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
Authors: ARMSTRONG W. , HEALY M. T. , LYTHE S. 
Published: May 2006

Summary

The influence of primary and lateral root respiration and radial oxygen leakage, upon oxygen concentrations in the primary pea root apex have been identified and in varying degrees quantified. The experiments were conducted on intact plants with the roots bathed in anoxic media and apical oxygen concentrations were deduced from measurements of radial oxygen loss made using cylindrical platinum electrodes.

The plants were from 2 to 10 days old, primary root lengths ranging from 3 to 13 cm; the mean numbers and total lengths of laterals ranged from 0 to 26 and 0 to 140 cm respectively. The results clearly show that the apical oxygen concentration in the root declines with growth if respiratory demand for oxygen can be met only by internal gas‐phase diffusion from the shoot. However the decline in the concentration departs markedly from that predicted for roots of uniform radius porosity and respiratory demand. There are strong grounds for believing that declining respiratory activity in sub‐apical parts of the primary root not only arrests the decline in apical oxygen concentration but can also reverse it. In primary roots, which were lacking laterals, and jacketed in solid agar to minimize lateral leakage of oxygen to a freshly anoxic bathing medium, the apical concentration declined from ca. 8 % in roots 3 cm in length to ca. 2.5 % when roots had grown to a length of 6.5 cm. Further growth was accompanied by a slight increase in concentration rather than the further decline which might have been anticipated. In roots from which laterals had not been excised the apical oxygen concentration had fallen to 1.9% in 6.5 cm roots and reached its lowest point, 0.3 %, in roots 9.5 cm in length. It was predicted that without the sink effect of the electrode sensor the highest concentration (l= 3 cm) would have been ≥ 9 % oxygen and the lowest (l= 95 cm) ca, 0.5 % oxygen. The influence of lateral roots was apparently related more to their number than to their total biomass.

In other batches of roots the agar jacket was removed to encourage the radial diffusive leakage of oxygen from sub‐apical parts to the freshly anoxic but static bathing medium. Leakage had a significant effect on the oxygen regime in the primary root apex and this was at its most striking in roots bearing laterals: so far as could be ascertained these roots became anoxic at a length of between 7 and 8 cm. It was pointed out that the oxygen sink activities of flooded soil would probably induced anoxia in even shorter pea roots. Unjacketed roots from which laterals had been excised had apical concentrations ca. 7.5 % (l= 3 cm) and this had declined to ca. 2 % in roots ranging from 7 to 11 cm in length.

Effective diffusive resistances for the roots were also calculated. The changes were noted by plotting against root length and were compared with the changes in pore space resistance previously measured. Resistances were many times greater than the pore space resistance and became very large indeed under the influence of lateral root respiration and leakage.