Richard D. Spence

Ozone alters carbon allocation in loblolly pine: assessment with carbon-11 labeling.

Loblolly pine (Pinus taeda L.) seedlings were exposed to 0.120 micromol mol(-1) (ppm) ozone for 7 h per day, 5 days per week for 12 weeks. No visible damage resulted from this regime. A short-lived radioisotope of carbon ((11)C) was used to characterize changes in plant physiology caused by ozone, the first time this technique has been used for ozone exposure studies. In comparison to plants kept in charcoal-filtered air, pines exposed to ozone exhibited reductions in photosynthesis (16%), speed of phloem transport (11%), phloem photosynthate concentration (40%) and total carbon transport toward roots (45%). Photosynthate not transported to the roots appeared to accumulate in the stems. Primary branches of pines exposed to ozone were some 50-60% heavier than those of control pines. Ozone was thus shown to have a significant short-term impact on phloem transport processes that results in a shift in allocation of photosynthate favoring stems.

Plant cell wall elasticity. II: Polymer elastic properties of the microfibrils

Plant cell walls are composed essentially of two polymer constituents, a network of highly structured microfibrils embedded in a relatively unstructured matrix. The first polymer analysis by Wu et al. (1985; Plant, Cell Environ. 8, 563) was adequate to describe elastic properties of the matrix only, but further analysis is required to describe the contribution of the microfibrils in maintaining cell wall integrity, restraining cell expansion, and regulating the direction of expansion. In this further analysis of plant cell wall elasticity, as the walls stretch, the microfibrils come under stress and resist further cell expansion. The structural and mechanical integrity of the cell wall is maintained despite increasing internal cell pressure. A test of the nonlinear pressure-volume relationship makes use of previously published plant-water relations data depicted on Hofler diagrams.

An integrated simulation model for a semi-arid agroecosystem in the loess plateau of northwestern China

Abstract We apply a resource integration approach to address increasing crop and forage production in a semi-arid agroecosystem in northwestern China. A holistic modeling approach was employed to study improvement in agricultural practices with respect to ecosystem conservation. Using a proposed rotational scheme for winter wheat and alfalfa production, portions of arable land are allocated for raising animal forage in order to improve soil productivity. Animal manures are returned to the soil to stabilize the structure further and to maintain fertility. Regional climate and topography are included in the simulation model. A series of 40-yr simulation runs for ravine tablelands and hilly topography were conducted to investigate model behavior. Long-term application of nitrogen fertilizer as an alternative management practice was evaluated using the model, but detrimental effects on primary and secondary production from such a practice were predicted by simulation results. The impact of population pressure on agricultural systems was also considered.