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In O(2)-free media, as in wetlands, the COPR for roots is likely to be quite low, dependent upon the respiratory demands, dimensions and diffusion characteristics of the stele/stelar meristem and the enzyme kinetics of cytochrome oxidase. Roots of non-wetland plants may not differ greatly in their COPRs from those of wetland species. There is a possibility that trace amounts of O(2) may still be present in stelar 'anaerobic' cores where fermentation is induced at low cortical OPPs.

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SUMMARYFor roots growing in anoxic media the intercellular pore-space of the root cortex provides a channel for the diffusive flow of respiratory gases between shooot and root. Three methods were used in an attempt to quantify the diffusive resistance of this pore space (Rp) in pea. Two of the methods were based on polarographic measurements of oxygen flow through the roots: the 'cooling' and 'gas-mixture' methods; the third was based upon measurements of porosity and root geometry ('shape'). The last gave results which agreed closely with those obtained using gas-mixtures. The relationship between i?p and root length followed a curvilinear form in which there was little change in R^ in the earlier stages of growth (3 to 65 cm). The cooling method also produced a curvilinear pattern, but numerically the resistances were considerably higher. Also, resistance rose significantly in the shorter roots. It was found that respiratory activity in the roots at 3 °C could account for these discrepancies; an electrical analogue of the root programmed for resistance (gas-mixture/root geometry values) and 3 °C respiratory activity, gave results which matched those obtained by the cooling method. It was concluded that the gas-filled pore space in the pea root forms a relatively non-tortuous diffusion path and that resistance is very strongly influenced by root shape. As roots extend beyond a length of 7 cm there is an accelerated rise in Rp. An original intention, that of obtaining a measure of diffusive resistance across the root wall, was thwarted by the variability in the data.

Polarography, using cylindrical platinum electrodes, proved suitable for measuring changes in the internal apical 02 concentration of the primary root of pea (Pisum sativum L. cv Meteor) effected by KCN and/or salicylhydroxamic acid (SHAM) in the bathing medium. An electrical rootaeration analog was used to help evaluate some of the results. Concentrations of KCN -0.05 millimolar had no significant effect. In response to 0.1 millimolar KCN, the 02 concentration rose substantially for approximately 2 hours, then declined, and after 10 hours had frequently fallen below the pretreatment level. Such changes suggest an initial inhibition of cytochrome oxidase-mediated 02 uptake followed by an induction of the alternative, cyanide-resistant respiratory pathway. These treatments proved nonlethal. Changes in 02 concentration similar to those described for 0.1 millimolar KCN were observed in response to 1 and 10 millimolar KCN but these treatments were lethal and the root apex became soft and often appeared flooded. Roots survived and showed no signifiant responses when treated with SHAM at concentrations -5 millimolar. However, when the alternative pathway had been (apparently) induced by 0.1 millimolar KCN, the addition of 5 mIllimolar SHAM to the bathing medium caused a substantial and persistent rise in the root apical 02 concentration, suggesting that this (nonlethal) concentration of SHAM could indeed inhibit 02 uptake via the cyanide-resistant pathway.It is concluded that while 02 uptake normally occurs by the cytochrome pathway in the primary pea root, the alternative, cyanide-resistant pathway can be induced by 0.1 millimolar KCN.Cyanide is a known inhibitor of Cyt oxidase yet the capacity to respire in its presence appears to be widespread throughout the plant kingdom (3, 5). There is now general agreement that cyanide-insensitive respiration is the result of an electron flow which bypasses the classical, Cyt pathway at the position of ubiquinone (20,21), 02 being ultimately reduced by an 'alternative' oxidase. Although the nature of the alternative oxidase has yet to be identified, it is known that electron flow to this terminal oxidase is insensitive to cyanide but sensitive to the hydroxamic acid group of compounds, e.g. SHAM2 (16).Despite the apparently widespread distribution of cyanide-insensitive respiration, its physiological significance remains uncertain. Several areas in which such respiration might be of importance have been reviewed by Lambers (7) and these include roles in heat production, fruit ripening, respiration ofplants that contain high levels ofcyanogenic glycosides producing HCN upon wounding, oxidation of NADH, and ion transport and osmoregulation.'To whom reprint requests should be sent. ' Abbreviations and trivial name: SHAM, salicylhydroxamic acid; ROL, radial oxygen loss; disulfiram, tetraethylthiuram disulfide.Also, Lambers suggests that cyanide-insensitive respiration might play an important role (at least in roots) in the oxidation of sugars that have been translocated to...