|Davis, Micheal - AUBURN UNIVERSITY|
|Boyd, Robert - AUBURN UNIVERSITY|
Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2000
Publication Date: April 1, 2001
Citation: Davis, M.A., Pritchard, S.G., Boyd, R.S., and Prior, S.A. 2001. Developmental and induced responses of nickel-based and organic defenses of the nickel-hyperaccumulator shrub, Psychotria douarrei. New Phytologist 150:49-59. Interpretive Summary: Plants that accumulate large quantities of metals in their leaves are termed "hyperaccumulators". These plants are economically useful for remediating metal-polluted soils. Little information exists, however, about why these plants accumulate metals. Evidence suggests that hyperaccululators use accumulated metals to defend themselves from herbivores and pathogens. We compared the metal-based (Ni-complexes) and organic defenses (tannins) of a hyperaccumulating shrub and a non- hyperaccumulating shrub to determine if these defenses responded similarly when exposed to simulated herbivory of young and old leaves. Neither Ni or tannins responded to simulated herbivory. Greater levels of tannins were found in young leaves, whereas old leaves had the highest concentration of Ni. Data suggested that tannins may function to complex Ni in the hyper accumulator shrub. Like tannins, Ni-based defenses were determined not to be induced by herbivory, but should be considered constitutive defenses instead.
Technical Abstract: Developmental and inducible changes in metal-based (nickel (Ni)) and organic defences (phenolics) are compared in the Ni-hyperaccumulating shrub, Psychotria douarrei. Young and old leaves of P. douarrei shrubs, subjected to different degrees of simulated herbivory, were analysed for metals, tannins, macronutrients and total carbon, and compared with a co-occurring nonhyperaccumulator shrub, Ficus webbiana. Leaf age affected both Ni-based and organic defences in P. douarrei; foliar metal concentrations were higher in mature leaves, whereas organic defences were higher in young leaves. Neither metal-based nor organic defences were increased by simulated herbivore damage, implying non-inducibility, although some organic defence compounds were significantly reduced. P. douarrei had a greater percentage of total phenolics, condensed tannins and protein precipitation ability than F. webbianai. Since total carbon content did not differ between species, Psychotria invests more of its leaf carbon budget in organic defences that does Ficus. Data suggest that P. douarrei foliage is well-protected by Ni, but tannins have multiple functions. The high concentrations of tannins in Psychotria leaves might function as a detoxification mechanism for elevated cytoplasmic metal concentrations, in addition to providing defensive benefits.