In the current density functional theory of linear and nonlinear time-dependent phenomena, the treatment of exchange and correlation beyond the level of the adiabatic local density approximation is shown to lead to the appearance of viscoelastic stresses in the electron fluid. Complex and frequency-dependent viscosity/elasticity coefficients are microscopically derived and expressed in terms of properties of the homogeneous electron gas. As a first consequence of this formalism, we provide an explicit formula for the linewidths of collective excitations in electronic systems. [S0031-9007 (97) Time-dependent density functional theory (TDFT) [1] is frequently invoked as a tool for studying the dynamics of many-particle systems. This theory maps the difficult problem of interacting electrons in a time-dependent external potential V ͑ r, t͒ to the simpler one of noninteracting electrons in an effective time-dependent potential V eff ͑ r, t͒ V ͑ r, t͒ 1 y H ͑ r, t͒ 1 y xc ͑ r, t͒ [where y H is the Hartree potential, and y xc is the exchange-correlation (xc) potential] yielding the same density n͑ r, t͒. In order to obtain a practical computational scheme, the xc potential is usually approximated as a function of the instantaneous local density,

The zero-temperature response of an interacting electron liquid to a time-dependent vector potential of wave vector q and frequency , such that qӶq F , qv F Ӷ ӶE F /ប ͑where q F , v F , and E F are the Fermi wave vector, velocity, and energy, respectively͒, is equivalent to that of a continuous elastic medium with nonvanishing shear modulus , bulk modulus K, and viscosity coefficients and . We establish the relationship between the viscoelastic coefficients and the long-wavelength limit of the ''dynamical local-field factors'' G L(T) (q, ), which are widely used to describe exchange-correlation effects in electron liquids. We present several exact results for , including its expression in terms of Landau parameters, and practical approximate formulas for , , and as functions of density. These are used to discuss the possibility of a transverse collective mode in the electron liquid at sufficiently low density. Finally, we consider impurity scattering and/or quasiparticle collisions at nonzero temperature. Treating these effects in the relaxation-time ( ) approximation, explicit expressions are derived for and as functions of frequency. These formulas exhibit a crossover from the collisional regime ( Ӷ1), where ϳ0 and ϳnE F , to the collisionless regime ( ӷ1), where ϳnE F and ϳ0. ͓S0163-1829͑99͒02632-6͔

Comparative analysis of a number of studies in drought-stressed maize (Zea mays L.) reporting quantitative trait loci (QTLs) for abscisic acid concentration, root characteristics, other morpho-physiological traits (MPTs) and grain yield (GY) reveals their complex genetic basis and the influence of the genetic background and the environment on QTL effects. Chromosome regions (e.g. near umc11 on chromosome 1 and near csu133 on chromosome 2) with QTLs controlling a number of MPTs and GY across populations and conditions of different water supply have been identified. Examples are presented on the use of QTL information to elucidate the genetic and physiological bases of the association among MPTs and GY. The QTL approach allows us to develop hypotheses accounting for these associations which can be further tested by developing near isogenic lines (NILs) differing for the QTL alleles. NILs also allow for a more accurate assessment of the breeding value of MPTs and, in some cases, may allow for the map-based cloning of the gene(s) underlying the QTL. Although QTL analysis is still time-consuming and resource-demanding, its integration with genomics and post-genomics approaches (e.g. transcriptome, proteome and metabolome analyses) will play an increasingly important role for the identification and validation of candidate genes affecting MPTs and GY.

§ All authors contributed equally to this work.Martensitic transformations are diffusionless solid-to-solid phase transitions characterized by a rapid change of crystal structure, observed in metals, alloys, ceramics, and proteins 1,2 . Phenomenologically, they come in two widely different classes. In steels, quenching generates a microstructure which remains essentially unchanged upon subsequent loading or heating; the transformation is not reversible 1 . In shape-memory alloys on the other hand the microstructures formed on cooling are easily manipulated by loads and disappear upon reheating, and the transformation is reversible 3,4 . Here we explain these sharp differences on the basis of the change in crystal symmetry during the transition. In particular, we show that martensitic transformations fall into two categories. In one case the energy barrier to plastic deformation (via lattice-invariant shears, as in twinning or slip) is no higher than the barrier to the phase change itself. These transformations are therefore irreversible, as observed in steels. In the other case, the energy barrier to lattice-invariant shears can be much higher than that pertaining to the phase change. Consequently, transformations of this type can occur with virtually no plasticity and can be reversible, as for shape-memory alloys.Martensitic transformations are at the basis of numerous technological applications. Most notable amongst these is in steel, where the transformation induced by quenching (fast cooling) is exploited for enhancing the alloy's strength 1 . Another is the fascinating shape-memory effect in alloys like Nitinol, used in medical and engineering devices 3 . Martensitic phase changes are also exploited to toughen structural ceramics 5 such as zirconia, and observed in biological systems such as the tail sheath of the T4 bacteriophage virus 6 . Ideas originating from the study of these transformations have led to improved materials for actuation (ferromagnetic shape-memory alloys 7,8 and ferroelectrics 9 ) and to candidates for artificial muscles 10 . Finally, the rich microstructure (distinctive patterns developed at scales ranging from a few nanometers to a few microns) that accompanies these transformations, has made this a valuable theoretical sand-box for the development of multi-scale modeling tools 11 .