Competition for light induces metal accumulation in a metal hyperaccumulating plant

Plants can respond to competition with a myriad of physiological or morphological changes. Competition has also been shown to affect the foraging decisions of plants belowground. However, a completely unexplored idea is that competition might also affect plants' foraging for specific elements required to inhibit the growth of their competitors. In this study, we examined the effect of simulated competition on root foraging and accumulation of heavy metals in the metal hyperaccumulating perennial plant Arabidopsis halleri, whose metal accumulation has been shown to provide allelopathic ability. A. halleri plants originating from both metalliferous and non-metalliferous soils were grown in a "split-root" setup with one root in a high-metal pot and the other in a low-metal one. The plants were then assigned to either simulated light competition or no-competition (control) treatments, using vertical green or clear plastic filters, respectively. While simulated light competition did not induce greater root allocation into the high-metal pots, it did result in enhanced metal accumulation by A. halleri, particularly in the less metal-tolerant plants, originating from non-metalliferous soils. Interestingly, this accumulation response was particularly enhanced for zinc rather than cadmium. These results provide support to the idea that the accumulation of metals by hyperaccumulating plants can be facultative and change according to their demand following competition.