Stress correlations in granular materials: an entropic formulation.

We study the response of dry granular materials to external stress using experiment, simulation, and theory. We derive a Ginzburg-Landau functional that enforces mechanical stability and positivity of contact forces. In this framework, the elastic moduli depend only on the applied stress. A combination of this feature and the positivity constraint leads to stress correlations whose shape and magnitude are extremely sensitive to the nature of the applied stress. The predictions from the theory describe the stress correlations for both simulations and experiments semiquantitatively.

Why do granular materials stiffen with shear rate? Test of novel stress-based statistics.

Recent experiments exhibit a rate dependence for granular shear such that the stress grows linearly in the logarithm of the shear rate, gamma. Assuming a generalized activated process mechanism, we show that these observations are consistent with a recent proposal for a stress-based statistical ensemble. By contrast, predictions for rate dependence using conventional energy-based statistical mechanics to describe activated processes, predicts a rate dependence of (ln(gamma))(1/2).

A small decrease of plastid transketolase activity in antisense tobacco transformants has dramatic effects on photosynthesis and phenylpropanoid metabolism.

Transketolase (TK) catalyzes reactions in the Calvin cycle and the oxidative pentose phosphate pathway (OPPP) and produces erythrose-4-phosphate, which is a precursor for the shikimate pathway leading to phenylpropanoid metabolism. To investigate the consequences of decreased TK expression for primary and secondary metabolism, we transformed tobacco with a construct containing an antisense TK sequence. The results were as follows: (1) a 20 to 40% reduction of TK activity inhibited ribulose-1,5-bisphosphate regeneration and photosynthesis. The inhibition of photosynthesis became greater as irradiance increased across the range experienced in growth conditions (170 to 700 micromol m(-2) sec(-1)). TK almost completely limited the maximum rate of photosynthesis in saturating light and saturating CO(2). (2) Decreased expression of TK led to a preferential decrease of sugars, whereas starch remained high until photosynthesis was strongly inhibited. One of the substrates of TK (fructose-6-phosphate) is the starting point for starch synthesis, and one of the products (erythrose-4-phosphate) inhibits phosphoglucose isomerase, which catalyzes the first reaction leading to starch. (3) A 20 to 50% decrease of TK activity led to decreased levels of aromatic amino acids and decreased levels of the intermediates (caffeic acid and hydroxycinnamic acids) and products (chlorogenic acid, tocopherol, and lignin) of phenylpropanoid metabolism. (4) There was local loss of chlorophyll and carotene on the midrib when TK activity was inhibited by >50%, spreading onto minor veins and lamina in severely affected transformants. (5) OPPP activity was not strongly inhibited by decreased TK activity. These results identify TK activity as an important determinant of photosynthetic and phenylpropanoid metabolism and show that the provision of precursors by primary metabolism colimits flux into the shikimate pathway and phenylpropanoid metabolism.

Acclimation of Arabidopsis leaves developing at low temperatures. Increasing cytoplasmic volume accompanies increased activities of enzymes in the Calvin cycle and in the sucrose-biosynthesis pathway.

Photosynthetic and metabolic acclimation to low growth temperatures were studied in Arabidopsis (Heynh.). Plants were grown at 23 degrees C and then shifted to 5 degrees C. We compared the leaves shifted to 5 degrees C for 10 d and the new leaves developed at 5 degrees C with the control leaves on plants that had been left at 23 degrees C. Leaf development at 5 degrees C resulted in the recovery of photosynthesis to rates comparable with those achieved by control leaves at 23 degrees C. There was a shift in the partitioning of carbon from starch and toward sucrose (Suc) in leaves that developed at 5 degrees C. The recovery of photosynthetic capacity and the redirection of carbon to Suc in these leaves were associated with coordinated increases in the activity of several Calvin-cycle enzymes, even larger increases in the activity of key enzymes for Suc biosynthesis, and an increase in the phosphate available for metabolism. Development of leaves at 5 degrees C also led to an increase in cytoplasmic volume and a decrease in vacuolar volume, which may provide an important mechanism for increasing the enzymes and metabolites in cold-acclimated leaves. Understanding the mechanisms underlying such structural changes during leaf development in the cold could result in novel approaches to increasing plant yield.

Human Eye Movements During Visually Guided Stepping.

Visually guided locomotion was studied in an experiment in which human subjects (N = 8) had to accurately negotiate a series of irregularly spaced stepping-stones while infrared reflectometry and electrooculography were used to continuously record their eye movements. On average, 68% of saccades made toward the next target of footfall had been completed (visual target capture had occurred) while the foot to be positioned was still on the ground; the remainder were completed in the first 300 ms of the swing phase. The subjects' gaze remained fixed on a target, on average, until 51 ms after making contact with it, with little variation. A greater amount of variation was seen in the timing of trailing footlift relative to visual target capture. Assuming that subjects sampled the visual cues as and when they were required, visual information appeared most useful when the foot to be positioned was still on the ground.