Accumulation of zinc and its role in plant defence

The elemental defence hypothesis was first put forward by Boyd and Martens in 1992. It suggested that plants concentrate high levels of heavy metals into their biomass to act as a defence against herbivory. This thesis focused on testing this theory using zinc (Zn) as the accumulated element. Several plant species, including T. caerulescens, are known as hyperaccumulators of Zn and can contain concentrations greater than 10,000 mg kg-1. The research used a novel technique to assess the levels of Zn in this plant species in the field. Other plant species, which contain more than 2000 mg Zn kg-1 but less than hyperaccumulators, are known as accumulators. They have received increased amounts of attention because research has shown that these concentrations can still have a negative impact on herbivores (Coleman et al., 2005). One of these accumulators, B. juncea, is a fast growing species, well suited to test the defensive qualities of Zn against herbivores and so was the focus of the rest of the thesis. B. juncea was tested to see if the uptake of Zn was inducible by assessing if there was an increased uptake of Zn after plants were subjected to attack by the herbivore H. aspersa. Previous research (Hodge et al, 2000) has shown that many plants are capable of increasing their concentration of defence compounds following herbivore attack. However, damage did not increase Zn uptake in B. juncea, so this elemental defence does not appear to be inducible. The preference and performance of herbivores (juvenile and adult H. aspersa and larvae of P. brassicae) on leaves of B. juncea containing high and low concentrations of Zn was tested. It found that the growth rate of both juvenile snails and larvae of P. brassicae was reduced by a diet of leaves high in Zn concentration and both species selected leaves low in Zn in preference tests, suggesting that Zn is an effective defence. In the final experimental chapter, the thesis evaluates the defensive properties of Zn in B. juncea plants grown in the field. In contrast to the earlier experiments, this field experiment found that plants containing elevated concentrations of Zn were significantly more damaged than those that had been grown on a control treatment, a result which goes against the elemental defence hypothesis. The thesis concludes by suggesting that although increased concentrations of Zn may have an impact on the growth and behaviour of herbivores in the laboratory, these effects do not necessarily occur under field conditions.