Influence of thiobencarb on nitrate reductase, nitrite reductase and DCPIP photoreduction in rice and Echinochloa crus-galli (L.) (barnyardgrass)

Summary. The influence of thiobencarb at 1500 ppm on levels of nitrate reductase, nitrite reductase and on 2,6-dichlorophenolindophenol (DCPIP) photoreduction was studied in rice ( Oryza sativa L.) and Echinochloa crus-galli (L.) (barnyardgrass) to understand the different sensitivities of these two plants to the herbicide. The herbicide treatment did not cause appreciable inhibition of nitrate reductase and nitrite reductase in rice leaf sections. In barnyard grass, the treatment strongly lowered nitrite reductase activity. DCPIP photoreduction by isolated chloroplasts of the treated rice leaves was not greatly affected. On the other hand DCPIP photoreduction by both mesophyll and bundle sheath chloroplasts from treated barnyardgrass was inhibited by 70%. Herbicide caused accumulation of nitrite in treated barnyard grass leaves when compared to rice. Thiobencarb phytotoxicity to barnyard grass may be due to impairment of the Hill reaction that in turn caused accumulation of nitrite in the leaf tissue.     

Effect of some substituted benzonitriles on photosynthetic activity of spinach and wheat in vivo and in vitro

The inhibitory effects of nine nitro and/or bromo-substituted benzonitrile compounds on the photosynthetic electron flow in isolated chloroplasts and on the in vivo CO₂ fixation of spinach (Spinacia oteracea L.) and wheat (Triticum aesticum L. cv. Bezoslaya) were investigated. Bromoxynil and 3-nitro-5-hromo-4-hydroxy-bcnzonitrile were the strongest and equally effective inhibitors of Ihe in vivo CO₂ fixation of spinach, hut in wheat the nitro-bromo-compound is ineffective and 3-nitro-benzonitrile is even more inhibitory than bromoxynil. None of the substances affected DCPIPH → methylviologen reduction. In the inhibition of the DCPIP reduction only the 3,5-disuhstituted 4-hydroxy-derivatives were effective. The fact that these compounds affect only the PS II reaction with both H₂O and DPC as electron donors suggests a site of inhibition on the reducing side of PS II, between Ihe PS II reaction centre and ihe DCPIP Site. It is suggested that in the inhibition of the DCPIP reduction only steric factors are important and the different electron configuration of the sterieally similar molecules may be involved only in the absorption and translocation processes of the compounds.     

Reversal of the inhibition of photosynthesis by herbicides affecting hydroxyphenylpyruvate dioxygenase by plastoquinone and tocopheryl derivatives in Chlamydomonas reinhardtii

Isoxaflutole or pyrazolate inhibition of tocopherol and plastoquinone biosynthesis in the green alga Chlamydomonas reinhardtii Dang leads to the inactivation of photosystem II and the degradation of its reaction centre D1 protein when exposed to strong light. Cell-permeable short-chain derivatives of plastoquinone and tocopherol were tested in the reversal. Addition of decyl-plastoquinone reverses herbicide-induced inhibition of photosynthesis and inactivation of photosystem II in short-time (1 h) exposure of the algae to high light. In high light longer than 1 h, decyl-plastoquinone alone loses effectiveness, but a synthetic permeable tocopheryl derivative retards the inhibitory effects on photosystem II and on the degradation of the D1 protein. This indicates that tocopherol deficiency induced by the herbicides makes a major contribution to their secondary mode of action in high light stress.     

Inhibition of photosystem II in spinach chloroplasts by trachyloban-19-oic acid

The effect of trachyloban-19-oic acid isolated from Iostephane heterophylla (Asteraceae), was investigated on several photosynthetic activities in spinach thylakoids. The results indicated that this compound inhibited ATP synthesis and uncoupled electron transport, as well as basal and phosphorylating electron flow. Therefore, trachyloban-19-oic acid acts as Hill reaction inhibitor. This compound did not affect photosystem I activity but inhibited uncoupled photosystem II electron flow from H₂O to DCPIP, and has not effect on electron flow from H₂O to SiMo, indicating that the site of inhibition of this compound is at the level of Q_A-Q_B. Chlorophyll a fluorescence measurements confirm the behavior of this diterpene as Q_A-Q_B inhibitor, and in the other hand, this results indicate that a perturbation in the thylakoid membranes at the level of LHC II occurs.     

Changes in carbohydrate metabolism in coconut palms infected with the lethal yellowing phytoplasma

ABSTRACT Lethal yellowing (LY), a disease caused by a phytoplasma, is the most devastating disease affecting coconut (Cocos nucifera) in Mexico. Thousands of coconut palm trees have died on the Yucatan peninsula while plantations in Central America and on the Pacific coast of Mexico are severely threatened. Polymerase chain reaction assays enable identification of incubating palm trees (stage 0+, phytoplasma detected but palm asymptomatic). With the development of LY, palm trees exhibit various visual symptoms such as premature nut fall (stage 1), inflorescence necrosis (stages 2 to 3), leaf chlorosis and senescence (stages 4 to 6), and finally palm death. However, physiological changes occur in the leaves and roots prior to onset of visual symptoms. Stomatal conductance, photosynthesis, and root respiration decreased in stages 0+ to 6. The number of active photosystem II (PSII) reaction centers decreased during stage 2, but maximum quantum use efficiency of PSII remained similar until stage 3 before declining. Sugar and starch concentrations in intermediate leaves (leaf 14) and upper leaves (leaf 4) increased from stage 0- (healthy) to stages 2 to 4, while root carbohydrate concentrations decreased rapidly from stage 0- to stage 0+ (incubating phytoplasma). Although photosynthetic rates and root carbohydrate concentrations decreased, leaf carbohydrate concentrations increased, suggesting inhibition of sugar transport in the phloem leading to stress in sink tissues and development of visual symptoms of LY.     

Uncoupling and inhibition properties of 3,4-seco-friedelan-3-oic acid isolated from Maytenus imbricata

3,4-Seco-friedelan-3-oic acid was isolated from Maytenus imbricata (Celastraceae). At low concentrations it inhibited non-cyclic electron transport and ATP synthesis in spinach chloroplasts, i.e., it behaved as a Hill reaction inhibitor, and at high concentrations it acts as an uncoupler by enhancing uncoupled electron transport and Mg²⁺-ATPase activity. 3,4-Seco-friedelan-3-oic acid did not inhibit PSII electron transport from DPC to DCPIP_ox and photosystem I activity, but it enhanced from TMQH₂ to MV, corroborating its action as uncoupler. It inhibits electron flow through PSII from water to sodium silicomolybdate. The whole results indicate that the 3,4-seco-friedelan-3-oic acid target is at the OEC complex enzyme, the donor side of PSII. The fluorescence decay data shows the formation of the K-band, which match this result, acting as inhibitor at the donor side of PSII and it as an uncoupler.     

Structure-activity relationships of triazinone herbicides on resistant weeds and resistant Chlamydomonas reinhardtii

Weeds resistant to the s-triazine herbicide atrazine also show resistance to the triazinone herbicide metribuzin. However, with highly lipophilic triazinones, thylakoids isolated from atrazine-resistant Amaranthus retroflexus (mutation at position Ser₂₆₄ of the photosystem II D-1 reaction centre protein) in general show a higher pI₅₀ value in photosystem II electron transport than those from the wild type (i.e. negative cross-resistance; ‘supersensitivity’). A quantitative structure–activity relationship (QSAR) can be established, wherein the lipophilicity of the compound plays a major role. In in-vivo experiments, it was found that the triazinone DRW2698 killed resistant Amaranthus retroflexus and Chenopodium album whereas the wild type was almost unaffected. Triazinones were further investigated in five different mutants of Chlamydomonas rheinhardtii (mutations in the D-1 protein at positions Ser₂₆₄, Ala₂₅₁, Leu₂₇₅, Phe₂₅₅, and Val²¹⁹). Inhibitory activity of all triazinones was generally enhanced in the Phe₂₅₅ mutant but decreased in the Val₂₁₉ mutant. In the other mutants, biological activity was decreased when position 3 of the triazinone was substituted by CH₃, OCH₃, SCH₃, NHCH₃ or N(CH₃)₂. However, negative cross-resistance was again observed when this position was occupied by free thiol. It is therefore suggested that these two groups of triazinones orient themselves differently within the herbicide binding niche of the photosystem II D-1 protein.     

Effects of a new inhibitor K-23 on electron transport in photosystem II of higher plants

The effects of inhibitor K-23 on variable fluorescence, oxygen evolution and DCIP photoreduction were investigated. K-23 promotes the oxygen evolution and DCIP photoreduction at low concentration and inhibits them at relatively high concentrations, while an efficient inhibition at low concentration is found in variable fluorescence. These data further confirm that the inhibitor K-23 action is based on its redox interaction rather than quenching effect. Addition of DPC could not restore the DCIP photoreduction activity. It is suggested that the inhibitory site is at the acceptor side. Using ferricyanide as electron acceptor, the effect of K-23 and DCMU on the oxygen evolution of trypsin-treated thylakoids was investigated. It is found that oxygen evolution of trypsin-treated thylakoids was insensitive to DCMU, whereas became more sensitive to K-23 and also the promotion of K-23 at low concentration disappeared. This strongly indicates that trypsin treatment modified the binding site of K-23 and increased its accessibility to K-23 target site. From the comparison of K-23 with DCMU, we conclude that the binding site of K-23 is different from that of DCMU even though they both bind at the acceptor side.     

Effect of a nonhost-selective toxin from Alternaria alternata on chloroplast-electron transfer activity in Eupatorium adenophorum

AAC-toxin, a putative nonhost-selective phytotoxin, was obtained from Alternaria alternata causing a brown leaf spot disease of Crofton weed ( Eupatorium adenophorum ). The effect of AAC-toxin on the electron transfer reaction of chloroplasts showed that the activity of photosystem II, but not photosystem I, was completely inhibited by the toxin. AAC-toxin affected the following chlorophyll fluorescence parameters: coefficient of photochemical quenching ( q _P), the half-time value of fluorescence rise, and the O–J–I–P fluorescence induction kinetics curve, but not the ratio values of F _v/ F _m (the quantum yield of photosystem II) and the half-time value of fluorescence quenching. It was concluded that the toxin inhibited electron transfer from Q_A to Q_B (primary and secondary quinine acceptors of photosystem II) in photosystem II by competing with Q_B for the binding site in D1 protein on the thylakoid membrane.     

Mode of action of new diethylamines in lycopene cyclase inhibition and in photosystem II turnover

The new bleaching herbicidal compound N,N‐diethyl‐N‐(2‐undecynyl)amine (NDUA) is identified here as an inhibitor of lycopene cyclase and is compared with the known cyclase inhibitors N,N‐diethyl‐N‐[2‐(4‐chlorophenylthio)ethyl]amine (CPTA) and N,N‐diethyl‐N‐[2‐(4‐methylphenoxy)ethyl]amine (MTPA). HPLC separation of chloroplast pigments shows lycopene accumulation in NDUA treated tissue. Variation in chain length of the undecynylamine moeity of NDUA from 7 to 21 C atoms reveals an optimum of 11 to 14 C atoms for herbicidal activity. A series of seven further analogues of NDUA and CPTA reveals the structural elements necessary for inhibition of lycopene cyclase. The effect of NDUA derivatives on photosynthesis has been studied in Chlamydomonas reinhardtii. Photosynthesis is highly sensitive, particularly towards the C14 and longer chain length analogues at nanomolar concentrations. It is shown that the breakdown of photosynthesis by NDUA is due to interference with the turnover of the D1 protein of the photosystem II reaction centre that requires the continous biosynthesis of the two reaction‐centre β‐carotene moieties in the reassembly phase. The D1 protein disappearance is most marked under strong light conditions. The depletion of photosystem II occurs before total pigment bleaching. This newly recognized mechanism in herbicidal activity is also the basis for the mode of action of other lycopene cyclase inhibitors as well as phytoene desaturase inhibitors. © 2001 Society of Chemical Industry     

Herbicidal action of 2-hydroxy-3-alkyl-1,4-naphthoquinones

The main mode of herbicidal activity of 2-hydroxy-3-alkyl-1,4-naphthoquinones is shown to be inhibition of photosystem II (PSII). The herbicidal and in vitro activities have been measured and correlated with their (Log)octanol/water partition coefficients (Log Ko/w). The length of the 3-n-alkyl substituent for optimal activity differed between herbicidal and in vitro activity. The maximum in vitro activity was given by the nonyl to dodecyl homologues (Log Ko/w between 6.54 and 8.12), whereas herbicidal activity peaked with the n-hexyl compound (Log Ko/w = 4.95). The effect of chain branching was also investigated using isomeric pentyl analogues substituted at position 3. All exhibited similar levels of in vitro activities but herbicidal activities differed, albeit moderately, with the exception of one analogue that was much less phytotoxic. Other modes of action were also investigated using two representative compounds. They did not show any activity on photosystem I or mitochondrial complex I, or generate toxic oxygen radicals by redox cycling reactions. Only moderate activity was found against mitochondrial complex III from plants, in contrast to much higher corresponding activity using an insect enzyme.     

Effect of different photosystem II inhibitors on chloroplasts isolated from species either susceptible or resistant toward s-triazine herbicides

In chloroplasts isolated from susceptible and atrazine-resistant Amaranthus retroflexus, the inhibition of photosynthetic electron transport by various classes of herbicides has been investigated. Resistance of mutant Amaranthus is not restricted to s-triazines but also extends to uracils, 1,2,4-triazine-5-ones, and ureas. For 1,2,4-triazin-5-ones and chloroplasts of both biotypes, a correlation between inhibition of photosynthetic electron transport and the partition coefficient could be established. In the case of phenolic herbicides only modestly decreased or even higher sensitivity of chloroplasts from the resistant biotype as compared to the susceptible one could be observed. These results are confirmed by binding of radioactively labeled herbicides to chloroplasts of both plants. Specific binding of atrazine or metribuzin to resistant chloroplasts is completely abolished, and that of diuron or phenisopham diminished as compared to susceptible chloroplasts. In contrast, binding of phenolic herbicides generally is enhanced in resistant chloroplasts. Photoaffinity labeling of thylakoids from both biotypes by 2-azido-4-nitro-6-[2′,3′-³H]isobutylphenol yields almost identical labeling patterns. These results are consistent with a recently proposed model (W. Oettmeier, K. Masson, and U. Johanningmeier, Biochim. Biophys. Acta679, 376 (1982) of two different herbicide binding proteins at the reducing side of photosystem II: a 32- to 34-kdalton protein responsible for binding of triazines, triazinones, ureas, and related herbicides and a photosystem II reaction center protein for binding of phenolic herbicides.     

Mode of action of meturine

The effect of meturine on the light processes of photosynthesis was studied.Meturine is a herbicide for weed control in potato and cotton crops. It is a N-phenyl—N-hydroxy—N′-methylurea.The experiments were carried out on isolated pea and spinach chloroplasts.When examining photosystem I, reduced DPIP was used as an electron donor, whereas methyl-viologen served as an electron acceptor. When examining photosystem II, DPIP represented the electron acceptor.The obtained experimental results have pointed to the absence of the effect of meturine upon the photoreaction I.Unlike N-phenyl—N′, N′-dimethylureas (CMU, DCMU) meturine has been a very weak inhibitor of photoreaction II.The authors explain the photoreaction II inhibition of chloroplasts from plants treated with herbicidal doses of meturine by conversion of N-phenyl—N-hydroxy—N′-methylurea into Hill reaction inhibitor(s). N-Phenyl—N′-methylurea can be one of such meturine metabolites.Meturine herbicidal action is accounted for by meturine transformation into Hill reaction inhibitor(s) in the plant tissues.     

Herbicidal interference with the photochemical activities of chloroplasts (in vivo and in vitro) of certain crop and weed species

The independent modes of action of diuron and atrazine on the photochemical activities of chloroplasts (In vivo and in vitro) from the leaves of crop plants Pisum sativum and Pennisetum typhoides and the weeds Amaranthus viridis and Cyperus rotundus were investigated. Hill reaction activity (DCPIP photoreduction) of in vivo chloroplasts (chloroplasts isolated from herbicide-sprayed plants) was unaffected by treatment at sublethal or intermediate levels of diuron or atrazine while that of in vitro chloroplasts (chloroplasts incubated in the required herbicidal concentration) was severely inhibited. The ferricyanide catalyzed noncyclic photophosphorylation was markedly reduced in both the in vivo and in vitro chloroplast systems. N-Methyl phenozonium sulfate (PMS)-mediated cyclic photophosphorylation was inhibited in the in vivo system while a pronounced enhancement of activity was noticed in the in vitro chloroplasts. The rate of NADP⁺ photoreduction was severely inhibited in the in vitro chloroplasts. The unaffected in the in vivo system. The herbicidal effects on the photoreactions of isolated chloroplasts were compared with chloroplasts isolated from herbicide-sprayed plants.     

Phytoplasmas change the source–sink relationship of field-grown sweet cherry by disturbing leaf function

Changes in the function of field-grown sweet cherry leaves infected with phytoplasma were evaluated through the analysis of photosynthesis, respiration, carbohydrates and hormones. Phytoplasmal infection caused witches' broom, small leaves, leaf yellowing and leaf rolling. The photosynthesis of infected leaves was considerably reduced, and they were unable to produce sufficient carbohydrates for their own needs. In contrast, the starch content of infected leaves was significantly increased. These results demonstrate the change in the role of infected leaves from sources to sinks. Further analysis revealed that the photosynthetic decline was related to a significant decrease in photosynthetic pigments and to marked inactivation of photosystem II (PSII). Furthermore, the loss of PSII function was due to a decrease in chlorophyll content, reduction and closure of active reaction centers, and decline in photochemical efficiency.     

Phytogrowth- and photosynthesis-inhibiting properties of nostoclide analogues

Six nostoclide analogues were synthesised from 3-benzyl-2(5H)-furanone in one step, with yields ranging from 10 to 71%, and subjected to several biological assays. The two most active of these, 5d and 5e, were shown to be phytogrowth inhibitors of the radicle of Lolium multiflorum Lam, while enhancing the root growth of Physalis ixocarpa Brot. Both compounds inhibited electron flow (basal, phosphorylating and uncoupled) from water to methylviologen (MV); both acted as Hill reaction inhibitors, since the synthesis of ATP was prevented. The uncoupled electron transport from photosystem II (PSII) (water to 2,6-dichlorophenol-indophenol (DPIP)) and photosystem I (PSI) (2,6-dichlorophenol-indophenol reduced (DPIPred) to MV) was inhibited with 500 microM of 5d by 22 and 14% respectively. In addition, 400 microM of 5d inhibited PSI (from tetramethyl-p-benzohydroquinone (TMQH(2)) to MV) by 40%. Thus 5d inhibited electron transport at the b(6)f complex. Finally, 500 microM of 5e inhibited electron flow (basal and phosphorylating) by 25%, and 300 microM of 5e enhanced light-activated membrane-bound Mg(2+)-ATPase by 66%. Thus 5e behaved as a weak Hill reaction inhibitor and an uncoupler. In general, the phytotoxicity of the synthetic lactones was only weakly related to inhibition of photosynthesis.     

Herbicidal cyanoacrylates with antimicrotubule mechanism of action

The herbicidal mode of action of the new synthetic cyanoacrylates ethyl (2Z)-3-amino-2-cyano-4-ethylhex-2-enoate (CA1) and its isopropyl ester derivative CA2 was investigated. For initial characterization, a series of bioassays was used indicating a mode of action similar to that of mitotic disrupter herbicides such as the dinitroaniline pendimethalin. Cytochemical fluorescence studies including monoclonal antibodies against polymerized and depolymerized tubulin and a cellulose-binding domain of a bacterial cellulase conjugated to a fluorescent dye were applied to elucidate effects on cell division processes including mitosis and microtubule and cell wall formation in maize roots. When seedlings were root treated with 10 microM of CA1 or CA2, cell division activity in meristematic root tip cells decreased within 4 h. The chromosomes proceeded to a condensed state of prometaphase, but were unable to progress further in the mitotic cycle. The compounds caused a complete loss of microtubular structures, including preprophase, spindle, phragmoplast and cortical microtubules. Concomitantly, in the cytoplasm, an increase in labelling of free tubulin was observed. This suggests that the herbicides disrupt polymerization and microtubule stability, whereas tubulin synthesis or degradation appeared not to be affected. In addition, cellulose labelling in cell walls of root tip cells was not influenced. The effects of CA1 and CA2 were comparable with those caused by pendimethalin. In transgenic Arabidopsis plants expressing a green fluorescent protein-microtubule-associated protein4 fusion protein, labelled arrays of cortical microtubules in living epidermal cells of hypocotyls collapsed within 160 min after exposure to 10 microM CA1 or pendimethalin. Moreover, a dinitroaniline-resistant biotype of goosegrass (Eleusine indica (L) Gaertn) with a point mutation in alpha-tubulin showed cross-resistance against CA1 and CA2. The results strongly indicate that the cyanoacrylates are a new chemical class of herbicide which possess the same antimicrotubule mechanism of action as dinitroanilines, probably including interaction with the same binding site in alpha-tubulin.     

Growth, photosynthetic pigments and photosynthetic activity during seedling stage of cowpea (Vigna unguiculata) in response to UV-B and dimethoate

UV-B (0.4 W m⁻²) irradiation and dimethoate (100 and 200 ppm) treatments, singly and in combination, declined the growth, photosynthetic pigment contents and photosynthesis (O₂ evolution and CO₂-fixation) of cowpea (Vigna unguiculata). Contrary to this, low concentration of dimethoate (50 ppm) caused stimulation on these parameters, while together with UV-B it showed inhibitory effects. Carotenoids (Car) showed varied responses. It was found that carbon-fixation (¹⁴CO₂) was more sensitive to both the stresses than photosynthetic oxygen evolution. Photosynthetic electron transport activity was reduced by both the stresses, however, 50 ppm dimethoate besides inhibiting photosystem II (PSII) and whole chain activity, showed slight stimulation in photosystem I (PSI) activity. The individual effect of two stresses on PSII activity was probably due to interruption of electron flow at oxidation side of PSII which extended to its reaction center following simultaneous exposure. A similar trend was also noticed in case of CO₂ liberation (measured as ¹⁴CO₂ release) in light and dark. Results suggest that dimethoate (100 and 200 ppm) and UV-B alone caused heavy damage on pigments and photosynthetic activity of cowpea, leading to the significant inhibition in growth. Further, the interactive effects of both the stresses got intensified. However, low concentration (50 ppm) of dimethoate showed stimulation, but in combination, it slightly recovered from the damaging effect, caused by UV-B.     

Long-term effects of herbicides on the photosynthetic apparatus II. Investigations on bentazone inhibition

Non-cyclic electron transport by isolated chloroplasts from the alga Bumilleriopsis is inhibited by bentazone, while the activity of photosystem I is not affected. Chloroplast material isolated from cells grown for several days in the presence of bentazone also shows inhibition of photosystem II activity, similar to the decreased cellular photosynthesis. A slow partial recovery is possible, whereas it is fast and complete after exposure of cells or isolated chloroplasts to the herbicide for some hours only. Irreversibility of longterm inhibition could be a metabolic process which brings about the binding of bentazone (or an active derivative) to the thylakoids in vivo, but not in vitro.     

Effects of short-term low temperatures on photosystem Ⅱ function of samara and leaf of Siberian maple (Acer ginnala) and subsequent recovery

Samara is the reproductive organ (seed) for many tree species in arid land in northwestern China. It is ecologically important in population development due to its dispersal function. However, information on its photosynthesis and effect of environmental stresses on its photosynthesis is still very limited. In the present study, responses of photosystem II (PSII) activity in samara and leaf of Siberian maple to short-term chilling/freezing and subsequent recovery potential were comparatively investigated by using polyphasic fluorescence test. The samara had more efficient photosynthesis (Fv/Fm and PIABS) and more efficient electron transport (φEo) but lower energy dissipation (DIo/RC) than leaf. Generally, the PSII performance and the electron transport for both samara and leaf were inhibited under low temperature stress, accompanied by an increase of energy dissipation in PSII reaction centers (RCs). PSII of both samara and leaf was not markedly affected by chilling and could acclimate to chilling stress. Short-term freezing could completely inhibit PSII activity in both samara and leaf, indicated by the drop of values of Fv/Fm, PIABS, φEo to zero. PSII functional parameters of short-term dark frozen samara could be largely recovered whereas those of frozen leaf could not be recovered. The higher tolerance of samara to short-term low temperature stress than leaf is of great ecological significance for seed development, population establishment of Siberian maple.