Screening Of Heavy Metal-Tolerant Energy Plants And Their Adaptability To Metal Stress

Combining phytoremediation with energy crop cultivation offers attractive economic alternatives, with the view of achieving low price decontamination of soil by the production of biodiesel. In order to screen potential energy plants that can be planted in heavy metal contaminated area for biodiesel, cadmium (Cd), zinc (Zn) and copper (Cu) tolerance and accumulation capacity of eight energy plants, including peanut (Arachis hypogaea), hemp (Cannabis sativa), flax (Linum usitatissimum), caster (Ricinus communis), soybean (Glycine max), sunflower (Helianthus annuus), rapeseed (Brassica rapa) and safflower (Carthamus tinctorius) were evaluated by pot experiments. On the basis of this, peanut, a metal toleranted energy plant widely cultivated in many countries for traditional food oil production, were chosen for further studies, the following questions are posed:(a) how do peanut leaves respond to distinct heavy metal in terms of morphology, anatomy and physiology; and whether the plasticity in response to heavy metal stress was adaptive; (b) do Cd or Zn toxicity affect photosynthetic performance and anatomic structure of leaves, and what relationships between these characteristics; and (c) what kind of mechanisms do peanut plants cope with Cd or Zn toxicity. Furthermore, the roles of exogenous substances, such as salicylic acid (SA) and silicon, in alleviating Cd toxicity were also studied. The results are showed as follows:The pot study conducted with Cd (50 to 200 mg Cd kg-1 sands), Zn(200 to 800 mg Cd kg-1 sands), and Cu (200 to 800 mg Cd kg-1 sands) indicated that all plant species tested initially have ability to withstand Cd and Zn stress, whereas the capacity of Cu tolerance are relatively low. Among these plants, hemp, flax, caster and peanut exhibited a higher level of Cd tolerance, while hemp, flax, and rapeseed had a strong tolerance to high Zn concentrations, and these plants could be cultivated in Cd or Zn-contaminated soils for biodiesel production. Metal accumulation in plants were metal specific and species specific. As for metals, Zn content in tissues was the most, followed which is Cd, and Cu the least. In respect for the plant species, hemp, flax, and peanut showed a high ability of Cd accumulation, while peanut and soybean exhibited higher Zn concentrations in shoots. These energy plants, therefore, are good candidates for the implementation of this new strategy of cultivating biodiesel crops for phytoremediation of Cd or Zn-contaminated soils.Phenotypic plasticity in morphological, anatomical and physiological traits of peanut leaves was tested at four different concentrations of Cd, Cu and Zn under greenhouse conditions. Among 18 characteristics tested, nine were found to be the most sensitive and demonstrate the greatest phenotypic plasticity. These were:the leaf area (LA), the leaf mass per area (LMA), chlorophyll a content (Chl a), chlorophyll b content (Chl b), total chlorophyll content (Chl t), the effective quantum yield of photosystem II (ΦPS II), stomatal density of upper epidermis (SDU), palisade thickness (PT), and palisade to spongy thickness ratio (P/S). The plasticity of chlorophyll content and fluorescence parameters may be maladaptive and reflects metal toxicity to leaves, whereas the anatomical plasticity is adaptive, indicative of a tradeoff between the physiological and anatomic plasticity. Both Cd and Zn treatments caused an inhibition in the net photosynthetic rate (Pn) of peanut (Arachis hypogaea) plants, due to the reduction of stomatal conductance (Gs) and photosynthetic pigment content, as well as the alteration in leaf structure. The decrease of the transpiration rate (E) and Gs might result from the Cd or Zn-induced xerophyte anatomic features of leaves (i.e. thick lamina, upper epidermis, palisade mesophyll, high palisade to spongy thickness ratio, as well as abundant and small stomata). The decline of Pn seems to be independent of the impairment in PSⅡ.Peanut plants had a strong tolerance to high Cd and Zn stress, and also accumulated a certain amount of Cd and Zn in the tissues. At the level of plant tissues, most of Cd and Zn absorbed by the plants were retained in the roots. At subcellular level, most of Cd and Zn in leaf and root cells were fixed in the cell wall fractions, whereas the most of Cd and Zn in the soluble fraction were compartmented in the vacuole. In respect of physiological characters, the obtained result showed that the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR) were inhibited by low Cd treatment, while at high level, SOD and GR were increased. Similarly, under high Zn condition, the activities of SOD and APX were also increased. Furthermore, a Cd induced Cd binding protein and a innate Zn binding protein was found in root and leaf soluble fraction, respectively. It is concluded that the tolerance of peanut plant to Cd and Zn toxicity was resulted from the metal exclusion stratedge, in which the fixation of Cd and Zn in cell wall, compartmentation of the vacuole, sequestration by metal binding protein, as well as an efficient antioxidant systems were involved.In order to assess cadmium tolerance of hemp, and whether salicylic acid (SA) pretreatment regulate the growth and photosynthetic capacity of hemp under Cd stress, a pot experiment was conducted under greenhouse conditions. Exposure of hemp plants to low Cd (25 mg kg-1) had stimulatory effects on plant growth, whereas it was inhibited at high Cd stress (50 and 100 mg kg-1). Cd exposure showed little inhibition in photosynthetic pigment, chlorophyll fluorescence, as well as photosynthetic performance. These results demonstrated that hemp has innate capacity to tolerant Cd stress. SA pretreatment counteracted the Cd-induced growth inhibition in hemp plants; this was more obvious under high Cd stress (100 mg kg-1). SA affect on alleviating Cd toxicity in hemp seedlings was associated with reduced Cd uptake and improved photosynthetic capacity due to stomatal limitations other than photosynthetic pigments.Silicon (Si) is generally considered a beneficial element for the growth of higher plants, especially for those grown under stressed environments. Recently, the mitigating role of Si in cadmium (Cd) stress has received some attention. However, its mechanisms involved remain poorly understood. We studied the effects of Si on tissue and subcellular distribution of Cd with two contrasting peanut cultivars (Luhua 11 and Luzi 101) differing in their Cd tolerance. The results showed that Cd exposure alone depressed plant growth for both cultivars, and this toxicity was more obvious in Cd-sensitive cultivar (Luhua 11) than in Cd-tolerant cultivar (Luzi 101). Si supply significantly alleviated the toxicity of Cd in peanut seedlings. In contrast, the alleviation of Cd toxicity was more significantly in Cd-sensitive cultivar (Luhua 11) than in Cd-tolerant cultivar (Luzi 101). The mechanisms of Si amelioration of Cd stress were cultivar dependent. In comparison to Luzi 101, Cd content in shoots, translocation factor of Cd from root to shoot, and Cd content in cell organelle fractions of leaves in Luhua 11 were more significantly inhibited by Si, indicating Si-mediated inhibition of Cd transport from roots to shoots, and reduction of Cd content in cell organelle fractions of leaves might be responsible for the role of Si in alleviating Cd toxicity in Luhua 11 seedlings.