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
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 ...view middle of the document...
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...