PROTON DENSITY AND APOPLASTIC DOMAINS WITHIN SOYBEAN NODULES IN RELATION TO THE OXYGEN DIFFUSION BARRIER

A spin-echo pulse sequence was used to obtain H-1 nuclear magnetic res onance microimages of soybean nodules, with resolution to 40 mm and 3D presentation, At an acquisition time (TR) of c. 270 ms and a short ec ho time (TE) of 8.3 ms, a high proton intensity was detected in infect ed tissue and vascular strands, relative to the cortex, A longer TE of 15.5 ms was associated with a relatively low proton intensity in infe cted tissue, Thus protons in infected tissue were characterized by a s hort T-2. Following detopping of the plant, or treatment of the root s ystem with Ar:O-2, proton intensity (TE 5.9, TR 120 ms) decreased expo nentially in a zone corresponding to the inner cortex-outer infected r egion, This result is interpreted as being caused by a loss of water m obility (i.e. increase in viscosity) or a filling of intercellular air spaces with fluid (i.e. loss of air-water interfaces), The membrane i mpermeant tracer lucifer yellow was observed to infiltrate from the rh izosphere through the nodule cortex into the infected region in intact nodules of detopped and Ar:O-2-treated, but not control, plants, This result is consistent with the filling of air spaces with fluid follow ing the imposed treatments, allowing infiltration of aqueous tracers, Variation in intercellular air space volume within the inner cortex ha s been suggested as a mechanism to allow a variable rate of diffusion of O-2 into soybean nodules.