The Interaction between Leaf Allelopathy and Symbiosis with Rhizobium of Ulex europaeus on Hawaii Island

The objective of this study was to assess the magnitudes of the leaf allelopathy of Ulex europaeus in two different habitats, and discuss the driver of the differences, including rhizobia. The magnitudes of leaf allelopathy of the samples collected in two different habitats were assessed by comparing the hypocotyl and radicle lengths of the lettuce seeds tested on the samples. One habitat was in and adjacent to an Acasia koa forest, while the other was more than 50 m away. A. koa is indigenous to Hawaii and known to have a close symbiotic relationship with Bradyrhizobium for nitrogen-fixing. Within the past three years, U. europaeus has newly invaded both sampling sites, whereas the A. koa forest has been there for several decades. The combined result of both hypocotyl and radicle lengths of the lettuce seeds tested on both sites by linear model and multicomparison analyses showed no significant difference. But the radicle lengths of the lettuce seeds tested on U. europaeus sampled in and adjacent to the A. koa forest were significantly longer than those of the samples more than 50 m away, as measured by t-test (p = 0.05). This result suggested that the magnitude of the leaf allelopathy of U. europaeus depended on the distance of the habitat from the A. koa forest.