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D F Parkhurst

Publications and source records attributed to D F Parkhurst.

5 recordsLinked to original sources
PubMed1990

Intercellular Diffusion Limits to CO(2) Uptake in Leaves : Studies in Air and Helox.

We studied plants of five species with hypostomatous leaves, and six with amphistomatous leaves, to determine the extent to which gaseous diffusion of CO(2) among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO(2) and water vapor are 2.3 times higher than in air. For fixed estimated CO(2) pressure at the evaporating surfaces of the leaf (p(i)), assimilation rates in helox ranged up to 27% higher than in air for the hypostomatous leaves, and up to 7% higher in the amphistomatous ones. Thus, intercellular diffusion must be considered as one of the processes limiting photosynthesis, especially for hypostomatous leaves. A corollary is that CO(2) pressure should not be treated as uniform through the mesophyll in many leaves. To analyze our helox data, we had to reformulate the usual gas-exchange equation used to estimate CO(2) pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO(2) and H(2)O are known. Finally, we describe a diffusion-biochemistry model for CO(2) assimilation that demonstrates the plausibility of our experimental results.

Journal Article↗
PubMed1988

Gradients of Intercellular CO(2) Levels Across the Leaf Mesophyll.

Most current photosynthesis models, and interpretations of many wholeleaf CO(2) gas exchange measurements, are based on the often unstated assumption that the partial pressure of CO(2) is nearly uniform throughout the airspaces of the leaf mesophyll. Here we present measurements of CO(2) gradients across amphistomatous leaves allowed to assimilate CO(2) through only one surface, thus simulating hypostomatous leaves. We studied five species: Eucalyptus pauciflora Sieb. ex Spreng., Brassica chinensis L., Gossypium hirsutum L., Phaseolus vulgaris L., and Spinacia oleracea L. For Eucalyptus, maximum CO(2) pressure differences across the leaf mesophyll were 73 and 160 microbar when the pressures outside the lower leaf surface were 310 and 590 microbar, respectively. Using an approximate theoretical calculation, we infer that if the CO(2) had been supplied equally at both surfaces then the respective mean intercellular CO(2) pressures would have been roughly 12 and 27 microbar less than the pressures in the substomatal cavities in these cases. For ambient CO(2) pressures near 320 microbar, the average and minimum pressure differences across the mesophyll were 45 and 13 microbar. The corresponding mean intercellular CO(2) pressures would then be roughly 8 and 2 microbar less than those in the substomatal cavities. Pressure differences were generally smaller for the four agricultural species than for Eucalyptus, but they were nevertheless larger than previously reported values.

Journal Article↗
PubMed1986

Critical radius of endotherms.

The critical radius effect for insulation, well known in the engineering literature, was used by other authors to explain the lack of insulation on newborn endotherms. If that effect existed in small animals, they would lose less heat if nude than if fur or feathers were present. We show 1) that the previous analysis, although incomplete, yields the same result as a solid insulation model with the required sophistication and 2) that a proper model of fur is a porous media model. Neither of two porous media versions yield a critical radius effect. No critical radius effect occurs because simultaneous heat transfer by conduction and radiation makes it impossible to obtain the required logarithmic increase in thermal resistance with increasing insulation radius in a porous medium.

Animals↗