Elemental defense in seedlings of a Ni hyperaccumulator, Alyssum murale (Brassicacae)

Background/Question/Methods

Many recent studies explore the role of metals in “elemental defense” of hyperaccumulators, but few studies focus on this phenomenon in seedlings, the most vulnerable life stage. We designed field and laboratory studies to compare herbivory in seedlings of a Ni-hyperaccumulator, Alyssum murale (Brassicaceae) with different shoot Ni levels. Seeds of Alyssum murale from serpentine soil in Bulgaria were germinated in commercial soil with two treatments: Ni+ and Ni-.   Ni+ soil contained 2000 ppm Ni (1:1 Ni acetate:Ni sulfate); no Ni was added to Ni- soil. We used AA spectroscopy to determine that shoot nickel in seedlings grown in Ni+ soil was significantly higher (Ni+: 45.28 ppm, Ni-: 1.68 ppm; p=0.000).  

Herbivory under field conditions was assessed in an array of 4 week old potted seedlings set out in a meadow for 2.5 weeks at UVA’s Blandy Research Station in Boyce, VA in summer 2008. Herbivory under laboratory conditions was assessed using first instar Pieris rapae larvae.   In a “choice design” study, Pieris rapae larvae were provided with A. murale leaves from Ni+ and Ni- seedlings for 3 days. For a “no-choice design” experiment we used a set of Ni+ and Ni- seedlings that were initially matched for size by height and leaf number. Pieris rapae larvae were enclosed with either Ni+ or Ni- potted seedlings for 1 week.

Results/Conclusions

Both Ni+ and Ni- seedlings showed high herbivory in the field (100% herbivore attack, 60-70% mortality), and there was no difference in final root or shoot biomass of Ni+ and Ni- seedlings. The choice experiment showed no difference in herbivory on Ni- and Ni+ leaves over the 3 day interval. The “no-choice” study showed a similar initial pattern with no difference in herbivory on Ni+ and Ni- seedlings for the first 3 days, followed by a significant decline in leaf damage to Ni+ seedlings (p=0.001). Final shoot weight of Ni+ seedlings in the “no choice” study was significantly greater (Ni+: 12.56 mg, Ni-: 7.96 mg; p=0.000).

Nickel does little to prevent herbivory in A. murale seedlings under field conditions. Generalist herbivores in the field may experience low toxicity due to the variety of foods in their diet. Lab experiments with a specialist herbivore suggest that toxicity may be more important than deterrence since P. rapae fed indiscriminately on Ni- and Ni+ leaves initially. This study underscores the importance of examining traits such as elemental defense at all stages of life history.