Zinc Hyperaccumulation

Zinc hyperaccumulation in Thlaspi caerulescens


as a chemical defence against herbivory




ABSTRACT




Thlaspi caerulescens is one of several plant species known to


accumulate heavy metals in excess of 2% of their above ground plant


biomass.   The reasons for hyperaccumulation are unknown, but


several studies conclude that it may be a plant chemical defence.


This has been of interest to biologists because these metals are


usually toxic.   The accumulation of these metals may serve as a


model for coevolution. We examined the effects of zinc


hyperaccumulation in Thlaspi on Xanthomonas campestris and found


that the plants containing zinc thrived when inoculated with this


bacteria, while plants not containing zinc showed signs of


deterioration.








INTRODUCTION


There are several wild plant species that have the ability to


accumulate high quantities of heavy metals in their above ground


biomass, up to three percent or more. Many of these plants are


found in the Brassicaceae family throughout Europe and the British


Isles.   These plants thrive on mineral outcrops with calamine and


serpentine soils rich with high levels of zinc, cadmium, and nickel


(Baker et al, 1994).   Several theories have been advanced on the


reasons for this hyperaccumulation.   Boyd and Martens propose that


it could be a form of drought resistance, inadvertent uptake,


interference, tolerance or disposal of metal from the plant, or a


chemical defence against herbivory or pathogens.


Several studies have supported the chemical defence


hypothesis.   Martens and Boyd (1994 and Boyd and Martens, 1994)


showed that nickel hyperaccumulation is an effective defence


against insect herbivores in two different feeding experiments.  


Boyd et at (1994) also demonstrated that nickel...