NewPhytol. (1997), 135, 655-658
Deterrence of herbivory by zinc hyperaccumulation in Thlaspi caerulescens (Brassicaceae) BY A. JOSEPH POLLARD^* AND ALAN J. M. BAKERS ^ Department of Biology, Furman University, Greenville, South Carolina 29613, USA ^ Department of Animal and Plant Sciences, University of Sheffield, Sheffield SIO 2TN, UK (Received 17 August 1996 ; accepted 3 January 1997) SUMMARY
Plants known as byperaccumulators take up and sequester bigb concentrations of potentially toxic elements from metalliferous soils. We tested tbe hypothesis tbat zinc hyperaccumulation benefits plants by deterring berbivory. In laboratory feeding trials, tbree species of herbivores were allowed to choose between Thlaspi caerulescens (Brassicaceae) plants grown in low-Zn and Zn-amended culture solution. Locusts (Schistocerca gregaria), slugs (Deroceras caruanae), and caterpillars (Pieris brassicae) all showed significant preferences for plants witb lower foliar Zn concentrations. Such differential feeding could result in selection pressures favouring tbe evolution of byperaccumulation. Tbe findings are also relevant to current proposals to exploit byperaccumulation as a means of remediating metal-contaminated soils. Key words: Zinc, byperaccumulation, berbivory, defence, Thlaspi caerulescens. (species endemic to metalliferous sites), accumulate INTRODUCTION
Certain plant species growing on metalliferous soils accumulate exceptionally high concentrations of metallic elements in their tissues (Baker & Brooks, 1989). Recent interest in metal hyperaccumulation has been stimulated by the potential for phytoremediation of polluted soils, i.e. using metalaccumulating plants to cleanse soils of contaminants (Baker et al., 1994«; Brown et al., 1994; U.S. Department of Energy, 1994; Nanda Kumar ef a/., 1995; Salt et al., 1995). This interest has led to intensive studies of the physiological mechanisms responsible for metal uptake and accumulation (Baker & Walker, 1990; Bernal et al., 1994; Brown et al., 1995; Kramer et al., 1996). However, further research is needed into the underlying question of why plants have evolved mechanisms to accumulate potentially toxic elements to concentrations many times higher than those found in soil or predicted under models of passive uptake. The definition of hyperaccumulation (Baker & Brooks, 1989) is based on comparative surveys indicating that, on metalliferous soils, most plants accumulate low concentrations of metals in their shoots, while a few species, usually metallophytes „„ , , , , , , , , J *
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d l higher amounts. Based on these discontinuities, proposed criteria (on a d. wt basis) are 100/^gg"^ for Cd; 1000/igg"^ for Co, Cu, Ni and P b ; and 10000/*gg"^ for M n and Zn. Approximately 400 plant taxa worldwide are now known to hyperaccumulate at least one metal according to these criteria, the great majority being hyperaccumulators of Ni, from outcrops of serpentine minerals (Brooks, 1987). A diverse range of plant families is included, especially in tropical regions (Brooks, 1987; Brooks et al. 1990). In Europe, many hyperaccumulators belong to the Brassicaceae ( = Cruciferae), in genera such as Thlaspi, Cochlearia and Alyssum (Reeves & Brooks, 1983; Reeves, 1988). The broad phylogenetic and biogeographic distribution across which the phenomenon of hyperaccumulation occurs implies that some adaptive function or functions have selected for its independent evolution in many taxa and localities. A range of hypotheses to explain the origins of hyperaccumulation has been summarized by Boyd & Martens (1993), including the possibility that metals in leaves might defend them against herbivores and pathogens. Thlaspi caerulescens J. & C. Presl (formerly T. .
alpestre L.) IS a European m e m b e r of the Brassicaceae
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A. y. Pollard and A. J. M. Baker
(Ingrouille & Smirnoff, 1986) that hyperaccumulates Zn, Cd and Ni, both in the field and in laboratory cultivation (Baker, Reeves & Hajar, 19946). We tested the hypothesis that hyperaccumulation of Zn by T. caerulescens deters feeding by desert locusts, Schistocerca gregaria (Forsk.) (Orthoptera: Acrididae); garden slugs, Deroceras carwanae (Pollonera) (Pulmonata: Limacidae); and caterpillars of the Large White butterfly, Pieris brassicae L. (Lepidoptera: Pieridae). T h e first two of these species are broad dietary generalists, whereas the third feeds exclusively on Brassicaceae. S. gregaria is native to Africa, and thus does not encounter T. caerulescens under natural conditions. D. caruanae and P. brassicae, although native to Britain, are not known to feed on T. caerulescens in the field. Our choice of these species was based on preliminary screenings which showed their willingness to eat T. caerulescens grown under low-Zn conditions. Our intention was to use the herbivores as a bioassay of the potential defensive role of Zn hyperaccumulation. This does not imply that these particular species have ever represented a selective force on the evolution of T. caerulescens. It is a basic paradox in experiments on plant defence mechanisms that, if one chooses to work with herbivores known to eat the plants under study in the wild, one might be focusing on a coevolved system in which the plant's defences have been overcome by adaptations in the herbivore (Rhoades, 1979). Therefore, our experiments utilized animals representative of three large groups of common and destructive herbivores, with a variety of feeding modes and taxonomic affiliations, in order to provide a general assessment of defensive potential.
plants. Each plant for study was transferred to a 25ml vial, with its roots immersed in the same culture solution (basal or Zn-amended) in which it had grown. Fourth-instar S. gregaria nymphs were obtained from a commercial biological supplier. D. caruanae (1—3 cm long) were collected from the University of Sheffield greenhouses. Both species were maintained on a lettuce diet (supplemented with bran for S. gregaria) until immediately before testing. Experimental design for these herbivores involved replicated {n — 10) pairwise choices between similar-sized plants, one grown in basal medium and the other in Zn-amended solution. For S. gregaria, one locust was placed together with two vials in a transparent box (internal volume 750 ml) for 4 h. For D. caruanae, the shoot portions of two plants were inserted through the floor of a 350 ml chamber containing three slugs, which were allowed to feed for 1 wk. The number of leaves on each plant was counted before and after feeding; for partially eaten leaves, the fraction of the leaf consumed was estimated. This simple method of data collection proved satisfactory because of the uniform architecture of the hydroponically grown plants: small rosettes of 5—15 leaves, each with a long petiole and circular-obovate blade 1-2 cm long, P. brassicae eggs were obtained commercially and allowed to hatch on cabbage leaves. Feeding trials were carried out in 9-cm-diameter Petri dishes, each containing two T. caerulescens leaves removed from 24-wk-old plants (one grown on basal medium; the other on Zn-amended medium, 22 wk after commencement of Zn amendment), along with a small reservoir of water to maintain humidity in the dish, A single first-instar larva (6—8 mm long) was placed in each dish. Larvae fed for 24 h, after which the fraction of the leaf lamina consumed was assessed visuallv. Ten replicate dishes were used.
MATERIALS AND METHODS
Seeds of T. caerulescens were collected at Clough Wood, Derbyshire, U K (Baker et al. 19946; Pollard & Baker 1996). Plants were grown hydroponically in tenth-strength Rorison solution (Hewitt, 1966), with FeClg substituted for FeNa-EDTA to avoid metal chelation, and p H adjusted to 6-5 using acetic acid. Seeds were germinated on rafts of polyethylene beads floating on this basal medium. Two weeks after germination, seedlings were transferred to rafts of polyurethane foam and polyethylene sheeting constructed to support 40 plants in a 1-5 1 polystyrene tank. At the time of transfer, the solution in half the tanks was amended to contain lOmgl"^ Zn (as ZnSO4.7H2O). Solutions were replaced every 3 d; plants were maintained under conditions of 16-h days (20 °C, PPFR of 80 /imol m"^ s~^ over the waveband of 400-700 nm), 8-h nights (15 °C). Herbivore choice experiments using locusts and slugs began approx. 12 wk after seed germination, i.e. 10 wk after the addition of Zn to the high-Zn
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After the feeding trials, uneaten portions of plants were analysed for foliar Zn concentration. Samples were ashed overnight at 500 °C, dissolved in 10 ml of 2 M HCl, and assayed by flame atomic absorption spectrophotometry.
Plants for locust and slug tests had foliar Zn concentrations of 14045 + 891/tg g"^-^ (mean + SE) when grown on Zn-amended medium, and 1474 + 451/ig g"^ when grown on basal medium, Mean Zn concentrations of leaves presented to caterpillars were 7432 + 732/*g g"^ and 528 + 63/ig g~^, respectively. There were no visible differences between the two groups of plants, and no signs of metal-induced toxicity. Initial number of leaves per rosette did not differ significantly between high-Zn and low-Zn plants presented to locusts and slugs. In similar experiments (A. J. Pollard & R.Norman, unpublished) there have been no significant
Zinc hyperaccuw-ulation and herbivory
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explained by the fact that plants for caterpillar experiments were much larger, yet intervals between solution changes had not changed; thus, the ratio of available Zn to plant biomass might have effectively o 8 E declined. en Slugs and locusts ate small quantities of high-Zn 8 6 plants before rejecting them. Such sampling be03
