[Influence of planting density and precipitation on N2O emission from a winter wheat field].

To investigate the impact of plant density on N2O emission from winter wheat field and the cause of seasonal variation in the emission, field experiment with four planting rates of 0, 90, 180 and 270 kg/ha was conducted at the Jiangning County near Nanjing during 1999-2000 winter wheat growing season. Data of the field measurements indicated that the N2O emission rates during the season from planting to overwintering were not influenced by the plant density, while the emission was positively correlated with the planting density during the season from turning green to maturity. The emissions from the field plots with planting rates of 0 and 90 kg/ha were not found to be significantly different. A further analysis suggested that the seasonal variation of N2O emission be mainly influenced by precipitation, which could be quantitatively described by an exponential function of a weighted average precipitation of 6-day period before measurement.

Three-dimensional structure of the Z band in a normal mammalian skeletal muscle.

The three-dimensional structure of the vertebrate skeletal muscle Z band reflects its function as the muscle component essential for tension transmission between successive sarcomeres. We have investigated this structure as well as that of the nearby I band in a normal, unstimulated mammalian skeletal muscle by tomographic three-dimensional reconstruction from electron micrograph tilt series of sectioned tissue. The three-dimensional Z band structure consists of interdigitating axial filaments from opposite sarcomeres connected every 18 +/- 12 nm (mean +/- SD) to one to four cross-connecting Z-filaments are observed to meet the axial filaments in a fourfold symmetric arrangement. The substantial variation in the spacing between cross-connecting Z-filament to axial filament connection points suggests that the structure of the Z band is not determined solely by the arrangement of alpha-actinin to actin-binding sites along the axial filament. The cross-connecting filaments bind to or form a "relaxed interconnecting body" halfway between the axial filaments. This filamentous body is parallel to the Z band axial filaments and is observed to play an essential role in generating the small square lattice pattern seen in electron micrographs of unstimulated muscle cross sections. This structure is absent in cross section of the Z band from muscles fixed in rigor or in tetanus, suggesting that the Z band lattice must undergo dynamic rearrangement concomitant with crossbridge binding in the A band.

Two structural states of the vertebrate Z band.

Ultrastructural analysis of the vertebrate Z band suggests that two reversible states of a single intricate lattice are essential for the contractile process. The two structural states of the Z band lattice (ss and bw) have been described in cross section in skeletal and cardiac muscle in different physiological states. The lattice responds to active tension but resists passive deformation. Changes in Z band form and dimension are correlated with cross-bridge binding. Two-dimensional image processing techniques show enhanced structural features that vary with the observed changes in lattice dimension. All projected images from all lattices show an approximate four-fold symmetry. Each image reveals differences in the appearance of axial filaments which enter from opposite sides of the Z band and cross-connecting filaments of similar curvature which appear to connect each axial filament to four nearest axial filaments. In the ss images, the apparent diameter of cross-cut axial filaments and the Z band interaxial filament spacing are smaller than in bw images. Cross-connecting filaments appear to overlap in the region half-way between axial filaments in ss images. We conclude that the Z band is an essential and dynamic part of the sarcomere, uniquely suited to transmit tension while maintaining dimensions appropriate for cross-bridge interaction.

The Z-band lattice in skeletal muscle in rigor.

Previous work with tetanized and relaxed muscle has shown a correlation between active tension and the structure of the Z-band. This suggests that there is a correlation between the cross-bridge binding in the A-band and the structure of the Z-band. Using electron microscopy and optical diffraction we have examined this correlation in glycerinated muscle in rigor and in unstimulated intact muscle. We have found that the Z-bands of muscles in rigor always show the basketweave form, while those of the unstimulated muscles always show the small square form. The basketweave form found in rigor muscles is similar in form and dimension to that found in tetanized muscle. Thus it appears that the small square form of the Z-band is found in physiological states with little cross-bridge binding and the basketweave form is found in states with a high degree of cross-bridge binding.

Two structural states of Z-bands in cardiac muscle.

We have compared the form and dimensions of the Z-band lattice in rat papillary muscle fixed at rest with and without ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) using electron microscopy and optical diffraction. In unstimulated muscle, the Z-band lattice form called basket weave predominated, and the Z-spacing (defined as the repeat distance of a tetragonal array of cross-cut thin filaments from the same sarcomere) was 23.93 +/- 0.37 nm. Muscles exposed to EGTA exhibited the small square-lattice form, and the Z-spacing was 20.50 +/- 0.19 nm. The Z-spacings in the two lattice forms were similar in cardiac and skeletal muscles such that the decrease in Z-spacing in the transition from basket weave to small square in this study was similar to the increase in Z-spacing previously demonstrated in skeletal muscle in the transition from small square to basket weave. The Z-lattice form and dimensions in unstimulated cardiac muscle resembled those in tetanized skeletal muscle. These findings are consistent with the higher resting tension in cardiac muscle and suggest that Ca2+ may be important for the maintenance of the expanded Z-lattice form.

Structural states in the Z band of skeletal muscle correlate with states of active and passive tension.

In skeletal muscle Z bands, the ends of the thin contractile filaments interdigitate in a tetragonal array of axial filaments held together by periodically cross-connecting Z filaments. Changes in these two sets of filaments are responsible for two distinct structural states observed in cross section, the small-square and basketweave forms. We have examined Z bands and A bands in relaxed, tetanized, stretched, and stretched and tetanized rat soleus muscles by electron microscopy and optical diffraction. In relaxed muscle, the A-band spacing decreases with increasing load and sarcomere length, but the Z lattice remains in the small-square form and the Z spacing changes only slightly. In tetanized muscle at sarcomere lengths up to 2.7 micron, the Z lattice assumes the basketweave form and the Z spacing is increased. The increased Z spacing is not the result of sarcomere shortening. Further, passive tension is not sufficient to cause this change in the Z lattice; active tension is necessary.

Z band dynamics as a function of sarcomere length and the contractile state of muscle.

The Z band in skeletal muscle has two distinct structural states--a relaxed (small square or ss) form and a maximally activated (basket weave or bw) form. We have examined by electron microscopy and optical diffraction Z lattice forms and dimensions and A band spacings in relaxed, tetanized, stretched, and stretched-and-tetanized rat soleus muscle. We have tested the independent contributions of passive load, active tension, and sarcomere length to Z band state. As the A band spacing decreased with increasing load and increasing sarcomere length in the untetanized muscles, the Z lattice remained in the ss form and the Z spacing changed only slightly. Computer-enhanced images from digitized electron micrographs showed that the ss Z lattice resisted deformation regardless of load or method of stretching. In contrast, when the muscle was tetanized at sarcomere lengths of up to 2.7 microns, the Z lattice assumed the bw form and the Z spacing was increased by 20%. Regardless of lattice form, Z spacing did not vary significantly with sarcomere length. Images from freeze-substituted preparations showed both lattice forms comparable to those in images from glutaraldehyde-fixed muscles. Thus, Z band state appears to be a function of the presence (or absence) of active tension. Our previous three-dimensional model is compatible with these observations and with the sub-structures revealed by computer-enhanced images of both lattice forms.

The Z-band lattice in skeletal muscle before, during and after tetanic contraction.

Electron micrographs and optical diffraction patterns of the Z-band were studied in rat soleus muscle fixed before, during, and after tetanic contraction. We compared the morphology (small square or basketweave pattern) and dimensions of the Z-lattice of control and tetanized muscles near rest length. Z-bands of muscle fixed at rest and of muscle allowed to rest after a tetanic contraction exhibited the small square pattern. Z-bands from muscle fixed during tetanic contraction exhibited the basketweave pattern. Concomitant with the transition to basketweave, we observed an average increase of 20% in spacing between the axial filaments of the Z-lattice. Optical diffraction measurements of the A-band d10 spacing revealed that the Z/A ratio remained constant during the transition. We have modelled the small square to basketweave transformation as resulting from a change of curvature of constant length cross-connecting Z-filaments when the axial filaments increase their separation.

Distribution and characterization of mineral-binding phosphoprotein particles in Bivalvia.

Representative species of four bivalve subclasses were examined for the presence of mineral-binding phosphoprotein particles in the physiological fluids. The particles were identified in Heterodont bivalves only, and particles from nine different Heterodont species were isolated and characterized. All phosphoprotein particles are internally cross-linked via histidinoalanine residues. In all species over 80% of the amino acid residues in the particles are aspartic acid, phosphoserine (and/or phosphothreonine), and histidine. These amino acids are probably the only residues directly related to mineral ion binding, since all phosphoprotein particles bind mineral irrespective of the minor amino acid content, which is species dependent. In their native state the phosphoprotein particles contain large amounts of calcium, magnesium, and inorganic phosphate ions (up to 45 metal ions and 8 phosphate ions per 100 amino acid residues) and trace amounts of transition elements. Evidence for the presence of calcium phosphate complexes in the native phosphoprotein particles was obtained by observing a concomitant increase in the inorganic phosphate and calcium ion content of the particles with pH in vivo.

Phosphoprotein particles: calcium and inorganic phosphate binding structures.

Phosphoprotein particles were isolated in their native state from the physiological fluid of the estuarine clam Rangia cuneata , and the characteristics of the mineral ion-protein complex which constitutes the native particle were investigated by using mineral ion binding and mineral ion exchange techniques. The particles are aspartic acid rich, highly phosphorylated proteins containing calcium, magnesium, and inorganic phosphate ions and covalently cross-linked via histidinoalanine residues. Twenty-nine percent of the amino acid residues are phosphorylated. In their native state, the particles contain a protected pool of calcium and inorganic phosphate ions and an exchangeable pool of calcium and magnesium ions. The Ca/PO4 ratio in the protected pool is about 2.5. The number of binding sites for the protected mineral is unknown, but on the average, the native particles contain about 0.2 inorganic phosphate ion per organic phosphate residue. There is 1.0 exchangeable metal ion binding site per organic phosphate residue, and there is probably a phosphoserine residue at each site. These sites bind calcium with an apparent binding constant (KCa) of 4 X 10(3) M-1 at 50% saturation under physiological conditions, and KCa/ KMg is about 1.6. In vivo, about 85% of the exchangeable sites are occupied. The total number of calcium ion binding sites (N) in the phosphoprotein particles is related to the number of organic phosphate residues (Po) and the number of bound inorganic phosphate ions (Pi) by the equation N = Po + 2. 5Pi . The phosphoprotein particles probably serve as both the transporter and reserve source of calcium ions for shell development.

Ntau- and Npi-histidinoalanine: naturally occurring cross-linking amino acids in calcium-binding phosphoproteins.

Two isomeric amino acid cross-links, Ntau-histidinoalanine and Npi-histidinoalanine have been isolated from calcium-binding phosphoprotein particles derived from the extrapallial fluid of the estuarine clam Rangia cuneata. The cross-links were identified and compared by 13C and 1H NMR spectroscopy and mass spectroscopy. In the phosphoprotein particles, 6% of the amino acid residues are involved in cross-linkages. This is the first demonstration of the occurrence of the isomer Npi-histidinoalanine.

The Z-band lattice in a slow skeletal muscle.

Structural features of the Z-lattice were examined in cross-sections of rat soleus muscle. Optical diffraction analysis of individual myofilament bundles revealed highly ordered, tightly packed tetragonal lattice regions with abrupt shifts in orientation. Optical reconstructions of cross-sections of the Z-lattice in regions having the same axial filament spacing showed both the basket weave and the small square lattice forms, as well as intermediate structures. Analysis of these regions in three dimensions and in two-dimensional projections suggests that a change in diameter together with a change in curvature of the cross-connecting filaments can explain the different appearances of the Z-lattice at a given sarcomere length.

Aragonite twinning in the molluscan bivalve hinge ligament.

Molluscan bivalve hinge ligaments are composed of long needle-shaped aragonite crystals embedded in a protein matrix. These crystals are twinned and, in general, the twin forms a thin lamella through the center of the crystal.

The Z lattice in canine cardiac muscle.

Filtered images of mammalian cardiac Z bands were reconstructed from optical diffraction patterns from electron micrographs. Reconstructed images from longitudinal sections show connecting filaments at each 38-nm axial repeat in an array consistent with cross-sectional data. Some reconstructed images from cross sections indicate two distinctly different optical diffraction patterns, one for each of two lattice forms (basket weave and small square). Other images are more complex and exhibit composite diffraction patterns. Thus, the two lattice forms co-exist, interconvert, or represent two different aspects of the same details within the lattice. Two three-dimensional models of the Z lattice are presented. Both include the following features: a double array of axial filaments spaced at 24 nm, successive layers of tetragonally arrayed connecting filaments, projected fourfold symmetry in cross section, and layers of connecting filaments spaced at intervals of 38 nm along the myofibril axis. Projected views of the models are compared to electron micrographs and optically reconstructed images of the Z lattice in successively thicker cross sections. The entire Z band is rarely a uniform lattice regardless of plane of section or section thickness. Optical reconstructions strongly suggest two types of variation in the lattice substructure: (a) in the arrangement of connecting filaments, and (b) in the arrangement of units added side-to-side to make larger myofilament bundles and/or end-to-end to make wider Z bands. We conclude that the regular arrangement of axial and connecting filaments generates a dynamic Z lattice.

Optical diffraction of the Z lattice in canine cardiac muscle.

Optical diffraction patterns from electron micrographs of both longitudinal and cross sections of normal and anomalous canine cardiac Z bands have been compared. The data indicate that anomalous cardiac Z bands resembling nemaline rods are structurally related to Z bands in showing a repeating lattice common to both. In thin sections transverse to the myofibril axis, both electron micrographs and optical diffraction patterns of the Z structure reveal a square lattice of 24 nm. This lattice is simple at the edge of each I band and centered in the interior of the Z band, where two distinct lattice forms have been observed. In longitudinal sections, oblique filaments visible in the electron micrographs correspond to a 38-nm axial periodicity in diffraction patterns of both Z band and Z rod. We conclude that the Z rods will be useful for further analysis and reconstruction of the Z lattice by optical diffraction techniques.