|Davis, Michael - UNIVERSITY OF ALABAMA|
|Boyd, Robert - UNIVERSITY OF ALABAMA|
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., Prior, S.A. 2001. Developmental and induced responses of nickel-based and organic defenses of the nickel-hyperaccumulating 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 hyperaccumulators 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 be induced by herbivory, but should be considered constitutive defenses instead.
Technical Abstract: Developmental and inducible changes in metal-based (nickel (Ni)) and organic defenses (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 analyzed for metals, tannins, macronutrients and total carbon, and compared with a co- occurring nonhyperaccumulator shrub, Ficus webbiana. Leaf age affected bot nickel Ni-based and organic defenses in P. dourarrei; foliar metal concentrations were higher in mature leaves, whereas organic defenses were higher in young leaves. Neither metal-based nor organic defenses were increased by simulated herbivore damage, implying noninducibility, although some organic defense compounds were significantly reduced. P. dourarrei had a greater percentage of total phenolics, condensed tannins and protein precipitation ability than F. webbianai. Since total carbon content did not tdiffer between species, Psychotria invests more of its leaf carbon budget in organic defenses than does Ficus. Data suggest that P. dourarrei 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.