Glucose and Pyruvate Metabolism in Daucus carota Cells: The Effect of the Ammonium Ion

In Daucus carota cells cultivated in vitro, the ammonium ionstimulates the incorporation of radioactivity from labelledglucose and labelled pyruvate into CO₂ and into the residueinsoluble in 60 per cent (v/v) ethanol. There is a higher ¹⁴CO₂production from [6-¹⁴C₂] glucose than from [6-¹⁴C] glucose.These results suggest a possible stimulation of glycolysis bythe ammonium ion.     

The Translocation of 14C-Photoassimilate from Normal and Mutant Leaves to the Pods of Pisum sativum L.

In experiments using radioactive carbon dioxide (¹⁴CO₂) a comparisonwas made of the ¹⁴C-photoassimilate translocation potentialsof two normal leaved (genotype AfAfTlTl) and two mutant formsof Pisum sativum (pea). A ¹⁴CO₂ administration method is describedthat permitted ¹⁴C-translocation studies to be conducted underfield conditions. One of the mutants available produced tendrils in place of leaves(afafTlTl). The other mutant studied was without tendrils buthad a much branched petiole with numerous relatively minuteleaflets (afaftltl). These mutants and the normal-leaved cultivarswith which they were compared were not isogenic lines. Lengthybackcrossing would be required before full assessment couldbe made of the possible agronomic value of such mutations. An interim evaluation of these mutants was based on ¹⁴C-distributionassays that were conducted 48 h after feeding ¹⁴CO₂, to specifiedleaves. The indication was that in translocation terms the leafand pod had a well defined respective source and sink relationshipthat was independent of leaf morphology. In each case the podswhich constituted the major ¹⁴C sinks depended on which leafhad been fed ¹⁴CO₂. With regard to sink specific activity asdefined by the quantity of ¹⁴C incorporated per unit dry weightof pod, the mutants were not significantly different from normal. The implication of these findings was that fundamental changesin pea leaf morphology could be made genetically without a markedeffect on the photoassimilate export potential of the leaf.     

Effect of leaf age on light and dark 14CO2 fixation in a CAM plant, Bryophyllum calycinum

Leaves of different ages from B. calycinum were exposed to ¹⁴CO₂in light during day and night. The labelling pattern on thechromatogram differed with leaf age. Young leaves had similarpatterns to those of C3 plants during both day and night. Matureleaves showed high incorporation of ¹⁴C into C4 acids, especiallyat night. In contrast, no significant difference with leaf agewas observed in the pattern of dark ¹⁴CO₂ fixation products.Study of the enzyme activity and the content of titratable acidat each leaf age suggested that high incorporation of ¹⁴C inC4 acids during the night was due to the simultaneous absorptionof CO₂ by both enzymes RuDPcarboxylase and PEPcarboxylase. (Received November 24, 1977; )     

Photosynthetic metabolism of 14CO2 in the process of the "glucose-bleaching" of Chlorella protothecoides

Changes in photosynthetic carbon metabolism during the glucosebleaching of Chlorella protothecoides cells were investigatedusing NaH¹⁴CO₃ as tracer. Several hours after incubating thegreen algal cells in the glucose medium in the dark, the ratesof ¹⁴C-incorporation into glucose polymers and sucrose decreasedand the incorporation into the lipid fraction (fatty acids)greatly increased. At this stage, the rate of photosynthetic¹⁴CO₂ fixation and the chlorophyll content were practicallythe same as in the starting green cells. Afterwards, the photosyntheticcapacity and chlorophyll content continued to decrease throughoutthe experimental period. In contrast, when photosynthetic ¹⁴CO₂fixation of green cells was carried out in the medium containingglucose, the rate of ¹⁴C-incorporation into glucose polymersincreased, though there was no change in the incorporationsinto sucrose and the lipid fraction. ¹Part of this investigation was reported at the Conference "ComparativeBiochemistry and Biophysics of Photosynthesis" (Japan-U.S. CooperativeScience Program) held at Hakone, Japan in 1967. ²Present address: Faculty of Agriculture, Tamagawa University,Machida-shi, Tokyo, Japan. (Received June 10, 1974; )     

Further studies on the metabolism of glucose in the process of "glucose-bleaching" of Chlorella protothecoides

Previous studies have demonstrated that when cells of Chlorellaprotothecoides are incubated in a medium containing glucosebut no nitrogen source, they are profoundly bleached with degenerationof chloroplast structure and photosynthetic activity. When anitrogen source (urea) is added to the glucose medium, bleachingof algal cells is greatly suppressed. In this work the metabolismof glucose in the process of glucose-induced bleaching was studiedusing ¹⁴C-glucose as tracer. Changes in algal cell activityfor ¹⁴CO₂-evolution and ¹⁴C-incorporation into various cellularsubstances from ¹⁴C-glucose were followed. Most conspicuouswere increases in cellular activities for assimilating ¹⁴C-glucoseinto lipids (fatty acids) and glucose polymer. When urea wasadded to the glucose medium, the incorporation of ¹⁴C by algalcells into fatty acids was greatly reduced, while the assimilationof ¹⁴C into glucose polymer was increased. These and previous observations suggest that the formation oflarge amounts of lipids (fatty acids) probably is causally relatedto the induction of algal cell bleaching. (Received March 5, 1969; )     

Modulated Degradation of Ribulose Bisphosphate Carboxylase in Leaves on Top-Pruned Shoots of the Mulberry Tree (Morus alba L.)

Yamashita, T. 1987. Modulated degradation of ribulose ftisphosphatecarboxylase in leaves on top-pruned shoots of the mulberry tree(Morus alba L.).—J. exp. Bot. 38: 1957–1964. The effects of pruning shoot tops on the synthesis and degradationof ribulose 1,5–Wsphosphate carboxylase (RuBPCase) inleaves on remaining shoots were investigated in mulberry trees.Leucine labelled with ¹⁴C was fed to leaf discs from field-grownmulberry trees and ¹⁴C incorporation into RuBPCase was examined.Proportion of ¹⁴C in RuBPCase to leucine–¹⁴C absorbedby leaf discs was remarkably lowered by top-pruning, thoughoccasionally a slight increase was observed soon after pruning.Yet RuBPCase content in leaves on top-pruned shoots became progressivelyhigher than that in leaves on intact shoots. Changes in ¹⁴Cin Ru1BPCase in leaves of mulberry saplings previously fed ¹⁴CO₂were followed. Following ¹⁴CO₂ feeding, the attainment of themaximal level of ₁₄C in RuBPCase was retarded by top-pruning.The highest level of ₁₄C in RuBPCase was maintained in leaveson top-pruned shoots but decreased in leaves on intact shoots.Specific radioactivity in RuBPCase continued to increase inleaves on top-pruned shoots even after attaining a maximum levelin the control leaves. These facts suggest that the increasein RuBPCase by top-pruning results from a cessation of its degradationfor the remobilization of nitrogen for newly developing leaveson shoot tops. Key words: RuBP carboxylase, shoot pruning, mulberry (Morus alba)     

Comparison of carbon dioxide fixation and the fine structure in various assimilatory tissues of Amaranthus retroflexus L

The pattern for primary products of CO₂-fixation and the chloroplaststructure of Amaranthus retrqflexus L., a species which incorporatescarbon dioxide into C₄ dicarboxylic acids as the primary productof photosynthesis, were compared in various chlorophyll containingtissues,i.e., foliage leaves, stems, cotyledons and pale-greencallus induced from stem pith. Despite some morphological differencesin these assimilatory tissues, malate and aspartate were identifiedas the major compounds labelled during a 10 sec fixation of¹⁴CO₂ in all tissues. Whereas, aspartate was the major componentin C₄-dicarboxylic acids formed in foliage leaves, malate predominatedas the primary product in stems, cotyledons and the pale-greencallus. The percentage of ¹⁴C-radioactivity incorporated intoPGA and sugar-P esters increased and ¹⁴C-sucrose was detectedin the prolonged fixation of ¹⁴CO₂ in the light, not only infoliage leaves, but also in stems and cotyledons. ¹ This work was supported by a Grant for Scientific ResearchNo. 58813, from the Ministry of Education, Japan. ² Present address: Institute of Applied Microbiology, Universityof Tokyo, Tokyo, Japan. ³ Present address: Department of Biochemistry, University ofGeorgia, Athens 30601. Georgia, U. S. A. (Received July 10, 1971; )     

The Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat 1. Partitioning During Transport to the Grain

When ¹⁴CO₂ was fed to flag leaf laminae at 20 d post-anthesis,the transport organs between the leaf and the grains containedappreciable ¹⁴C in glutamine, glutamate, serine, alanine, threonineand glycine. Smaller amounts of ¹⁴C were present in gamma-aminobutyricacid (GABA), aspartate and cysteine. Other amino acids whichwere labelled in the source leaf were not labelled in the transportorgans. The export of labelled glutamine, serine, glycine andthreonine from the source leaf was favoured in comparison tothe other amino acids mentioned. Threonine accumulated, andwas subsequently metabolised, in the rachis. [¹⁴C]GABA alsoaccumulated in the rachis. In the grains, the relative amountof soluble [¹⁴C]alanine increased with chase time. This wasprobably due to de novo synthesis and reflected the specialrole of alanine in grain nitrogen metabolism. Wheat, Triticum aestivum, ¹⁴CO₂, amino acids, transport, carbon metabolism     

Synthesis of Citrulline and Arginine in Chlorella pyrenoidosa

The distribution of radioactivity in Chlorella during dark ¹⁴CO₂fixation was investigated either (a) in normal cells with andwithout added ammonium chloride, or (b) in nitrogen-starvedcells supplied with intermediates of the Krebs-Henseleit ureacycle. In the control experiments almost all the activity was presentin compounds of or associated with, the tricarboxylic acid cycle. The amino-acids citrulline and arginine became radioactive onlyin the presence of ammonia or ornithine where initially theycomprised 40–60 per cent. of the total activity, reactionsof the Krebs–Henseleit urea cycle being implicated intheir formation. No evidence could be found for a complete ureacycle. Unidentified compounds deriving their radioactivity fromthe C₄ carbon of citrulline and/or arginine were detected andformed up to 40 per cent. of the total ¹⁴CO₂ incorporated after25 min.     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

Modifications of Translocation Rate in Studies of Apple Leaf Efficiency

M.7 apple rootstocks were used during the peak period of shootextension for comparisons of dry-matter production per unitleaf area between intact plants and others which had been partiallydefoliated. Dry-matter increment per unit leaf area over a 16-dayinterval was some 70 per cent higher in partially defoliatedplants than in controls. ¹⁴CO₂ was supplied to designated leaves of comparable age andposition. Sample discs were taken from the ‘fed’leaves at intervals up to 9 days from supplying ¹⁴CO₂. Translocationrates were estimated by comparison with leaves on a third setof plants whose petioles were steamed to prevent translocationimmediately on removal of the ¹⁴CO₂ feeding chambers. Translocationrates in partially defoliated plants were enhanced some 30 percent compared with controls. It is suggested that features of the plant outside the studiedleaves may have contributed to the overall efficiency of assimilateproduction and utilization. Malus sylvestris L., apple, dry matter production, leaf efficiency, defoliation, translocation, assimilate distribution, sorbitol, sucrose     

Measurements of 14C Incorporation by Illuminated Intact Leaves of Coffee Plants from Gas Mixtures Containing 14CO2

A study was made of the incorporation of ¹⁴C by intact leavesof Coffea arabica (cultivars Mundo Novo, Catuai, 1130–13,and H 6586–2) and Coffea canephora (cultivar Guarini)supplied with gas mixtures containing ¹⁴CO₂ under controlledconditions. Samples of the leaves were combusted and the ¹⁴Cin the CO₂ produced measured using a liquid scintillation counter.The results were used to estimate photosynthetic rates. Theeffects of changing the partial pressures of O₂ and CO₂ on thephotosynthetic rate were studied and estimates made of the CO₂compensation point and photorespiration. The data obtained show differences between the mean net photosyntheticrates of the C. arabica cultivars (6·14 mg CO₂ dm^–2h^–1) and the mean rate for the C. canephora cultivar (3·96mg CO₂ dm^–2 h^–1). The cultivar of the latter speciesphotorespired more rapidly than the cultivar Catuai of C. arabica.Rates of photosynthesis in coffee measured using the ¹⁴CO₂ methodwere similar to rates obtained by others using an infrared gasanalyser. The ¹⁴CO₂ method proved to be reliable for photosyntheticmeasurements and the apparatus is suitable for use in fieldconditions.     

Relationship between photosynthetic carbon metabolism and essential oil biogenesis in peppermint under Mn-stress

Changes in growth and yield parameters, and ¹⁴CO₂ and (U-¹⁴C)sucrose incorporation into the primary metabolic pool, and essentialoil have been investigated under Mn-deficiency and subsequentrecovery in Mentha piperita, grown in solution culture. UnderMn-deficiency, CO₂ exchange rate, total chlorophyll, total assimilatoryarea, plant dry weight, and essential oil yield were significantlyreduced, whereas chlorophyll a/b ratio, leaf area ratio andleaf stem ratio significantly increased. In leaves of Mn-deficientplants, ¹⁴CO₂ incorporation into the primary metabolic pool(ethanol-soluble and -insoluble) and essential oil were significantlylower, whereas (U-¹⁴C) sucrose incorporation into these componentswas significantly higher as compared to the control. Among theprimary metabolites, the label was maximum in sugars, followedby organic acids and amino acids. A higher label in these metaboliteswas, in general, observed in stems of Mn-deficient plants ascompared to the control. Mn-deficient plants supplied with completenutrient medium for 3 weeks exhibited partial recovery in growthand yield parameters, and essential oil biogenesis. Thus, underMn-deficiency and subsequent recovery, the levels of primaryphotosynthetic metabolites and their partitioning between leafand stem significantly influence essential oil biogenesis. Key words: Mentha piperita, Mn-stress, ¹⁴CO₂ and [U-¹⁴C] sucrose incorporation, oil accumulation, primary photosynthetic metabolites     

Utilization Modes of Inorganic Carbon for Photosynthesis in Various Species of Chlorella

Rates of CO₂ and HCC₃^– fixation in cells of various Chlorellaspecies in suspension were compared from the amounts of ¹⁴Cfixed during the 5 s after the injection of a solution containingonly ¹⁴CO₂ or H¹⁴CO₃^–. Results indicated that irrespectiveof the CO₂ concentration during growth, Chlorella vulgaris 11h and C. miniata mainly utilized CO₂, whereas C. vulgaris C-3,C. sp. K. and C. ellipsoidea took up HCO₃^– in additionto CO₂. Cells of C. pyrenoidosa that had been grown with 1.5%CO₂ (high-CO₂ cells) mainly utilized CO₂, whereas those grownwith air (low-CO₂ cells) utilized HCO₃^– in addition toCO₂. Cells that utilized HCO₃^– had carbonic anhydrase(CA) on their surfaces. The effects of Diamox and CA on the rates of CO₂ and HCO₃^–fixation are in accord with the inference that HCO₃^– wasutilized after conversion to CO₂ via the CA located on the cellsurface. CA was found in both the soluble and insoluble fractions;the CA on the cell surface was insoluble. Independent of the modes of utilization, the apparent K_m (NaHCO₃)for photosynthesis was much lower in low-CO₂ cells than in high-CO₂ones. The fact that the CA in the soluble fraction in C. vulgarisC-3 was closely correlated with the K_m(NaHCO₃) indicates thatsoluble CA lowers the K_m. ¹ Dedicated to the late Professor Joji Ashida, one of the foundersand first president of the Japanese Society of Plant Physiologists. ⁴ On leave from Research and Production Laboratory of Algology,Bulgarian Academy of Sciences, Sofia. (Received September 14, 1982; Accepted March 1, 1983)     

Effect of CO2 Concentration on Growth, Carbon Distribution and Loss of Carbon from the Roots of Maize

The effects of three ranges of CO₂ concentration on growth,carbon distribution and loss of carbon from the roots of maizegrown for 14 d and 28 d with shoots in constant specific activity¹⁴CO₂ are described. Increasing concentrations of CO₂ led toenhancement of plant growth with the relative growth rate (RGR)of the roots affected more than the RGR of the shoots. Between16% and 21% of total net fixed carbon (defined as ¹⁴C retainedin the plant plus ¹⁴C lost from the root) was lost from theroots at all CO₂ concentrations at all times but the amountsof carbon lost per unit weight of plant decreased with time.Possible mechanisms to account for these observations are discussed. Key words: Growth, Roots, Carbon loss, [CO₂]     

Photosynthetic Assimilation of 14CO2 by Waterstressed Sunflower Leaves at Two O2 Concentrations and the Specific Activity of Products

Attached leaves of sunflower (Helianthus annuus L. var. Mennonite)with water potentials of –5 to –18 ? 10⁵ Pa, wereexposed for different times to 300 vpm CO₂ containing ¹⁴CO₂and 21 or 1.5% O₂. ¹⁴C accumulated linearly with time in bothO₂ concentrations and at all stresses. 3-Phosphoglyceric acidwas saturated with ¹⁴C after 10 min in unstressed plants atboth O₂ concentrations but with increasing stress the rate ofaccumulation and the specific activity decreased. With decreasingleaf water potential there was accumulation of radioactivityin the glycolate pathway intermediates glycine and serine. Otheramino acids contained a slightly larger proportion of assimilatedcarbon as water potential decreased. The specific activitiesof all compounds were smaller with stress. In contrast to theamino acids less radioactivity accumulated in sugars, organicacids, and sugar phosphates and their specific activities decreasedwith stress. The radioactive labelling patterns and specificactivity measurements are interpreted as showing increased carbonflux in the glycolate pathway and inhibition of the metabolismof serine to sucrose. These changes are related to previousresults showing that with stress photo respiration increasesas a proportion of photosynthesis. Lowering the O₂ concentrationto 1.5% decreased the accumulation of radioactivity in glycineand stopped photorespiration. It increased the amount of radioactivityin serine and sucrose but did not greatly change specific activities.Oxygen effects were independent of water stress. Glycolate pathwaymetabolism is discussed in relation to photorespiration andthe effects of water stress.     

Operation of the reductive pentose phosphate cycle daring the induction period of photosynthesis in Chlorella

When Chlorella oulgaris ll h cells grown in air containing 4%CO₂ (high-CO₂ cells) were given low concentrations of¹⁴CO₂ (<150ppm), the initial rate of photosynthetic ¹⁴CO₂ fixation wasvery low and linear ¹⁴CO₂ fixation was observed after an inductionperiod which lasted for ca. 45 min. No such induction period was observed when high-CO₂ cells weregiven high concentrations of ¹⁴CO₂ (10,000 ppm) or when IOW-CO₂cells were given either low or high concentrations of ¹⁴CO₂,supporting the observations by Briggs and Whittingham (l). However,irrespective of CO₂ concentrations during growth and of ¹⁴CO₂concentrations during the experiments, most of the ¹⁴C was incorporatedinto phosphate esters during the initial periods of photosynthetic¹⁴CO₂ fixation. These results are in sharp contrast to the reportby Graham and Whittingham (4). ¹ Requests for reprints should be addressed to S. Miyachi, RadioisotopeCentre, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan. (Received June 30, 1979; )     

Effects of temperature and CO2 concentration on photosynthetic CO2 fixation by Chlorella

The rates of photosynthetic ¹⁴CO₂ fixation by Chlorella vulgarisllh, grown under high CO₂, were determined between 4 to 37°Cwith air containing from 300 to 13,000 ppm ¹⁴CO₂. When the CO₂level was increased, both the rate of photosynthesis and theoptimum temperature for maximum photosynthesis increased. Themaximum photosynthetic rate was reached at 12°C with 300ppm ^l4CO₂. Among the photosynthetic products fromed at 300 ppm ¹⁴CO₂, glycolatedecreased greatly when the temperature was raised from 20 to30°C. At 3,000 ppm ¹⁴CO₂ an insignificant amount of glycolatewas formed at all temperatures, whereas ¹⁴C-incorporation intothe insoluble fraction, sucrose, and the lipid fraction wassignificantly higher than at 300 ppm ¹⁴CO₂. The ¹⁴C in sucrosewas greatly increased and the radioactivity in the insolublefraction decreased when the temperature was raised from 28 to36°C. (Received April 8, 1980; )     

Energetic Costs of Tissue Construction in Yellow-poplar and White Oak Trees Exposed to Long-term CO2Enrichment

Two methods were used to estimate construction costs for leaves,stems, branches and woody roots of yellow-poplar (LiriodendrontulipiferaL.) trees grown at ambient (35 Pa) and elevated (65Pa) CO₂for 2.7 years and trees of white oak (Quercus albaL.)grown at these same CO₂partial pressures for 4 years. Samplecombustion in a bomb calorimeter combined with measurementsof ash and nitrogen content provided the primary method of estimatingtissue construction costs (W_G; g glucose g^-1dry mass). Thesevalues were compared with a second, simpler method in whichcost estimates were derived from tissue ash, carbon and nitrogencontent (V_G). Estimates of W_Gwere lower for leaves, branchesand roots of yellow-poplar and for leaves of white oak grownat elevated compared with ambient CO₂partial pressures. TheseCO₂-induced differences in W_Granged from 3.7% in yellow-poplarroots to 2.1% in white oak leaves. Only in the case of yellow-poplarleaves, however, were differences in V_Gobserved between CO₂treatments.Leaf V_Gwas 1.46 g glucose g^-1dry mass in ambient-grown treescompared with 1.41 g glucose g^-1dry mass for CO₂-enriched trees.Although paired-estimates of W_Gand V_Gclustered about a 1:1 linefor leaves and branches, estimates of V_Gwere consistently lowerthan W_Gfor stems and roots. Construction costs per unit leafarea were 95 g glucose m^-2for yellow-poplar trees grown at ambientCO₂and 106 g glucose m^-2for trees grown at elevated CO₂partialpressures. No differences in area-based construction costs wereobserved for white oak. Whole-plant energy content was 1220g glucose per tree in ambient-grown white oak compared with2840 g glucose per tree for those grown at elevated CO₂partialpressures. These differences were driven largely by CO₂-inducedchanges in total biomass. We conclude that while constructioncosts were lower at elevated CO₂partial pressures, the magnitudeof this response argues against an increased efficiency of carbonuse in the growth processes of trees exposed to CO₂enrichment. Bomb calorimeter; construction costs; elevated CO₂; energy allocation; global change; growth respiration; heat of combustion; respiration; Liriodendron tulipifera; Quercus alba     

Stimulation of Net Photosynthesis in Cucumber Leaf Discs: Effect of K-Glyoxylate and KCL

Incubation of leaf discs of Cucumis sativus in 15 mol m^–3K-glyoxylate (pH 4.6) doubled the rate of net photosynthesisat limiting CO₂ or HCO^–₃ compared with discs floated ondistilled water. When both control and treated discs were incubatedin Mes-TMAOH buffer at pH 5.0, K-glyoxylate still increasednet photosynthesis by as much as 70%. The tetramethylammoniumsalt (TMA-glyoxylate) was without effect but KCl enhanced netphotosynthesis. Both KCl and K-glyoxylate increased stomatalaperture at pH 5.0. At pH 7.5 (Mops-TMAOH), neither stomatalaperture or photosynthesis was altered by K-glyoxylate, KClor TMA-glyoxylate. None of these salts was found to stimulatephotosynthesis in isolated cucumber mesophyll cells over a rangeof pH values although the cells incorporated as much ¹⁴C-glyoxylateas did leaf discs. The data suggest that enhanced photosynthesisin leaf discs is not due directly to a stimulation of mesophyllcell photosynthesis but rather is a consequence of increasedCO₂ availability and decreased stomatal resistance at low pHin the presence of potassium. Key words: Cucumber, Photosynthesis, Potassium glyoxylate