Effects of Extended Periods of Osmotic Stress on Water Relationships of Pepper

Pepper plants grown to uniform size in a controlled environment were subjected to an osmotic stress for periods of 1 to 10 days. Polyethylene glycol 400 was used as the osmotic agent. Leaf area of the plants, grown under uniform conditions, was proportional to the weight of the plants. This relationship was not altered by reduction in rate of growth due to a decrease in osmotic potential of the nutrient solution. The rate of transpiration of the pepper plants decreased as the osmotic potential of the nutrient solution was decreased. The reduction in rate of transpiration was most rapid when the osmotic potential was reduced from ?0.5 to ?7.5 bars. There was continued reduction in the rate of transpiration with change in potential to ?12.5 bar but this change was less than that at the higher potentials. The rate of transpiration remained at a reduced rate for as long as the plants were growing in the solution with low osmotic potential. Alternating the osmotic potential of the nutrient solution between ?0.5 and ?5.0 bar did not change the response to the ?5.0 tension. The reduction in rate of transpiration resulting from the lowering of the osmotic potential by addition of NaCl was similar to that produced by addition of polyethylene glycol. Water potential, osmotic potential, relative water content and stomatal movement were all in dynamic equilibrium with the water content of the leaves. The water content of the leaves was regulated by the supply and demand. In these investigations the demand remained constant. The supply was altered by decreasing the difference in water potential between leaf and substrate and by an increase in resistance to flow of water in the roots as a result of the decrease in osmotic potential of the nutrient solution.     

Targeting KRAS Mutant Cancers with a Covalent G12C-Specific Inhibitor

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Packing constraints and electrostatic surface potentials determine transmembrane asymmetry of phosphatidylethanol

The energetic determinants of the distribution of anionic phospholipids across a phosphatidylcholine (PtdCho) bilayer with different packing constraints in the two leaflets were studied, using (13)CH2-ethyl-labeled phosphatidylethanol (PtdEth) as a (13)C NMR membrane probe. PtdEth is unique in exhibiting a split (13)CH2-ethyl resonance in sonicated vesicles, the two components originating from the inner and outer leaflets, thus permitting the determination of the PtdEth concentration in each leaflet. Small and large unilamellar PtdEth-PtdCho vesicles were prepared in solutions of different ionic strengths. A quantitative expression for the transbilayer distribution of PtdEth, based on the balance between steric and electrostatic factors, was derived. The transbilayer difference in packing constraints was obtained from the magnitude of the PtdEth signal splitting. The electrostatic contribution could be satisfactorily described by the transmembrane difference in Gouy-Chapman surface potentials. At low (0.1-0.25%) PtdEth levels and high (up to 500 mM) salt concentrations, PtdEth had a marked fivefold preference for the inner leaflet, presumably because of its small headgroup, which favors tighter packing. At higher PtdEth content (4.8-9.1%) and low salt concentrations, where electrostatic repulsion becomes a dominant factor, the asymmetry was markedly reduced and an almost even distribution across the bilayer was obtained. In less curved, large vesicles, where packing constraints in the two leaflets are approximately the same, the PtdEth distribution was almost symmetrical. This study is the first quantitative analysis of the balance between steric and electrostatic factors that determines the equilibrium transbilayer distribution of charged membrane constituents.     

Multiple forms of ADP-glucose pyrophosphorylase from tomato fruit.

ADP-glucose pyrophosphorylase (AGP) was purified from tomato (Lycopersicon esculentum Mill.) fruit to apparent homogeneity. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis the enzyme migrated as two close bands with molecular weights of 50,000 and 51,000. Two-dimensional polyacrylamide gel electrophoresis analysis of the purified enzyme, however, revealed at least five major protein spots that could be distinguished by their slight differences in net charge and molecular weight. Whereas all of the spots were recognized by the antiserum raised against tomato fruit AGP holoenzyme, only three of them reacted strongly with antiserum raised against the potato tuber AGP large subunit, and the other two spots (with lower molecular weights) reacted specifically with antisera raised against spinach leaf AGP holoenzyme and the potato tuber AGP small subunit. The results suggest the existence of at least three isoforms of the AGP large subunit and two isoforms of the small subunit in tomato fruit in vivo. The native molecular mass of the enzyme determined by gel filtration was 220 +/- 10 kD, indicating a tetrameric structure for AGP from tomato fruit. The purified enzyme is very sensitive to 3-phosphoglycerate/inorganic phosphate regulation.