Developmental and induced responses of nickel-based and organic defences of the nickel-hyperaccumulating shrub, Psychotria douarrei

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 analyzed for metals, tannins, macronutrients and total carbon, and compared with a co-occuring nonhyperaccumulator shrub, Ficus webbiana. Leaf age affected both nickel 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 noninducibility, 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 than 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.