Elemental allelopathy by an arsenic hyperaccumulating fern, Pteris vittata L.

Abstract

The functional advantages of arsenic (As) hyperaccumulation by plants are poorly understood. One proposed benefit, termed elemental allelopathy, occurs when hyperaccumulated As is cycled from the plant back into the top layer of soil, allowing As hyperaccumulators to gain an advantage over intolerant species by increasing soil As concentrations ([As]) underneath their canopy. To date, there are no studies that detail the presence of increased soil [As] associated with As hyperaccumulators. In this study, we documented variation in the soil [As] associated with the Chinese brake fern, Pteris vittata L. and also compared the effects of environmentally relevant soil and solution [As] on competitor plant growth. Four populations of P. vittata were identified in central Florida, USA. P. vittata tissue samples and soil samples were collected at the base of and at 3 m away from ferns in each population (n = 36). Five sample locations were randomly selected from each site, and soils from the base and 3 m away from each fern were collected to examine the effects of naturally occurring soil [As] on the germination and growth of a potential competitor plant (Oxalis stricta). Solutions with increasing [As] were also used to examine the threshold for negative effects of [As] on O. stricta growth. [As] were measured using inductively coupled plasma mass spectrometry (ICP-MS). Overall, soil [As] from the base of ferns was nearly twice that of soil 3 m away indicating that ferns hyperaccumulate As. However, ferns and their associated soil, contained different [As] depending on their collection site, indicating that these populations accumulate and use [As] differently. O. stricta growth decreased and germination was delayed as solution and soil [As] increased. However, the relative distance from the fern that the soil was collected from did not affect growth, which would be expected with elemental allelopathy. Our results show that P. vittata is associated with higher soil [As] and these concentrations are sufficient to inhibit growth of competitors. However, the absence of a strong inhibitory relationship associated with proximity to the fern across all locations suggests that the possible functional advantages of elemental allelopathy may depend on site specific characteristics.