The Spatial Distribution and Ecology of Zoochlorellae-Bearing Ciliates in a Productive Pond1

ABSTRACT. The spatial distributions of five zoochlorellae-bearing ciliate species (Euplotes daidaleos, Frontonia vernalis, Acaryophrya sp., Disematostoma bütschlii, and Stokesia vernalis) were investigated in a productive freshwater pond. The vertical profiles of all species were compared to the levels of O₂, CO₂, light, temperature, dissolved inorganic nitrogen, and the availability of particulate food sources. In the stratified water column, Stokesia remained close to the water surface: all other species reached peak abundance close to the oxic-anoxic boundary. The latter behavior probably accommodated the ciliates’requirements for aerobic respiration and particulate food and their dependance on the essential resources of light and dissolved nutrients, which came from opposite directions. With the collapse of stratification, Euplotes and Frontonia returned to the sediment where they reverted to a heterotrophic nutrition although they retained some zoochlorellae. Acaryophrya and Disematostoma also became heterotrophic, but they remained evenly distributed in the water column and they lost their zoochlorellae. Stokesia disappeared, presumably because it encysted. There was some evidence for vertical spatial separation of the five species in the water column.     

Effect of oxygen pressure on synthesis and export of nitrogenous solutes by nodules of cowpea

Nodules of cowpea plants (Vigna unguiculata (L.) Walp. cv. Vita 3 :Bradyrhizobium CB756) cultured for periods of 23 d with their root systems maintained in atmospheres containing a range of partial pressures of O₂ (pO₂; 1–80%, v/v, in N₂) formed and exported ureides (allantoin and allantoic acid) as the major products of fixation at all pO₂ tested. In sub-ambient pO₂ (1 and 2.5%) nodules contained specific activities of uricase (urate: O₂ oxidoreductase; EC 1.7.3.3) and allantoinase (allantoin hydrolyase; EC 3.5.2.5) as much as sevenfold higher than in those from air. On a cell basis, uninfected cells in nodules from 1% O₂ contained around five times the level of uricase. Except for NAD: glutamate synthase (EC 1.4.1.14), which was reduced in sub-ambient O₂, the activities of other enzymes of ureide synthesis were relatively unaffected by pO₂. Short-term effects of pO₂ on assimilation of fixed nitrogen were measured in nodules of air-grown plants exposed to subambient pO₂ (1, 2.5 or 5%, v/v in N₂) and¹⁵N₂. Despite a fall in total¹⁵N₂ fixation, ureide synthesis and export was maintained at a high level except in 1% O₂ where formation was halved. The data indicate that in addition to the structural and diffusional adaptations of cowpea nodules which allow the balance between O₂ supply and demand to be maintained over a wide range of pO₂, nodules also show evidence of biochemical adaptations which maintain and enhance normal pathways for the assimilation of fixed nitrogen. This work was supported by a grant from the Australian Research Council (to C.A.A.) and an Australian Development Assistance Bureau postgraduate fellowship (to F.D.D.).     

Leaf gas films of Spartina anglica enhance rhizome and root oxygen during tidal submergence

Gas films on hydrophobic surfaces of leaves of some wetland plants can improve O₂ and CO₂ exchange when completely submerged during floods. Here we investigated the in situ aeration of rhizomes of cordgrass (Spartina anglica) during natural tidal submergence, with focus on the role of leaf gas films on underwater gas exchange. Underwater net photosynthesis was also studied in controlled laboratory experiments. In field experiments, O₂ microelectrodes were inserted into rhizomes and pO₂ measured throughout two tidal submergence events; one during daylight and one during night‐time. Plants had leaf gas films intact or removed. Rhizome pO₂ dropped significantly during complete submergence and most severely during night. Leaf gas films: (1) enhanced underwater photosynthesis and pO₂ in rhizomes remained above 10 kPa during submergence in light; and (2) facilitated O₂ entry from the water into leaves so that rhizome pO₂ was about 5 kPa during darkness. This study is the first in situ demonstration of the beneficial effects of leaf gas films on internal aeration in a submerged wetland plant. Leaf gas films likely contribute to submergence tolerance of S. anglica and this feature is expected to also benefit other wetland plant species when submerged.     

Effect of pO(2) on the Formation and Status of Leghemoglobin in Nodules of Cowpea and Soybean

Nodulated cowpea (Vigna unguiculata [L.] Walp. cv Vita 3: Bradyrhizobium strain CB756) and soybean (Glycine max [L.] Merr. cv White Eye: Bradyrhizobium strain CB1809) were grown with their root systems maintained in a flowing gas stream containing a range of pO₂ (1-80%, v/v) in N₂ for up to 28 days after planting. At the extremes of sub- and supra-ambient pO₂, the levels of leghemoglobin (Lb) in nodules were reduced. However, neither the proportional composition of Lb component proteins (eight in soybean, three in cowpea) nor their oxidation state was affected by pO₂. Short-term changes in pO₂ (transferring plants grown with sub- or supra-ambient pO₂ in the rhizosphere to air or vice versa) caused a significant decline in Lb content and, in cowpea but not soybean, where pO₂ was increased, a higher percentage of oxidation of Lb. Combining data on changes in Lb level of cowpea nodules grown in sub-ambient pO₂ with those for their structural adaptation to an under supply of O₂ indicated that, despite the nodules having a lower level of Lb, the amount per infected cell was increased by up to twofold and per bacteroid up to fivefold (in those from 1% O₂) compared to those grown in air. Progressive decline in pO₂ resulted in a progressive increase on this basis, indicating a close relationship between Lb content and the adaptation of nodule functioning to external O₂ level.     

Regulatory O2 tensions for the synthesis of fermentation products in Escherichia coli and relation to aerobic respiration

In an oxystat, the synthesis of the fermentation products formate, acetate, ethanol, lactate, and succinate of Escherichia coli was studied as a function of the O₂ tension (pO₂) in the medium. The pO₂ values that gave rise to half-maximal synthesis of the products (pO_0.5) were 0.2–0.4 mbar for ethanol, acetate, and succinate, and 1 mbar for formate. The pO_0.5 for the expression of the adhE gene encoding alcohol dehydrogenase was approximately 0.8 mbar. Thus, the pO₂ for the onset of fermentation was distinctly lower than that for anaerobic respiration (pO_0.5≤ 5 mbar), which was determined earlier. An essential role for quinol oxidase bd in microaerobic growth was demonstrated. A mutant deficient for quinol oxidase bd produced lactate as a fermentation product during growth at microoxic conditions (approximately 10 mbar O₂), in contrast to the wild-type or a quinol-oxidase-bo-deficient strain. In the presence of nitrate, the amount of lactate was largely decreased. Therefore, under microoxic conditions, the pO₂ appears to be too high for (mixed acid) fermentation to function and too low for aerobic respiration by quinol oxidase bo. Received: 7 February 1997 / Accepted: 2 May 1997     

The effects of photosynthetically raised pH and light on some ciliated Protozoa in a eutrophic pond

Some ciliated Protozoa (e.g. Loxodes magnus, L. striatus, Spirostomum teres, S. ambiguum and Frontonia leucas) are abundant during summer in the hypolimnion of a eutrophic pond in north-west England but are absent from the epilimnion. The work described in this paper was begun with the aim of investigating the suggestion that high pH values caused by phytoplankton photosynthesis contributed to the exclusion of these ciliates from the epilimnion. In July 1973, phytoplankton photosynthesis and high pH were found only in the epilimnion, hence conditions were compatible with the above suggestion. Ciliates were, therefore, kept in the laboratory in hypolimnion water and were exposed to phytoplankton photosynthesis, both with and without pH increase. It was found that Loxodes died under both treatments hence there is no evidence that high pH is lethal to Loxodes. It seemed possible, therefore, that either light or toxins released by algae during photosynthesis are lethal to Loxodes. L. magnus was, therefore, exposed to light in the absence of phytoplankton (in filtered hypolimnion water) both in the laboratory and in the pond and it was found that light was lethal. High light intensities might, therefore, contribute to the exclusion of at least Loxodes species from the surface water of the pond, although other adverse factors are probably operative since Loxodes species do not migrate into the epilimnion at night.     

Adaptation of Nodulated Soybean (Glycine max L. Merr.) to Growth in Rhizospheres Containing Nonambient pO(2)

Nodulated soybean (Glycine max L. Merr. cv White Eye inoculated with Bradyrhizobium japonicum strain CB 1809) plants were cultured in the absence of combined N from 8 to 28 days with their root systems maintained continuously in 1, 2.5, 5, 10, 20, 40, 60, or 80% O₂ (volume/volume) in N₂. Plant dry matter yield was unaffected by partial pressure of oxygen (pO₂) and N₂ fixation showed a broad plateau of maximum activity from 2.5 to 40 or 60% O₂. Slight inhibition of nitrogenase activity occurred at 1% O₂ and as much as 50% inhibition occurred at 80% O₂. Low pO₂ (less than 10%) decreased nodule mass on plants, but this was compensated for by those nodules having higher specific nitrogenase activities. Synthesis and export of ureides in xylem was maintained at a high level (70-95% of total soluble N in exudate) over the range of pO₂ used. Measurements of nitrogenase (EC 1.7.99.2) activity by acetylene reduction indicated that adaptation of nodules to low pO₂ was largely due to changes in ventilation characteristics and involved increased permeability to gases in those grown in subambient pO₂ and decreased permeability in those from plants cultured with their roots in pO₂ greater than ambient. A range of structural alterations in nodules resulting from low pO₂ were identified. These included increased frequency of lenticels, decreased nodule size, increased volume of cortex relative to the infected central tissue of the nodule, as well as changes in the size and frequency of extracellular voids in all tissues. In nodules grown in air, the inner cortex differentiated a layer of four or five cells which formed a band, 40 to 50 micrometers thick, lacking extracellular voids. This was reduced in nodules grown in low pO₂ comprising one or two cell layers and being 10 to 20 micrometers thick in those from 1% O₂. Long-term adaptation to different external pO₂ involved changes which modify diffusive resistance and are additional to adjustments in the variable diffusion barrier.     

SEASONAL VARIATIONS OF PHOTOSYNTHETIC PIGMENTS, TOTAL C, N, AND P CONTENT, AND PHOTOSYNTHESIS IN PHYLLARIOPSIS PURPURASCENS: (PHAEOPHYTA) FROM THE STRAIT OF GIBRALTAR

Photosynthetic pigments, C, N, and P tissue composition, and photosynthetic rate were measured from April to October in the brown alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) growing at a 30-m depth in the Strait of Gibraltar. Ir-radiance reaching the population ranged from 13.5 to 27.5 mol.m^-2.mo^-1. The available light for this species, expressed as a percentage of the irradiance above the water, was 1.8%. Dissolved inorganic nitrogen forms, NO3-and NH₄+, were constant from April to October, whereas phosphate was depleted in August. Chlorophyll a decreased from 520.0 ± 165.0 to 199.6 ± 159.9 μg.g^-1 dry weight; in contrast, chlorophyll c and carotenoids did not change until September but increased threefold in October. C:N and N:P ratios changed in the same way and in the same range. They were constant until July but increased from 15–17 up to 42 (C:N) and from 14 to 40 (N:P) in October, suggesting a severe P limitation of growth of this species. The dark respiration rate and the light compensation point were constant from April to October (0.5 ± 0.1 μmol O₂. m^-2.s^-1 and 6.5 ± 0.2 μmol.m^-2. s^-1, respectively), whereas the maximum rate of apparent photosynthesis, light onset saturation parameter, and half saturation constant for light were maximum in April to May (3.7 μmol O₂. m^-2.s^-1and 40 and 41.5 μmol.m^-2. s^-1, respectively) and October (3.6 μmol O₂. m^-2.s^-1 and 50 and 53.7 μmol.m^-2. s^-1, respectively). They were minimum in August (1.2 μmol O₂.m^-2.s^-1 and 11.3 and 12 μmol.m^-2.s^-1, respectively). These minimum figures yielded a negative carbon budget in August and 0 in September, whereas it was positive the rest of the year. Photosynthetic efficiency, estimated by the ratio between maximum apparent photosynthesis and light half saturation constant, showed a strong agreement with productivity measured by means of an independent method. These results indicate that lamina expansion in this species is controlled by photosynthetic efficiency.     

Effects of various partial pressures of oxygen and carbon dioxide on different stages of somatic embryogenesis in Picea abies

Effects of various partial pressures of oxygen (5, 20 and 45 kPa) and carbon dioxide (0.03 and 6 kPa) on initiation, proliferation and maturation of somatic embryos in Picea abies were studied. The pO₂ had a significant effect on the initiation of embryogenic tissue from mature zygotic embryos. However, the effect of pO₂ was dependent on the strength of the basal medium. Low pO₂ stimulated the formation of embryogenic tissue when the zygotic embryos were incubated on full strength medium, but was inhibitory when half-strength medium was used. Proliferation of embryogenic tissue was stimulated by higher partial pressures of both CO₂ and O₂. The effect of the gas phase on maturation of somatic embryos varied between different cell lines. However, there was a general tendency for 5 kPa O₂ and 6 kPa CO₂ to stimulate maturation.Abbreviations ABA abscisic acid - BA benzyladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - ET embryogenic tissue     

The influence of dissolved oxygen concentration on phosphofructokinase and the glucose metabolism ofEscherichia coli K-12

The glucose metabolism and the response of phosphofructokinase activity to oxygen were investigated using glucose-limited chemostat cultures ofE. coli K-12. With a dilution rate of 0.2 hr^–1 and a glucose input concentration of 0.83 g/litre, 10 steady states were obtained ranging from 320 to 0 mm HgO₂. Dissolved oxygen reached zero level at a pO₂ of 25.8 mm Hg. The specific phosphofructokinase activity was constant above 28 mm Hg O₂ and increased linearly at lower pO₂ levels until it reached highest activity at 0 mm Hg O₂. Cell dry weight also started to decrease linearly from 28 to 5.9 mm Hg O₂, and fell sharply thereafter. Acid production rate did not start before pO₂ reached 25.6 mm Hg, increased progressively with an additional sharp increase below 5.9 mm Hg O₂. The main endproducts formed were acetic acid and ethanol with lactic acid appearing below 5.9 mm Hg O₂. The results suggest an effect of oxygen on phosphofructokinase synthesis rather than an ATP inhibition of the enzyme.This work was supported by a grant from the Australian Research Grant Commission.     

Oxygen limitation of N2 fixation in stem-girdled and nitrate-treated soybean

The effects of increasing rhizosphere pO₂on nitrogenase activity and nodule resistance to O₂diffusion were investigated in soybean plants [Glycine max (L.) Merr. cv. Harosoy 63] in which nitrogenase (EC 1.7.99.2) activities were inhibited by (a) removal of the phloem tissue at the base of the stem (stem girdling), (b) exposure of roots to 10 mM NO₃over 5 days (NO₃-treated), or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O₂(O₂-inhibitcd). In control plants and in plants which had been treated with 100 kPa O₂, increasing rhizosphere O₂concentrations in 10 kPa increments from 20 to 70 kPa did not alter the steady-state nitrogenase activity. In contrast, in plants in which nitrogenase activities were depressed by stem girdling or by exposure to NO₃, increasing rhizosphere pO₂resulted in a recovery of 57 or 67%, respectively, of the initial, depressed rates of nitrogenase activity. This suggests that the nitrogenase activity of stem-girdled and NO₃-treated soybeans was O₂-limited. For each treatment, theoretical resistance values for O₂diffusion into nodules were estimated from measured rates of CO₂exchange, assuming a respiratory quotient of 1.1 and 0 kPa of O₂in the infected cells. At an external partial pressure of 20 kPa O₂, the stem-girdled and NO₃^--treated plants displayed resistance values which were 4 to 8.6 times higher than those in the nodules of the control plants. In control and O₂-inhibited plants, increases in pO₂from 20 to 70 kPa in 10 kPa increments resulted in a 2.5- to 3.9-fold increase in diffusion resistance to O₂, and had little effect on either respiration or nitrogenase activity. In contrast, in stem-girdled and NO₃^--treated plants, increases in external pO₂had little effect on diffusion resistance to O₂, but resulted in a 2.3- to 3.2-fold increase in nodule respiration and nitrogenase activity. These results are consistent with stem-girdling and NO₃^--inhibition treatments limiting phloem supply to nodules causing an increase in diffusion resistance to O₂at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external pO₂.     

Photobehavior of the Ciliated Protozoon Loxodes: Taxic,Transient, and Kinetic Responses in the Presence and Absence of Oxygen1

ABSTRACT. Behavioral responses to light at different oxygen tensions were studied in the ciliate Loxodes striatus. In the absence of O₂ it does not react to light. In the presence of O₂ it reacts to light as if the pO₂ had been further increased, with the induction of positive geotaxis, a transient phobic response, and finally with a permanent kinetic response (increased swimming velocity and a decreased rate of tumbling). Cells treated with cyanide behave as cells in an anoxic environment and do not react to light. It is concluded that the light response is due to the photochemical production of oxygen radicals and that the sensory receptors for O₂ and for light are identical. The three types of behavioral response (geotaxis, transient, and kinetic responses) are discussed in terms of their adaptive significance for the orientation of Loxodes in the natural environment.     

Der Einfluß von Licht auf die Atmung von Chlorella bei gehemmter Photosynthese

Summary On illumination with blue light the O₂-uptake of Chlorella pyrenoidosa (211-8b) in which photosynthetic O₂-liberation has been suppressed by 10^-5M DCMU initially decreases, but in the course of 5–10 min increases over that in preceding darkness (Fig. 1). Whereas an enhancement of O₂-uptake is already induced by traces of blue radiation and saturated at about 1.5x10^-10einsteins cm^-2sec^-1, the initial inhibition of O₂-uptake can be measured only after application of more than 1.5×10^-10einsteins cm^-2sec^-1 (Fig. 2).The long induction time that passes before a steady enhancement in O₂-uptake is reached, the low energy requirement of the enhancement, and its spectral dependence with greatest efficiency of wavelengths around 455 nm and 375 nm and no effect of wavelengths beyond 520 nm (Fig. 3) resemble the corresponding data found earlier for an enhancement of respiration by light in a chlorophyll-free, carotenoidcontaining Chlorella mutant. It is therefore likely that the increased O₂-uptake in DCMU-poisoned cells of wild type Chlorella depends on an increase in respiration. The pigment involved is not known, but from the action spectrum it could be a flavin or a cis-carotenoid.In contrast to the increase the initial decrease in O₂-uptake does not show up in strong blue light only, but is also present in red light in which it stays constant throughout the period of measurement of 20 min (Fig. 4). Its intensity dependence is similar in blue and in red light; the lower efficiency of blue, which appears in Fig. 5, is at least partially due to the time interval of 5 min chosen for its determination: in these first 5 min after the beginning of blue illumination the slow increase in respiration already begins. The spectral dependence of the decrease in O₂-consumption in the red part of the visible spectrum yields greatest activity around 680 nm, a slow drop towards 525 nm and a steep one towards 743 nm (Fig. 6). From that and the absence of any after-effect of red light on the O₂-consumption in following darkness (Fig. 8), which might be expected if phytochrome action were involved, we think chlorophyll to be the pigment responsible for light-dependent inhibition of O₂-uptake. A mutant of Scenedesmus, Bishop's Nr. 11, which is unable to evolve photosynthetic oxygen, behaves just like DCMU-poisoned Chlorella (Fig. 7). We therefore consider the decreased O₂-consumption in the light to result from a partial inhibition of respiration and not from remaining photosynthesis unaffected by 10^-5M DCMU. As photosystem I still operates in Bishop's mutant 11 as well as in DCMU-poisoned Chlorella, illumination might lead to an accumulation of ATP by cyclic photophosphorylation and thus to a lowering of the cellular ADP level. This could result in a slowing down of glycolysis and consequently of respiratory O₂-uptake.     

The effect of oxygen partial pressure in bioreactors on cell proliferation and subsequent differentiation of somatic embryos of Cyclamen persicum

The effect of the relative oxygen partial pressure (pO₂) in bioreactors on cell proliferation and subsequent differentiation of somatic embryos from suspension cultures of Cyclamen persicum Mill. was investigated. The growth rate of cell line 3738-VIII in growth-regulator containing medium in bioreactors at 5% pO₂ was slightly reduced in comparison to 10% and 20% pO₂. Cultures growing at 40% pO₂ had a lower growth rate, a markedly reduced cell viability and showed a decrease of the medium pH to 3.5. Because a pH-control with a setpoint of 3.3 caused cell death within 4 days, it was assumed, that the reason for the poor cell proliferation and viability in the cultures at 40% pO₂ was an effect of medium acidification rather than of the high O₂ partial pressure. A significantly higher number of germinating embryos was obtained from the cultures grown at 40% pO₂ than from those grown in flasks or in bioreactors at 5%, 10% and 20% pO₂. These results were specific for cell line 3738-VIII. Another cell line, 3736-12, did not show marked differences in cell proliferation, viability, pH or subsequent regeneration of somatic embryos when grown at different O₂ partial pressures. This revised version was published online in June 2006 with corrections to the Cover Date.     

RESPONSE OF CATALASE ACTIVITY TO ENVIRONMENTAL STRESS IN THE FRESHWATER DINOFLAGELLATE PERIDINIUM GATUNENSE1

Catalase activity increased in Peridinium gatunense (formerly P. cinctum fa. westii) cells during the decline of the seasonal spring bloom period in Lake Kinneret. This was correlated with the low ambient total CO₂ concentration. The relationship was confirmed in laboratory experiments where maximum catalase activity occurred under an atmosphere composed of 30% O₂ and 0.003% CO₂. Conversely, high CO₂ concentrations inhibited catalase activity. The rise in catalase activity was not directly due to increasing environmental pH, as in vitro and in vivo measurements showed a characteristic broad pH curve with a constant activity from pH 6–10 for catalase. Photoinhibition of catalase occurred above 250 μmol photons · m^?2· s^?1. However, at high photoinactivating irradiances, photoinhibition was ameliorated under high pO₂/pCO₂. Such conditions prevail in the Kinneret at the end of the spring. We propose that the enhancement of photorespiration (under high pO₂/pCO₂) induces a temporary burst in catalase activity despite the progressively photoinhibitory conditions of early summer.     

Photosynthesis and respiration in a fast growing strain of Gigartina exasperata (Harvey and Bailey)

Photosynthesis and respiration rates of blades from a selected, fast growing strain of the marine red alga. Gigartina exasperata Harvey and Bailey, a carrageenan producer, were measured with an oxygen electrode and compared with rates similarly obtained from wild material of the same species. The measurements, expressed as μl O₂ · mg chl a^?1, min^?1. were made over a light intensity range from 5 to 800 μE · m^?2 · sec^?1 and a temperature range of 6 to 16°C. The photosynthesis light intensity data are best described by hyperbolic functions.     

CYTOCHROME f LOSS IN ASTAXANTHIN-ACCUMULATING RED CELLS OF HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE): COMPARISON OF PHOTOSYNTHETIC ACTIVITY, PHOTOSYNTHETIC ENZYMES, AND THYLAKOID MEMBRANE POLYPEPTIDES IN RED AND GREEN CELLS

Photosynthetic activity, chloroplast enzymes, and poly-peptides were compared in green and red (ketocarotenoid-containing) cultures of the microalga Haematococcus pluvialis Flotow. Green cultures, grown at 80 μmol pho-tons.m^-2. s^-1 in an acetate-containing medium, had a mean generation time of 27 h. Ketocarotenoid accumulation was induced by transfer of green cultures to PO₄-deficient medium and exposure to 250 μmol photons.m^-2. s^-1. Under these conditions, there was no increase in cell number, and the cultures turned red. Relative amounts of enzymes and thylakoid polypeptides in red and green cells were ascertained by immunoprobing with standardization on a chlorophyll (Chl) basis. In red cultures, the level of cytochrome f was greatly decreased (< 1% of green cell level), which is expected to greatly impair the linear electron flow from photosystem (PS) II to PS I. Also, the levels of apoproteins in red cells, namely, of CPI, D2, CP47, LHC I, and ribulose-1, 5-bisphosphate carboxylase were reduced to 15, 18, 29, 48, and 80%, respectively, of those in green cells. Only adenosine triphosphate syn-thase exhibited no significant change in the two types of cultures. The respiration rate of red cultures was much higher (100 μmoles O₂. mg Chl^-1.h^-1) than that of green cells (16 μmoles O₂. mg Chl^-1.h^-1). Conversely, net O₂ evolution (at P_max in green cultures was 80 μmoles O₂. mg Chl^-1.h^-1 but was —40 μmoles O₂. mg Chl^-1.h^-1 in red cultures. PS II activity was demonstrated in broken cells of both green and red cultures, showing activity of 40 and 15 μmoles DCPIP-mg Chl^-1.h^-1 (with DPC as electron donor), respectively. In contrast, PS I activity measured by the Mehler reaction showed that red rather than green cells had a greater activity (64 vs. 46 μmoles O₂. mg Chl^-1.h^-1, respectively). Thus, in spite of the decline of O₂ evolution in red cells, the photosystems were still functional. We postulate that the decline of O₂, evolution in red cells is largely attributable to an increase in the respiration rate and the impairment of linear electron flow from PS II to PS I and, to some extent, to a decrease in components of the photosystems.     

Nodule structure and nitrogenase activity ofCoriaria arborea in response to varying pO2

When excised root nodules ofCoriaria arborea are assayed for nitrogenase activity at various pO₂ they show a broad optimum between 20 and 40 kPa O₂, with some evidence for adaptation. Continuous flow assays of nodulated root systems of intact plants indicate that Coriaria shows an acetylene induced decline in nitrogenase activity. When root systems were subject to step changes in pO₂ nitrogenase activity responded with a steep decline followed by a slower rise in activity both at lower and higher than ambient pO₂. Thus Coriaria nodules are able to adapt rapidly to oxygen levels well above and well below ambient. Measurement of nodule diffusion resistance showed that the adaptation is accompanied by rapid increase in resistance at above ambient pO₂ and decrease in resistance at below ambient pO₂. Plants grown with root systems at pO₂ from 5–40 kPa O₂ did not differ in growth or nodulation. The anatomy of Coriaria nodules shows they have a dense periderm which encircles the nodule and also closely invests the infected zone. The periderm is both thicker and more heavily suberised in nodules grown at high pO₂ than at low pO₂. Vacuum infiltration of India ink indicates that oxygen diffusion is entirely through the lenticel and via a small gap adjacent to the stele.     

Quenching Effect of Tocopherols on the Methyl Linoleate Photooxidation and Their Oxidation Products

The quenching effect of α-, γ- and δ-tocopherols on the methylene blue sensitized photo- oxidation of methyl linoleate was investigated, and the ¹O₂. quenching ability of tocopherols was determined. The ¹O₂ quenching rate constants for α-, γ- and δ-tocopherols in ethanol were estimated to be 2.6 × 10⁸ m^?1 sec^?1, 1.8 × 10⁸ m^?1 sec^?1 and 1.0 × 10⁸ m^?1 sec^?1, respectively. And the rate constants for the chemical reaction between each tocopherol and ¹O₂ were 6.6 × 10⁶ m^?1 sec^?1, 2.6 × 10⁶ m^?1 sec^?1 and 0.7 × 10⁶ m^?1 sec^?1 for α-, γ- and δ-tocopherols, respectively. The results show that α-tocopherol is the most effective compound toward ¹O₂ among the three tocopherols. The photooxidation of each tocopherol produced two peroxides which, after chemical reduction, were identified to be tocopherol hydroquinone by gas chromatography-mass spectrometry analysis. The photooxidation mechanism of these tocopherols was assumed to be different from that of autoxidation.