Anatomy of the legume nodule cortex: Species survey of suberisation and intercellular glycoprotein

Nodules of 29 species from 23 legume genera were examined for suberisation and glycoprotein deposits. Extensive suberisation of the nodule outer cortex to form a periderm was considered a primitive feature, common to non-legume and caesalpinioid nodules. The periderm was less extensive in nodules of Mimosoideae and Papilionoideae. Vascular bundles within the nodule were always surrounded by a vascular endodermis, defined by the presence of suberin on radial walls. Suberisation of the tangential walls of this endodermis was considered to be a primitive feature (present in all species examined of Caesalpinioideae and Mimosoideae, and in 10 out of 21 Papilionoideae) which may limit solute import to and export from the nodule. Glycoprotein was observed in the apoplast of the cortex in the three papilionoid species examined, but was absent in the caesalpinioid species examined. The common endodermis was recognised as an advanced feature, present only in certain species of the subfamily Papilionoideae (5 of 7, and 11 of 15 species of indeterminate and determinate nodule growth respectively). A membrane impermeant dye (lucifer yellow-CH), supplied in the rhizosphere under a mild vacuum, was observed to infiltrate through the cortex and into the infected zone in caesalpinioid nodules, and as far as the inner cortex in mimosoid and papilionoid nodules. Thus the common endodermis does not serve as an apoplastic barrier, and is unlikely to serve as a significant oxygen 'diffusion barrier'.