Diurnal changes in allocation and partitioning of recently assimilated carbon in loblolly pine seedlings

We examined diurnal fluctuations in acquisition and partitioning of recently assimilated ¹⁴CO₂, and in subsequent allocation and partitioning to roots of loblolly pine (Pinus taeda L.) seedlings. Nonmycorrhizal seedlings were grown under optimal nutrient conditions in continuously flowin solution culture. Shoots of 15-week-old loblolly pine seedlings were labeled with ¹⁴CO₂ for 30 min at four separate labeling times: 1000, 1200, 1400 and 1600 h. Six whole plant harvests were conducted during a 48 h chase period, i.e. 0, 4, 8 12, 24 and 48 h after the end of the labeling and evacuation periods. Although assimilation of ¹⁴CO₂ was constant between 1000 and 1400 h, there were significant differences in partitioning of ¹⁴C-labeled assimilate in needles of all age classes. The highest percentage of recently assimilated ¹⁴CO₂ in the ethanol-soluble fraction of photosynthesizing tissue was observed near the beginning and end of the photoperiod. Partitioning of ¹⁴C in the ethanol-soluble fraction declined between the 1000 and 1400 h labeling eriods, and was accompanied by an increase in partitioning of recently assimilated ¹⁴CO₂ toward starch and a decrease in respiratory losses. These data suggest that most of the ¹⁴CO₂ assimilated at 1000 h was used to support shoot metabolic activities and possibly restore soluble sugar reserves. Peak starch accumulation in needles during the 1400 h labeling period, concomitant with minimal respiratory loss, indicated that photosynthate production exceeded demand and export out of source leaves. A possible feedback regulation of photosynthesis by starch and/or sugar accumulation may be responsible for the observed decline in assimilation of ¹⁴CO₂ during the 1600 h labeling period. Net accumulation of recently assimilated ¹⁴CO₂ in roots was correlated with assimilation rate of ¹⁴CO₂, but independent of partitioning of recently assimilated carbon in photosynthetic tissue. However, the percentage of total seedling ¹⁴C allocated to roots was essentially the same throughout the 48 h chase, regardless of time of labeling and assimilation rate. The data suggest a strong diurnal regulation of starch and soluble sugars synthesized from recently assimilated carbon in needles of loblolly pine seedlings that was independent of assimilation rate. Allocation and transport of recently assimilated carbon to roots of loblolly pine seedlings were not subject to short-term fluctuations in supply and demand.     

Metabolism of arginine by aging and 7 day old pumpkin seedlings

The metabolism of arginine by etiolated pumpkin (Cucurbita moschata) seedlings was studied over various time and age intervals by injecting arginine-U-¹⁴C into the cotyledons. At most, 25% of the ¹⁴C was transported from the cotyledon to the axis tissue and the amount of this transport decreased with increasing age of the seedlings. The cotyledons of 25 day old plants contained 60% of the administered ¹⁴C as unmetabolized arginine. Little ¹⁴C was in sugars and it appeared that arginine was the primary translocation product. Time course studies showed that arginine was extensively metabolized and the labeling patterns suggest that different pathways were in operation in the axis and cotyledons. The amount of arginine incorporated into cotyledonary protein show that synthesis and turnover were occurring at rapid rate. Only 25% of the label incorporated into protein by 1.5 hr remained after 96 hr. The label in protein was stable in the axis tissue. By 96 hr 50% of the administered label occurred as ¹⁴CO₂ and it appeared that arginine was metabolized, through glutamate, by the citrio acid cycle in the cotyledons. The experiments showed that an extensive conversion of arginine carbon into other amino acids did not occur.     

N2-Fixation and Seedling Growth Promotion of Lodgepole Pine by Endophytic Paenibacillus polymyxa

We inoculated lodgepole pine (Pinus contorta var. latifolia (Dougl.) Engelm.) with Paenibacillus polymyxa P2b-2R, a diazotrophic bacterium previously isolated from internal stem tissue of a naturally regenerating pine seedling to evaluate biological nitrogen fixation and seedling growth promotion by this microorganism. Seedlings generated from pine seed inoculated with strain P2b-2R were grown for up to 13 months in a N-limited soil mix containing 0.7 mM available N labeled as Ca(¹⁵NO₃)₂ to facilitate detection of N₂-fixation. Strain P2b-2R developed a persistent endophytic population comprising 10²–10⁶?cfu?g^?1 plant tissue inside pine roots, stems, and needles during the experiment. At the end of the growth period, P2b-2R had reduced seedling mortality by 14 % and ¹⁵N foliar N abundance 79 % and doubled foliar N concentration and seedling biomass compared to controls. Our results suggest that N₂-fixation by P. polymyxa enhanced growth of pine seedlings and support the hypothesis that plant-associated diazotrophs capable of endophytic colonization can satisfy a significant proportion of the N required by tree seedlings growing under N-limited conditions.     

Use of anin situ labeling technique for the determination of seasonal14C distribution in Ponderosa pine

A¹⁴C labeling apparatus was developed to permit the labeling of four-year-old Ponderosa pine with¹⁴CO₂ in the field. The labeling system is a completely closed canopy system with¹⁴CO₂ monitored by a GM tube ratemeter apparatus. The level of¹⁴CO₂ corresponding to ambient levels is monitored by a microloggercomputer which controls a¹⁴CO₂ generating system. The generated¹⁴CO₂ is mixed in the canopy by circulating the atmosphere with 12V diaphram pumps. The portable system requires little operator attention. At approximately monthly intervals over a one-year period two four-year-old Ponderosa pine trees were labeled for three to five days using this labeling apparatus. After an assimilate distribution period, one tree was excavated and analyzed for¹⁴C distribution. During late spring and early summer most of the carbon assimilated (>60%) was found in the active growing tips and new needles, with little being allocated to the roots (<10%) or woody material (<20%). During mid to late fall there was an increase in root labeling along with an increase in carbon going to woody material. Over the winter period, most of the fixed carbon (65%) resided in the older leaves. The early spring labeling period showed another pulse of root labeling along with some labeling of woody tissues.     

Mechanism of C4 photosynthesis in Chloris gayana: pool sizes and kinetics of 14CO2 incorporation into 4-carbon and 3-carbon intermediates.

In one group of C₄ species, including Chloris gayana, C₄ acids are decarboxylated via phosphoenolpyruvate carboxykinase to give phosphoenolpyruvate as the initial C₃ product. This paper presents an analysis of the kinetics of labeling of various photosynthetic intermediates in Chloris gayana leaves exposed to ¹⁴CO₂, and the pool sizes of these intermediates, primarily to provide information about the subsequent metabolism of phosphoenolpyruvate. Saturation labeling of the C-4 of aspartate and malate, and the C-1 of 3-phosphoglycerate, indicated photosynthetically active pools of 0.45, 0.22, and 0.95 μol/mg chlorophyll, respectively. For aspartate and 3-phosphoglycerate, the total leaf pools and the photosynthetic pools were of similar size, but the total pool of malate was about 100 times larger than the photosynthetically active pool. From the relative rates of labeling of phosphoenolpyruvate, pyruvate, alanine, and C-1, C-2 plus C-3 of aspartate, during steady-state ¹⁴CO₂ assimilation, relative pool sizes were calculated to be about 10:11:78:100, respectively. Pulse/chase labeling of leaves provided estimates of relative photosynthetic pool sizes in the ratio of about 6:15:90:100, respectively, where aspartate is arbitrarily assigned a value of 100 in both cases. Notably, labeling of alanine was consistent with its derivation from the C-1, C-2 plus C-3 carbons of aspartate, and the alanine pool was at least eight times larger than the phosphoenolpyruvate pool that showed similar labeling kinetics. Results were consistent with the view that at least most of the phosphoenolpyruvate produced by C₄ acid decarboxylation is metabolized via alanine.     

The homogentisate ring-cleavage pathway in the biosynthesis of acetate-derived naphthoquinones of the droseraceae

Photosynthesis experiments with ¹⁴CO₂ established that of 16 Droseraceae species tested Drosophylum lusitanicum incorporated the highest amount of label into plumbagin (2-methyl-5-hydroxy-1,4-naphthoquinone). Tyrosine-[β-¹⁴C] fed to Drosophyllum was shown to label plumbagin efficiently (20% incorporation). Extensive chemical degradation of the labeled naphthoquinone showed, however, that the incorporation of tyrosine was indirect, the label being distributed throughout the molecule. It was established that plumbagin and the closely related 7-methyljuglone are biosynthesized via the acetate-polymalonate pathway. Tyrosine is broken down to acetate in this tissue via the homogentisate pathway, which was demonstrated by feeding and incorporation of label into plumbagin of intermediates such as homogentisate-[¹⁴C], maleyl- and fumarylacetoacetate-[¹⁴C]. Simultaneous application of tyrosine-[β-¹⁴C] and α,α′-bipyridyl, an inhibitor of the homogentisate oxigenase, led to an accumulation of homogentisate-[¹⁴C] within the tissue. The degradation of tyrosine to acetate by Drosophyllum is not due to epiphytic bacteria since ring cleavage of tyrosine and formation of plumbagin from breakdown products occurred both within sterile grown plants and sterile cell suspension cultures. In tissue kept in darkness, plumbagin undergoes a slow turnover with a half life of about 400 hr.     

The Transpiration Rate of Unhardened, Hardened, and Dehardened Seedlings of Spruce and Pine

Tissue injuries in winter are sometimes interpreted as caused by drought damages. The possibility that the tolerance of conifers of winter and spring conditions is increased through decrease in transpiration rate has been little investigated. The transpiration rate of 3 or 6 months old unhardened, hardened, and dehardened seedlings of Pinns silvestris L. and Picea abies (L.) Karst. was assessed gravimetrically, at 20°C. The transpiration rates of seedlings of spruce and pine which had been hardened for 3 months were about ¹/₂ the rates of dehardened seedlings. Transpiration during a dehardening period increased at different rates in pine and spruce. After a dehardening period of 3–5 days the transpiration rate of spruce reached a maximum, whereas pine reached the maximum transpiration rate after a dehardening period of 10–14 days. Transpiration of seedlings of spruce and pine hardened for 3 months showed only a very slight reaction to light and darkness. This indicated that stomata of hardened seedlings remained closed. The results are discussed in connection with frost and drought injuries in pine and spruce during early spring.     

Retention of intact concanavalin A and Lens culinaris lectin in transformed lymphoblasts during mitogenic response.

The fate of cell bound mitogens during mitogenic response of mouse cortisone resistant thymocytes (CRT) was studied using ¹²⁵I-concanavalin A (Con A) and ¹²⁵I-Lens culinaris lectin (LcH). A majority of autoradiographic grains derived from ¹²⁵I-lectins bound to a CRT population were distributed in a single broad peak. Pulse labeling of CRT with ¹²⁵I-Con A or ¹²⁵I-LcH in the initial 60 min of incubation followed by 48 hr of culture with unlabeled mitogens revealed that transformed lymphoblasts carried over half of cell-bound ¹²⁵I-mitogens. ¹²⁵I-Con A and ¹²⁵I-LcH found in the lymphoblasts in the above pulse experiments were electrophoretically identical to the native mitogens. A significant loss of cell-bound ¹²⁵I-mitogens was observed only after extensive cell division.     

On the Reason for the Different Photosynthetic Rates of Seedlings of Pinus silvestris and Betula verrucosa

The growth and net photosynthetic properties of seedlings of Pinus silvestris L. and Betula verrucosa Ehrh., grown under identical conditions in a controlled environment chamber, were compared. The relative growth rate of birch was about twice that of pine. The rates of in situ net photosynthesis were 1.50 and 2.30 micromoles CO₂ meter⁻² second⁻¹ and the photosynthetic quantum yields under light-limiting conditions were 0.022 and 0.032 for pine and birch, respectively. The total leaf surface areas were used for calculating the CO₂ flux densities. The difference in the rates of in situ net photosynthesis depended equally on morphological and metabolic factors. It was assumed that a pronounced mutual shading and an unfavorable leaf inclination made the pine seedlings less efficient in absorbing the unidirectional light of the climate chamber than the broadleaved seedlings of birch. Both pine and birch were adapted to the growth conditions so the flux densities of absorbed quanta were rate-limiting for in situ net photosynthesis. It was concluded that the difference in the photosynthetic quantum yields (i.e. the linear slope of the photosynthetic light curve) of the two species defined the metabolically controlled part of the difference in the rate of in situ net photosynthesis. The quantum yield of pine was lower than that of birch and was partly explained by pine having a higher rate of photorespiration than birch. The remaining difference was most likely controlled by the properties of the chloroplast thylakoids, e.g. energy transfer efficiency between pigments, photosynthetic electron transport, or coupling between electron transport and photophosphorylation.     

Ozone and water deficit reduced growth of Aleppo pine seedlings

The effects of ambient and elevated ozone (O₃) levels on photosynthesis, growth, pigment, biomass and element contents of Aleppo pine (Pinus halepensis Mill.) were studied for two growing seasons (1997, 1998). Two-year-old seedlings were exposed to elevated O₃ in open-top chambers. The treatments were charcoal-filtered air and non-filtered air + 50 nl l^–1 O₃ (24 h per day, 7 days per week). In summer 1998, half of the seedlings were drought-stressed (leaf water potential down to approximately –2 MPa), while the other half were kept well-watered. At the beginning of the season (1998), current (c) and previous-year (c + 1) needles under O₃ stress showed an increase in stomatal conductance and net photosynthesis. During the drought period, only stomatal conductance increased in both needle age-classes, whereas the net photosynthesis decreased. At the end of the measuring period, both parameters were reduced in the O₃ treatment. Both O₃ and drought decreased chlorophyll a and b concentrations, growth and biomass. A carry-over effect of O₃ on pigments was also observed. Needle K content was increased in the O₃ treatment. Drought protected Aleppo pine against O₃ (less chlorotic mottle and less decrease of stem and branch biomass).     

DNA synthesis in developing two-cell mouse embryos

Mated CF1 (Carworth) female mice were sacrificed at 2 hr intervals between 29 and 43 hr after human chorionic gonadotrophin (HCG) administration. One- and two-cell eggs were incubated in [³H]thymidine for 1 hr. Labeled two-cell embryos were first observed at 31 hr and reached a maximum number at 35 hr. The S period is approximately 6 hr in duration. Although both blastomeres were labeled in most cases, embryos with only one labeled blastomere were more numerous at later times. In vitro labeling was corroborated by injecting [³H]thymidine directly into the isthmic portion of the oviduct. Embryos usually complete the second cleavage division 18–20 hr after onset of DNA synthesis. The cell cycle at the two-cell stage is thus characterized by a G₁ of close to 1 hr, a 6 hr S, and a G₂ of about 12 hr.Embryos developing in vitro frequently fail to progress beyond the two-cell stage. The block is not due to absence of DNA synthesis since these embryos were found to incorporate [³H]thymidine.     

Biosynthesis of the pyrrolidine ring of nicotine in Nicotiana glutinosa

The biosynthesis of the pyrrolidine ring of nicotine has been studied using short-term steady-state exposures of Nicotiana glutinosa seedlings to ¹⁴CO₂. The pyrrolidine ring of the labeled nicotine has been degraded in a systematic manner to ascertain the radioactivity at each carbon, and a new method has been developed for obtaining C-2′ with complete radiochemical integrity. Some of the labeling patterns obtained were symmetrical while others were clearly unsymmetrical. The duality of the labeling patterns found in these ¹⁴CO₂ biosyntheses, together with other data on pyrrolidine ring biosynthesis which are critically examined, is best rationalized by postulating two biosynthetic pathways for formation of the pyrrolidine ring, one involving a symmetrical precursor and the other an unsymmetrical one.     

The hormonal control of betacyanin synthesis in Amaranthus caudatus

Gibberellic acid (GA₃) inhibits amaranthin synthesis whereas the growth retardant, phosphon D, enhances pigment levels in A. caudatus seedlings exposed to light. No effect was observed on chlorophyll and carotenoid synthesis. Radioactive tyrosine and DOPA were incorporated into amaranthin. The specific activity of amaranthin synthesised in the presence of ¹⁴C-tyrosine or ¹⁴C-DOPA in seedlings treated with GA₃ is higher than water controls. The specific activity of pigment from phosphon D treated tissue is relatively low. GA₃ treated tissue has lower active tyrosine and DOPA pools compared to phosphon treated seedlings. Tyrosine and DOPA-oxidase activity increases in GA₃ treated and H₂O control seedlings exposed to light. Kinetin stimulates the synthesis of amaranthin in dark-grown seedlings and this is not overcome by simultaneous GA₃ application. Dark-grown seedlings treated with different kinetin concentrations and incubated in ¹⁴C-tyrosine synthesise radioactive amaranthin of similar specific activity. Kinetin treatment of dark-grown seedlings brings about an increased tyrosine and DOPA-oxidase activity. The results indicate that GA₃ controls the production and/or availability of tyrosine whereas kinetin can mimic light treatment and controls the utilisation of tyrosine probably by bringing about the synthesis or activation of tyrosine and DOPA-oxidase protein.     

Non-uniform labelling of geraniol biosynthesized from 14CO2 in Pelargonium graveolens

Geranium (Pelargonium graveolens) cuttings were exposed to a 2 hr pulse of ¹⁴CO₂ then allowed to metabolize the label in circulating air for an additional 22 hr. Geraniol isolated from cuttings 2, 4, 8, 12, 16, 20 and 24 hr after the start of the experiment revealed the label in this compound to suffer substantial turnover. Chemical degradation of the labelled geraniol to yield the C₃-isopropylidene fragment showed the distribution of label favored the isopentenyl pyrophosphate-derived half of the molecule. Between 2 and 12 hr of the time-course the distribution of label between the halves of the molecule showed the proportion of label associated with the isopentenyl pyrophosphate-derived half to increase to 78 %. From 12 to 24 hr this preferential labelling declined and approached an equal distribution between the halves. Hypotheses presented to rationalize these observations include the existence of a dimethylallyl pyrophosphate pool and multiple compartments of isoprenoid biosynthesis.     

BIOSYNTHESIS IN ISOLATED ACETABULARIA CHLOROPLASTS : I. Protein Amino Acids

The ability of chloroplasts isolated from Acetabulana mediterranea to synthesize the protein amino acids has been investigated. When this chloroplast isolate was presented with ¹⁴CO₂ for periods of 6–8 hr, tracer was found in essentially all amino acid species of their hydrolyzed protein Phenylalanine labeling was not detected, probably due to technical problems, and hydroxyproline labeling was not tested for The incorporation of ¹⁴CO₂ into the amino acids is driven by light and, as indicated by the amount of radioactivity lost during ninhydrin decarboxylation on the chromatograms, the amino acids appear to be uniformly labeled. The amino acid labeling pattern of the isolate is similar to that found in plastids labeled with ¹⁴CO₂ in vivo. The chloroplast isolate did not utilize detectable amounts of externally supplied amino acids in light or, with added adenosine triphosphate (ATP), in darkness. It is concluded that these chloroplasts are a tight cytoplasmic compartment that is independent in supplying the amino acids used for its own protein synthesis. These results are discussed in terms of the role of contaminants in the observed synthesis, the "normalcy" of Acetabularia chloroplasts, the synthetic pathways for amino acids in plastids, and the implications of these observations for cell compartmentation and chloroplast autonomy.     

Ribulose bisphosphate carboxylase and glycollate oxidase from jack pine: Effects of sulphur dioxide fumigation

Ribulose bisphosphate (RuBP) carboxylase and glycollate oxidase were partially purified from jack pine (Pinus banksiana Lamb.) needles. Preincubation of RuBP carboxylase with HCO₃^? and Mg²⁺ markedly stimulated its activity. RuBP carboxylase showed hyperbolic reaction kinetics with respect to HCO₃^?, Mg²⁺, and RuBP. Both SO₃^2- and SO₄^2- inhibited RuBP carboxylase, but SO₃^2- was more inhibitory than SO₄^2-. The SO₃^2- inhibition was competitive with respect to HCO₃^? (whether SO₃^2- was present during activation or was added to the activated enzyme), while the SO₄^2- inhibition was non-competitive with respect to HCO₃^?. Glycollate oxidase was inhibited more severely by low concentrations of SO₃^2- than by SO₄^2-. Fumigation of jack pine seedlings with 0.34 ppm sulphur dioxide for 24 and 48 hr produced a considerable decline in the activities of these enzymes, but 1 hr of fumigation produced no effect. During the longer exposures the sulphur content of the needles increased considerably, although the needles showed no visible injury. It is suggested that the accumulation of SO₃^2- and SO₄^2- in the needles following sulphur dioxide exposure influenced the enzyme activities.     

A bioassay of the availability of residual 15N fertilizer eight years after application to a forest soil in interior British Columbia

Although a high proportion of fertilizer N may be immobilized in organic forms in the soil, no studies have examined the long-term availability of residual fertilizer ¹⁵N in forestry situations. We investigated this by growing lodgepole pine (Pinus contorta) seedlings in surface (0–10 cm) soil sample eight years after application of ¹⁵N-urea, ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ to lodgepole pine in interior British Columbia. After nine months of growth in the greenhouse, seedlings took up an average of 8.5% of the ¹⁵N and 4.6% of the native N per pot. Most of the mineral N in the pots without seedlings was in the form of nitrate, while pots with seedlings had very low levels of mineral N. In contrast to the greenhouse study, there was no significantuptake of ¹⁵N by trees in the field study after the first growing season, although half of the soil organic ¹⁵N was lost between one and eight years after fertilization. This indicates the need to understand the mechanisms which limit the uptake of mineral N by trees in the field, and the possible mismatch of tree demand and mineral N availability.     

The synthesis of acidic chromosomal proteins during the cell cycle of HeLa S-3 cells. II. The kinetics of residual protein synthesis and transport

The kinetics of acidic residual chromosomal protein synthesis and transport were studied throughout the cell cycle in HeLa S-3 cells synchronized by 2 mM thymidine block and selective detachment of mitotic cells. Pulse labeling the cells with leucine-³H for 2 min and then "chasing" the radioactive proteins for up to 3 hr showed that the amount of protein synthesized, transported, and retained in the acidic residual chromosomal protein fraction is greater immediately after mitosis and later in G₁ than in the S or G₂ phases of the cell cycle. During S, only 20–25% of the proteins synthesized and transported to the acidic residual chromosomal protein fraction are chased during the first 2 hr after pulse labeling, whereas up to 40% of the material entering the residual nuclear fraction in mitosis, G₁, and G₂ leaves during a 2 hr chase. Polyacrylamide gel electrophoretic profiles of these proteins, at various times after pulse labeling, reveal that the turnover of individual polypeptides within this fraction has kinetics of synthesis and turnover which are markedly different from one another and undergo stage-specific changes.     

Assimilation of glucose carbon in subcellular rat brain particles in vivo and the problems of axoplasmic flow

1. Rats were injected with [U-¹⁴C]glucose and the content of ¹⁴C in proteins and lipids of the cerebral P₁ (`nuclear'), P<sub>2</sub> (`mitochondrial'), P₃ (`microsomal') and high-speed supernatant fractions was measured 7, 22 and 93hr. after injection of labelled glucose. 2. The crude brain mitochondrial fractions (P<sub>2</sub>) were subfractionated on continuous sucrose gradients (0·32–1·8m-sucrose) and the <sup>14</sup>C content of the proteins and lipids of about 20 subfractions was measured. 3. About 40–50% of the <sup>14</sup>C assimilated by brain proteins was found in the P<sub>2</sub> (`mitochondrial') fraction. About 68–70% of the ¹⁴C assimilated by brain lipids was also recovered from the lipids of the P₂ fraction. 4. Between 22 and 93hr. after injection of [U-¹⁴C]glucose both the amount of ¹⁴C in the protein of the P₂ (`mitochondrial') fraction and the specific activity of this protein increased. The specific activity of the protein of all other particulate fractions (P₁, P₂ and P₃) and subfractions (obtained from sucrose-density-gradient subfractionation of fraction P₂) when related to the specific activity of the high-speed supernatant protein also increased during 93hr. after injection of [U-¹⁴C]glucose. The amount of ¹⁴C in the protein of the high-speed supernatant and the specific activity of this protein decreased during the same period. 5. The distribution of ¹⁴C in the lipids of all subcellular particulate fractions remained unchanged during the period 22–93hr. after injection of [U-¹⁴C]glucose. 6. It was concluded that a diffusion occurs of some supernatant proteins into subcellular particulate matter of the cerebrum and no significant preference for any subcellular particulate matter was observed. The lipids occur in the cerebrum mainly in a non-diffusible state, which is consistent with the view that they form almost entirely a part of the structure of the cerebrum. 7. The data obtained do not lend further support to the concept of axoplasmic flow within the cerebrum or the concept of a one-directional flow of mitochondria or other subcellular particles within the cerebrum.     

On the intracellular site of biosynthesis of α-tocopherol in Hordeum vulgare

When barley seedlings were exposed to ¹⁴CO₂ for 1–3 hr, ¹⁴C was preferentially incorporated into plastoquinone, but only in low amounts i