The water permeability of, and the transport of tritiated water and ³⁵S-labelled sulfate through mycorrhizal and nonmycorrhizal root systems of Monterey pine (Pinus radiata)

Abstract

Each 6-8 month old seedling was detopped, the root system enclosed in a chamber holding labelled (THO, THO and ³⁵S-labelled sulfate) medium, 1.5 bars pressure applied to the medium, and root system exudates collected for up to 116 min., on a series of filter paper discs.

Exudate flux rates were generally increasing within about the first 10 min. of collection, then levelled off or dropped slightly. Average flux rates over the interval 12th-48th min. of exudate collection were taken as a basis for comparison. Mean values were; mycorrhizal, 20.62, and nonmycorrhizal, 15.98 mg H₂O/g dry root/min/1.5 bars, the difference not statistically significant.

Root system permeabilities, computed from the average flux rates over the 12th-48th min. interval, were highly variable, Coefficient of Variation = 75.94%. There was a highly significant inverse correlation (r = -.5859) between root system permeability and dry weight. Mycorrhizal root systems tended to be more permeable; mean values were: mycorrhizal, .822, and nonmycorrhizal, .638; after adjusting for root dry weight by covariance analysis, mycorrhizal, .842, and nonmychorrhizal, .604 g H₂O/g dry root/hour/bar.

The increase with time of THO-C/Co (C/Co is the ratio of radioisotope concentration in the exudate over that in the root medium) was slow; linear in the first hour, then tapering off. THO-C/Co was still increasing at the latest exudate collection. Mean THO-C/Co at the end of the first hour was 0.38, at the 116th min., 0.50. The increase in THO-C/Co with time was probably controlled by exchange and not exudate flux rate.

There was a significant inverse correlation (r = -.4674) between time of arrival of THO at the cut end and root system permeability. In the absence however, of a compact radioisotope front, THO appearance at the cut end was not adopted as a reliable indicator of permeability. The presence of mycorrhizas did not substantially alter THO transport into the root system exudates.

The increase with time of 35S-C/Co was initially more rapid; the linear increase then tapering off were more rapid and before the end of the 116 min. exudate collection period, ³⁵S-C/Co had levelled off. Appearance of ³⁵S at the cut end and the rate of its transport into the root system exudates did not relate with permeability. ³⁵S- C/ Co in the exudates from mycorrhizal root systems levelled off sooner and at a lower level. Mean plateau ³⁵S-C/Co values were: mycorrhizal, .38, and nonmycorrhizal, .55, the difference statistically significant.

Supplementary experiments with killed (by boiling) mycorrhizal root systems confirmed that dead root systems are more permeable. THO-C/Co in the exudates from dead root systems levelled off at about 0.3, before the end of the 116 min. exudate collection period.