Abstract:
A study was conducted in which two strains of the fast – growing bean rhizobia- PV 1 and
PV 2 and two others of the slow – growing bradyrhizobia for cowpeas and soybean- CP 1 &
GM 8 , respectively, were used to test their ability to: - (a) proliferate in copper
contaminated liquid media, (b) survive in copper contaminated soil, (c) nodulate
respective host legumes and (d) fix nitrogen under increasing copper levels both in vitro
and in vivo. Known population sizes of each of the strains were exposed to copper
concentrations in the range: 0, 20, 40 60, 80 and 100 ppm in either Yeast - extract
Mannitol Broth or in modified Leonard Jar assemblies or potted soil. Most Probable
Number (MPN) studies were also done to estimate populations of the native strains of the
fast - and slow - growing rhizobia in a Cu -contaminated soil. Results indicated that
copper was more toxic to the slow – growing bradyrhizobia than to the fast – growing
rhizobia. Reduction in population sizes in vitro was more significant (p = 0.05) for
bradyrhizobial than for rhizobial strains. Reductions in fresh nodule volume, fresh nodule
mass and total shoot nitrogen were more significant (p = 0.05) in the bradyrhizobial – than
in rhizobial – legume associations. Copper depressed the populations of slow – growing
strain CP 1 and GM 8 in vivo more than those of the fast – growing strains PV 1 and PV 2 .
There was no statistically significant (p = 0.05) effect of 82.5 mg Cu/kg soil on numbers
(MPN) of native rhizobial strains, probably due to adsorption of Cu by soil colloids. This
study showed that based on the solution culture, potted soil and soil survival experiments,
slow - growing rhizobia were more susceptible to Cu toxicity than were the fast - growing
rhizobia.
