Zinc hyperaccumulation in Thlaspi caerulescens
as a chemical defence against herbivory
Thlaspi caerulescens is one of several plant species known to
accumulate heavy metals in excess of 2% of their above ground plant
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
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
Isles. These plants thrive on mineral outcrops with calamine and
(Baker et al, 1994). Several theories have been advanced on the
reasons for this hyperaccumulation. Boyd and Martens propose that
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 hyperaccumulating
plants resisted pathogens including Xanthomonas campestris.
Thlaspi caerulescens J. and C. Presl (Brassicaceae) is a
hyperaccumulating plant found in the British Isles. It has been
shown to accumulate 10,000 ppm (>1%) of its biomass in zinc (Baker
et at, 1994), and Pollard and Baker (1997) suggest that this is an
effective defence against herbivory for this species. This paper
explores the effects of zinc hyperaccumulation in Thlaspi as a
defence against Xanthomonas campestris.
MATERIALS AND METHODS
Thlaspi caerulescens seeds were collected in Cloughwood, U. K.
These seeds germinated on polyester beads supported in expanded
polystyrene rafts floating on one-tenth strength Rorison’s solution
(Hewitt, 1966). These containers were placed in a Conviron E-15
environmental growth chamber at the following settings: 20 C, 90%
RH, 16 hr day, and 8 hr night. After three weeks, twenty seedlings
were transferred to 4 rafts composed of expanded styrene on
polyethylene, each supporting five plants individually. Ten
plants floated on one-tenth strength Rorison’s, and ten plants
floated on a solution containing Rorison’s and 10ppm zinc, as ZnSO
The solutions were freshened every four days to inhibit any
possible algal growth.
After twenty days, each plant was transferred to an individual
beaker containing 25ml of solution. The ten Rorison’s plants
retained the same solution as did the zinc plants. Parafilm held
the plants in place. The plants were then inoculated with three
different strains of Xanthomonas campestris, a bacteria known to
harm plants. Each plant had each strain inoculated on three
different leaves. The plants grew in the growth chamber for one
week and then were examined.
The plants were analyzed with a ranking scale based on
appearance by three people who did not know which plants contained
1= Healthy, green leaf with no brown, small puncture hole
2= Longer puncture hole with some white spots
3= Some leaf discoloration, expanded hole, some shriveling leaves
4= Shriveling of several leaves, whole plant not thriving
5= Many leaves dead, small shriveled plants
Plant Solution Average
Rank 1 Non-zinc 5.00 2 Zinc 1.67 3 Zinc 2.00 4 Non-zinc 4.67
5 Non-zinc 2.67 6 Non-zinc 3.67 7 Zinc 2.67 8 Non-zinc 2.00 9
Non-zinc 4.00 10 Zinc 1.33 11 Non-zinc 1.67 12 Non-zinc 5.00 13
Zinc 2.33 14 Zinc 2.00 15 Zinc 1.00 16 Zinc 1.33 17 Zinc
2.00 18 Non-zinc 1.67 19 Zinc 1.67 20 Non-zinc 1.00
The rankings showed that the zinc plants were on average
healthier than were the non-zinc plants after being inoculated.
However, the non-zinc plants did show a variety of rankings, but
they were statistically unhealthier than the zinc plants. The
are 97% confident that the results were significant. The zinc
plants showed a healthier response to the bacteria than did the
These results demonstrate that zinc hyperaccumulation in
Thlaspi caerulescens is an effective defence against the pathogen
Xanthonomas campestris. Since the experiment was a double blind
investigation, the results were not biased.
Our results were consistent with other studies in this area.
Boyd et al. found that nickel accumulation in S. polygaloides was
an effective defence against pathogens such as Xanthomonas. Of the
plants inoculated with this bacteria, the nickel accumulating
plants inhibited the growth of a powdery mildew. Growth of the
fungus Alternaria brasssicola was also inhibited by nickel
concentration in the plants.
Martens and Boyd (1993) showed in feeding experiments that
nickel accumulation is an effective defence against insect
herbivory. The insects fed non nickel bearing leaves survived or
showed weight gain while the insects fed nickel bearing leaves did
not. The nickel accumulation is effective because of broad
toxicity, low cost, and high lethality.
Pollard and Baker (1997) conducted studies showing preferences
of locusts (Schistocerca gregaria), slugs (Deroceras caruanae), and
caterpillars (Pieris brassicea) to Thlaspi caerulescens grown in
low zinc and zinc amended solutions. They all showed preferential
feeding on plants with low zinc concentrations.
This is an important finding because zinc in these quantities
is normally lethal to a plant. Boyd and Martens (1993) suggest
that it is reasonable to assume that the hyperaccumulated metals,
especially nickel, might also be toxic to pathogens and herbivores
since they are widely used in fungicides and bactericides. Studies
show that hyperaccumulators are more susceptible to fungi when
grown on non-serpentine soil. Studies by Martens and Boyd (1993)
suggest that metal accumulation may provide a useful example of the
coevolution of defence mechanisms because of the increased fitness
it allows for these plants. Herbivores were given a choice between
accumulating and non-accumulating S. polygaloides, and the fitness
of non-hyperaccumulating plants was 0.42 of that of the
hyperaccumulators. Selective pressures could favor the evolution
of these plants.
All of these findings suggest that plants accumulating heavy
metals may be utilizing an effective defence against herbivory and