Studies On Cadmium Accumulation And Tolerance In Two Canna Species

Cadmium (Cd) pollution in soil has been one of the most important environmentalconcerns in the world. Phytoremediation, used to clear Cd pollution, has gained significantprogress in recent decades as cost-effective, eco-friendly technologies. Scientists arededicated to find new plants with ecomomic value or landscape value used in Cd pollutionarea. Some canna species might be an interesting alternative for use in phytoremediationtechnology with their high tolerance to Cd, high biomass production and fast-growingadvantage. Canna edulis is a new energy plants with ornamental value, its rhizome can beused to produce industrial alcohol due to high starch content. But the absorptioncharacteristics and remediation ability of C. edulis to Cd have not repored until now. In thispaper, three edible canna cultivars (PLRF, Xingyu-1and Xingyu-2) which are the mostcommon cultivars in China, and a C. generalis cultivar (Australia) were used as materials.This study mainly focused on the Cd accumulation characteristics in two Canna species andthe effect on their growth under Cd stress, discussed the tolerance mechamisns of Canna fromphotosynthetic physiology, antioxidative stress, micro-environment change in the rhizospheresoil and absorption and distribution of mineral elements under Cd stress by pot experientment.The main results are described as follows:1. Canna plants could be used for the remediation in situ decontamination of soilspolluted with Cd because of their high biomass, high tolerance and accumulation of Cd andfast-growing advantage, but they were not hyperacculmulators because they didn’t meet thestandard of Cd huperaccumulators. There were differences between C. edulis and C. generalis,the biomatter, tolerance and accumulation of C. edulis were higher than those of C. generalis,so the remediation effect of C. edulis was better than C. generalis. There were differencesamong the cultivars of C. edulis as well. The accumulation ability of the Xingyu-1was thehighest among three cultivars, followed by PLRF and the Xingyu-2. The remedationefficiency of Xingyu-1or PLRF was better than Xingyu-2in Cd polluted soil. However theXingyu-2had higher security as food crops for cultibation because it belonged to a relativelylower accumulator in three cultivars. 2. The distribution of Cd in canna plants was non-uniform.70%to80%of Cd presentedin the underground organs (roots and rhizomes). The retention of Cd in the roots played animportant role in detoxifying Cd.3. Low concentration of Cd treated in seedling stage promoted the growth of Canna, buthigh concentration of Cd inhibited the growth of Canna. The inhibited effects showeddifference between two treatment stages and two species, the mature stage was lower thanseedling stage, C. edulis was lower than the C. generalis.4. Cd stress decreased the photosynthetic pigment contents of canna plants, resulting tothe declines of net photosysthesis rate (Pn). At the same time, the stomatal conductance (Gs)and intercellular CO2concentration (Ci) of edible canna were also inhibited under Cd stress,but Cd stress had no significant effect on the Gs and Ci of C. generalis. So the Pn declines ofedible canna were due to both stomatal and non-stomatal inhibiting factors, while that of C.generalis was only dedicated to the non-stomatal inhibiting factors. Cd stress significantlydecreased the transpiration rate (Tr) of canna plants, led to the reduction of transpiration pulland limitment of the Cd uptake and transfication from the underround organ to aerial organ,which was another protection mechanism to reduce Cd toxicity.5. Low concentrations of Cd did not damage PSII of canna plant, but high concentrationsof Cd significantly reduced the maximum quantum yield (Fv/Fm), the actual quantum yield,energy capturing and the energy involved in the photochemical reaction, damagedphotosystem II (PSII), blocked photosynthetic electron transfer rate (ETR), resulted in Pndeclines. C. generalis reduced toxicity of high concentrations Cd by improvingnon-photochemical quenching (qN). However, there were no such protection mechanisms inC edulis.6. Oxidative stress was induced by Cd treatment in both leaves and roots. The changes ofH2O2contents were stronger and earlier than the relative electric conductivity (REC) and thecontents of malondialdehyde (MDA). The degree of oxidative stress in the roots was strongerthan in the leaves, in C. generalis than in C edulis. At the same time, canna plants started theantioxidant enzymes protection system and non-enzymes antioxidant system in response toCd stress. High Cd-tolerant species or cultivar, such as C edulis or Xingyu-1, had higherantioxidant enzymes activity and antioxidant contents than the low Cd-tolerance ones, such asC. generalis cultivar Australia. Peroxidase (POD) was responsible for the peroxidedetoxification in roots of C. generalis, while catalase (CAT) was responsible for the peroxidedetoxification in roots of C edulis.7. Cd treatment changed the soil microenvironment of canna plants rhizosphere,improved the quantity of microbial community, reduced the soil enzyme activities, and affected the activation and utilization of the soil mineral elements in the rhizosphere. Lowconcentration Cd had little effect on the soil microenvironment of canna plants rhizosphere,but high oncentrations of Cd stress increased numbers of fungi and actinomycetes in soilrhizosphere and had little effect on the numbers of bacteria. Cd stress decreased urease,protease and invertase activity in rhizosphere soil of canna plants. The decline of protease wasthe largest, followed by urease, sucrase was the smallest. The inhibition effect in seedlingstage was less than in mature stage. Cd stress also increased soil catalase activity.8. Low concentration of Cd promoted abosorption of the mineral and nitrogen nutrientelements of edible canna, and changed the distribution ratio of mineral elements inabove-ground organ and underground organ. However, high contentration Cd inhibited theuptake and the translation of mineral elements in organ of edible canna. The variation ofdifferent mineral elements showed the significant difference among the different cultivars.The effects of Cd stress on macroelements were less than that of microelements in C edulis.Cd stress significantly inhibited abosorption and accumulation of the mineral and nitrogennutrient elements in C. generalis Australia. The promoted effect of Cd on the mineralelements in edible canna was related to the changes of soil microenvironment in rhizosphere,whereas the inhibition effect of high concentration Cd strss on the mineral elements in cannaplants was mainly due to the reduction of active absorption of mineral elements by reducingphotosynthetic energy and enzyme activity in metabolism, of passive absorption bydestruction ion channael and transporter protein. Cd stress reduced the transpiration pull andblocked the translocation of the mineral elements from the underground organ to arieal organtoo. According to the results of mineral elements, declines of chlorophyll content andchlorosis of the cannal plants induced by Cd were not caused by Fe, Mg, Cu, Mn, Ca and Kdeficit.