Inhibition of phosphorus and water passage across intact roots by polyethylene glycol and phenylmercuric acetate

Application of polyethylene glycol or phenylmercuric acetate to intact bean (Phaseolus vulgaris L., cv. Red Wade) roots inhibited passage of phosphorus across the roots to the xylem. The same results occurred for foliar application of phenylmercuric acetate when time was allowed for absorption and distribution of the chemical in the plant. For both chemicals the inhibition of phosphorus was proportional to or greater than any accompanying restriction on water flow across the root.     

Inhibition of chloroplast photochemical reactions with 2-chloromercuri 4,6-dinitrophenol.

2-Chloromercuri 4,6-dinitrophenol inhibited photosystem I mediated photochemical reactions of Euphorbia hirta chloroplasts. The compound inhibited cyclic photophosphorylation and NADP reduction (in the presence of dichlorophenol indophenol and ascorbate couple) at concentrations as low as 10^?6m. At higher concentrations (above 10^?4m), however, it affected all NADP reductions but still showed negligible effect on ferricyanide reduction or noncyclic photophosphorylation. The compound may be used as an inhibitor of cyclic photophosphorylation.     

Inhibition of chloroplast electron transport reactions by trifluralin and diallate

The herbicides trifluralin (alpha,alpha,alpha-trifluoro-2,6-dinitro-N, N-dipropyl-p-toluidine) and diallate (S-[2,3-dichloroallyl] diisopropylthiocarbamate) inhibit electron transport, ATP synthesis, and cytochrome f reduction by isolated spinach (Spinacia oleracea L.) chloroplasts. Both compounds inhibit noncyclic electron transport from H(2)O to ferricyanide more than 90% in coupled chloroplasts at concentrations less than 50 mum. Neither herbicide inhibits electron transport in assays utilizing only photosystem I activity, and the photosystem II reaction elicited by addition of oxidized p-phenylenediamine or 2,5-dimethylquinone is only partially inhibited by herbicide concentrations which block electron flow from H(2)O to ferricyanide. Inhibition of ATP synthesis parallels inhibition of electron flow in all noncyclic assay systems, and cyclic ATP synthesis catalyzed by either diaminodurene or phenazine metho-sulfate is susceptible to inhibition by both herbicides. These results indicate that trifluralin and diallate both inhibit electron flow in isolated chloroplasts at a point in the electron transport chain between the two photosystems.     

Responses of stomata of barley and maize to phenylmercuric acetate

A reduction of stomatal aperture in light was found in leaves of maize after they had been treated with 10“3-5 m phenylmercuric acetate (PMA). Complete closure of the stomata in darkness was prevented, whilst there was total closure in the controls. Higher PMA concentrations had bigger effects. The relative water content (RWC) of barley tissues was slightly reduced 12 hours after treatment with PMA. The transpiration rate observed on PMA-treated barley plants was lower in light and higher in darkness than in untreated plants. Water saturation deficit (WSD) was higher by about 5%, and water holding capacity (WHC) lower (25%) than in untreated plants. The results suggest that the concentration of PMA normally applied as an antitranspirant is unfavourable for healthy growth of maize and barley.     

The effect of phenylmercuric acetate on salt tolerance in wheat

Summary Sodium chloride reduced the growth of the wheat cultivar Armada. Although phenylmercuric acetate (PMA) also reduced the growth under non-saline conditions, in the presence of sodium chloride (100 mM) fresh weight, dry weight and leaf area per plant were promoted particularly when sprayed with PMA at 50 M. Both net photosynthesis and transpiration were reduced by PMA but the reduction was greater in the absence rather than the presence of NaCl. Furthermore PMA lowered shoot sodium contents and promoted the selectivity for K over Na under saline conditions. Any beneficial effects of PMA are the consequences of improved water relations, lowered ion content and increased leaf area for photosynthesis.     

Inhibition of chloroplast development by tentoxin

Light-dependent chloroplast development in detached pea shoots was measured in terms of chlorophyll synthesis and the synthesis of Fraction 1 protein. Both synthetic processes were inhibited more than 90% by the fungal metabolite, tentoxin (1 or 10 μg/ml). These results place Pisum sativum in the class of tentoxin-sensitive higher plants. Tentoxin, actinomycin D, lincomycin, D-threo-chloramphenicol and carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) were compared in their ability to inhibit RNA and protein synthesis by isolated pea chloroplasts. Energy for the synthetic reactions was supplied either by light or by added ATP. Only CCCP gave the same pattern of inhibition as tentoxin, i.e. inhibition of both RNA and protein synthesis in the light-driven system but no inhibition in the ATP-driven system. It is concluded that chloroplast developmental processes are inhibited by tentoxin through the inhibition of photophosphorylation.     

Inhibition of chloroplast coupling factor by naphthylglyoxal

The trypsin-activated Ca²⁺ -ATPase of spinach chloroplast membranes was completely inhibited by treatment with naphthylglyoxal, a fluorescent compound that should bind covalently to arginine residues. The inhibition followed apparent first-order kinetics. The apparent order of reaction with respect to inhibitor concentration gave values near unity, suggesting that inactivation is a consequence of modifying one arginine residue per active site. Partial protection against naphthylglyoxal was afforded by ADP and ATP, with either less or no protection by other nucleotide bases. At inhibition levels less than complete, the K_m for ATP was not affected but the V_max of the enzyme was diminished. The light-dependent exchange of tightly bound nucleotides on the membrane-bound enzyme was not inhibited by naphthylglyoxal treatment, indicating significant retention of the conformational response of the enzyme to the membrane high-energy state. Using [³H]naphthylglyoxal, the extent of inhibition was a linear function of the amount of naphthylglyoxal bound up to 60% inhibition. The curves extrapolated to 2 mol naphthylglyoxal bound, associated with complete inhibition of ATPase. The radioactive naphthylglyoxal was distributed equally between α- and β-subunits.     

Concentration-dependent effects of salicylaldoxime on chloroplast reactions

Salicylaldoxime has been found to have a variety of concentration-dependent effects on chloroplast activities. At low concentrations (< 10 mM), salicylaldoxime reversibly inhibits all reactions which involve Photosystem II. Since the DCMU-insensitive silicomolybdate Hill reaction is also inhibited, one site of inhibition is definitely located before the DCMU-sensitive site, possibly before the photoact. The inhibition kinetics and the response of chloroplast fluorescence may indicate another site in the DCMU-sensitive region. At almost exactly the same concentrations (< 10 mM), salicylaldoxime uncouples phosphorylation reversibly, whether it is supported by Photosystem II or by Photosystem I. At higher concentrations (approx. 20 mM) salicylaldoxime inhibits Photosystem II irreversibly, uncouples irreversibly, and begins to cause changes in chloroplast light scattering which could be manifestations of membrane damage. At very high concentrations (approx. 45 mM) salicylaldoxime irreversibly inhibits Photosystem I activity in the region of plastocyanin. This is indicated by the ability of salicylaldoxime to inhibit the photooxidation of cytochrome f but not the photooxidation of P-700.     

Evidence against an intermediary role of abscisic acid in stomatal closure induced by phenylmercuric acetate and faresol

The hypothesis was tested that stomatal closure induced by the antitranspirants phenylmercuric acetate and farnesol is mediated by an increase in abscisic acid content. Isolated leaves of Spinacia oleracea L. cv. Müma, Tradescantia X andersoniana Ludw. et Rohw. and Commelina communis L. were incubated in solutions of phenylmercuric acetate (10⁻³ M ) and of farnesol (2 × 10⁻³ M ). In all three species, a reduction in stomatal aperture was observed. The extent and velocity of the reaction differed between the species and also between the two compounds applied. The abscisic acid content of the leaves was determined after stomatal closure had been achieved. No significant increase in abscisic acid level was found in treated leaves of Spinacia and Commelina . In Tradescantia , on the contrary, a reduction to about 50% was observed after 120 min treatment. Visible damage of the treated leaves and membrane alterations observable by electron microscopy occurred, especially after treatment with farnesol. These changes in membrane structure suggest a connection with the reduction in stomatal aperture.     

Effects of nitrofen on chloroplast coupling factor-dependent reactions

Nucleotides induce a conformational change in the proteins of the CF₀-CF₁ complex. They give rise to reduced proton permeability of the thylakoid membranes. This reaction is paralleled by an enhanced yield of the steady-state proton uptake and a reduced nonphosphorylating electron-transport rate. Nitrofen acts as an energy-transfer inhibitor. It inhibits the rate of nucleotide exchange on CF₁ both at ‘loose’ and ‘tight’ binding sites. During illumination the percentage of nucleotide-free CF₀-CF₁ complex seems to be enhanced in the presence of nitrofen. This results in a prevention of the described ADP effects on proton uptake and electron transport. These similar effects of nitrofen on loose and tight nucleotide-binding sites correspond with the idea that both types are different states of identical sites.     

Melittin: An inhibitor of chloroplast photochemical reactions

Melittin, a polypeptide component of bee venom, is an inhibitor of photochemical reactions in chloroplasts isolated from higher plants. At concentrations around 5 μM, melittin acts as an uncoupler of photophosphorylation and abolishes the 518 nm light induced absorbance changes. At higher concentrations (30–50 μM), melittin abolishes both the light-induced photooxidation of cytochrome $\text{f}$, and partially inhibits other reactions of photosynthetic electron transfer, without causing lysis of the membrane. The observed inhibitions appear to be due to changes in the properties of the membrane lipid bilayer, caused by penetration of melittin molecules.     

Stimulatory effect of phenylmercuric acetate and benzene on the growth of a broad spectrum mercury-resistant strain of Bacillus pasteurii

A broad spectrum mercury-resistant organism, Bacillus pasteurii DR₂, was isolated from the effluents of Durgapur Steel Plant, India. In the presence of phenylmercuric acetate or benzene the rate of glucose uptake and the level of some metabolic enzymes increased significantly. Inhibition of the glucose oxidation rate by these chemicals in lysozyme-treated cells indicated that these compounds facilitated the transport of glucose across the cell wall and thereby stimulated growth.     

Chlamydomonas chloroplast ferrous hemoglobin. Heme pocket structure and reactions with ligands

We report the optical and resonance Raman spectral characterization of ferrous recombinant Chlamydomonas LI637 hemoglobin. We show that it is present in three pH-dependent equilibrium forms including a 4-coordinate species at acid pH, a 5-coordinate high spin species at neutral pH, and a 6-coordinate low spin species at alkaline pH. The proximal ligand to the heme is the imidazole group of a histidine. Kinetics of the reactions with ligands were determined by stopped-flow spectroscopy. At alkaline pH, combination with oxygen, nitric oxide, and carbon monoxide displays a kinetic behavior that is interpreted as being rate-limited by conversion of the 6-coordinate form to a reactive 5-coordinate form. At neutral pH, combination rates of the 5-coordinate form with oxygen and carbon monoxide were much faster (>10(7) microM-1 s-1). The dissociation rate constant measured for oxygen is among the slowest known, 0.014 s-1, and is independent of pH. Replacement of the tyrosine 63 (B10) by leucine or of the putative distal glutamine by glycine increases the dissociation rate constant 70- and 30-fold and increases the rate of autoxidation 20- and 90-fold, respectively. These results are consistent with at least two hydrogen bonds stabilizing the bound oxygen molecule, one from tyrosine B10 and the other from the distal glutamine. In addition, the high frequency (232 cm-1) of the iron-histidine bond suggests a structure that lacks any proximal strain thus contributing to high ligand affinity.