TRIPARTITE ASSOCIATION OF ROBINIA PSEUDOACACIA, RHIZOBIUM, AND VESICULAR ARBUSCULAR MYCORRHIZAL FUNGI

ROBERT JOHN REINSVOLD, Purdue University

Abstract

Black locust (Robinia pseudoacacia L.) is a fast growing leguminous tree capable of forming symbiotic associations with N$\sb2$-fixing bacteria and mycorrhizal fungi. The objectives of this study were to: (1) collect isolates of Rhizobium spp and VA mycorrhizal fungi from three black locust plantings, (2) screen the microorganisms for their symbiotic effectiveness, (3) determine the effect of soil N and P on the symbioses, and (4) determine if selective pairings of rhizobial and fungal isolates influence growth of black locust seedlings. Rhizobia were isolated from root nodules exhibiting acetylene reduction rates in the top 3 percentile for three black locust plantings. Among these isolates, differences were observed in their abilities to nodulate black locust, their nitrogenase activities, and the growth yields of the host plants. Spores of Endogonaceous fungi were extracted from soil around the roots of the black locust. Eight species of Glomus, two species of Gigaspora, and one species of Sclerocystis were identified in association with black locust at the three plantings. The species composition varied from planting to planting. Attempts to culture these fungi with black locust Zea mays failed so five alternative fungal species from established pot cultures were screened. Although capable of colonizing the root, inoculation did not significantly affect the growth of 60-day-old black locust. To determine the combined influence of different levels of added nitrogen and phosphorus on nodulation, N$\sb2$-fixation, and growth of the host, seedlings were grown in pots for 30, 60, and 105 days in 16 nutrient combinations (4 levels of P x 4 levels of N). As the level of P was increased, nodulation and growth of seedlings also increased. In contrast, nodulation was inhibited at low and high levels of added N as compared to the medium levels. The effect of 36 combinations of fungal and rhizobial isolates (including controls) on the growth of seedlings were examined at 55, 95, and 135 days. The rhizobial isolate significantly affected seedling growth and nutrient concentrations in the tissue. A 30-fold difference was observed between nodulated and nonnodulated seedlings. Among nodulated seedlings, differences also were observed yet not as great. Inoculation with the mycorrhizal fungi did not significantly affect plant growth. There was no significant interaction between fungal and rhizobial inoculation at any age.

Degree

Ph.D.

Subject Area

Forestry

COinS