DELIVERY OF O-2 TO BACTEROIDS IN SOYBEAN NODULE CELLS - CONSIDERATIONOF GRADIENTS OF CONCENTRATION OF FREE, DISSOLVED O-2 IN AND NEAR SYMBIOSOMES AND BENEATH INTERCELLULAR SPACES

Based on a simulation model of the structure of and distribution of O- 2 within infected cells of soybean nodules, gradients of concentration of dissolved O-2 ([O-2]) have been calculated within and between symb iosomes embedded in host cytoplasm, through which the flux of O-2 to t he symbiosomes is facilitated by leghaemoglobin. As a consequence of f acilitation, gradients of [O-2] in cytoplasm between symbiosomes are v ery small. Within symbiosomes, from which leghaemoglobin is considered to be absent, respiration by bacteroids generates steeper gradients o f [O-2], thus restricting respiration and N-2 fixation. However, if ba cteroid mass is considered to be randomly distributed within a symbios ome, about 80% of this mass lies within about 0.6 mu m of the surface (the peribacteroid membrane). Consequently, respiration within a symbi osome was calculated to be between 65% and 92% of that attained if bac teroids were directly in contact with the cytoplasm. For N? fixation, the corresponding values were 44% to 91%. In cytoplasm, near the surfa ce of a symbiosome, there is a boundary layer in which equilibrium bet ween O-2, leghaemoglobin and oxyleghaemoglobin is perturbed by O-2 con sumption within. Calculations of the thickness of the boundary layers gave values of only 3.65 to 3.75 x 10(-9) m, thus they had little effe ct on calculated gradients of [O-2] in cytoplasm. In contrast, perturb ations of the leghaemoglobin oxygenation equilibrium affected layers o f cytoplasm beneath intercellular spaces to a depth of 0.15 to 0.3 x 1 0(-6) m in the physiological range of volume average [O-2]. This affec ted gradients of [O-2] in the cytoplasm near intercellular spaces. Rev isions have been made to the model cell, incorporating these new calcu lations. Results suggest that infected nodule cells may be able to wit hstand 1-2 mu M O-2 in the outermost layers of cytoplasm without inhib ition of Nr fixation caused by excessive O-2 within the symbiosomes.