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Zinc hyper accumulation in Thlaspi caerulescens as a chemical defence against herbivore 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 hyper accumulation 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 hyper accumulation 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 hyper accumulation. 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 herbivore or pathogens. Several studies have supported the chemical defence hypothesis. Martens and Boyd (1994 and Boyd and Martens, 1994) showed that nickel hyper accumulation is an effective defence against insect herbivores in two different feeding experiments.
Boyd et at (1994) also demonstrated that nickel hyper accumulating plants resisted pathogens including Xanthomonas campestris. Thlaspi caerulescens J. and C. Press (Brassicaceae) is a hyper accumulating 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 herbivore for this species. This paper explores the effects of zinc hyper accumulation in Thlaspi as a defence against Xanthomonas campestris. MATERIALS AND METHODS Thlaspi caerulescens seeds were collected in Clough wood, 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 Conv iron 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 10 ppm zinc, as Zn SO 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 25 ml of solution. The ten Rorison's plants retained the same solution as did the zinc plants. Para film 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 zinc. RESULTS Scale: 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 Mann-Whitney U Test showed a one-tailed probability of 0. 031. We are 97 % confident that the results were significant. The zinc plants showed a healthier response to the bacteria than did the non-zinc plants. DISCUSSION These results demonstrate that zinc hyper accumulation in Thlaspi caerulescens is an effective defence against the pathogen Xanthomonas 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 herbivore. 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 gregorian), slugs (Deroceras caruanae), and caterpillars (Paris brassica) 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 hyper accumulated metals, especially nickel, might also be toxic to pathogens and herbivores since they are widely used in fungicides and bactericides.
Studies show that hyper accumulators 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-hyper accumulating plants was 0. 42 of that of the hyper accumulators. 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 herbivore and pathogens.
They may also be good examples of the methods of coevolution.
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