Ribulose Diphosphate Carboxylase Research Articles

Effect of Glyoxylate on the Sensitivity of Net Photosynthesis to Oxygen (the Warburg Effect) in Tobacco

The addition of glyoxylate to tobacco (Nicotiana tabacum) leaf discs inhibited glycolate synthesis and photorespiration and increased net photosynthetic (14)CO(2) fixation. This inhibition of photorespiration was investigated further by studying the effect of glyoxylate on the stimulation of photosynthesis that occurs when the atmospheric O(2) level was decreased from 21 to 3% (the Warburg effect). The Warburg effect is usually ascribed to the increased glycolate synthesis and metabolism that occurs at higher O(2) concentrations. Photosynthesis in control discs increased from 59.1 to 94.7 micromoles of CO(2) per gram fresh weight per hour (a 60% increase) when the O(2) level was lowered from 21 to 3%, while the rate for discs floated on 15 millimolar glyoxylate increased only from 82.0 to 99.7 micromoles of CO(2) per gram fresh weight per hour (a 22% increase). The decrease in the O(2) sensitivity of photosynthesis in the presence of glyoxylate was explained by changes in the rate of glycolate synthesis under the same conditions.The rate of metabolism of the added glyoxylate by tobacco leaf discs was about 1.35 micromoles per gram fresh weight per hour and was not dependent on the O(2) concentration in the atmosphere. This rate of metabolism is about 10% the amount of stimulation in the rate of CO(2) fixation caused by the glyoxylate treatment on a molar carbon basis. Glyoxylate (10 millimolar) had no effect on the carboxylase/oxygenase activity of isolated ribulose diphosphate carboxylase. Although the biochemical mechanism by which glyoxylate inhibits glycolate synthesis and photorespiration and thereby decreases the Warburg effect is still uncertain, these results show that cellular metabolites can regulate the extent of the Warburg effect.

Changes in the Ability of Photophosphorylation and Activities of Surface‐Bound Adenosine Triphosphatase and Ribulose Diphosphate Carboxylase of Chloroplasts Isolated from the Barley Leaves Senescing in Darkness

AbstractDark‐induced aging of detached primary leaves of 11‐day‐old barley seedlings brings about a significant decline in the rates of ferricyanide [Fe(CN)6]3− reduction and photophosphorylations of isolated chloroplasts. Ferricyanide‐supported noncyclic photophosphorylation is somewhat more susceptible to leaf aging than phenazine methosulfate (PMS)‐supported cyclic phosphorylation. Non‐latent membrane‐bound adenosine triphosphatase (ATPase) and ribulosediphosphate carboxylase (RuDPCase) lose about half of their initial activities after 24 h, while during this period the electron transport and photophosphorylation activities are much less affected. Also, the loss of RuDPCase is almost complete, while chloroplasts still exhibit a significant level of [Fe(CN)6]3− reduction and photophosphorylations after 7 days of dark incubation. This would suggest that the enzymatic dark reactions are more sensitive to aging stress than the primary photochemical reactions of chloroplasts.Studies on the effect of divalent cations such as Mg2+ and Ca2+ on non‐latent ATPase activity revealed that the dark stressed aging of detached leaves brings about a time dependent alteration in the response of this enzyme to Mg2+, but not to Ca2+. The former showed inhibitory as well as stimulatory response, whereas the latter always caused the usual stimulation. Addition of kinetin (50 μM) ensured retention of [Fe(CN)6]3− reduction, photophosphorylations and RuDPCase activity in chloroplasts during leaf aging, but it failed to preserve the initial loss in the activity of the non‐activated membrane‐bound ATPase.

MOLECULAR, ENZYMATIC AND ULRASTRUCURE CHARACTERIZATION OF THE PYRENOID OF THE SCALY GREEN MONAD MICROMONAS SQUAMATA1,2

ABSTRACTPyrenoid material of micromonas squamata Manton & Parke was obtained free of cell and subcellular particle conamination by differential centrifugation of brei from osmoically lysed cells. The isolated pyrenoid particles were characterized by transmission and scanning electron microscopy. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of pyrenoid extracts revealed a compled polypeptide composition with major components of 12, 54 and 66 kilodalton mol wt. Whole pyrenoids possessed the enzymatic properties of ribulose diphosphate carboxylase and fixed carbon dioxide with specific activity 10 times greaer than that of a pyrenoid‐free high speed supenaant fracion of cell brei. Energy dispersive X‐ray microanalysis revealed he presence of copper in masses of cryo‐impacted pyrenoid material. Ultrastrucural cytochemistry was employed o determine he chemical nature of the reserve carbohydrate shell. Also, the pyrenoid of the intact cell was characterized by transmission electron microscopy.

Effect of abscisic acid on differentiation and ribulose diphosphate carboxylase of chloroplasts

Effect of abscisic acid on differentiation and ribulose diphosphate carboxylase of chloroplasts

Metabolic Regulation of Carbon Flux during C4 Photosynthesis I. Evidence for Parallel CO2 Fixation by Mesophyll and Bundle Sheath Cells in situ

Summary CO2 fixation by isolated mesophyll protoplasts, bundle sheath strands and thin leaf slices of three types of C4 plants Digitaria sanguinalis (NADP-malic enzyme type), Panicum miliaceum (NAD-malic enzyme type) and Eriochloa borumensis (phosphoenolpyruvate-carboxykinase [PEP-CK] type) was investigated in the presence of appropriate inhibitors of mesophyll and bundle sheath photosynthesis. The inhibitors used were: malonate and maleate for PEP carboxylation, and D,L-glyceraldehyde for ribulose 1,5-diphosphate (RuDP) generation in all the three species; oxalate for C4 acid decarboxylation through NADP-malic enzyme in D. sanguinalis, and 3-mercaptopicolinic acid for C4 acid decarboxylation through PEP-CK in E. borumensis. In the absence of inhibitors, the C4 leaf slices fixed CO2 at very high rates at both 0.4 mM and 6 mM HCO3−, and O2 (0–100 %) had no effect. The 14C label was equally distributed between C4 acids and Calvin cycle compounds. In the absence of functional PEP carboxylation and C4 acid decarboxylation, the C4 leaf slices fixed CO2 directly into Calvin cycle compounds (the rates were 14–17 % of the control). At low HCO3− (0.4 mM) concentration, the direct fixation of CO2 by RuDP carboxylase in C4 leaf slices treated with maleate and appropriate C4 acid decarboxylase inhibitor was progressively inhibited by 21 % and 100% O2 and the label readily appeared in photorespiratory intermediates. The O2 inhibition was effectively reversed by increasing the added HCO3− concentration. Similar effects of O2 and HCO3− were observed during CO2 fixation by isolated bundle sheath stands. Pyruvate + oxaloacetate-dependent CO2 fixation (PEP carboxylation) in leaf slices treated with D,L-glyceraldehyde was increased by 2 fold at 100% O2 and the HCO3− concentration had no effect. Similar enhancement by O2 was observed with isolated mesophyll protoplasts. O2 inhibited bundle sheath photosynthesis competitively with respect to HCO3−, and reversal of O2 inhibition by CO2 was also competitive with respect to O2. The Km(CO2) for bundle sheath photosynthesis was about 11–13 µM at pH 8.2 and 37C; and increased roughly by 5-fold at 21 % O2 and 10-fold at 100 % O2. Both the observed and calculated Vmax for cell photosynthesis were similar except for D. sanguinalis in which the observed Vmax was 33 % lower than the calculated Vmax. The results indicate that 14–17 % of atmospheric CO2 entering the leaf is directly fixed by the Calvin cycle without the intervention of C4 cycle. The significance of these observations is discussed in relation to the mechanism and function of C4 photosynthesis and the C4 control of photorespiration in vivo.

Effect of Glycidate, an Inhibitor of Glycolate Synthesis in Leaves, on the Activity of Some Enzymes of the Glycolate Pathway

Under conditions where glycolate synthesis was inhibited at least 50% in tobacco (Nicotiana tabacum L.) leaf discs treated with glycidate (2,3-epoxypropionate), the ribulose diphosphate carboxylase activity in extracts and the inhibition of the activity by 100% oxygen were unaffected by the glycidate treatment. [1-(14)C]Glycidate was readily taken into leaf discs and was bound to leaf proteins, but the binding occurred preferentially with proteins of molecular weight lower than ribulose diphosphate carboxylase. Glycidate added to the isolated enzyme did not inhibit ribulose diphosphate carboxylase activity or affect its inhibition by 100% O(2). Thus, glycidate did not inhibit glycolate synthesis by a direct effect on ribulose diphosphate carboxylase/oxygenase.NADH-glyoxylate reductase and phosphoglycolate phosphatase activities were also unaffected in extracts of leaf discs supplied with glycidate, but NADPH-glyoxylate reductase was inhibited about 35% in such extracts. Addition of 10 mm glycidate to isolated NADPH-glyoxylate reductase inhibited the activity about 25% after 15 minutes of reaction.The small inhibition of NADPH-glyoxylate reductase by glycidate may help to explain the increase in glyoxylate concentration found in glycidate-treated leaf discs. Increasing the glyoxylate pool size in leaf discs has been shown to effectively block glycolate synthesis and photorespiration and increase net photosynthesis. Thus, the similar effects brought about by glycidate in leaf discs can be attributed to indirect effects of metabolic regulation.

Photosynthetic carbon metabolism during leaf ontogeny in Zea mays L.: Enzyme studies

The activities of several enzymes, including ribulose-1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured as a function of leaf age in Z. mays. Mature leaf tissue had a RuDP-carboxylase activity of 296.7 μmol CO2 g(-1) fresh weight h(-1) and a PEP-carboxylase activity of 660.6 μmol CO2 g(-1) fresh weight h(-1). In young corn leaves the activity of the two enzymes was 11 and 29%, respectively, of the mature leaves. In senescent leaf tissue, RuDP carboxylase activity declined more rapidly than that of any of the other enzymes assayed. On a relative basis the activities of NADP malic enzyme (EC 1.1.1.40), aspartate (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2), and NAD malate dehydrogenase (EC 1.1.1.37) exceeded those of both PEP and RuDP carboxylase in young and senescent leaf tissue. Pulse-chase labeling experiments with mature and senescent leaf tissue show that the predominant C4 acid differs between the two leaf ages. Labeling of alanine in senescent tissue never exceeded 4% of the total (14)C remaining during the chase period, while in mature leaf tissue alanine accounted for 20% of the total after 60 s in (12)CO2. The activity of RuDP carboxylase during leaf ontogeny in Z. mays parallels the development of the activity of this enzyme in C3 plants.

Light versus Dark Carbon Metabolism in Cherry Tomato Fruits

The photosynthetic properties of the internal and peripheral tissues of the cherry tomato fruit (Lycopersicum esculentum var. cerasiforme Dun A. Gray) were investigated. Whole fruit and their isolated tissues evolve large amounts of CO(2) in darkness. In the light, this evolution decreases but nevertheless remains a net evolution; 3-(3,4-dichlorophenyl)-1,1-dimethylurea abolishes the effects of light.Incorporation of (14)CO(2) by leaves and fruit tissues demonstrates that the outer region of the fruit has the highest photosynthetic efficiency on a chlorophyll basis; the internal fruit tissue, richer in chlorophyll, has a much lower efficiency. The identification of intermediates following short term incubations with (14)CO(2) shows that in darkness the fruit accumulates the majority of label in malate. In the light, leaf tissue exhibits a pattern of incorporation characteristic of C-3 metabolism, whereas fruit tissue exhibits a decreased labeling of malate with a concomitant appearance of label in Calvin cycle intermediates. This is in agreement with the levels and types of carboxylating activities demonstrated in vitro; especially noteworthy is the very low ribulose diphosphate carboxylase activity in the internal fruit tissue.The photosynthetic potential, phosphoenolpyruvate carboxylase activity, and quantities of malate accumulated by fruit tissues are parallel to their chlorophyll content during growth and maturation.

Photosynthetic Pod Wall of Pea (Pisum sativum L.)

The pod wall of pea (Pisum sativum L.) was shown to contain two distinct photosynthetic layers. The outer, comprising chlorenchyma of the mesocarp, captured CO(2) from the outside atmosphere; the inner, a chloroplast-containing epidermis lining the pod gas cavity, was involved in photoassimilation of the CO(2) released from respiring seeds.Structural features of the pod included the thick cuticle and stomata of the outer epidermis, the inward projecting veinlets of the vascular network in the mesocarp, the sparsity of air spaces, the fiber and parenchyma layers of the endocarp, and the abundant chloroplasts, thin cuticle, and rounded outer contours of cells of the inner epidermis.The inner epidermis showed high specific activities of ribulose 1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31), contained up to 20% of the pod's chlorophyll, and was capable of fixing 66% of the CO(2) released during the photoperiod to the pod gas space by the seeds of a fully grown fruit.The in vitro carboxylation capacity of the pod exceeded the estimated gross photosynthesis of the fruit for all but the last few days of development. Chlorophyll content and carboxylation activity declined more markedly in the outer photosynthetic layers than in the inner epidermis.The ratio of activities of RuDP carboxylase to PEP carboxylase in pod extracts varied from 2.4:1 to 12:1 as against 48:1 to 156:1 in extracts of leaves.Structural and physiological properties of the pod were related to its capacity to conserve respired CO(2) and provide photosynthate to developing seeds.

Comparative Studies on C4 and C3 Photosynthetic Systems: Effect of Metabolic Inhibitors and Biochemical Intermediates on Carbon Metabolism

The effect of several metabolic inhibitors and biochemical intermediates on photosyntheticcarbon metabolism was examined in Setaria italica (malate utilising C 4 plant), Amaranthus paniculatus (asparate utilising C 4 plant) and Rumex vesicarius (C 3 plant). The inhibition by MgCl 2 , KNC, PGA and 6-phosphogluconate and stimulation by ribulose diphosphate (RuDP) in Setaria and Amaranthus (and not in Rumex ) affirmed the role of RuDP carboxylase in mediating the movement of label in C 4 plants. The negligible effet of salicylaldoxime in all three plants suggested the independence of label-movement from cyclic photophosphorylation. The suppression of '4C-movement by ouabain indicated the possible role of an ATPase in active transport of C 4 acids during C 4 photosynthesis. The negligible effect of DCMU on 14 C-transfer in Setaria in contrast to remarkable inhibition of the same in Amaranthus suggested the existence of an alternative source of reducing power (NADPH 2 ) in Setaria , which is presumed to be NADP malic enzyme.

Development of the cation-induced stacking capacity during the biogenesis of higher plant thylakoids

We studied the capacity of the thylakoid membrane to form grana stacks in the presence of cations, monovalent or divalent, added to N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine “low-salt” disorganized plastids during their greening. Grana stacking was monitored by the yield of heavy subchloroplast fractions separated by differential centrifugation after digitonin disruption of plastids (J. H. Argyroudi-Akoyunoglou, 1976, Arch. Biochem. Biophys., 176, 267–274) . Primary thylakoids of the agranal protochloroplasts formed in periodic light do not show the cation-induced stacking capacity of the mature green chloroplast thylakoids. Similarly, the cation effect saturates at lower cation concentrations in mature chloroplasts than in plastids of the early stages of greening. The capacity for cation-induced stacking and for saturation of the effect at low cation concentrations appears gradually after exposure to continuous light and parallel to the appearance of chlorophyll b and the polypeptides of the 25,000–30,000 molecular weight range of lipid-free thylakoids, probably derived from the chlorophyll b-rich chlorophyll protein Complex II. The thylakoid peripheral stroma proteins ribulosediphosphate carboxylase and the coupling factor protein are not involved in the cation-induced stacking, since their removal (H. Strotmann, H. Hesse, and K. Edelmann, 1973, Biochim. Biophys. Acta, 314, 202–210) does not affect the thylakoid aggregation.

Development of a photosynthesis model with an emphasis on ecological applications : III. Carbon dioxide and oxygen dependencies.

A theoretical description of the simultaneous processes of photosynthesis and photorespiration in a single leaf is developed, based on the hypothesis that carbon dioxide and oxygen compete for the active site of ribulose diphosphate carboxylase. Michaelis-Menten kinetics and competitive inhibition at the end of a diffusion path provide the basic structure of the model. Data of Ludwig (1972) from sunflower are analyzed according to the formulation. This description is part of a more general physiological-ecological model of photosynthesis presented previously (Tenhunen et al., 1976a, b) and continues to elaborate sub-processes in terms of physiologically meaningful parameters. The description is considered a working hypothesis. Data on photorespiration from the literature are reviewed as they relate to this working hypothesis. Several lines of investigation are thereby suggested that will help clarify the role of photorespiration in whole leaf photosynthesis and determine the over-all utility of this modeling approach.

Water Relations and Physiological Activity of Potatoes1

Abstract The effects of water stress on internal water potential components and specific physiological processes were investigated in field grown potatoes (Solanum tuberosum L. cv. Viking). Leaf water potential (ψleaf) as estimated by the pressure chamber, was not directly related to soil water potential (ψsoil) until a specific minimum ψsoil was attained. Subsequently ψleaf did not increase in response to increases in ψsoil. Water stress affected physiological processes such as stomatal resistance, photosynthesis and enzyme activity. A decline in ψleaf was apparently responsible for increased stomatal resistance and decreases in photosynthetic rates. The activities of ribulose diphosphate carboxylase and phosphoenolpyruvate carboxylase decreased as ψleaf declined. The relationship between water stress and physiological processes and the inability of ψleaf to respond to increases in ψsoil after a maximum stress may partially explain the extreme sensitivity of potatoes to even mild water stress.

Changing Ribulose Diphosphate Carboxylase/Oxygenase Activity in Ripening Tomato Fruit

Tomato fruit (Lycopersicum esculentum Mill) from green, pink, and red stages were assayed for changes in the activity of ribulose diphosphate carboxylase and oxygenase, phosphoenolpyruvate carboxylase, changes in the levels of glycolate and respiratory gas exchange. The ribulose diphosphate carboxylase activity decreased as the fruit ripened. By comparison, the ribulose diphosphate oxygenase activity increased during the transition from the green to the pink stage, and declined afterward. The changes in the endogenous glycolate levels and the respiratory gas exchange, as observed at different stages of ripening, resembled the changes in the ribulose diphosphate oxygenase activity. The utilization of glycolate in further metabolic activity may result in the formation of peroxidases required for the onset of ripening.

Purification and Properties of Phosphoenolpyruvate and Ribulose Diphosphate Carboxylases from C4 and C3 Plants

Summary Ribulose diphosphate (RuDP) and phosphoenolpyruvate (PEP) carboxylases were partially purified from leaves of three C4 plants: Setaria, Pennisetum and Amaranthus and two C3 plants: Oryza and Rumex. RuDP carboxylase from C4 plants had a lower Km for RuDP and HCO3− than that of the enzyme from C3 type. Their reaction velocities were similar in all cases, reflecting the potential of RuDP carboxylase in C4 plants as well as in C3 plants. PEP carboxylase from C4 plants possessed higher Km values for PEP and Mg++ than that from C3 plants, but the Km for HCO3− was similar in both the categories. The maximum reaction velocities of PEP carboxylase from C4 plants were nearly 15–20 times higher than those from C3 plants, suggesting the basically different nature of PEP carboxylases from C3 and C4 plants. It is envisaged that in C4 plants PEP carboxylase helps in rapid fixation of CO2 into C4 acids at a faster rate under atmospheric CO2 concentration and these C4 acids move into bundle sheath and are decarboxylated creating a much higher concentration of CO2 than outside. At thus elevated levels of CO2 in bundle sheath of C4 plants RuDP carboxylase operates effectively.

Effect of SO 2 polluted air upon enzyme activity in plants originating from areas with different annual mean atmospheric SO 2 concentrations

Plants of the broad-leaved dock ( rumex obtusifolius), originating from areas with different annual mean atmospheric SO 2 concentrations, were exposed to charcoal filtered air or 0·2 ppm SO 2 (⋍520 μg SO 2/m 3) for 11 days. Comparisons were made of the levels of activity of ribulose-diphosphate carboxylase (RuDPC), glutamate-pyrurate transaminase (GPT), glutamate-oxaloacetate transaminase (GOT) and peroxidase in leaf extracts from the plants. The effects of the sulphur dioxide fumigation varied, depending on the age of the plant leaves and the area of origin of the plants. The differential response to SO 2 exposure between the plants from the high mean SO 2 area and the low mean SO 2 area suggested that adaptation to the polluted conditions prevailing in the high mean SO 2 area may have occurred in the plants originating from that area.

Carbon isotope composition of biochemical fractions isolated from leaves of Bryophyllum daigremontianum berger, a plant with crassulacean acid metabolism: Some physiological aspects related to CO2 dark fixation

Isotope analysis of the biochemical fractions isolated quantitatively from young and mature leaves of Bryophyllum daigremontianum Berger have been carried out before and after a dark period of accumulation of organic acids. The mature leaf is enriched in (13)C compared to the young leaf. The δ(13)C values of the different leaf constituents vary between the δ(13)C values of C4 plants (-11‰) and those of C3 plants (-27‰). During the dark period, the two types of leaves store organic acids with δ(13)C values of ≃-15‰ and lose insoluble sugars, including starch with a δ(13)C value of ≃-12‰. Furthermore, young leaves store phosphorylated compounds with δ(13)C values of ≃-11‰ and lose weakly polymerised sugars with δ(13)C values of ≃-18‰. These results led to the formulation of a hypothesis of the origin of the two substrates of β-carboxylation: phosphoenolpyruvate arises from the glycolytic breakdown of the insoluble sugars rich in (13)C, and the major portion of the CO2 is the result of the complete breakdown (respiration) of the soluble sugars rich in (12)C. The existence of two independent sugar pools leads to the assumption that there are two separate glycolytic pathways. The (13)C enrichment of the stored products of the young leaves in the day seems to be the result of a weak discrimination for (13)C by ribulose diphosphate carboxylase, which reassimilates to a great extent the CO2 released from malate accumulated in the night.

Changes in RuDP Carboxylase Activity of Rice Leaf Blade at Various Growing Stages

Ribulose-1, 5-diphosphate (RuDP) carboxylase, catalase and acid phosphatase activitics, and chlorophyll and soluble protein contents expressed in term of unit leaf area were investigated at various stages of leaf growth in pots. The following results were obtained : (1) When the leaf blade was divided into three equal parts, RuDP carboxylase activity of each part of young leaf blade immediately after it had fully expanded showed the ranking order: middle≥top<base. The activity of the aged old leaf showed the ranking order: base<middle<top. But the mean activity in the whole leaf blade was considerably lower in older leaf than in expanded younger one. Changes in RuDP carboxylase activity also had a similar pattern to soluble protein content, but not to chlorophyll content in all leaf parts. (2) When RuDP carboxylase activity in different position of leaf blades was measured at various stages of leaf growth, the results were obtained that the mature top leaf which completely expanded had the highest activity and as the leaf positions dropped down, RuDP carboxylase activity of each leaf decreased drastically. RuDP carboxylase activity of developing immature leaf was as low as that of lower leaf position. Chlorophyll content of leaf blade attained its maximum at the lower leaf than the mature top leaf and their differences among various leaf positions were observed after young panicle formation stages, but not at early growing stages. (3) After heading date, RuDP carboxylase activity of flag leaf was maintained at relatively higher levels untill toward the yellowing stages. During these stages, soluble protein content, chlorophyll content, and catalase activity of each leaf changed parallel with RuDP carboxylase activity, respectively, but acid phosphatase activity increased with the ageing of expanded leaf blades. (4) The correlation evident between RuDP carboxylase activity and soluble protein content observed through the growing stages (table 3 and 4.) indicated that there was a parallel correlation between RuDP carboxylase activity and photosynthetic rate when we referd to the past some observations that protein-nitrogen content of rice leaf was closely correlated with the photosynthetic rate. Chlorophyll content also changed parallel with the RuDP carboxylase activity through growing stages except for the early stages. But the degree of the parallelism between the two was lower than that between RuDP carboxylase and soluble protein content.

The retention of photosynthetic activity by senescing chloroplasts of oat leaves

The retention of photosystems I and II and or RuDP carboxylase activity in chloroplasts isolated from the first leaves of Victory oat (Avena sativa L.) seedlings was followed as the chloroplasts senesced in darkness. Both photosystems (PS) I and II retained their full activity after 3 days at 1°C, while even after 7 days at 1°C around 80% of the activity was still present. After 3 days at 25°C, PS I lost only 20% and PS II 50% of the initial activity. Acid pH increased the rate of decay of both systems, PS II falling almost to zero after 3 days at pH 3.5 (at 25°C). The preparations were almost bacteria-free, and addition of antibiotics not only did not improve their stability, but accelerated the rates of loss of photosynthetic activity. This is held to indicate that the enzymes are undergoing some turnover even in isolated chloroplasts. If the leaves were allowed to senesce in the dark first and the chloroplasts then isolated, their photosynthetic activities had greatly decreased, showing that senescence is more rapid in situ than in isolation. Under these conditions PS I decayed more rapidly than PS II. Ribulosediphosphate carboxylase, as measured by CO2 fixation, declined more rapidly than the photosystems, though the addition of kinetin and indole-3-acetic acid somewhat decreased the rate of loss, at least for the first 24 h. When the intact (detached) leaves were held in the dark, the rate of oxygen evolution declined rapidly, but in monochromatic blue light (450 nm) at 25°C about 30% of the initial rate was retained after 72 h.

Sulfite action on ribulosediphosphate carboxylase in the lichen Pseudevernia furfuracea.

Ribulosediphosphate carboxylase can be extracted from Pseudevernia furfuracea, either by Triton X-100 or by pretreatment with liquid nitrogen. With respect to HCO 3- the K m is 33 mM. The enzyme is competitively inhibited by sulfite, as is the case with spinach chloroplasts. The K i value (15.5-18.5 mM sulfite) indicates that in lichens the enzyme is not more sensitive than that in spinach. Thus the extreme sensitivity of lichens to SO2 is not based on low tolerance at the enzymatic level, but is due to a low degree of avoidance.