The literature on heavy metal toxicity to vascular plants is reviewed. Special attention is given to forest plant species, especially trees, and effects at low metal concentrations, including growth, physiological, biochemical and cytological responses. Interactions between the metals in toxicity are considered and the role of mycorrhizal infection as well. Of the metals reviewed, Zn is the least toxic. Generally plant growth is affected at 1000 μg Zn L−1 or more in a nutrient solution, though 100 to 200 µg L−1 may give cytological disorders. At concentrations of 100 to 200 μg L−1, Cu and Cd disturb metabolic processes and growth, whereas the phytotoxicity of Pb generally is lower. Although a great variation between plant species, critical leaf tissue concentrations affecting growth in most species being 200 to 300 μg Zn g−1 dry weight, 15 to 20 μg Cu g−1 and 8–12 μg Cd g−1. With our present knowledge it is difficult to propose a limit for toxic concentrations of Zn, Cu, Cd and Pb in soils. Besides time of exposure, the degree of toxicity is influenced by biological availability of the metals and interactions with other metals in the soil, nutritional status, age and mycorrhizal infection of the plant.

Toxicity of heavy metals (Zn, Cu, Cd, Pb) to vascular plants

CHAPTER 3 – Effects of Flooding on Growth and Metabolism of Herbaceous Plants

Chromium (Cr) is the second most common metal contaminant in ground water, soil, and sediments due to its wide industrial application, hence posing a serious environmental concern. Among various valence states, Cr(III) and Cr(VI) are the most stable forms. Cr(VI) is the most persistent in the soil and is highly toxic for biota. Since Cr is a non-essential element for plants, there is no uptake mechanism; Cr is taken up along essential elements such as sulfate through sulfate transporters. Cr accumulation in plants causes high toxicity in terms of reduction in growth and biomass accumulation, and Cr induces structural alterations. Cr interferes with photosynthetic and respiration processes, and water and minerals uptake mechanism. Various enzymatic activities related to starch and nitrogen metabolism are decreased by Cr toxicity either by direct interference with the enzymes or through the production of reactive oxygen species. Cr causes oxidative damage by destruction of membrane lipids and DNA damage. Cr may even cause the death of plant species. Few plant species are able to accumulate high amount of Cr without being damaged. Such Cr-tolerant, hyperaccumulator plants are exploited for their bioremediation property. The present review discusses Cr availability in the environment, Cr transfer to biota, toxicity issues, effect on germination and plant growth, morphological and ultrastructural aberrations, biochemical and physiological alterations, effect on metabolic processes, Cr-induced alterations at the molecular level, Cr hyperaccumulation and Cr detoxification mechanism, and the role of arbuscular mycorrhizae in Cr toxicity, in plants.

Chromium toxicity and tolerance in plants

Toxicity and Tolerance in the Responses of Plants to Metals

Effect of two Ni concentrations (10 and 200 μM) on growth, Ni accumulation, chlorophyll and proline contents, relative water content (RWC) as well as the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and glutathione S-transferase (GST) were studied in shoots of wheat plants. Treatments caused a considerable accumulation of Ni in the shoots. However, exposure of plants to 10 μM Ni did not lead to significant alterations in shoot growth except for a slight increase in fresh mass. The other parameters studied were not affected by treatment of plants with 10 μM Ni. In contrast, 200 μM Ni caused inhibition of shoot growth, a decline in RWC and chlorophyll content, accumulation of proline and occurrence of visible symptoms of Ni toxicity. The activities of SOD and CAT decreased in response to 200 μM Ni. Conversely, several-fold enhancements of POD and GST activities were observed following the 3rd day of 200 μM Ni treatment.

Effect of nickel on antioxidative enzyme activities, proline and chlorophyll contents in wheat shoots

Exposure of white bean seedlings to phytotoxic burdens of Co, Ni, or Zn reduced the export of 14C-photoassimilates from the nearly fully expanded unifoliate leaves. Little 14C reached the major sink areas, the young trifoliate leaves and the root tips, of seedlings exposed to metal. The unifoliate leaves accumulated sucrose, reducing sugars, and starch. These effects were evident within 1 or 2 days.

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Carbohydrate Levels and Photoassimilate Export from Leaves of Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc.

Vein loading in unifoliate leaves of white bean seedlings exposed to excess Co2+, Ni2+, or Zn2+ for 1 to 4 days was studied by incubating leaf discs in [14C]sucrose. The discs from plants exposed to metal exhibited an increased total uptake of radiosucrose but reduced vein loading. Differences between treatments due to infiltration of disc margins were eliminated by analyzing 7-millimeter discs cut from the center of incubated 14-millimeter discs. Uptake of radiosucrose was greater particularly in discs from seedlings exposed to excess Ni2+ and Zn2+. The effect increased as exposure of the seedlings to metal increased up to 4 days. Autoradiographs showed vein loading in control leaf tissues with most of the radiosucrose accumulating in minor veins and little remaining in the mesophyll. In discs from metal-treated plants, most of the sucrose remained in the mesophyll without accumulating preferentially in the minor veins. This effect was evident within 24 hours of exposure to excess metal and intensified with longer exposures to metal. The inhibition of vein loading was also evident in situ. Both the preferential accumulation of sucrose into the minor veins of control tissues and the accumulation into mesophyll of metal exposed tissues were sensitive to 2,4-dinitrophenol and the blockage of sulfhydryl groups. It is concluded that the inhibition of vein loading contributes markedly to the observed toxicological effects of reduced photoassimilate export and of accumulation of carbohydrates in fully expanded leaves of bean seedlings exposed to excess metal ions.

/pdf/vein-loading-in-seedlings-of-phaseolus-vulgaris-exposed-to-3byf17tpnq.pdf

Vein Loading in Seedlings of Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc

Petiole growth in the celery-leaved crowfoot (Ranunculus sceleratus L.): Effects of auxin-transport inhibitors

In the presence of the leaf blades, excised petioles of the semiaquatic celery-leaved buttercup Ranunculus sceleratus L. elongate when submerged, or when treated in air with indoleacetic acid (IAA), gibberellin A3 (GA3), or C2H4 alone or in combination, responses that are similar to those obtained using intact plants. Isolated petiole sections do not respond to C2H4, but are responsive in the presence of low levels (10−6 mol L−1) IAA. Gibberellin A3 is less active than IAA in promoting growth or in sensitizing the petiole tissue to C2H4.

Petiole growth in Ranunculus sceleratus: the role of growth regulators and the leaf blade

The semiaquatic celery-leaved buttercup, Ranunculus sceleratus L. flowers under 18-h photoperiods (long days (LD)) but remains vegetative under 8-h photoperiods (short days (SD)). A minimum of 20-30 LD are required for floral bud initiation which occurs prior to the elongation of the flowering stalk. The effects of LD on flowering and the subsequent growth of the flowering stem can be mimicked with gibberellin (GA3) under SD. When plants which have been induced to flower by 28 LD are submerged, the flower stalk elongates rapidly over the next 48 h. Submergence effects can be mimicked by C2H4, but not by GA3 or indole-3-acetic acid (IAA) applications.

Ananda B. Samarakoon

Papers

Carbohydrate Levels and Photoassimilate Export from Leaves of Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc.

Vein Loading in Seedlings of Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc

Petiole growth in the celery-leaved crowfoot (Ranunculus sceleratus L.): Effects of auxin-transport inhibitors

Petiole growth in Ranunculus sceleratus: the role of growth regulators and the leaf blade

Flowering and stem growth in the celery-leaved buttercup, Ranunculus sceleratus