| In recent years, there have been many reports about contaminated rice in China whichtells us that Cd pollution has been endangering the people’s daily life. Among all the methodsfor cleaning up Cd-contaminated soil, phytoremediation is particularly attractive because ofits special characteristics. Compared with other plants that could be used forphytoremediation purposes, maize is highly regarded by researchers around the world becauseof its excellent characteristics that include availability of many genotypes, ease of cultivation,fast growth, high biomass yield, versatile adaptation and low concentration of pollutant thatends up in seeds.The life cycle of maize includes two key phases: the vegetative phase and thereproductive phase; and two key periods: seedling time and pollination time. Therefore, westudied the mechanism of cadmium tolerance on maize seedlings and the effect of pollinationon the accumulation of Cd in maturing plants. Also, by covering the ear silk in order toprevent the pollination process, we studied the effect of pollination on the Cd accumulationand distribution in every organ of the plant. Finally, we tested our lab results by using thetechnology to clean up contaminated soil in a trail field. The primary results from the presentstudy are presented below:1. We have studied the mechanism by which maize seedlings tolerate Cd in soil duringthe seedling phase. Maize seeds, genotype CT38, were grown in pots containing variousamounts of Cd: CK(Blank) with a background level0.13mg Cd/kg soil and1.00,5.00,20.00,50.00,100.00and200.00mg/kg of soil. We determined the germination rate andinvestigated the growth conditions of maize seedlings during the transition period from theendosperm stage to living solely by photosynthesis. We analyzed the Cd content in theguttation. The germination success of the maize CT38seeds were not statistically affected bythe Cd, except at the highest level of the200mg Cd/kg soil (n=3, p=0.05). Besides theintercept in the root, the tolerance mechanism was also related to the maize’s ability toexclude Cd by guttation. When the Cd content in soil was below5mg/kg, the maize couldendure the pollution without Cd appearing in guttation. When the Cd content in soil was from20mg/kg to100mg/kg, the maize could still endure the pollution while excluding Cd fromguttation，in which the highest Cd concentration was no more than10.4mg/L. However,when the Cd content in the soil was200mg/kg，the capability of the maize to exclude Cdweakened, Cd concentration in the guttation was only8.8mg/L, and the plant could notendure the high Cd pollution any more. The dry mass weight of shoots and roots of maize plants in200mg Cd/kg soil were only25.4%and48.3%, respectively, of that in the blankcontrol. What’s more，there were some dead plants among those grown in the200mg Cd/kgtreated soil. If the maize seedlings had not excluded Cd from guttation, the Transfer Factor(TF) would have become smaller with the increasing of Cd content in soil. Once the maizeplant exclude Cd from guttation, the TF was about equal to0.5and did not change withincreasing Cd content in the soil. Furthermore, there was a strong liner relationship betweenthe Cd content in the maize root and the Cd content in soil after the Cd appeared in the maizeseedling’s guttation. The Cd concentration in the root is11.2times higher than that in the soil.In short, the phenomenon of excluding Cd from guttation is an important feature of the maizeseedlings’ ability to endure and accumulate Cd in soil.2. We investigated the effect of different Cd concentrations in soil on the growth andphysiological features of maize plant. Maize seeds were planted in a series pots with differentlevels of Cd; i.e.,0,1,5,20,50,100mg Cd (as CdCl2/kg of soil). After pollination in thepustulation period, we measured the height and leaf area of the plants and determined thechlorophyll content. In addition, we recorded changes in the activity of three antioxidantenzymes (i.e., SOD, CAT and POD) and the content of malonaldehyde (MDA). We alsoexamined the condition of the plants’ pollen by the Scanning Electron Microscopy (SEM).The results showed that the height and leaf area of maize plant during the postulation perioddid not significantly increase from the blank to the5mg/kg level, but decreased significantlyfrom50to100mg/kg. The chlorophyll content of the maize during the postulation periodincreased at first, reaching a maximum in the5mg/kg treatment plant and then decreased.The Cd content in the soil did not affect the concentration of chlorophyll a/b in the plants. Asthe concentration of Cd in the soil increased, the activity of SOD initially increased and thendecreased with increasing Cd, but the activity of CAT and POD increased in all treatmentgroups. The content of MDA in the plants also increased in all treatments, especially at thelevels of20-100mg Cd/kg soil. Comparing this behavior with the response of the seedlings,we concluded that the maize plant after the elongation phase cannot tolerate the Cd stress aswell as the seedlings did. In the seedling phase, maize could endure100mg Cd/kg soilwithout any obvious ill effects, but after entering the pollination phase after elongation, themaize plants exhibited the serious shorten symptom. What’s more, the reproductive organs ofmaize plant have been affected, and the germ pore of the maize pollen changed with theincreasing of Cd content in the soil. The operculum of pollen germ hole became smaller anddeeper at50mg Cd/kg soil treatments and, more seriously, there was no pollen among plantssubjected to the100mg Cd/mg soil treatments. 3. We have studied the accumulation and distribution of Cd in maize plants. For thematuring maize plant, we determined the dry mass and Cd concentration in every organ,including vegetative organs and reproductive organs. The vegetative organs are the root, stemand leaf, while the reproductive organs include the flower and ear that is divided into bract,ear stalk, ear cob, silk and seeds. From the original data on the dry mass weight andconcentration, we calculated the amount of Cd accumulated and the percentage in every organ.The results showed that low Cd treatments can increase the dry mass weight slightly, whilehigh Cd concentrations in the soil can cause the dry mass weight to decrease significantly.However, the percentage of dry mass weight found in the reproductive organs is almost thesame under all treatment conditions. Further, even though the Cd concentration in every organincreased with increasing amounts of Cd in soil, the distribution of Cd in vegetative andreproductive organs is nearly the same under all of the treatment levels. Specifically, thepercentage of Cd accumulated in vegetative organs was about78.9~88.5%, which shows thevegetative organs are the major sink for absorbed Cd. Only a much lower percentage Cdentered the flower and the ear. In the maize ear the non-edible organs, including bracket, coband ear stalk, absorbed more than50%of Cd that accumulated in the reproductive organs.We conclude that the presence of the non-edible organs can prevent the Cd from entering theseeds, which are the most important and worthy organs of the whole maize plant. In summary,after considering all of the sinks for Cd in the maize plant, we find that the percentage of Cdin the seeds is not more than6%of the Cd accumulated by the plant.4. We studied the effect of pollination on the Cd accumulation in maize. For the maizegrowing on the clean soil, we irrigated them in a solution of CdCl2once before pollination inorder to make the Cd content in pot soil5,50,100mg Cd/kg soil. When the maize plantsfinished their life period, we determined the Cd concentration in every organ and comparedthe results between the plants that were allowed to pollinate and those that were preventedfrom being pollinated. The comparison showed that even though the dry mass weight ofun-pollinated plants was16.4~22.4%lower than that of the pollinated plants, the Cdaccumulation in un-pollinated plant is more46.2%higher than in the pollinated plant.However, the distribution between vegetative organs and reproductive organs did not changeunder different pollination conditions. This result shows that the physiology of the maizeduring pollination plays an important role in the accumulation of Cd by the plants.5. We field-tested our lab results on vegetable fields in four villages in the BaiyunDistrict of Guangzhou City where the soil is heavily polluted by heavy metal. The soils wereanalyzed for five heavy metals; i.e., Ni, Cu, Zn, Cd and Pb. The heavy metal conditions were assessed using single pollution indices for each contaminant and two kinds of integratedpollution indices, including the arithmetic mean and the Nemerow pollution index. The resultsshowed that the Cd pollution is the most serious condition in the investigated soils and thatthe levels of the other metals (i.e., Ni, Cu, Zn and Pb) in the soil are safe. The Cdcontamination in the fields of the four villages decreased in the following order: Hengli＞Nanpu＞Fangshi＞Daling. Based on these results, we chose the vegetable fields in Henglivillage as the location for phytoremediation.6. The phytoaccumulation ability of maize CT38was tested in the field trail. Building onour results obtained in our laboratory, we studied the optimal amount and time for addingnitrilotriaceticacid (NTA) that would results in the maximum Cd accumulation in the wholemaize plant. Furthermore, we prevented pollination by covering the silk with a plastic bag inorder to maximize the Cd accumulation that we saw. The Cd concentrations before and afterphytoremediation of the fields were used to generate maps (drawn by Geological Statisticssoftware (GS+, Version5.0)) to visualize the overall effect of phytoremediation. The resultsshowed that the maize CT38accumulated more Cd than did the native maize species, HZ.Without the addition of NTA, CT38accumulated25.9%more Cd than HZ did. Further, afterthe addition of25mmol of NTA, CT38accumulated39.3%more Cd than did HZ. AddingNTA after pollination, although the amount of Cd accumulated increased, the relativedistribution of Cd within the organs of the plant did not change. While adding NTA beforepollination cannot increase the Cd accumulation but can change the percentage of every organ.The percentage of Cd in the leaf increased and the percentage in the stem decreased when theNTA was added. By using phytoremediation engineering techinques in the field, we haveverified that the un-pollinated maize can accumulate more Cd than pollinated maize. Throughun-pollinated phytoremediation, the Cd content in field decreased from0.4~0.5mg Cd/kg soilto0.24mg/kg or less, while the decrease with pollinated phytoremediation was from0.4~0.5mg Cd/kg of soil to0.24~0.28mg/kg. Most importantly, the Cd concentration in seedsin the field phytoremediation study was in the range of0.03~0.062mg Cd/kg of seeds, whichwas below the Chinese government’s permitted concentration (0.1mg Cd/kg seeds) in coarsecereals (GB2072-2012). Therefore, this study shows that it is possible to conduct maizephytoremediation of Cd-contaminated soil while, at the same time, harvesting a crop, forsubsequent consumption by animals or humans. |
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