A comparative study of nodule morphology, nodule anatomy, microbiology and cross infection relationships between Albizia lebbeck (L.) Benth., Pithecellobium dulce (Roxb.) Benth., Samanea saman (Jacq.) Merr. and Dalbergia sissoo Roxb. was made. Root nodules were distributed singly as well as in clusters on the primary and secondary roots of A. lebbeck, P. dulce and S. saman Nodules were present in the axils of lateral roots of Dalbergia sissoo. Mature nodules of Mimosoid trees were oblate, elongated, branched and coralloid but they were globose in D.,sissoo. Rhizobia entered the root via root hair and formed infection threads in all the four species. Rhizobia also entered the root through ruptured epidermis in A. lebbeck. Intercellular movement of rhizobia was observed in A. lebbeck and P. dulce nodules. Movement of rhizobia was also noticed through xylary elements in S. saman and through pits in A. lebbeck nodules. Root nodule primordia developed with the initiation of mitotic divisions in the cortical cells of roots in all the four species. Bacteria showed intercellular movement in A. lebbeck and P. dulce. Both determinate and indeterminate type of nodules occurred in A. lebbeck and P. dulce while indeterminate type of nodules were found in S. saman and determinate type in D. sissoo. Both determinate and indeterminate nodules had a similar anatomical structure. They could be differentiated into a nodule meristem, nodule cortex, vascular tissues and bacteroid region. Nodule meristem Was multilayered, comprising of thin walled, tightly packed and actively dividing cells. Nodule cortex was mainly composed of parenchymatous tissue but patches of sclereids were found scattered in the cortex of P. dulce and S. saman Sclereids could not be observed in the cortex of A. lebbeck. A continuous layer of sclereids was found in the cortex of D. sissoo.
Distinct periderm was present in all the mimosoid species but it was absent in D. sissoo. In all the four species amphicribral vascular bundles could be observed which were arranged around the bacteroid region. In addition to amphicribral vascular bundles collateral vascular bundles were also observed in P. dulce. Bacteroidregion showed both the infected and non-infected (interstitial) cells. Infected cells were vacuolated in P. dulce and non-vacuolated in A. lebbeck, S. saman and D. sissoo. Starch grains were found in the. cytoplasm of interstitial cells of A. lebbeck, P. dulce and D. sissoo whereas tannin was observed in the cytoplasm of interstitial cells of S. saman Infected cells wee loosely arranged in D. sissoo. Persistent infection threads were observed in the bacteroid region of S. saman, P. dulce and D. sissoo. Bacteroids of S. saman were spherical and rod-shaped enclosed in a beribacteriod membrane. Pleomorphic form of bacteroids were found in the nodules of D. sissoo, A. lebbeck and P. dulce.
Rhizobial isolates from P. dulce and D. sissoo grew after 24 hours of incubation at 28°-30°C, whereas isolates from A lebbeck and S. saman grew after 48 hours of incubation at 28°-30°C. Rhizobial isolates from all the four species produced translucent and gummy colonies on YMA medium. They did not absorb congo red, they were gram negative bacilli, non-sporing and motile. Isolates obtained from nodules of A. lebbeck and S. saman were slow growers having monotrichous flagella, formed 'serum zone with slow change with litmus milk and showed alkaline reaction on YMA medium with bromothymol blue. Isolates of P. dulce showed amphitrichous flagella while isolates of D. sissoo showed lophotrichous flagella. They were fast-growers showing acidic reaction on YMA medium with bromothymol blue.
Rhizobial isolates of D. sissoo and P. dulce utilized all the eleven sugars used as carbon source in the culture medium, whereas isolates of A: lebbeck Utilized eight and isolates of S. saman utilized seven out of eleven sugars used.
The isolates from the four legume species showed varied sensitivity against different antibiotics used in the present study. All isolates belonging to Mimosoideae were susceptible against Gentamycin, Neomycin and Tetracycline. All of them were resistant against Cephalexin. They showed varied reaction against other antibiotics. Isolates of D. sissoo were resistant against Amoxicillin, Ampicillin, Choxacillin, Erythromycin, Neomycin and Sulphamethoxazole Trimethoprim and susceptible against Gentamycin and Tetracycline. Isolates from nodules of. A. lebbeck, P. dulce, S. saman and D. sissoo produced nodules on each other host plants.
Shape and colour of nodules in cross-inoculated plants of D. sissoo were found host determined. However shape and colour of nodules in cross inoculated plants of A. lebbeck, P. dulce and S. saman were not host determined. In cross inoculated plants rhizobial isolates from D. sissoo and S. saman induced substantial increase in nitrogen content and dry matters of P. dulce, S. saman and D. sissoo, while in A. lebbeck total nitrogen content increased with strain specific.