Observation of the Oxygen Diffusion Barrier in Soybean (Glycine max) Nodules with Magnetic Resonance Microscopy.

The effects of selected gas perfusion treatments on the spinlattice relaxation times (T(1)) of the soybean (Glycine max) nodule cortex and inner nodule tissue were studied with (1)H high resolution magnetic resonance microscopy. Three gas treatments were used: (a) perfusion with O(2) followed by N(2); (b) O(2) followed by O(2); and (c) air followed by N(2). Soybean plants with intact attached nodules were placed into the bore of a superconducting magnet and a selected root with nodules was perfused with the gas of interest. Magnetic resonance images were acquired with repetition times from 50 to 3200 ms. The method of partial saturation was used to calculate T(1) times on selected regions of the image. Calculated images based on T(1) showed longer T(1) values in the cortex than in the inner nodule during all of the gas perfusions. When nodules were perfused with O(2)-O(2), there was no significant change in the T(1) of the nodule between the two gas treatments. When the nodule was perfused with O(2)-N(2) or air-N(2), however, the T(1) of both the cortex and inner nodule increased. In these experiments, the increase in T(1) of the cortex was 2- to 3-fold greater than the increase observed in the inner nodule. A similar change in T(1) was found in detached live nodules, but there was no change in T(1) with selective gas perfusion of detached dead nodules. These observations suggest that cortical cells respond differently to selected gas perfusion than the inner nodule, with the boundary of T(1) change sharply delineated at the interface of the inner nodule and the inner cortex.