Symbiotic performance of selectedCyclopia Vent. (honeybush) rhizobia under nursery and field conditions

Three newly selected strains ofCyclopia rhizobia together with an inoculant strain, which has never been tested in the field with adequate experimental design, was assessed under both nursery and field conditions for symbiotic performance. The three new test strains were initially selected for their superior N₂-fbcing abilities under glasshouse conditions, and then evaluated in this study for field performance.Cyclopia subternata Vogel andCyclopia genistoides (L.) R. Br., which have the potential for producing high quality honeybushtea, were used as host plants in both the nursery and field studies. The effect of seedling inoculation at the nursery level was also examined for the four test strains under nursery conditions. The inoculation of cuttings under nursery conditions produced.significant increases in shoot biomass, shoot %N and shoot N content. More specifically, inoculatingC. subternata with strains UCT44b and UCT61a significantly increased shoot biomass and N content relative to strain PPRICI3. Strains UCT44b and UCT61a also showed better nodulation withC. subternata cuttings compared to strains UCT40a and PPRICI3. Field inoculation ofCyclopia increased all growth parameters relative to the uninoculated control, except for leaf %N.Cyclopia subternata inoculated with strains UCT44b, UCT40a and UCT61a produced significantly lower δ¹⁵N values than the uninoculatedC. subtemata reference plant. Using the¹⁵N natural abundance method,C. subternata was estimated to be gaining about half of its N from N₂ fixation, whileC. genistoides obtained less than half of its N from symbiotic nutrition.