Provenance-specific photosynthetic and physiological sensitivity of Robinia pseudoacacia L.-rhizobia association to low phosphorus availability

In terrestrial ecosystems, the mechanisms that allow plants to adapted to low P availability are largely not understood, particularly for woody legume woody. Here, we investigated the responses of rhizobia inoculation and low phosphorus (P) availability on black locust (Robinia pseudoacacia L.) from different geographic origins of China. For this purpose, black locust seedlings from two provenances with distinct climatic and soil background, i.e., the Gansu Province (GS) of Northwest China and the Dongbei region (DB) of Northeast China, were exposed to sufficient (0.5 mM) and low P availability (0.5 µM) with and without rhizobia inoculation in a greenhouse for two months. The rhizobia strain used for inoculation was obtained from a black locust forest stand of 50-years-old in the Shandong Province of East China. CO2 and H2O gas exchange parameters as well as foliar physiological traits were measured. Our results indicated significantly different provenance-specific responses to inoculation and low P availability. Compared to sufficient P supply, inoculated DB Robinia showed a 57.6% decrease in whole plant biomass and a 49.2% decrease in the number of nodules at low P supply. However, low P availability had no effects on biomass accumulation of GS Robinia and increased its nodule formation by 240%. Still, gas exchange parameters and foliar physiological traits of GS Robinia plants were more responsive to low P supply than in DB plants. When inoculated with rhizobia at low P treatment, Robinia plants seem to use P preferentially for nodule production, thus weakening photosynthesis (86%) and reducing total biomass (58%) compared to non-inoculated plants. From these results it is concluded that GS Robinia may constitute a feasible candidate provenance for future afforestation programs of vulnerable terrestrial ecosystems. However, long-term systemic studies are needed in future to elucidate the physiological mechanisms of its adaption to low P availability.