Ultrastructural Studies on Root Nodules of Pithecellobium dulce (Roxb.) Benth. (Fabaceae)

Ultrastructural studies were conducted on Pithecellobium dulce (Roxb) Benth. root nodules collected from trees growing under natural conditions. Rhizobial infection on root surface of P. dulce started with curling of root hair. Both curled and straight root hairs were observed. Th e internal structure of a mature nodule showed an epidermis, cortex, vascular region and a bacteriod region. Vascular bundles were amphicribral. A distinct periderm consisted of sclereid tissue could be observed in the cortex outside the vascular tissue. Th e bacteroid region contained infected and un-infected (interstitial) cells intermingled with each other. Infected cells of developing nodules as well as of mature nodules were vacuolated. Infection threads were also observed in the bacteroid zone. Th e rhizobia were released from the infection thread into the host cytoplasm form rhizobial (unwalled) droplets. Rhizobia were also observed in the intercellular spaces between infected cells. Starch grains were observed in the interstitial cells.