Photosynthetic and light-enhanced dark fixation of 14CO2 from the ambient atmosphere and 14C-bicarbonate infiltrated through vascular bundles in maize leaves

1. The capacity of light-enhanced dark fixation of ¹⁴CO₂ from theambient atmosphere decayed following time-course characteristicsof a first-order reaction (half-life, 1–2 min). The levelof phosphoenolpyruvate in maize leaves under CO₂-free air didnot decrease in the dark subsequent to preillumination. Theseresults indicate that phosphoenolpyruvate carboxylase is activatedin light and quickly inactivated in the following darkness.
2. Removal of oxygen from the atmosphere did not exert any effecton the products of light-enhanced dark fixation of ¹⁴CO₂ providedfrom the atmosphere, the major labeled compounds being malateand aspartate. This confirms that the transfer of carboxyl carbonof C₄-acids to form 3-phosphoglycerate is light-dependent.
3. WhenNaH¹⁴CO₃ solution was vacuum-infiltrated through vasculartissuesof maize leaves, the main initial photosynthetic ¹⁴CO₂fixationproducts were phosphate esters. This indicates thatby thistechnique, ¹⁴CO₂ could be directly provided to the bundlesheathcells, and was fixed via the reductive pentose phosphatecycle.On the other hand, the main initial ¹⁴CO₂-fixation productswere malate and aspartate even when ¹⁴CO₂ was provided throughvascular tissues in the dark immediately following preillumination.The possible regulatory mechanisms underlying the above findingsare discussed.

¹ This work was reported at the 4th International Congress onPhotosynthesis, Reading, September 1977. Request for reprintsshould be addressed to S. Miyachi, Institute of Applied Microbiology,University of Tokyo, Bunkyo-ku, Tokyo 113, Japan ² Present address: Okinawa Branch of Tropical Agriculture ResearchCenter, Ishigaki-shi, Okinawa 907, Japan. (Received October 28, 1977; )     

The Effect of 2-Chloro-4-ethylamino-6-isopropyl-amino-s-triazine (Atrazine) on Distribution of 14C-Compounds Following 14CO2 Fixation in Excised Kidney Bean Leaves

Excised leaves of kidney bean plants treated with various concentrationsof atrazine for different periods were allowed to fix ¹⁴CO₂CO₂ fixation was inhibited by atrazine. The ¹⁴C-labelling patternof atrazine-treated leaves resembled dark Co₂-fixation patterns.The carbon-I-carboxyl group of ¹⁴C-aspartic acid from atrazine-treatedand ‘dark’ leaves showed no significant differencesin total radioactivity. Although atrazine disrupted the photosyntheticapparatus, it seemed to have no effect on non-photosyntheticCO₂ fixation.     

Protoplasts as a tool for isolating functional chloroplasts from leaves

Leaf protoplasts from various grasses can be used for isolatingchloroplasts with high photosyndietic activity. The protoplastswere stable for more than 20 hr during which time chloroplastscould be isolated from protoplasts without any loss of originalCO₂ fixation capacity (100–157 µmoles/mg chl-hr).Using Triticum aestiuum to optimize assay conditions, the pHoptimum for CO₂ fixation by the chloroplasts isolated from protoplastswas between 8.2 and 8.6. Magnesium (0.75 mM) was required formaximum CO₂ fixation by the isolated chloroplasts and sodiumascorbate in the medium allowed a more linear increase in CO₂fixation with time. Based upon ¹⁴CO₂ fixation and ferricyanide-dependentoxygen evolution as criteria of intactness, chloroplasts fromprotoplasts exhibited a high degree of intactness compared tothose obtained by mechanical grinding. Chloroplasts isolatedfrom grass leaves by mechanical grinding had a relatively lowcapacity for endogenous CO₂ fixation and required addition ofribose-5-phosphate and ADP for maximum activity. (Received September 8, 1975; )     

Photosynthetic Characteristics of Chloroplasts Isolated from Euglena gracilis Z Grown Photoautotrophically

Photosynthetically competent chloroplasts were isolated fromcells of Euglena gracilis Z grown photoautotrophically in 1.5%CO₂. The isolated chloroplasts were intact and substantiallyfree from cytosolic, mitochondrial and microbody materials.The effects of some compounds on the activity of photosynthetic¹⁴CO₂ fixation were examined. The optimal pH and sorbitol concentrationwere 8.0 and 0.33 M, respectively. The chloroplasts requireda high level of P, (5 to 20 mM) for the maximal rate of photosynthesis.They were insusceptible to 10 mM of free Mg²⁺. ATP, ADP andAMP at 1 to 5 mM notably stimulated photosynthesis, althoughhigh concentrations of AMP were unfavorable. In the assay mediumdeveloped for this study, the chloroplasts exhibited photosyntheticactivity of 120µmoles-mg^–1 Chl-h^–1 at 30?C. Chloroplasts could also be isolated from cells grown under ordinaryair. The rate of photosynthetic ¹⁴CO₂ fixation at 1 mM NaH^l4CO₃was higher in these chloroplasts than in those isolated fromcells grown in 1.5% CO₂, whereas at 10 mM NaH^l4CO₃, the ratesof the two types of chloroplasts were nearly the same. Theseresults suggest that the CO₂ concentration given during growthof the algal cells affects the affinity for dissolved inorganiccarbon at the chloroplast level. (Received March 30, 1987; Accepted August 17, 1987)     

Effects of growth temperature on photosynthetic carbon metabolism in green plants III. Differences in structure, photosynthetic activities and activities of ribulose diphosphate carboxylase and glycolate oxidase in leaves of wheat grown under varied temperatures

The rate of ferricyanide photoreduction in broken chloroplastsisolated from leaves of wheat acclimatized to a low temperature(mean temperature, 5–7?C) was similar to that in chloroplastsfrom wheat acclimatized to a high temperature (20–25?C). There was no practical difference in glycolate oxidase activityin leaf extracts of wheat plants grown at low and high temperatures.In contrast, the ribulose diphosphate carboxylase activity ofchloroplasts from low temperature sample was less than halfthat for the high temperature sample. Chloroplasts having a high rate of photosynthetic CO₂-fixationwere obtained from wheat acclimatized to a low temperature,whereas the CO₂-fixation activity in chloroplasts isolated fromhigh temperature-acclimatized wheat was very low. Electron microscopy revealed that chloroplasts in high temperature-acclimatizedwheat were ellipsoidal, electron dense and contained starchgranules. Those in low temperature-acclimatized leaves wereround and did not contain starch granule. ²Present address: Department of Botany, Faculty of Science,University of Tokyo, Tokyo, Japan (Received August 7, 1973; )     

The Effect of Salinity on Light and Dark CO2-Fixation of Salt-Adapted and Unadapted Cell Cultures of Atriplex and Tomato

The effect of salinity on light and dark CO₂,-fixation was determinedin cells of A triplex portulacoides and tomato (Lycopersiconesculenturn Mill.) grown in culture. CO₂,-fixation of tomatocells was also determined in cultures adapted to mannitol andpolyethylene glycol (PEG). Salinity up to 400 mM NaCI in thecase of A triplex and up to 50 mM in the case of tomato enhancedthe rate of light-induced CO₂,-flxation in unadapted cells.Higher salt concentrations led to a marked decline in CO₂-flxationin both species. In salt-adapted A triplex cells no declinein the rate of light CO²,-flxation was seen even at 500 mM NaCl.Dark CO₂,-fixation was approximately 40% and 80% of the lightfixation in control cell cultures of A triplex and tomato, respectively.No enhancement in dark CO₂,-flxation was seen as salinity wasincreased, but a decline was found at similar salt concentrationsthat decreased fixation in the light. Mannitol-and PEG-adaptedtomato cells fixed CO₂, at somewhat lower rates than the controlcells in the light but not in the dark. Key words: Salinity, CO₂-fixation, cell cultures, Atriplex, tomato     

Characterization of the Type of Photosynthetic Carbon Dioxide Fixation in Jute (Corchorus olitorius L.)

Activities of photosynthetic and photorespiratory enzymes viz.,ribulose bisphosphate carboxylase, phosphoenol pyruvate carboxylaseand glycolate oxidase from jute (Corchorus olitorius L.; cv.JRO 632) leaves were compared with those from maize (C₄) andsunflower (C₃) leaves. The photosynthetic CO₂ fixation products,the release of ¹⁴CO₂ in light and dark following photosynthesisin ¹⁴CO₂, chlorophyll a: b ratio, gross leaf photosyntheticrate and dry matter production rate were also studied. The resultsshow that jute is a C₃ plant. Key words: Jute, Corchorus olitorius, C₃ photosynthesis     

Excretion of glycolate during synchronous culture of Ankistrodesmus braunii in the presence of disalicylidenepropanediamine or hydroxypyridinemethanesulfonate

The rate of excretion of glycolate by the unicellular greenalga Ankistrodesmus braunii changes during its life cycle. Itis high in the main growth phase during the light period witha maximum 6 hr after the start of illumination, and low duringthe period of cell division in the dark. The glycolate excretion is stimulated by DSPD and HPMS, whilethe total ¹⁴CO₂-fixation is inhibited by DSPD and enhanced byHPMS. Changes in the effects of DSPD and HPMS on glycolate excretionas well as on photosynthetic ¹⁴CO₂-fixation during the courseof the algal life cycle were followed using the technique ofsynchronous culture. How far the change of glycolate excretion is due to a changeof glycolate oxidase activity during the life cycle and to achange of C₂-supply from the carbon reduction cycle is discussed.The effect of DSPD on glycolate excretion suggests a participationof ferredoxin in the glycolate pathway. (Received August 10, 1968; )     

Effects of dark preincubation and chloramphenicol on blue light-induced CO2 incorporation in Chlorella cells

Chlorella cells incubated in the dark longer than 12 hr showedpronounced blue light-induced ¹⁴CO₂ fixation into aspartate,glutamate, malate and fumarate (blue light effect), whereasthose kept under continuous light showed only a slight bluelight effect, if any. 2) During dark incubation of Chlorellacells, phosphoenolpyruvate carboxylase activity and the capacityfor dark ¹⁴CO₂ fixation decreased significantly, whereas ribulose-1,5-diphosphatecarboxylase activity and the capacity for photosynthetic ¹⁴CO₂fixation (measured under illumination of white light at a highlight intensity) did not decrease. 3) In cells preincubatedin the dark, intracellular levels of phosphoenolpyruvate and3-phosphoglycerate determined during illumination with bluelight were practically equal to levels determined during illuminationwith red light. 4) The blue light effect was not observed incells incubated widi chloramphenicol, indicating that blue light-inducedprotein synthesis is involved in the mechanism of the effect. (Received April 9, 1971; )     

Effects of growth temperature on photosynthetic carbon metabolism in green plants II. Photsoynthetic 14CO2-incorporation in plants acclimatized to varied temperatures

During photosynthetic ¹⁴CO₂-fixation, leaves of plants suchas wheat, the broad bean and spinach, which had been acclimatizedto high temperature (20–25?C), incorporated much moreradioactivity into sucrose, and less into glycine and serinein comparison with similar plants grown in the cold (mean temperature,5–7?C). Radioactivities incorporated into glycine and serine greatlydescreased on the addition of -hydroxyethylsulfonate or on theremoval of oxygen from the atmosphere, indicating that thesecompounds are synthesized through the glycolate pathway. In leaves of wheat grown under low temperatures, relativelyhigh radioactivity was detected in ribulose 1,5-diphosphateamong the photosynthetic ¹⁴CO₂-fixation products, whereas practicallyno radioactivity was detected in this compound in leaves ofwheat which had been acclimatized to high temperatures. We assumedthat the carboxylation reaction of ribulose 1,5-diphosphateis suppressed in plants acclimatized to low temperatures. It was further inferred that the C-2 and C-2 moiety of ribulose1,5-diphosphate accumulating as a result of suppression of carboxylationis converted to glycine and serine through the glycolate pathway. The possibility was also discussed that during photosyntheticCO₂-fixation in wheat leaves at least a part of the C₆-compoundformed by the carboxylation of ribulose 1,5-diphosphate is directlyconverted to sugar phosphate. ¹Part of this investigation was reported at the 2nd InternationalCongress on Photosynthesis Research at Stresa, Italy, June 1971.This paper is based on a dissertation submitted by S.S. to theFaculty of Science, the University of Tokyo, in partial fulfilmentof the requirements for a Ph.D. degree. ²Present address: Department of Botany, Faculty of Science,University of Tokyo, Tokyo, Japan. (Received July 20, 1973; )     

Carbon dioxide fixation in the light and in the dark by isolated spinach chloroplasts

Factors affecting CO₂ fixation in the spinach (Spinacia oleracea) chloroplast were investigated. Free magnesium ions are shown to be highly inhibitory for photosynthetic CO₂ fixation in isolated intact spinach chloroplasts. The pH optimum for CO₂ fixation is about 8.5 but is dependent upon the reaction medium. Conditions are defined under which chloroplasts illuminated in the absence of CO₂ accumulate ribulose 1,5-diphosphate, and fix CO₂ in a subsequent dark period when high magnesium ion concentrations are provided. The regulation of photosynthetic CO₂ assimilation by these factors is discussed.     

Carbon dioxide assimilation and photosystem II deficiency in bundle sheath strands isolated from C4 plants

The activities of Hill reaction and photosynthetic ¹⁴CO₂ fixationin bundle sheath strands enzymatically isolated from millet(Panicum miliaceum) were 3–15 times as high as those observedin corn (Zea mays). In both preparations, 3-phosphoglyceratewas the initial ¹⁴CO₂ fixation product and the radioactivitywas incorporated into sucrose and insoluble compounds (glucose-polymers)during the later period. After 20 sec of photosynthetic ¹⁴CO₂fixation, the percent of ¹⁴C incorporated into sugar phosphatesin millet was about 3 times as high as that in corn, while incorn, the percent of ¹⁴C in 3-phosphoglycerate was higher thanthat observed in millet throughout the experimental period.When ¹⁴C-phosphoglycerate was added to the isolated bundle sheathstrands, the rates of transfer of the radioactivity to dihydroxyacetonephosphate and sugar diphosphates in millet were significantlyhigher than those in corn. These results indicate that in thebundle sheath strands isolated from corn in which photosystemII activity is deficient, the reductive pentose cycle is impairedat the reduction step of 3-phosphoglycerate to glyceraldehydephosphate due to the limited supply of NADPH through the photoelectrontransport system. In contrast, the bundle sheath strands isolatedfrom millet which have adequate photosystem II activity cancarry out normal photosynthetic CO₂ fixation. (Received January 23, 1975; )     

Effect of the age of tobacco leaves on photosynthesis and photorespiration

Relationships among the activities of enzymes related to photosynthesisand photorespiration, and ¹⁴CO₂ photosynthetic products wereinvestigated with individual tobacco leaves attached to thestalk from the bottom to the top. P-glycolate phosphatase ofthe chloroplasts and glycolate oxidase of the peroxisomes hadtheir maximum activities in the 25th leaf from the dicotyledons.Maximum photorespiration was similarly distributed. The highestratio of serine-¹⁴C to glycine-¹⁴C in the photosynthesates andmaximum glycolate formation were also observed in the 25th leaf.Glutamateglyoxylate aminotransferase, serine hydroxymethyltransferaseand glycine decarboxylase were more active in the upper leaves.RuDP carboxylase had nearly constant activity in all leaves,except for the youngest in which activity decreased. MaximumCO₂ photosynthesis and enzyme activity for the C₄ dicarboxylicacid cycle occurred in the upper, youngest leaf. Distributionof photosynthetic CO² fixation among the leaves did not coincidewith RuDP carboxylase activity. The photosynthetic capacityappeared to be better related to the distribution pattern forenzymes of the C₄ dicarboxylic acid pathway, i.e. PEP carboxylase,pyruvate Pi dikinase and 3-PGA phosphatase in the upper leaves.The results suggest that the C₄ dicarboxylic acid pathway participates,to some extent, in photosynthesis in young leaves of tobacco,a dicotyledonous plant. ¹This work was reported at the Annual Meeting (1970) of theJapanese Plant Physiologists in Kobe. ²The Central Research Institute, Japan Monopoly Corporation1-28-3, Nishishinagawa, Shinagawaku, Tokyo, 141 Japan. (Received November 2, 1972; )     

Role of Carbonic Anhydrase and Identification of the Active Species of Inorganic Carbon Utilized for Photosynthesis in Chora corallina

Carbonic anhydrase (CA, EC. 4.2.1.1 [EC] ) activity in air-grown Characorallina was detected mainly in the intracellular fraction,most of which composed of chloroplasts and cytoplasmic gel,and not on the cell surface. Only minor levels of CA activity,on the basis of equivalent volumes, were detected in the cellsap and the cytoplasmic sol. The maximum rate of photosynthetic O₂ evolution by air-grownChara corallina at pH 6.0 was twice that at pH 7.6, while theapparent K_m for external inorganic carbon (C_i) at pH 7.6 wasabout three times that at pH 6.0. However, the apparent K_m(CO₂)was about three times larger at pH 6.0 than at pH 7.6. The K_m(C_i)-valueat pH 7.6 increased severalfold in the presence of acetazolamide(AZA), an inhibitor of CA, but no inhibition was observed atpH 6.0. The pH-dependence may be due to differences in the permeabilityof AZA at the given pH values. Fixation of ¹⁴CO₂ at 20 µMand of H¹⁴CO₃^– at 200 µM over the course of 5 swas very similar at pH 7.4. Addition of CA significantly suppressedthe photosynthetic ¹⁴CO₂-fixation but it stimulated the H¹⁴CO₃^–-fixation.This result indicates that free CO₂ is an active species ofC_i that is incorporated into the cell during photosynthesis. These results together suggest the following: (1) Free CO₂ isutilized for photosynthesis, (2) CA is mainly located insidethe cell and functions to increase the affinity for CO₂ in photosynthesisby facilitating the supply of CO₂ from the plasmalemma to thesite of CO₂-fixation. ³Present address: Biological Laboratory, The University of theAir, Wakaba 2-11, Chiba, 260 Japan. (Received December 9, 1988; Accepted March 22, 1989)     

Photosynthetic carbon metabolism in photoautotrophically and photomixotrophically cultured tobacco cells

Labeling patterns of light and dark ¹⁴CO₂-fixation in photoautotrophicallyand photomixotrophically cultured tobacco cells were determined.During short term ¹⁴CO₂ fixation under light, malate(C₃–C₃carboxylation) was heavily labeled as were phosphoglyceric acidand sugar phosphates(C₁–C₅ carboxylation). Dark fixationcould not account for this high ¹⁴CO₂ incorporation into theC₄ compounds linked to PEPCase. Two carboxylation pathways linkedto the RuBPCase and PEPCase were indicated in ¹⁴CO₂-fixationin light in photoautotrophically and photomixotrophically culturedcells. (Received October 25, 1979; )     

Pre-illumination as a Factor in the Dark Fixation of Carbon Dioxide by Leaf Disks of Kalanchoe blossfeldiana var. Feuer Blute

Dark fixation of CO₂ by leaf disks or whole leaves taken fromplants of variety ‘Feuer Blute’ was measured using¹⁴CO₂. Results indicate that dark fixation by leaf disks isindependent of photoperiodic induction of the plant, but isquantitatively related to the amount of light, over a fairlywide range, to which the leaf is exposed in the single precedinglight period.     

Stimulation of Malate Synthesis by Glycolate in Tomato Leaves in Relation to CO2 Concentration

The mechanism by which malate synthesis from CO2 is increasedunder low concentrations of CO₂ was investigated in C₃ plants.A number of metabolites were administered to illuminated tomatoleaves, and their effects on the incorporation of ¹⁴CO₂ intomalate were determined. Compared with water as a control, glycolate,glyoxylate, D,L-glycerate, glycine, phosphoglycolate and L-serineincreased malate synthesis by factors of 6.8, 3.8, 3.3, 2.5,2.3 and 2.2, respectively. The effect of exogenous glycolateon malate synthesis from CO₂ was dependent on its concentrationup to 100 mu, but was independent of ambient CO₂ concentration.The feeding of l-¹⁴C-glycolate in the light indicated that glycolatestimulated the carbon flow from CO₂ to malate. The analysis of the products of ¹⁴CO₂ fixation in illuminatedleaves supplied with glycolate showed increases in malate andsugar and decreases in serine and phosphate esters. However,this stimulated malate synthesis ceased when malonate was suppliedsimultaneously with glycolate. Treatment with glycolate didnot affect the dark ¹⁴CO₂-fixation, but increased the ¹⁴C-malatesynthesis, with a corresponding decrease in ¹⁴C-aspartate and14^C-glutamate. These results suggest that exogenous glycolateactivates malate dehydrogenase in leaves, and that the increasedglycolate formation at low CO₂ concentrations is associatedwith the increased malate synthesis from CO₂. (Received January 12, 1981; Accepted May 20, 1981)     

Differential Stimulation of PEP-carboxylation in Guard Cells and Mesophyll Cells by Ammonium or Fusicoccin

Dark CO₂-fixation in guard cells of Vicia faba was much moresensitive to ammonium than in mesophyll cells. Addition of ammonium(5.0 mol m^–3; pH₀ 7.6) caused up to a 7-fold increasein dark CO₂-fixation rates in guard cell protoplasts (GCP),whereas in leaf slices, mesophyll cells, and mesophyll protoplaststhe increase was only about 1.4-fold. In both cell or tissuetypes, total CO₂-fixation rates were higher in the light (2–12-foldhigher in GCP and 28-fold in mesophyll); these rates were onlyslightly changed by ammonium treatment. However, separationof ¹⁴C-labelled products after fixation of CO₂ in the lightby GCP revealed a large ammonium-induced shift in carbon flowfrom starch and sugars to typical products of C₄-metabolism(mainly malate and aspartate). In contrast, in mesophyll cellsamino acid and malate labelling was only moderately increasedby ammonium at the expense of sucrose. The data suggest thatin vivo ammonium might facilitate stomatal opening and/or delaystomatal closing through an increased production of organicacids. Key words: PEP-carboxylation, guard cell protoplasts, ammonium, fusicoccin     

Green Light Drives CO2 Fixation Deep within Leaves

Maximal ^l4CO₂-fixation in spinach occurs in the middle of thepalisade mesophyll [Nishio et al. (1993) Plant Cell 5: 953],however, ninety percent of the blue and red light is attenuatedin the upper twenty percent of a spinach leaf [Cui et al. (1991)Plant Cell Environ. 14: 493]. In this report, we showed thatgreen light drives ¹⁴CO₂-fixation deep within spinach leavescompared to red and blue light. Blue light caused fixation mainlyin the palisade mesophyll of the leaf, whereas red light drovefixation slightly deeper into the leaf than did blue light.¹⁴CO₂-fixation measured under green light resulted in less fixationin the upper epidermal layer (guard cells) and upper most palisademesophyll compared to red and blue light, but led to more fixationdeeper in the leaf than that caused by either red or blue light.Saturating white, red, or green light resulted in similar maximal¹⁴CO₂-fixation rates, whereas under the highest irradiance ofblue light given, carbon fixation was not saturated, but itasymptotically approached the maximal ¹⁴CO₂-fixation rates attainedunder the other types of light. The importance of green lightin photosynthesis is discussed. ¹Supported in part by grants from Competitive Research GrantsOffice, U.S. Department of Agriculture (Nos. 91-37100-6672 and93-37100-8855).     

Effects of elevated CO2 concentrations on three montane grass species: III. Source leaf metabolism and whole plant carbon partitioning

Agrostis capillaris L.⁵, Festuca vivipara L. and Poaalpina L.were grown in outdoor open-top chambers at either ambient (340 3µmol mol^–1) or elevated (6804µmol mol^–1)concentrations of atmospheric carbon dioxide (CO₂) for periodsfrom 79–189 d. Photosynthetic capacity of source leaves of plants grown atboth ambient and elevated CO₂ concentrations was measured atsaturating light and 5% CO₂. Dark respiration of leaves wasmeasured using a liquid phase oxygen electrode with the buffersolution in equilibrium with air (21% O₂, 0.034% CO₂). Photo-syntheticcapacity of P. alpina was reduced by growth at 680 µmolmol^–1 CO₂ by 105 d, and that of F. vivipara was reducedat 65 d and 189 d after CO₂ enrichment began, suggesting down-regulationor acclimation. Dark respiration of successive leaf blades ofall three species was unaltered by growth at 680 relative to340 µmol mol^–1 CO₂. In F. vivipara, leaf respirationrate was markedly lower at 189 d than at either 0 d or 65 d,irrespective of growth CO₂ concentration. There was a significantlylower total non-structural carbohydrate (TNC) concentrationin the leaf blades and leaf sheaths of A. capillaris grown at680µmol mol^–1 CO₂. TNC of roots of A. capillariswas unaltered by CO₂ treatment. TNC concentration was increasedin both leaves and sheaths of P. alpina and F. vivipara after105 d and 65 d growth, respectively. A 4-fold increase in thewater-soluble fraction (fructan) in P. alpina and in all carbohydratefractions in F. vivipara accounted for the increased TNC content. In F. vivipara the relationship between leaf photosyn-theticcapacity and leaf carbohydrate concentration was such that therewas a strong positive correlation between photosynthetic capacityand total leaf N concentration (expressed on a per unit structuraldry weight basis), and total nitrogen concentration of successivemature leaves reduced with time. Multiple regression of leafphotosynthetic capacity upon leaf nitrogen and carbohydrateconcentrations further confirmed that leaf photosynthetic capacitywas mainly determined by leaf N concentration. In P. alpina,leaf photosynthetic capacity was mainly determined by leaf CHOconcentration. Thus there is evidence for down-regulation ofphotosynthetic capacity in P. alpina resulting from increasedcarbohydrate accumulation in source leaves. Leaf dark respiration and total N concentration were positivelycorrelated in P. alpina and F. vivipara. Leaf dark respirationand soluble carbohydrate concentration of source leaves werepositively correlated in A. capillaris. Changes in source leafphotosynthetic capacity and carbohydrate concentration of plantsgrown at ambient or elevated CO₂ are discussed in relation toplant growth, nutrient relations and availability of sinks forcarbon. Key words: Elevated CO₂, Climate change, grasses, carbohydrate partitioning, photosynthesis, respiration