Proton density and apoplastic domains within soybean nodules in relation to the oxygen diffusion barrier

Abstract

A spin-echo pulse sequence was used to obtain 1H nuclear magnetic resonance microimages of soybean nodules, with resolution to 40 mm and 3D presentation. At an acquisition time (TR) of c. 270 ms and a short echo time (TE) of 8.3 ms, a high proton intensity was detected in infected tissue and vascular strands, relative to the cortex. A longer TE of 15-5 ms was associated with a relatively low proton intensity in infected tissue. Thus protons in infected tissue were characterized by a short T2. Following detopping of the plant, or treatment of the root system with Ar:O2, proton intensity (TE 5.9, TR 120 ms) decreased exponentially in a zone corresponding to the inner cortex-outer infected region. This result is interpreted as being caused by a loss of water mobility (i.e. increase in viscosity) or a filling of intercellular air spaces with fluid (i.e. loss of air-water interfaces). The membrane impermeant tracer lucifer yellow was observed to infiltrate from the rhizosphere through the nodule cortex into the infected region in intact nodules of detopped and Ar:O2-treated, but not control, plants. This result is consistent with the filling of air spaces with fluid following the imposed treatments, allowing infiltration of aqueous tracers. Variation in intercellular air space volume within the inner cortex has been suggested as a mechanism to allow a variable rate of diffusion of O2 into soybean nodules.