Bibliothek

Notizen: The effects on magnetic-field-line structure of adding various static transverse magnetic fields to a Solov'ev-equilibrium field-reversed configuration are examined. It is shown that adding fields that are antisymmetric about the axial midplane maintains the closed field-line structure, while adding fields with planar or helical symmetry opens the field structure. Antisymmetric modes also introduce pronounced shear. © 2000 American Institute of Physics.

Notizen: We would like to respond to recent comments (Reynolds. Anaesthesia 2001: 56: 1129) on our epidural technique. In our practice we apply loss of resistance to air technique, which helps identify a nick in the dura with a small leak of CSF. On many occasions, only a few drops of CSF and not a constant leak helped identify dural puncture, which could have been missed with the use of saline. Upon inserting the epidural needle, we remove the stylet when the needle is engaged in the interspinous ligament or the ligamentum flavum. Very often, it is difficult for our residents to advance the epidural needle with one hand and apply constant pressure on the plunger with the other hand. Excessive pressure with one hand, by a resident, had caused dural puncture when the needle was pushed in too far. By reinserting the stylet with each advancement of the needle, we remove tissues that may enter and occlude the epidural needle. It is quite possible that constant plunger pressure with saline may also avert this problem.

Notizen: The effects of soil moisture distribution on water uptake of drip-irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady-state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.

Notizen: This study investigated the effect of the dwarfing M9, semi-dwarfing MM106 and local Hashabi rootstocks on the water use, canopy conductance (Gc) and hydraulic conductance (k) of apple orchards with the same scion, Golden Delicious. The average summer leaf area index (LAI) was 2·4, 2·7 and 1·7 for M9, MM106 and Hashabi, respectively. Irrigation in 1997 was less than water use until June, and excessive afterwards. In 1998, irrigation was doubled, and was excessive throughout the season. Sap flow (J) in June–August 1998 totalled 476, 682 and 606 mm (or 0·60, 0·86 and 0·76 of class A pan evaporation) for M9, MM106 and Hashabi, respectively. Maximum sap velocity in the three rootstocks (approximately 70 cm h−1) occurred in the outer 30–60% of the stem, and its decrease with depth was greater in M9 than in the other rootstocks. Midday Gc during both summers was least for M9, intermediate for Hashabi and greatest for MM106. The k value of M9 and MM106 for the soil to stem, stem to leaves and soil to leaves pathways were determined from daily courses of water potential of leaves, Ψl, stem, Ψstem and J. Specific k (ks, i.e. relative to stem sapwood area) did not significantly differ between the two rootstocks for soil to stem and soil to leaf pathways, but leaf specific k (kl) was greater for MM106 soil to stem (71% greater) and soil to leaf (63%) pathways, respectively. The inverse slopes of the relationships between midday canopy resistance (Rc) and vapour pressure deficit (D) for MM106 was 1·75 of that for M9, and the ratio of their Huber values, i.e. the ratio of sapwood to leaf area, was 1·6. These findings indicate that differences in water use are attributable to differences in kl, and not to differences in wood properties (ks). Application of a model relating Rc to orchard area specific k (kg) showed that the slope of the relationships between midday Rc and D for the 1998 data could be predicted using common values of ks (0·134 kg m−2 s−1 MPa−1) and midday Ψl (−1·34 MPa) for the three rootstocks. The implications of these findings, and the similarities in the differences between rootstocks of Gc, kl, kg and Huber values, are discussed with respect to rootstock water use and irrigation.