Oxygen Diffusion in Pea

Abstract

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.

The Anatomical Characteristics of Roots and Plant Response to Soil Flooding

“…The accumulation of diffusive resistance during root extension results in a decline in apical oxygen concentrations in the NWL species, pea (Armstrong, Healy & Webb, 1982;Armstrong, Healy & Lythe, 1983). Again, on a small sample of species it has been shown that rooting depth and FRP may be related (Yu, Stolzy & Letey, 1969).…”

Oxygen Diffusion in Pea Ii. Oxygen Concentrations in the Primary Pea Root Apex as Affected by Growth, the Production of Laterals and Radial Oxygen Loss

“…In Figure 6 these results have been plotted against root length. Also presented in the same figure for comparison is the relationship between pore-space resistance and length (Armstrong et al, 1982). The magnifying effect that respiration and leakage of primary root and laterals have on the diffusive resistance as observed from the electrode position is clearly Primory root length (cm) Fig.…”

“…Meteor) were reared under sterile conditions on a nutrient 1 % agar medium in 250 ml measuring cylinders. Details of sterilization, and transplantation have been described elsewhere (Healy, 1975;Armstrong, Healy and Webb, 1982). Plants were removed for an assessment of their ventilating powers as required by extracting agar and plant en bloc.…”

“…The effective diffusivity Dg is a function of the fractional porosity (e) and tortuosity of the diffusion path (T) and is related to D^, the diffusivity of oxygen in air, by the expression Dg = Dg er. Applying this to hypothetical pea roots having a respiratory demand M of 1 X 10"'' g cm~^ s~^ and in which e = 0-038 and T = 1 (Armstrong et al, 1982), growth would influence the apical oxygen concentration in the manner shown in Figure 1. The experimental results presented below (Figs 2 and 3) affirm a decline in apical oxygen concentration during the early stages of growth but they show significant departures from the simple model.…”

“…The lack of convexity in the initial decline might in part be due to the pore-space resistance characteristics of the pea root for, when pea roots are grown in agar, pore-space resistance does not increase in the linear fashion implicit in Eqn 4 and, in the early stages of growth, the increase in resistance is only slight (see Fig. 6 and Armstrong et al, 1982). It must also be remarked that if leakage had not been minimized to the degree anticipated this also could have a modifying infiuence which would accord with the nature, if not the scale, of the observations.…”