Larkum, AWD 1972, 'Frond structure and growth in laminaria hyperborea', Journal of the Marine Biological Association of the United Kingdom, vol. 52, no. 2, pp. 405-418.
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Age, frond weight, frond area, stipe length and other data have been recorded for plants of Laminaria hyperborea growing off the coast of south-west England and south-west Ireland. The ratio of total frond weight to total frond area (unit frond weight) has been found to be related to age, depth, type of community, and water turbulence. Decrease in unit frond weight appears to result from a decrease in the number of cell layers of the frond, which is most marked at the frond tip. At the frond tip differences between the thickest and thinnest fronds may be fivefold or more. It is concluded that water turbulence, particularly wave-action, is an important factor influencing the changes in frond structure. The importance of submarine irradiance as a factor is not clear from the present evidence. An intrinsic factor affecting frond structure may be the age of plants. © 1972, Marine Biological Association of the United Kingdom. All rights reserved.
Larkum, AWD & Bonner, WD 1972, 'Light-induced absorbance changes of cytochromes and other pigments in pea chloroplasts and chloroplast fragments. I. The effect of red and near far-red actinic light and of ruby laser flashes', Archives of Biochemistry and Biophysics, vol. 153, no. 1, pp. 241-248.
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Light-induced spectral changes in the region 540 to 580 nm were studied by means of dual-wavelength differential spectrophotometry and by a Q-switched ruby laser flash technique in intact chloroplasts, swollen chloroplasts, and chloroplast fragments. In intact chloroplasts the major spectral components are attributed to cytochrome f (photo-oxidized), an unknown pigment with a broad band (decrease in absorbance) centered around 572 nm, and possibly a b-type cytochrome (photo-oxidized). In swollen chloroplasts and chloroplast fragments the photo-oxidation of cytochrome f is modified, cytochrome b6 is photoreduced, and the P-572 response is diminished. The changeover from the intact to the swollen response is brought about by the addition to the suspension medium of certain cations of which sodium, with an effect beginning at about 10 mm, is the most potent. Laser flashes onto dense suspensions of high-salt chloroplasts produced fast (t 1 2, 300 μsec) oxidation of cytochrome f, even in the absence of ascorbate and slower (t 1 2, 5 msec) reduction of a b-type cytochrome. However, it is suspected that cations leaking from the chloroplasts in these dense suspensions modified the cytochrome responses. © 1972.
Larkum, AWD & Bonner, WD 1972, 'Light-induced absorbance changes of cytochromes and other pigments in pea chloroplasts and chloroplast fragments. II. The effect of inhibitors, uncouplers and ionophores', Archives of Biochemistry and Biophysics, vol. 153, no. 1, pp. 249-257.
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In high-salt intact chloroplasts the reversible cytochrome f photo-oxidation is relatively insensitive to 3,4(dichlorophenyl) 1,1′-dimethylurea (DCMU) and is accompanied by changes in the region 560 to 572 nm which are inhibited by n-heptyl-4-hydroxyquinoline-N-oxide and antimycin A. These results indicate a cyclic electron-transport system around photosystem I which may involve chtochrome b6. In intact chloroplasts (low-salt) treated with medium concentrations (1 μm) of carbonylcyanide p-trifluoromethoxy phenylhydrazone (FCCP), cytochrome b-559 is photo-oxidized in far-red light. This also occurs to a small extent with high concentrations of antimycin A and in aged chloroplast fragments. It is not found in chloroplasts uncoupled by valinomycin plus nigericin (or plus NH4Cl). These results are taken to mean that FCCP induces a secondary effect as well as its primary uncoupling action, causing structural changes which result in an unnatural connection of cytochrome b-559 to photosystem I. Other effects of FCCP are also discussed. Valinomycin alone, caused marked changes in the light-induced responses of cytochrome f and in the region of the b cytochromes but these were substantially different from the effect in combination with nigericin (or NH4Cl). The results are discussed in terms of a possible effect of an electrical potential difference across the functional photosynthetic membrane. © 1972.
Larkum, AWD & Bonner, WD 1972, 'Light-induced absorbance changes of P518 in intact chloroplasts', BBA - Bioenergetics, vol. 256, no. 2, pp. 396-408.
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The light-induced absorbance change centred at 518 nm (P518 response) is 20-30-fold greater in intact chloroplasts than in swollen chloroplasts. The various characteristics of this large P518 response, including induction effects and chromatic transients, were studied. 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) (2 μM) strongly inhibited the response and the uncoupler, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone, abolished it. Other uncouplers such as NH4Cl, methylamine, atebrin and nigericin were without effect. Valinomycin inhibited the response and valinomycin in combination with NH4Cl or nigericin abolished it. In the presence of DCMU the P518 response could be restored with 2,6-dichlorophenolindophenol (+ ascorbate) but not with N,N,N′,N′-tetramethyl-p-phenylenediamine (+ ascorbate). The results support a correlation between the P518 response and a membrane potential across the functional photosynthetic membrane, which may be produced by two mechanisms, one fast and one slow. It is suggested that an oxidation-reduction loop between the two photosystems, forms a significant component of the slower mechanism. © 1972.
Larkum, AWD & Bonner, WD 1972, 'Light-induced oxidation of cytochrome f in isolated chloroplasts of Pisum sativum', BBA - Bioenergetics, vol. 256, no. 2, pp. 385-395.
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Photooxidation of cytochrome f has been studied in a number of types of chloroplast preparations from pea leaves. On the basis of the dark reversibility after far-red illumination, the effect of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea and other criteria, it is possible to distinguish three basic types of response according to the type of preparation: (a) high-salt chloroplasts; (b) intact chloroplasts and unswollen chloroplast fragments, and (c) swollen chloroplasts and swollen chloroplast fragments. Types a and b were further characterised by relatively high oxidation of cytochrome f in red light. It is suggested that in type b preparations photooxidation of cytochrome f can be explained by (i) a large component of electron flow through an intermediate pathway between the two photosystems in which a large redox pool is linked through a coupling site to cytochrome f, and (ii) a small component of endogenous cyclic electron flow involving cytochrome f. In high-salt chloroplasts there may be in addition a pool of low molecular weight substance capable of reducing cytochrome f in the dark. © 1972.
Larkum, AWD & Bonner, WD 1972, 'The effect of artificial electron donor and acceptor systems on light-induced absorbance responses of cytochromes f and other pigments in intact chloroplasts', BBA - Bioenergetics, vol. 267, no. 1, pp. 149-159.
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An investigation has been made into the effect of low concentrations of 2,6-dichlorophenolindophenol (DCIP) and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), in the presence of ascorbate, on light-induced absorbance changes in the α-band region of cytochrome f and b-type cytochromes. Strong photo-oxidation of cytochrome f occurred with DCIP, in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (2 μM), but not with TMPD. However, with TMPD a large light-induced absorbance increase occurred due to a broad band centred at 566 nm. With DCIP, the uncoupler, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP), in conjunction with the electron acceptor 1,1′-ethylene-2,2′-bipyridylium dibromide (diquat), inhibited the cytochrome f response and an absorbance increase in the 560-575 nm region occurred similar to that with TMPD. Neither FCCP nor diquat had any great effect on the TMPD system. The results support a pathway of electron transport between the two photosystems in which (a) DCIP-ascorbate interacts with an intermediate on the Photosystem II side of a coupling site; (b) TMPD-ascorbate interacts after this site, and (c) cytochrome f is located on the Photosystem I side of the site. © 1972.
RAVEN, JA 1972, 'ENDOGENOUS INORGANIC CARBON SOURCES IN PLANT HOTOSYNTHESIS', New Phytologist, vol. 71, no. 6, pp. 995-1014.
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The quantity of CO2 arising from photorespiration in angiosperms and in algae has been estimated. Much of such CO2 arises from the metabolism of glycolate so that estimates are based on the rate of formation of glycolate and the rate at which it is metabolized (Table 1). The total CO2 production in leaves or algae in the light can then be taken as equal to the photorespiratory CO2 plus the CO2 produced by‘dark’ respiratory pathways. Methods of estimating CO2 release from illuminated green tissues are considered and the rate of decarboxylation deduced from biochemical evidence of substrate turnover is compared with the observed rate of CO2 release; the difference between these values is reassimilation (Table 1). In angiosperms with the PGA pathway, in air, reassimilation is never complete and ranges from o to 0.6 of the endogenous CO2 production. This implies that the resistance to refixation is as great or greater than the resistance to loss of endogenous CO2 to the medium and is in accord with the relative values of these resistances based on other evidence. In law oxygen environment less glycolate is produced and hence less photorespiratory CO2; the resistance to refixation is smaller and so fractional reassimilation in increased. Unicellular algae have the PGA pathway and some appear to have more efficient reassimilation than the corresponding angiosperms. The special problems of metabolism in four carbon acid plants are discussed. Such plants show essentially complete reassimilation since they do not lose CO2 when illuminated. In their leaves the resistance to escape of endogenously produced CO2 must be very much greater than the resistance to refixation. The low resistance to refixation is probably related to high levels of CO2 in chloroplasts which are generated by the‘CO2‐pump’ rather than to any peculiarities of carboxydismutase. The importance of reassimilation is discussed in relation to the influence of CO2 on various aspects of metabolism, to isotopic...