Plant tissues vary in elemental concentration. Some plants (termed accumulators) have unusually elevated levels of an element (often a metal), whereas others (termed hyperaccumulators) have still greater levels. Elemental hyperaccumulation may have several functions, including plant defense against natural enemies. A recent literature survey found that 34 studies of elemental plant defense, including 72 experimental tests, have been conducted.
Results/Conclusions
At least some tests have demonstrated defense by hyperaccumulated As, Cd, Ni, Se and Zn, although results for Zn are mixed. Defense by hyperaccumulated Ni has been shown for most leaf/root chewing herbivores and pathogens tested (20 of 26 tests) but not for herbivores of other feeding modes (1 of 8 tests). Most tests (5 of 6) using Ni concentrations below accumulator levels found no defensive effect, and the single test using plants in the accumulator range also found no effect. For Zn, mixed results have been reported for both hyperaccumulator (3 of 6 tests showed defense) and accumulator levels (3 of 4 tests showed defense). These tests have focused exclusively on leaf chewing/scraping herbivores: no herbivores of other feeding modes, or pathogens, have been tested. Both hyperaccumulator and accumulator concentrations of Se generally have shown defensive effects (12 of 14 tests). Most (75%) of these positive results used plants with accumulator Se concentrations. The three tests of Cd showed defensive effects in two cases, one for hyperaccumulator and one for sub-accumulator Cd concentrations. Arsenic has been tested only once, and was found effective against a leaf-chewing herbivore at a concentration much less than the hyperaccumulator level. Elemental defenses of hyperaccumulator plants may have community-level effects. For example, field surveys have identified insects that feed on Ni hyperaccumulator tissues and contain elevated whole-body Ni levels. These “high Ni insects” mobilize metals into local food webs and their high Ni levels may defend them against natural enemies. Other ecological processes, such as decomposition, can also be affected by hyperaccumulator leaf Ni levels. These initial studies suggest that elemental plant defenses deserve further exploration by ecologists.
