Cheruiyot, Dorothy .
Elemental Defense and the Joint Effects Hypothesis: An Initial Test Using Spodoptera exigua (Lepidoptera: Noctuidae).
Metal hyperaccumulation may be an elemental defense, in which high concentrations of a metal in plant tissues decrease herbivore survival or growth rate. The Joint Effects Hypothesis suggests that combinations of metals,or a combination of a metal with an organic defense compound, may have enhanced defensive effects. This enhancement may be additive or it may be synergistic(defined as enhancement exceeding additive expectations): in either case the concentration of metal for which a defensive benefit accrues to a plant will be lowered. I tested the Joint Effects Hypothesis using neonates of Spodopteraexigua fed artificial diet. Metal-metal experiments amended diet with pairs of metals, using four metals hyperaccumulated by some plants (Co, Cu, Ni andZn). I also conducted metal-organic compound experiments, pairing each metal(Co, Cu, Ni and Zn) with nicotine, mustard seed powder and tannic acid. Itested for joint effects using both lethal and sublethal concentrations of the chemicals tested: lethal concentration tests used LC20 levels and sublethal concentration tests used concentrations that inhibited larval weightby 10-25%. Of six metal-metal pairs, three (Co + Cu, Cu + Zn and Ni + Zn) had synergistic joint effects in lethal concentration tests and the other three pairs (Co + Ni, Co + Zn and Cu + Ni) had additive joint effects. For sublethal tests, all metal-metal pairs had additive joint effects except for Co + Cu (which was synergistic).For lethal tests of metal-organic combinations, synergistic joint effects were present for all metals + tannic acid, for Co, Cu, or Zn + mustard seed powder,and for Cu + nicotine. Additive joint effects were detected for Co, Ni or Zn +nicotine and for Ni + mustard seed powder. For sublethal tests, Co or Zn +mustard seed powder, Co + tannic acid, Cu + nicotine, and Zn + tannic acid had synergistic joint effects whereas Co + nicotine, Cu + mustard seed powder, Cu +tannic acid, and Ni with all three organic compounds had additive joint effects. The synergistic and additive joint effects documented here show that combinations of metals, or metals and organic compounds, enhance the defensive effect of a metal by allowing it to harm an herbivore at lower concentrations than if that metal were present singly. These results support the Joint Effects Hypothesis and suggest that defensive effects of metals may have contributed to the evolution of hyperaccumulation by some plants.
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1 - Auburn University, Biological Sciences, P. O Box 9532, Columbus, GA, 31908, USA
Presentation Type: Oral Paper:Papers for Sections
Location: Franklin B/Hyatt
Date: Wednesday, July 11th, 2012
Time: 10:45 AM