TIME-COURSE OF CHANGES INVOLVED IN THE OPERATION OF THE OXYGEN DIFFUSION BARRIER IN WHITE LUPIN NODULES

Nodulated white lupins (Lupinus albus L. cv. Multolupa) were subject t o either darkening for 12 h, followed by 24 h recovery in light, or to 50% O-2 for 30 min. For each treatment, nodules were harvested at int ervals for analysis by light and electron microscopy and determination of glycoprotein content using Enzyme Linked Immunosorbent Assays (ELI SA). This allowed for an analysis of the sequence of events causing an increase in intercellular space occlusion within the inner cortex. Th e temporal sequence in response to darkening appears to be: (1) an ini tial rapid increase in the detectable levels of intracellular glycopro tein, due to either a state change or de novo synthesis, (2) a concomi tant increase in the volume of thickened cell walls, causing a reducti on of intercellular space Volume and (3) after 1-3 h a release of glyc oprotein into the intercellular space network of the inner cortex, acc ompanied (and possibly spread) by the continued constriction of the sp aces due to cell wall and cell content expansion. The results for expo sure to 50% O-2 showed a similar, but much more rapid, sequence of eve nts, operating within 15-30 min. The main difference between the two s equences was the lack of expansion of thickened cell walls with increa sed pO(2). Also, it was possible to detect glycoprotein within cell wa lls following exposure to 50% O-2 but not following darkening. These o bservations are discussed in relation to proposed mechanisms for the o peration of a variable oxygen diffusion barrier in legume nodules.