Light dependence of quantum yields of Photosystem II and CO2 fixation in C3 and C4 plants

The light dependence of quantum yields of Photosystem II (_II) and of CO₂ fixation were determined in C₃ and C₄ plants under atmospheric conditions where photorespiration was minimal. Calculations were made of the apparent quantum yield for CO₂ fixation by dividing the measured rate of photosynthesis by the absorbed light [A/I=_CO2 and of the true quantum yield by dividing the estimated true rate of photosynthesis by absorbed light [(A+R_l)/I_a=_CO2·], where R_L is the rate of respiration in the light. The dependence of the _II/_CO2 and _II/_CO2 ^* ratios on light intensity was then evaluated. In both C₃ and C₄ plants there was little change in the ratio of _II/_CO2 at light intensities equivalent to 10–100% of full sunlight, whereas there was a dramatic increase in the ratio at lower light intensities. Changes in the ratio of _II/_CO2 can occur because respiratory losses are not accounted for, due to changes in the partitioning of energy between photosystems or changes in the relationship between PS II activity and CO₂ fixation. The apparent decrease in efficiency of utilization of energy derived from PS II for CO₂ fixation under low light intensity may be due to respiratory loss of CO₂. Using dark respiration as an estimate of R_L, the calculated _II/_CO2 ^* ratio was nearly constant from full sunlight down to approx 5% of full sunlight, which suggests a strong linkage between the true rate of CO₂ fixation and PS II activity under varying light intensity. Measurements of photosynthesis rates and _II were made by illuminating upper versus lower leaf surfaces of representative C₃ and C₄ monocots and dicots. With the monocots, the rate of photosynthesis and the ratio of _II/_CO2 exhibited a very similar patterns with leaves illuminated from the adaxial versus the abaxial surface, which may be due to uniformity in anatomy and lack of differences in light acclimation between the two surfaces. With dicots, the abaxial surface had both lower rates of photosynthesis and lower _II values than the adaxial surface which may be due to differences in anatomy (spongy versus palisade mesophyll cells) and/or light acclimation between the two surfaces. However, in each species the response of _II/_CO2 to varying light intensity was similar between the two surfaces, indicating a comparable linkage between PS II activity and CO₂ fixation.Abbreviations A measured rate of CO₂ assimilation - A+R_L true rate of CO₂ assimilation; e - CO₂ estimate of electrons transported through PSII per CO₂ fixed by RuBP carboxylase - f fraction of light absorbed by Photosystem II - F'_m yield of PSII chlorophyll fluorescence due to a saturating flash of white light under steady-state photosynthesis - F_s variable yield of fluorescence under steady-state photosynthesis; PPFD-photosynthetic photon flux density - I_a absorbed PPFD - PS II Photosystem II - R_d rate of respiration in the dark - R_I rate of respiration in the light estimated from measurement of R_d or from analysis of quantum yields - apparent quantum yield of CO₂ assimilation under a given condition (A/absorbed PPFD) - true quantum yield of CO₂ assimilation under a given condition [(A+R_L)/(absorbed PPFD)] - quantum yield for photosynthetic O₂ evolution - electrons transported via PS II per quantum absorbed by PS II Supported by USDA Competitive Grant 90-37280-5706.     

Cleavage of nucleotides by Ce4+ and the lanthanide metal complexes

The cleavage of adenosine-5-monophosphate (5-AMP) and guanosine-5-monophosphate (5-GMP) by Ce⁴⁺ and lanthanide complex of 2-carboxyethylgermanium sesquioxide (Ge-132) in acidic and near neutral conditions was investigated by NMR , HPLC and measuring the liberated inorganic phosphate at 37°C and 50°C. The results showed that 5-GMP and 5-AMP was converted to guanine (G), 5-monophosphate (depurination of 5-GMP), ribose (depurination and dephosphorylation of 5-GMP), phosphate and adenine (A), 5-monophosphate (depurination of 5-AMP), ribose (depurination and dephosphorylation of 5-AMP), phosphate respectively by Ce⁴⁺. In presence of lanthanide complexes, 5-GMP and 5-AMP were converted to guanosine (Guo) and phosphate and adenosine (Ado) and phosphate respectively. The mechanism of cleaving 5-GMP and 5-AMP is hydrolytic scission     

Mechanisms for controlling balance between light input and utilisation in the salt tolerant alga Dunaliella C9AA

The yield of photosynthetic O₂ evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters _{PS I} and _{PS II} . _{PS I} is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. _{PS II} is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O₂ evolution decreased more than predicted by either _{PS I} or _{PS II}. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.Abbreviations F₀ dark level of chlorophyll fluorescence yield (PS II centres open) - F_m maximum level of chlorophyll fluorescence yield (PS II centres closed) - F_v variable fluorescence (F_m-F₀) - PS I Photosystem I - PS II Photosystem II - P₇₀₀ reaction centre chlorophyll(s) of PS I - q_N coefficient of non-photochemical quenching of chlorophyll fluorescence - q_P coefficient of photochemical quenching of fluorescence yield - q_E high-energy-state quenching coefficient - _{PS I} yield of PS I - _{PS II} yield of PS II - _S yield of photosynthetic O₂ evolution - _P intrinsic yield of open PS II centres     

Response of excised embryos of rice (Oryza sativa L.) to X-rays

Summary The response of rice (Oryza sativa L.) embryos to X-rays (M₁ to M₃) was studied. By means of irradiating excised embryos, both chlorophyll and macromutation were successfully induced in three genotypes of rice. However, differential responses in terms of mutation frequency, mutation spectrum and optimal levels of X-rays required for induction of mutation (chlorophyll as well as morphological) were found to exist between cultivars. In Satika and Ashkhata, LD₅₀ values and maximum induced seed sterility are concomitant to optimum level of radiation required for triggering chlorophyll mutation. However, optimum dose for induction of macromutation in Satika and Kerangserang is independent of either LD₅₀ and/or induced seed sterility.Chances of obtaining both dominant and locus specific recessive mutations in the immediate X-ray treated generation (M₁) are large. This indicates the very high degree of effectiveness of the excised embryo irradiation technique with rice.     

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (_p), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (q_I).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (_exe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q _N and the efficiency of excitation capture (_exe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.Abbreviations F₀ basic fluorescence - F₀ basic fluorescence, thylakoid in energized state - F_m maximal fluorescence - F_m maximum fluorescence in energized state - F_s steady state fluorescence - F_v maximal variable fluorescence - PFD photon flux density - PS II_rc Photosystem II reaction center - q_F0 quenching of basic fluorescence - q_E energy related quenching - q_N non-photochemical quenching:-q_f-total quenching - q_I inhibition-related quenching - q_p photochemical quenching - q_r quenching due to state transition - R_d dark respiration - _p PS II efficiency of excitation capture of open PS II_rc - _pe extrapolated minimal value of _p - _p0 extrapolated maximal value of _p - _si quantum efficiency of linear electron transport, calculated from gas exchange measurements based on incident light - _sf quantum efficiency of linear electron transport, calculated from fluorescence measurements, based on incident measuring light     

Analogues of NADP+ as inhibitors and coenzymes for NADP+ malic enzyme from maize leaves

Structural analogues of the NADP⁺ were studied as potential coenzymes and inhibitors for NADP⁺ dependent malic enzyme from Zea mays L. leaves. Results showed that 1, N⁶-etheno-nicotinamide adenine dinucleotide phosphate ( NADP⁺), 3-acetylpyridine-adenine dinucleotide phosphate (APADP⁺), nicotinamide-hypoxanthine dinucleotide phosphate (NHDP⁺) and -nicotinamide adenine dinucleotide 2: 3-cyclic monophosphate (23NADPc⁺) act as alternate coenzymes for the enzyme and that there is little variation in the values of the Michaelis constants and only a threefold variation in V_max for the five nucleotides. On the other hand, thionicotinamide-adenine dinucleotide phosphate (SNADP⁺), 3-aminopyridine-adenine dinucleotide phosphate (AADP⁺), adenosine 2-monophosphate (2AMP) and adenosine 2: 3-cyclic monophosphate (23AMPc) were competitive inhibitors with respect to NADP⁺, while -nicotinamide adenine dinucleotide 3-phosphate (3NADP⁺), NAD⁺, adenosine 3-monophosphate (3AMP), adenosine 2: 5-cyclic monophosphate (25AMPc), 5AMP, 5ADP, 5ATP and adenosine act as non-competitive inhibitors. These results, together with results of semiempirical self-consistent field-molecular orbitals calculations, suggest that the 2-phosphate group is crucial for the nucleotide binding to the enzyme, whereas the charge density on the C₄ atom of the pyridine ring is the major factor that governs the coenzyme activity.Abbreviations NADP⁺ 1, N⁶-etheno-nicotinamide adenine dinucleotide phosphate - NHDP⁺ nicotinamide-hypoxanthine dinucleotide phosphate - APADP⁺ 3-acetylpyridine-adenine dinucleotide phosphate - SNADP⁺ thionicotinamide-adenine dinucleotide phosphate - AADP⁺ 3-aminopyridine-adenine dinucleotide phosphate - 23NADPc⁺ -nicotinamide adenine dinucleotide 2: 3-cyclic monophosphate - 3NADP⁺ -nicotinamide adenine dinucleotide 3-phosphate - 2AMP adenosine 2-monophosphate - 3AMP adenosine 3-monophosphate - 23AMPc adenosine 2: 3 monophosphate cyclic - A adenosine - RuBP ribulose 1,5-bisphosphate - SCF-MO Self-Consistent Field-Molecular Orbitals (method)     

Differential effects of foods traditionally regarded as ‘heating’ and ‘cooling’ on prostaglandin E2 production by a macrophage cell line

Some components of natural foods may enhance or inhibit prostaglandin formation and potentially affect the inflammation condition. A macrophage cell line, RAW264.7, was employed to examine the effects of foods traditionally regarded as heating or cooling on the production of PGE₂, a well-known proinflammatory mediator. Foods traditionally regarded as heating (litchi, longan, and dried longan) or cooling (chrysanthemum flower, bitter gourd, and lotus seed plumule) were extracted sequentially with water and ethyl acetate. The water extracts (WE) and ethyl acetate extracts (EAE) were applied to RAW264.7 macrophages in the presence or absence of LPS (lipopolysaccharide). In the absence of LPS, the WEs from the heating foods, litchi, longan, or dried longan had a dose-dependent enhancing effect on PGE₂ production, with respective EC₅₀s of 8.4, 16, and 11 mg/ml. This effect was accompanied by significant induction of COX-2 protein expression, as shown by Western blot analysis. In contrast, LPS-induced PGE₂ production was inhibited in a dose-dependent manner by the WEs of the cooling foods, chrysanthemum flower, bitter gourd, and lotus seed plumule, with respective IC₅₀s of 0.6, 0.13, and 0.08 mg/ml. At the concentrations tested, none of the EAEs had any effect on basal PGE₂ production, while LPS-induced PGE₂ production was inhibited or increased by the EAE from bitter gourd and longan, respectively. Water-soluble extracts of foods traditionally regarded as heating enhanced basal PGE₂ production, while those from cooling foods significantly inhibited LPS-induced PGE₂ production by the macrophage cell line. This subject merits further study to determine whether appropriate food selection may help patients suffering from chronic inflammatory conditions.     

Heterogeneous photosystem 2 activity in isolated spinach chloroplasts

A study was made of the fluorescence induction curves from gently-broken spinach chloroplasts inhibited with DCMU. It was found that there were four kinetically different phases associated with such curves of which only the fastest did not appear to follow exponential kinetics. A comparison of the effects of various concentrations of DCMU on the rate of oxygen evolution and on the fluorescence induction curve did not support the hypothesis that any of the kinetic phases was simply an artefact caused by incomplete inhibition of electron transport. It was also found that 5 min of dark incubation did not maximally oxidize the electron acceptors to photosystem 2 since some acceptors were only oxidized following far-red illumination, suggesting a heterogeneity among these acceptors with respect to their re-oxidation properties. Investigation of the effect of the Q₄₀₀ oxidation state on the fluorescence induction curve revealed that it only influenced the slowest kinetic phase and that Q₄₀₀ did not seem to be associated with the other phases.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1 - 1 dimethylurea - PS 1 photosystem 1 - PS2 photosystem 2 - HEPES N-2-Hydroxyethylpiperazine-N-2-ethanesulfonic acid - EDTA ethylene-diaminetetraacetic acid - F_max maximum yield of fluorescence emission - F₀ initial yield of fluorescence emission - F_v variable yield of fluorescence emission - N.E. non-exponential kinetics     

The relationship between Photosystem II intrinsic quantum yield and millisecond luminescence in thylakoids

The relationship between charge recombination at Photosystem II (PS II), as indicated by millisecond luminescence, and PS II quantum yield was studied in spinach thylakoids during electron flow to methylviologen. Under the low magnesium conditions used, a decrease in quantum yield was observed in the absence of non-photochemical excitation quenching, and therefore cannot be due to a restriction in excitation delivery to the reaction centre. It was found that the decrease of the parameter _p, which is a measure of the intrinsic quantum yield of open PS II centers, correlates with an increase in luminescence per open center. The relationship between these two parameters was the same whether _p was manipulated by dissipation of the transthylakoid pH gradient or of the electrical potential. This indicates that the mechanism by which _p decreases depends in the same way on the two components of the protonmotive force as does the charge recombination at PS II. Calculation of the yield of luminescence with respect to the back reaction will be necessary to determine whether the charge recombination occurs at a sufficiently high rate to be directly responsible for the _p decrease.     

Deconvolution in 3-D optical microscopy

Summary Fluorescent probes are becoming ever more widely used in the study of subcellular structure, and determination of their three-dimensional distributions has become very important. Confocal microscopy is now a common technique for overcoming the problem of out-of-focus flare in fluorescence imaging, but an alternative method uses digital image processing of conventional fluorescence images — a technique often termed deconvolution or restoration. This review attempts to explain image deconvolution in a non-technical manner. It is also applicable to 3-D confocal images, and can provide a further significant improvement in clarity and interpretability of such images. Some examples of the application of image deconvolution to both conventional and confocal fluorescence images are shown.     

Immunofluorescence localization of the epidermolytic toxin target in mouse epidermal cells and tissue

Summary An epidermolytic toxin target was observed in keratohyalin granules of sectioned epidermis by a direct fluorescence procedure using FTC-toxin, but not by an indirect procedure using sequential reaction with toxin, anti-toxin and FTC-secondary antibody. The investigation of the two procedures was extended to keratinocytes. A dispase digestion procedure yielded three fractions which corresponded to basal, spinous and granular cells according to biochemical and morphological criteria. It was shown that the direct and indirect procedures both detected the toxin target in the keratohyalin granules of granular cells, but that the indirect procedure was very insensitive. In control experiments, the profilaggrin of keratohyalin granules was detected readily in cells by a direct procedure using FTC-antiprofilaggrin but only weakly by an indirect double antibody procedure. Insensitivity to indirect procedures thus appears to be a particular property of the keratohyalin granule site. It was shown that the toxin target was readily accessible in permeable (trypsin-isolated) granular cells but inaccessible in impermeable (dispase-isolated) cells.     

Photoperiodism and thermoperiodism in the predatory mite Amblyseius potentillae are probably based on the same mechanism

Summary A comparative study was made of the photoperiodic and thermoperiodic induction of diapause in the phytoseiid mite Amblyseius potentillae. Sensitivity to thermoperiod was found to be highest during the protonymphal and deutonymphal stages, with some sensitivity still being present in the young adult. Summation of both photoperiodic and thermoperiodic cycles was shown to take place, which demonstrated the presence of a photoperiodic counter as well as a thermoperiodic counter in these mites. Vitamin A appeared to be necessary for some early step in the physiological mechanism of diapause induction and not just for the expression of the diapause response. The light sensitivity threshold for photoperiodic induction of diapause was found to be extremely low, viz. less than 0.02 W/cm². Moreover, the light sensitivity threshold appeared to be strongly temperature dependent in A. potentillae. Experiments in which the mites experienced various sequences of short-day photoperiods and short-day thermoperiods, applied either concurrently or in succession, showed that the information collected by the photoperiodic counter and the thermoperiodic counter is integrated into one induction sum. These results strongly suggest that photoperiodic and thermoperiodic induction of diapause in these mites is based on the same physiological mechanism.Abbreviations DD continuous darkness - LL continuous light - LD light-dark cycle (e.g. LD 16:8 is a cycle of 16 h of light and 8 h of darkness) - TC thermoperiodic cycle (e.g. TC 16:8 (27°: 15°) is a thermoperiod with a 16 h thermophase of 27 °C and an 18 h cryophase of 15°C)     

The effect of ionic stress on photosynthesis in Dunaliella tertiolecta

A comparison of the effects of ionic stress and an uncoupler on long-term fluorescence transients (the Kautsky effect) in the green alga Dunaliella tertiolecta indicated that the large quenching induced by ionic stress was caused by a pH gradient across the thylakoid membrane. This possiblity was given support by the increase in the slow phase of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in algae subjected to ionic stress. Low-temperature fluorescence emission spectra indicated that salt stress enhanced photosystem-I emission in the dark, and a comparison of simultaneous emissions at 695 and 720 nm at room temperature indicated a further increase in photosystem-I emission during the fluorescence transients. Taken together with the decrease in the fast phase of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced fluorescence relaxation in stressed algae, our results indicate that ionic stress stimulates cyclic electron flow, and that non-cyclic flow is inhibited. The effect of sucrose-induced osmotic stress was similar to, but less marked than, the effects of NaCl and KCl; the effect of decreasing the external salinity was small.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP carbonylcyanide p-trifluoromethoxyphenylhydrazone - PSI, II photosystem I, II     

Acclimation of barley to changes in light intensity: photosynthetic electron transport activity and components

Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 mole m^-2 s^-1) or low (L) intensity (55 mole m^-2 s^-1) white light. Plants were transferred from high to low (H L) and low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (Q_B), cytochrome f(Cytf) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. Q_B and Cytf were elevated in H plants compared with L plants. The acclimation of H L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H L plants was associated with a decrease in both Q_B and Cytf. In L H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L H electron transport was associated with increased levels of Q_B and Cytf. In contrast to the light intensity effects on Q_B levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment.Abbreviations Asc ascorbate - BQ 2,5-dimethyl-p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L low control plants grown under low light intensity - L H plants transferred from low to high light intensity - MV methyl viologen - P-700 photoreaction center of Photosystem I - Q_B atrazine binding site - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.     

Ganglioside-cholera toxin interactions: a binding and lipid monolayer study

Summary On t.l.c. plates ¹²⁵I-cholera toxin binds to a disialoganglioside tentatively identified as GD_lb with about 10 times less capacity than to ganglioside GM₁. Binding of labeled toxin to both gangliosides was abolished in presence of excess amounts of unlabeled B subunit. Ganglioside extracts from human or pig intestinal mucosa showed toxin binding to gangliosides GM₁ and GD_1b. In ganglioside-containing lipid monolayers the penetration of the toxin was independent of the ganglioside binding capacity.Abbreviations GM₂ Gal-NAc14Gal(3-2NeuAc)14G1c1Cer - GM₁ Gal3Ga1-NAc14Gal(32NeuAc)14G1c11Cer - GD_1a NeuAc23Ga113Gal-NAc14Gal(32NeuAc)14G1c11Cer - GD1b Gall3Gal-NAcl4Gal(32NeuAc82NeuAc)14Glc11Cer - GT_1b NeuAc23Ga113Ga1-NAcal4Gal(3-2NeuAc82NeuAc)14G1c11Cer - dpPC 1,2-hexadecanoyl-sn-glycero-3-phosphocholine - dpPE 1,2-hexadecanoyl-sn-glycero-3-phosphoethanolamine     

An elongated segment of DNA observed between two human α globin genes

Summary Detailed restriction enzyme analysis of the DNA from a Chinese female showed that one of her chromosomes had a >17.5 kb deletion of DNA, including the , 2, and 1 globin genes, which is present in many Southeast Asians with an -thalassemia-1 chromosome. Her normal chromosome had the expected cluster of -like globin genes (5----2-1-3), but the segment of DNA between the two globin genes was elongated by some 0.5–0.7 kb. Analyses of various restriction sites suggested that this normal variant of the human globin gene complex is due to a crossover between a normal chromosome with () and a chromosome with an -thalassemia-2 (–^3.7) and an -21-hybrid gene.     

Different Relationship Between Electron Transport and CO2 Assimilation in two Zea mays Cultivars as Influenced by Increasing Irradiance

Gas exchange and fluorescence parameters were measured simultaneously in two Zea mays L. cultivars (Liri and 121C D8) to assess the relationship between the quantum yield of electron transport (_PS2) and the quantum yield of CO₂ assimilation (_CO2) in response to photosynthetic photon flux density (PPFD). The cv. Liri was grown under controlled environmental conditions in a climate chamber (CC) while cv. 121C D8 was grown in CC as well as outdoors (OT). By exposing the two maize cultivars grown in CC to an increasing PPFD, higher photosynthetic and photochemical rates were evidenced in cv. Liri than in cv. 121C D8. In Liri plants the _PS2/_CO2 ratio increased progressively up to 27 with increasing PPFD. This suggests that the reductive power was more utilised in non-assimilatory processes than in CO₂ assimilation at high PPFD. On the contrary, by exposing 121C D8 plants to increasing PPFD, _PS2/_CO2 was fairly constant (around 11–13), indicating that the electron transport rate was tightly down regulated by CO₂ assimilation. Although no significant differences were found between _PS2/_CO2 of the 121C D8 maize grown under CC and OT by exposing them to high PPFD, the photosynthetic rate and photochemical rates were higher in OT maize plants.     

Measurement of frequency-dependent sexual activity in Drosophila Melanogaster

In experiments on sexual competition in Drosophila melanogaster, the course of mating succes with time is represented by a sigmoid curve. By logarithmic transformation such curves are changed into straight lines that can be compared by covariance analysis. This method allows discrimination of the behaviour of the two types in competition, and allows us to follow it in the course of time. From a sexual competition experiment between the wild type Canton S and the mutant white-ebony we conclude that sexual activity of males and females of both types is generally frequency dependent, with evidence of rare-female advantage as well as rare-male advantage.     

Fluorometric evidence for control of the activity of F1-adenosinetriphosphatase by ligand-induced conformation change

The effect of ATP on the fluorescence intensity of bovine heart F₁-adenosinetriphosphatase labeled at its essential Lys with 7-chloro-4-nitro-2,1,3-benzoxadiazole (N-NBD-F₁) has been examined in solutions containing different concentrations of ADP. The fluorescence of N-NBD-F₁ is unaffected by ATP in the absence of ADP. But when increasing amounts of ATP are added to a solution of N-NBD-F₁ containing 0.37 or 1.0 mM ADP, the fluorescence of N-NBD-F₁ first decreases and then increases continually as the concentration of ATP is further raised. Parallel measurements of the suppression of the fluorescence of N-NBD-F₁ and the inhibition of the ATPase activity of the unlabeled enzyme by ADP in the presence of ATP show a quantitative correlation between the changes in fluorescence and in ATPase activity. The data are consistent with the model for F₁-ATPase with one principal catalytic subunit for ATP hydrolysis and synthesis, and two auxiliary subunits which control the conformation and hence the catalytic activity of through interaction between all the subunits.     

Minimization of the Photon Energy Absorbed by ‘Closed’ Reaction Centers of Photosystem 2 as a Photoprotective Strategy in Higher Plants

Photoinactivation of photosystem 2 (PS2) results from absorption of so-called excessive photon energy. Chlorophyll a fluorescence can be applied to quantitatively estimate the portion of excessive photons by means of the parameter E = (F – F₀)/F_m, which reflects the share of the absorbed photon energy that reaches the reaction centers (RCs) of PS2 complexes with Q_A in the reduced state (closed RCs). Data obtained for cotton (Gossypium hirsutum), bean (Phaseolus vulgaris), and arabidopsis (Arabidopsis thaliana) suggest a linear relationship between the total amount of the photon energy absorbed in excess (excessive irradiation) and the decline in PS2 activity, though the slope may differ depending on the species. This relationship was sensitive not only to the leaf temperature but also to treatment with methyl viologen. Such observations imply that the intensity of the oxidative stress as well as the plant's ability to detoxify active oxygen species may interact to determine the damaging potential of the excessive photons absorbed by PS2 antennae. Energy partitioning in PS2 complexes was adjusted during adaptation to irradiation and in response to a decrease in leaf temperature to minimize the excitation energy that is trapped by closed PS2 RCs. The same amount of the excessive photons absorbed by PS2 antennae led to a greater decrease in PS2 activity at warmer temperatures, however, the delay in the development of non-photochemical and photochemical energy quenching under lower temperature resulted in faster accumulation of excessive photons during induction. Irradiance response curves of EF suggest that, at high irradiance (above 700 mol m^–2 s^–1), steady-state levels of this parameter tend to be similar regardless of the leaf temperature.