The Influence of Temperature on Nitrate Reductase Activity of Agrostis palustris and Cynodon dactylon

The influence of temperature was studied in relation to nitrate reductase activity of creeping bentgrass (Agrostis palustris Huds. cv. ‘Toronto’) a cool season grass and bermudagrass (Cynodon dactylon L. cv. ‘Tifgreen’) a warm season grass. Maximum nitrate reductase activity of both species occurred at 20°C. The nitrate reductase level in bentgrass leaves was reduced when grown at 35°C while bermudagrass leaves were relatively unaffected. The activity per se of the bentgrass enzyme preparation was inhibited rather than synthesis of the enzyme.     

Demonstration of an “Excitation-Contraction Recoupling” Mechanism in Mammalian Ventricular Myocardium

A PROCESS called “excitation-contraction coupling” has been generally accepted to take place only in the direction of excitation to contraction. Through this mechanism a propagated action potential initiates an active state in skeletal or cardiac muscle and the muscle contracts. We propose that, in the mammalian ventricular myocardium at least, the process is not unidirectional and an important reverse coupling between the contractile system and the excitable plasma membrane has been overlooked. Through this feedback interaction the mode of contraction (that is, isotonic or isometric) not only determines the instantaneous electrical state of the plasma membrane, but also influences the mechanical events of the subsequent beats. Thus when Kaufmann et al.¹ recorded intracellular action potentials from cat papillary muscle, the time course of the repolarization was altered depending on the mode of contraction. Some kind of contraction-excitation feedback has also been suggested by Stauch² and Lab^3,4. They showed a difference in the shape of the monophasic action potential, as recorded by a suction electrode, when comparing isotonic and isovolumic contraction of the intact ventricle. But their experimental conditions did not allow satisfactory analysis of the phenomenon.     

The role of ribonucleoproteins in the production of mitotic abnormalities

Summary Immersion of growing roots of onion and lily in aerated solutions of ribonuclease affected their pattern of growth and altered the structure and mitotic distribution of the chromosomes. Action of the enzyme on meristematic cells caused enlargement of nucleoli, excessive contraction, stickiness, adhesion, and clumping of chromosomes, and production of aneuploid and polyploid chromosome complexes, tripolar and multipolar spindles, binucleate and multinucleate cells. Very few cases of chromosome fragmentation were observed.Accumulation of abnormalities accompanied the passage of ribonuclease across the root as determined by alterations in stainability of the cells with pyronin and fast green. There was no visible modification of stainability of the chromosomes with methyl green or the Feulgen reagent.These results, when compared with those produced by control solutions, indicate that ribonuclease enters the living cell and degrades ribonucleoproteins essential for maintenance of structural and functional integrity. The implications of these results, with respect to the production of aberrations by other agents, are discussed.This study was supported by a research grant (RG-149) from the National Institutes of Health, U.S. Public Health Service.     

Environmental Effects on Transpiration and Leaf Water Potential in Avocado

A field study was conducted to determine how atmospheric and edaphic conditions influenced the water relations of avocado trees (Persea americana Mill. cv. Bacon). With high and low levels of incident photosynthetically active radiation (PAR, 400–700 nm wave length), and either wet or dry soil, leaf conductance decreased as the absolute humidity difference from leaf to air increased. For any water stress treatment, conductance was higher at high PAR than at low PAR. Both conductance and transpiration were higher in well-watered trees than in stressed trees, and in prestressed trees levels were intermediate to unstressed and stressed trees. A model for water flux through the soil-plant-atmosphere continuum was used to examine the relationship of leaf xylem pressure potential to transpiration in well-watered trees and in trees stressed by dry soil. There was a close linkage between leaf xylem pressure potential and transpiration in unstressed and previously stressed trees with high or low PAR, i.e. similar potentials occurred with equivalent transpiration regardless of previous treatment or time of day. In stressed trees, xylem pressure potential was lower than in unstressed trees both during the day and night, and at a given transpiration rate the potential was lower after 1400 h than before that time. The model indicated that in stressed trees xylem pressure potential was uncoupled from transpiration, presumably because of altered resistance in the soil-root portion of the transport system.     

Chick embryo fibroblasts produce two forms of hyaluronidase

Cultured chick embryo fibroblasts derived from skin and skeletal muscle exhibit hyaluronidase activity both associated with the cell layer and secreted into the medium. Although both forms of the enzyme have a number of similar characteristics (R.W. Orkin and B.P. Toole, 1980, J. Biol. CHem. 255), they differ in thermal stability at neutral pH and in behavior on ion-exchange chromatography. Both forms of the enzyme are equally stable at acidic pH for long intervals, but the cell-associated hyaluronidase is significantly less stable than the secreted froms at neutral pH and at temperatures more than or equal to 30 degrees C. Neither the presence of proteases nor inhibitors of hyaluronidase appear to be involved in the cell-asspcoated enzyme. Chromatography of the two forms of hyaluronidase on carboxymethyl cellulose reveals that most (60-90 percent) of the secreted form of the enzyme elutes at a lower ionic strength than the cell- associated enzyme. Treatment of the secreted form of hyaluronidase with neuraminidase shifts its elution profile on carboxymethyl cellulose toward that of the cell-associated form, and also decreases its thermal stability at neutral pH. In contrast, treatment of the secreted form of hyaluronidase with alkaline phosphatase has no detectable effect. These data suggest that the secreted hyaluronidase differs from the cellular form in possessing additional sialic acid residues which endow the former with increased stability in the extracellular milieu.