Plant physiology: Needles point to pollution

Plant Cell Environ. 26, 1929–1939 (2003)

The iconic shape of conifers — a single vertical main trunk with minor side branches — is distorted by exposure to air pollution. According to M. D. Collier and colleagues, the hormonal changes involved could provide an early warning of nitrogenous pollutants.

Conifer growth is an example of 'apical dominance', where a plant's growing tip prevents the development of major side branches. Gardeners make trees and shrubs more bushy by suppressing apical dominance through pruning; plants achieve the same effect themselves by using hormones called cytokinins.

Cytokinins have previously been linked to loss of apical dominance in spruce trees growing in polluted areas. To investigate this further, Collier et al. sprayed plantations of Sitka spruce with combinations of ammonium nitrate and sodium sulphate. Although the three-year study period was too short to show changes in growth patterns, the cytokinin content of the trees' needles was substantially increased after exposure to the nitrogenous pollutant.

These increases were masked by simultaneous spraying with acid sulphur. But this may not bar cytokinin from becoming a biological indicator of nitrogen pollution. In Europe at least, industrial sulphur emissions are dropping, whereas nitrogenous pollution is increasing. Such trends will need careful monitoring if the Christmas tree is not to become the Christmas bush.

Christopher Surridge

Neurobiology: Amnesia in ageing flies

Neuron 40, 1003–1011 (2003)

Like humans, fruitflies gradually lose their memory as they age. Takuya Tamura and colleagues show that ageing specifically impairs the fly's middle-term memory — a phase of memory formation that depends on the amnesiac gene.

Flies can learn to avoid odours when specific fragrances are coupled with electric shocks. Their memories can be separated into three distinct phases: short-term memory is present immediately after training, and fades away within 2 hours; middle-term memory lasts more than 5 hours; and an 'anaesthesia-resistant' memory lasts more than 24 hours. These three phases seem to form independently to some extent, as mutations in 'memory' genes affect each phase differently. Mutations in amnesiac, for instance, disrupt middle-term memory only.

Tamura and colleagues find that the memory defects of older flies are identical to those of flies with amnesiac mutations. Also, the mutant flies do not show any further memory decline as they age. Restoring amnesiac expression in the mutants brings back memories of smells and shocks — but also reinstates age-dependent memory decline.

Yet expression of amnesiac is not altered in normal old flies, nor can their memory be repaired by more copies of the gene. The authors propose that although age-related memory decline may result from disruption of the amnesiac pathway, this gene itself seems unaffected.

Marie-Thérèse Heemels

Nonlinear dynamics: Chaos relatively robust

Phys. Rev. Lett. 91, 231101 (2003)

The behaviour of chaotic systems is acutely sensitive to their initial conditions and to the way the trajectories of their components evolve in time. So it wouldn't seem surprising if a change in coordinate systems — the kind of transformation invoked in general relativity — were to perturb this behaviour. On the contrary, Adilson E. Motter finds that chaos seems in general to be indifferent to changes in coordinates. It is, in other words, relativistically invariant.

Chaos can be quantified by the Lyapunov exponent, a measure of how quickly small initial differences diverge as two near-equivalent systems evolve. A positive Lyapunov exponent is a signature of chaos. Motter finds that a change in time coordinates may alter the absolute value of Lyapunov exponents but does not alter their sign: chaos cannot be introduced or conjured away by a relativistic reparametrization of time.

This conflicts with an earlier suggestion that at least one model of quantum cosmology (an attempt to match quantum theory with relativity) can have either a positive or zero Lyapunov exponent, depending on the choice of coordinates. That outcome, says Motter, arises from an improper definition of the Lyapunov exponent.

Philip Ball

Meteors: Parallel lines

Credit: AGU

Geophys. Res. Lett. doi:10.1029/2003GL018312 (2003)

Meteors passing through the upper atmosphere can leave behind them glowing trails, like the contrails of aircraft. But this signature is sometimes written with a double pen: a single meteor leaves two parallel trails, which are subsequently twisted and distorted by winds. What causes these 'double trains'?

A long-standing idea is that double trains — recognized in meteor showers such as the Leonids since at least the beginning of last century — are the two sides of a hollow 'tube' ploughed out by the meteor, the darker interior of which is created by chemical depletion of the light-emitting species. But the brightness contrast is too big for this to be tenable.

Perhaps the meteor fragments into two? But if so, why never into three or four? M. C. Kelley et al. argue that, instead, the shape and dynamics of double trains are consistent with the idea that one train is due to excited gas from the meteor, and the other, lying below the first, comes from dust sedimenting under gravity.

Philip Ball