Intrinsic oxygen use kinetics of transformed plant root culture.

Root meristem oxygen uptake, root tip extension rate, and specific growth rate are assessed as a function of dissolved oxygen level for three transformed root cultures. The influence of hydrodynamic boundary layer was considered for all measurements to permit correlation of oxygen-dependent kinetics with the concentration of oxygen at the surface of the root meristem. Oxygen uptake rate is shown to be saturated at ambient conditions, and a saturation level of approximately 300 micromole O2/(cm(3) tissue.hr) was observed for all three of these morphologically diverse root types. In nearly all cases, the observation of a minimum oxygen pressure, below which respiration, extension, or root growth would not occur, could be accounted for as a boundary layer mass transfer resistance. The critical oxygen pressure below which respiration declines is below saturated ambient oxygen conditions. In contrast, critical oxygen pressures for root tip extension were much higher; extension was nearly linear for the two thicker root types (Hyoscyamus muticus, henbain; Solanum tuberosum, potato) above ambient oxygen levels. The performance of the thinnest root, Brassica juncea (Indian mustard) was consistent with reduced internal limitations for oxygen transport. Extension rates did not correlate with biomass accumulation. The fastest growing henbain culture micro = 0.44 day(-)(1)) displayed the slowest extension rate (0.16 mm/hr), and the slowest growing mustard culture (micro = 0.22 day(-)(1)) had the fastest tip extension rate (0.3 mm/hr). This apparent paradox is explained in terms of root branching patterns, where the root branching ratio is shown to be dependent upon the oxygen-limited mersitem extension rate. The implications of these observations on the performance of root culture in bioreactors is discussed.

Longitudinal distribution of ozone absorption in the lung: effect of continuous inhalation exposure.

The effect of continuous exposure to ozone on the absorption of ozone in the conducting airways of human lungs was investigated with a bolus-response method. Eleven healthy nonsmoking college students (8 males, 3 females) were exposed at rest for 2 h on 3 separate days to air containing 0 ppm, 0.12 ppm, and 0.36 ppm ozone. A personal inhalation chamber equipped with a head-only clear plastic dome was used for exposure. Every 30 min a subject removed the dome and orally inhaled a series of five ozone-air boluses, each in a separate breath. Penetration of the boluses distal to the lips was targeted in the range of 70-120 ml (corresponding to the central conducting airways). By integrating the inhaled and exhaled-ozone concentration curves, we obtained the absorbed fraction (lambda) and the dispersion (sigma2) of the ozone bolus for each test breath. In addition, the subtraction of baseline measurements made just before exposure enabled us to determine the changes in absorbed fraction (deltalambda) and in dispersion (deltasigma2) that resulted from exposure alone. Absorbed fraction decreased, but sigma2 increased during O3 exposure, and the differences in deltalambda and in deltasigma2 between breathing air and exposure to either 0.12 ppm or 0.36 ppm O3 were significant. We concluded that exposure of the conducting airways to O3 reduced their capacity to absorb O3, possibly by the depletion of biochemical substrates that are normally oxidized by O3.

Longitudinal distribution of O3 absorption in the lung: gender differences and intersubject variability.

Because the National Ambient Air Quality Standard for ozone (O3) is intended to protect the most sensitive individuals in the general population, it is necessary to identify sources of intersubject variation in the exposure-dose-response cascade. We hypothesize that differences in lung anatomy can modulate exposure-dose relationships between individuals, and this results in differences between their responsiveness to O3 at a fixed exposure condition. During quiet breathing, the conducting airways remove the majority of inhaled O3, so the volume of this region should have an important impact on O3 dose distribution. Employing the bolus inhalation method, we measured the distribution of O3 absorption with respect to penetration volume (Vp), and using the Fowler single-breath N2 washout method, we determined the dead space volume (VD) in the lungs of 10 men and 10 women at a fixed respiratory flow of 250 ml/s. On average, the women absorbed O3 at smaller Vp than the men, and the women had smaller VD than the men. When expressed in terms of Vp/VD, the absorption distribution of the men and women was indistinguishable. Moreover, an interpretation of the O3 distribution in terms of an intrinsic mass transfer parameter (Ka) indicated that differences between the O3 dosimetry in all subjects, whether men or women, could be explained by a unique correlation with anatomic dead space: Ka (in s-1) = 610 VD-105 (in ml). Application of this result to measurements of O3 exposure response indicated that previously reported gender differences may be due to a failure in properly accounting for tissue surface within the conducting airways.

A portable inhalation system for personal exposure to ozone.

A low-cost portable inhalation system was developed for exposing an individual subject to 60-600 parts per billion of ozone in a 30-l clear-plastic head dome. The inhalation system had the following novel features: a canister vacuum cleaner that supplied room air without the need for precleaning or humidification; a 7% oxygen-in-nitrogen feed to a commercial ultraviolet ozonator that avoided an excess production of ozone; a compact inline mixer that assured homogeneous mixing of the 200-300 liters per minute room air supply with the 0.5-1.0 liters per minute of ozonated gas flow, positioning of gas inlet and exhaust hoses on the head dome that provided fresh gas delivery in the vicinity of the mouth; a quick-disconnect neck seal that allowed rapid donning of the head dome by the subject, and mounting of most system components on a small mobile cart. Temperature, humidity, and ozone and carbon dioxide concentrations were measured inside the dome while a subject exercised on a bicycle ergometer. An air flow of 200 liters per minute between rest and light exercise created a suitable microenvironment in the dome. During moderate and heavy exercise, however, a higher flow of 300 liters per minute should be used to suppress the build-up of carbon dioxide and humidity.