sensitive fixation of CO₂. In vitro activity of phosphoglycolate phosphatase (EC 3.1.3.18) was found in all strains tested, with the highest activities noted for Gloeobacter violaceus 7.82 and Gloeothece 6909 and for young cultures of A. cylindrica . The lowest activities were found in Anabaena 7120 and Anacystis nidulans 625, strains excreting no or only minor quantities of glycolate.     

The inhibition of photosynthesis and photorespiration in isolated mesophyll cells of Phaseolus and Lycopersicon by reduced osmotic potentials

Mesophyll cells isolated from Phaseolus vulgaris and Lycopersicon esculentum show decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44 M (−1.08 MPa) to 0.77 M (−1.88 MPa). Photorespiration assayed by ¹⁴CO₂ release in CO₂-free air and by ¹⁴CO₂ release from the oxidation of [1–¹⁴C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO₂ compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I⁻¹1 at −1.08 MPa to 130 μl 1⁻¹ for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non- stomatal inhibition of CO₂ assimilation and glycolate pathway metabolism occurs simultaneously with stomatal closure.     

Involvement of photorespiration and glycolate pathway in carbonic anhydrase induction and inorganic carbon concentration in Chlamydomonas reinhardtii

Interaction between induction of carbonic anhydrase (CA) activity, induction of inorganic carbon (C_i) concentrating mechanisms and the photorespiratory glycolate pathway has been studied in wild type 6145c and photorespiratory mutant 18–7F (low in phosphoglycolate phosphatase activity) cells of C. reinhardtii . Cell transfer from high CO₂ (5%, v/v) to low CO₂ (0.03%) provoked an increase of extracellular and total (extracellular plus intracellular) CA in both wild type and mutant cells. During adaptation to low CO₂ conditions, both strains excreted ammonium to the medium at a similar rate in the presence of l -methionine- d-l -sulfoximine (MSX), an inhibitor of glutamine synthetase (GS). MSX also provoked ammonium excretion by air adapted wild type and mutant cells, even though both strains had high levels of CA activity and of C_i concentrating activities.
GS increased in both strains after transfer from high to low CO₂ conditions. However, this increase was abolished by aminooxyacetate, an inhibitor of the glyoxylate-serine aminotransferase, and by glycolaldehyde, an inhibitor of triose phosphate to ribulose 1,5-bisphosphate conversion. CA synthesis did not occur in the presence of either aminooxyacetate or glycolaldehyde. Algae grown in high CO₂ in the presence of aminooxyacetate did not induce C_i concentrating mechanisms. Integration of these three processes, i.e., CA synthesis, C_i-concentration, and photorespiratory glycolate pathway is proposed in the framework of carbon metabolism of the alga.     

Adjustments of net photosynthesis in Solanum tuberosum in response to reciprocal changes in ambient and elevated growth CO2 partial pressures

Single leaf photosynthetic rates and various leaf components of potato ( Solanum tuberosum L.) were studied 1–3 days after reciprocally transferring plants between the ambient and elevated growth CO₂ treatments. Plants were raised from individual tuber sections in controlled environment chambers at either ambient (36 Pa) or elevated (72 Pa) CO₂. One half of the plants in each growth CO₂ treatment were transferred to the opposite CO₂ treatment 34 days after sowing (DAS). Net photosynthesis (P_n) rates and various leaf components were then measured 34, 35 and 37 DAS at both 36 and 72 Pa CO₂. Three-day means of single leaf P_n rates, leaf starch, glucose, initial and total Rubisco activity, Rubisco protein, chlorophyll ( a + b ), chlorophyll ( a/b ), α -amino N, and nitrate levels differed significantly in the continuous ambient and elevated CO₂ treatments. Acclimation of single leaf P_n rates was partially to completely reversed 3 days after elevated CO₂-grown plants were shifted to ambient CO₂, whereas there was little evidence of photosynthetic acclimation 3 days after ambient CO₂-grown plants were shifted to elevated CO₂. In a four-way comparison of the 36, 72, 36 to 72 (shifted up) and 72 to 36 (shifted down) Pa CO₂ treatments 37 DAS, leaf starch, soluble carbohydrates, Rubisco protein and nitrate were the only photosynthetic factors that differed significantly. Simple and multiple regression analyses suggested that negative changes of P_n in response to growth CO₂ treatment were most closely correlated with increased leaf starch levels.     

The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

Glycolate metabolism in cyanobacteria. III. Nitrogen controls excretion and metabolism of glycolate in Anabaena cylindrica

The effect of nitrogen on excretion and metabolism of glycolate in Anabaena cylindrica (CCAP 1403/2a) was studied. Glycidate, an inhibitor of glutamate:glyoxylate aminotransferase (EC 2.6.1.4), reduced the L-methionine-DL-sulfoximine-induced NH₄⁺ release by ca 40%, while net CO₂ fixation and C₂H₂ reduction were not lowered. This indicates that at least a part of the glyoxylate synthesized in A. cylindrica is metabolized via glycine to serine. Addition of NH₄Cl or glutamate to the medium reduced the excretion of glycolate. At pH 9, under air, NH₄Cl reduced the excretion by 10–30% and under high pO2 (0.03 kPa CO₂ in O₂) by about 80–90%. At pH 7.5, under high pO₂, NH₄Cl and glulamate reduced the excretion by about 40 and 80%, respectively. Also, the presence of NH₄Cl stimulated the animation of glyoxylate under such conditions as shown by an increased glycine pool and a decreased glutamate pool. We suggest that nitrogen regulates the capacity of A. cylindrica to retain and recycle glycolate intracellularly and that glutamate serves as an amino donor in the conversion of glyoxylate to glycine.     

LIMITATIONS OF PHOTOSYNTHESIS IN DIFFERENT REGIONS OF THE ZEA MAYS LEAF

The progressive development of the photosynthetic apparatus occurring along the length of the Zea mays leaf offers a convenient system with which to examine the limitations to photosynthetic CO₂ assimilation during biogenesis of a C₄ leaf. Changes in light-induced O₂ evolution and CO₂ assimilation, chlorophyll content, activity of PEP-carboxylase, NADP-malic enzyme and the 'R5P system' (consisting of d -ribose-5-phosphate-keto isomerase, ATP- d -ribulose-5 phosphate 1-phosphotransferase and d -ribulose-1,5-bisphosphate carboxylase) and fluorescence emission characteristics were examined along the length of the second leaf of 7-day-old plants grown under a diurnal light regime. The results suggest that the major limitation to CO₂ assimilation in the leaf sheath lies within the chlorenchyma and is either energy supply for carboxylation or the capacity of key photosynthetic enzymes. In the leaf blade stomatal resistance to CO₂ diffusion constitutes a major fraction of the total leaf resistance to CO₂ assimilation implicating the stoma as the major limiting factor to photosynthetic CO₂ assimilation.     

CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum)

The interaction of CO₂ enrichment and drought on water status and growth of pea plants was investigated. Pisum sativum L. (cv. Alaska) plants were grown from seeds in growth chambers using 350 and 675 μl I1 CO₂, a photon flux density of 600 μmol M-2 S-1, a 16 h photoperiod and a temperature regime of 20/14°C. The drought treatment was started at the beginning of branch initiation and lasted for 9 or 11 days. The water status of the plants was monitored daily by measuring total leaf water potential and stomatal conductance. The total leaf water potential of well-watered plants was not affected by the CO₂ level. Under draughting conditions total leaf water potential decreased, with a slower decrease under the high CO₂ regime, due, at least in part, to reduced stomatal conductance. Upon rewatering, total leaf water potential and stomatal conductance recovered within one day. High CO₂ counteracted the reduction in height and, to some extent, leaf area that developed in low CO₂ unwatered plants. Additional CO₂ had no effect on branch number and did not prevent the complete inhibition of branch development that resulted from drought stress. Removing the drought conditions resulted in a rapid recovery of the internal water status and also a rapid recovery of most, but not all, plant growth parameters.     

CO2, light and temperature influence senescence and protein degradation in wheat leaf segments

Effects of environmental conditions influencing photosynthesis and photorespiration on senescence and net protein degradation were investigated in segments from the first leaf of young wheat ( Triticum aestivum L. cv. Arina) plants. The segments were floated on H₂O at 25, 30 or 35°C in continuous light (PAR: 50 or 150 µmol m⁻² s⁻¹) in ambient air and in CO₂‐depleted air. Stromal enzymes, including phosphoglycolate phosphatase, glutamine synthetase, ferredoxin‐dependent glutamate synthase, phosphoribulokinase, and the peroxisomal enzyme, glycolate oxidase, were detected by SDS‐PAGE followed by immunoblotting with specific antibodies. In general, the net degradation of proteins and chlorophylls was delayed in CO₂‐depleted air. However, little effect of CO₂ on protein degradation was observed at 25°C under the lower level of irradiance. The senescence retardation by the removal of CO₂ was most pronounced at 30°C and at the higher irradiance. The stromal enzymes declined in a coordinated manner. Immunoreactive fragments from the degraded polypeptides were in most cases not detectable. However, an insolubilized fragment of glycolate oxidase accumulated in vivo, especially at 25°C in the presence of CO₂. Detection of this fragment was minimal after incubation at 30°C and completely absent on blots from segments kept at 35°C. In CO₂‐depleted air, the fragment was only weakly detectable after incubation at 25°C. The results from these investigations indicate that environmental conditions that influence photosynthesis may interfere with senescence and protein catabolism in wheat leaves.     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

Factors Affecting Development of Peroxisomes and Glycolate Metabolism among Algae of Different Evolutionary Lines of the Prasinophyceae

Leaf-type peroxisomes are not present in the primitive unicellular Prasinophycean line of algae but are present in the multicellular algae Mougeotia, Chara, and Nitella, which are in the one evolutionary line, Charophyceae, that led to higher plants. Processes related to glycolate metabolism that may have been modified or induced with the appearance of peroxisomes have been examined. The algal dissolved inorganic carbon-concentrating mechanism and alkalization of the medium during photosynthesis were not lost when peroxisomes appeared in the members of the Charophycean line of algae. Therefore, it is unlikely that lowering of the CO2 concentration in the environment was a major factor in the evolutionary appearance of peroxisomes. Multicellular Mougeotia, early members of the Charophycean line of algae, have peroxisomes, but they excrete excess glycolate into the medium. The cytosolic pyruvate reductase for D-lactate synthesis and the glycolate dehydrogenase activity almost disappeared when peroxisomal glycolate oxidase, which also oxidizes L-lactate, appeared. These biochemical changes do not indicate what caused the induction of leaf-type peroxisomes in this evolutionary line of algae. The oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and glycolate oxidase require about 200 to 400 [mu]M O2 for 0.5 Vmax. These high-O2-requiring steps in glycolate metabolism would have functioned faster with increasing atmospheric O2, which might have been the causative factor in the induction of peroxisomes.     

Photosynthesis, growth and nutrient changes in non-nodulated Phaseolus vulgaris grown under atmospheric and elevated carbon dioxide conditions

The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol^-1, respectively) CO₂ concentrations ([CO₂]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO₂ had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO₂-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO₂], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO₂ enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO₂ occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO₂-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO₂ plants. Under enhanced CO₂, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO₂ grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.     

Photorespiratory nitrogen cycle – A critical evaluation

The photorespiratory nitrogen cycle proposed by Keys et al. (Nature 275: 741–743, 1978) involved formation of glycine by transamination of glyoxylate in the peroxisomes utilizing glutamate. Subsequently, glycine is oxidized to ammonia, serine and CO₂ in the mitochondria. The ammonia is reassimilated via the GS/GOGAT pathway generating glutamate. In this article, experimental evidence which suggests the occurrence of alternative mechanisms of glycolate and serine synthesis as well as of CO₂ and ammonia evolution is discussed. The problem of utilization of NADH coupled to ATP synthesis during photosynthesis is still unresolved, which complicates the glycine oxidation reaction in light. Further, factors are presented that determine the availability of amino donors in the peroxisomes and of amino acids viz., glycine, serine and glutamate for the operation of the photorespiratory N cycle. Recent evidence regarding the role of formate arising out of the reaction of glyoxylate with H₂O₂ in the regulation of photosynthetic electron flow in the Hill reaction, as well as of photorespiratory substrates functioning as carbon sources for the citric acid cycle in the light or for export to the growing tissues, suggests that the role of photo-respiration in plant metabolism needs to be reexamined.     

Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2-fixing alfalfa plants

Plant responses to elevated CO₂ can be modified by many environmental factors, but very little attention has been paid to the interaction between CO₂ and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m⁻² s⁻¹ photosynthetic photon flux (PPF), using a factorial combination of CO₂ concentration (400 µmol mol⁻¹ or 700 µmol mol⁻¹) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO₂ strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO₂ also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO₂ and low VPD. Moreover, plants grown under elevated CO₂ and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO₂ and high VPD. Elevated CO₂ significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO₂ were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO₂ as well as considered in the extrapolations of results to a warmer, high-CO₂ world.     

Drought-induced shifts in daily CO2 uptake patterns for leafy cacti

For cacti with persistent, relatively large leaves, most shoot CO₂ uptake under well-watered conditions occurs by the leaves using the C₃ pathway. For three species in the primitive subfamily Pereskioideae, droughts of 7 or 14 days decreased leaf daytime net CO₂ uptake by an average of 49 and 88%, respectively; these species always had a net CO₂ release at night by the leaves and both at night and during the day by the stems. For three leafy species in subfamily Opuntioideae, 7 and 14 days of drought reduced leaf daytime net CO₂ uptake by 90 and 100%, respectively. Although drought reduced the total CO₂ uptake over 24 h, the average percentage occurring at night by the leaves of these species increased from 5% under wet conditions to 71% after 7 days of drought to 99% after 14 days of drought. For two of the three species of Opuntioideae, 7 days of drought caused the small net CO₂ uptake by the sterns to shift from the daytime to the nighttime, while for the third species drought caused a reduction of its stem nocturnal net CO₂ uptake. Thus, shifts from predominantly daytime to predominantly nighttime net CO₂ uptake can be induced by drought for the leaves and the stems of leafy cacti in subfamily Opuntioideae, indicating a high degree of biochemical versatility.     

CO2 enrichment of soybeans. Effects of leaf/pod ratio

The effect of varying leaf number on response of soybean ( Glycine max (L.) Merr. cv. Fiskeby V) to CO₂ enrichment was studied. Plants were trimmed at pod set to 15 pods and 1 or 3 leaves (15:1 and 5:1 pod/leaf ratio) and placed in 350 or 1000 μl/l CO₂ growth chambers. Photosynthetic rates and dry weights were measured 6 times in all plants at each CO₂ concentration over a period of 39 days. Measured at treatment CO₂ concentration, photosynthetic rates deelined rapidly in enriched plants, but remained higher than those of non-enriched plants. When all plants were measured at the same CO₂ concentration, for most sampling dates, neither growth, CO₂ concentration or pod/leaf ratio significantly affected rates of photosynthesis per unit area of comparable leaves. CO₂ enrichment significantly increased total weights and pod weights in 15:1 but not 5:1 pod/leaf ratio plants. Plants with a 5:1 pod/leaf ratio had significantly higher total and pod weights than 15:1 ratio plants. Both the photosynthesis and dry weight data suggest that plants in the 5:1 ratio enriched treatment were sink-limited, but plants in all other treatments were source limited.     

Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2

1. We tested the hypothesis that the net partitioning of dry mass and dry mass:area relationships is unaltered when plants are grown at elevated atmospheric CO₂ concentrations.
2. The total dry mass of Dactylis glomerata, Bellis perennis and Trifolium repens was higher for plants in 700 compared to 350 μmol CO₂ mol^–1 when grown hydroponically in controlled-environment cabinets.
3. Shoot:root ratios were higher and leaf area ratios and specific leaf areas lower in all species grown at elevated CO₂. Leaf mass ratio was higher in plants of B. perennis and D. glomerata grown at elevated CO₂.
4. Whilst these data suggest that CO₂ alters the net partitioning of dry mass and dry mass:leaf area relationships, allometric comparisons of the components of dry mass and leaf area suggest at most a small effect of CO₂. CO₂ changed only two of a total of 12 allometric coefficients we calculated for the three species: ν relating shoot to root dry mass was higher in D. glomerata , whilst ν relating leaf area to total dry mass was lower in T. repens .
5. CO₂ alone has very little effect on partitioning when the size of the plant is taken into account.     

The effects of elevated atmospheric CO2 on lipid metabolism in leaves from mature wheat (Triticum aestivum cv. Hereward) plants

Winter wheat (Triticum aestivum L. cv. Hereward) plants were grown for 35 d either at 350 μ mol mol^–1 CO₂ or at 650 μ mol mol^–1 CO₂. Lipid synthesis was studied in these plants by incubating the 5th leaf on the main stem with [1–¹⁴C]acetate. Increased CO₂ concentrations did not significantly affect the total incorporation of radiolabel into lipids of whole leaf tissue, but altered the distribution for individual lipid classes. Most noticeable amongst acyl lipids was the reduction in labelling of diacylglycerol and a corresponding increase in the proportion of phosphatidylcholine labelling. In the basal regions, there were similar changes and, in addition, phosphatidylglycerol labelling was particularly increased following growth in an enriched CO₂ atmosphere. The stimulation of labelling of the mitochondrial-specific lipid, diphosphatidylglycerol, prompted an examination of the mitochondrial population in wheat plants. Mitochondria were localized in intact wheat sections by immunolabelling for the mitochondrial-specific chaperonin probe. Growth in elevated CO₂ doubled the number of mitochondria compared to growth in ambient CO₂. Fatty acid labelling was also significantly influenced following growth at elevated CO₂ concentrations. Most noticeable were the changes in 16C:18C ratios for the membrane lipids, phosphatidylcholine, phosphatidylglycerol and monogalactosyldiacylglycerol. These data imply a change in the apportioning of newly synthesized fatty acids between the 'eukaryotic' and 'prokaryotic' pathways of metabolism under elevated CO₂.