Effects Of Abscisic Acid Application And Nitrogen Forms On Decreasing Cadmium Accumulation In Plants

Cadmium (Cd) contamination of soils causes a severe reduction in crop yield and nutrition quality. As a result, Cd put human health at risk through food chain. Measures need to be developed to minimize Cd entering the human food chain from contaminated soils. Up to now, several options have been proposed to remediate the contaminated soils. However, physical and chemical methods are too expensive, while phytoremediation and breeding or selection of low-Cd crops are time consuming. Besides, GM crops have continuously generated unresolved controversies. Previous studies reported that pretreatment with abscisic acid (ABA) showed lesser Cd uptake and nitrogen (N) form has a significant effect on soil pH and Cd uptake of the root system. On the basis of these findings, in this thesis, Arabidopsis and Chinese cabbage (Brassica chinensis L.) were used to investigate the effects of of ABA application and N forms on decreasing Cd accumulation in plants in order to provide new strategies for safe production in the soils with slight or moderate Cd contamination. The results are summarized as following.(1) In hydroponic culture, application with low doses of (0.1-0.5 μM) abscisic acid (ABA) clearly inhibited Cd uptake by roots and decreased Cd level in Arabidopsis wild-type plants (Col-0). Expression of IRT1 in roots was also strongly inhibited by ABA treatment. Decrease in Cd uptake and the inhibition of IRT1 expression were clearly lesser pronounced in an ABA-insensitive double mutant snrk2.2/2.3 than in the Col-0 in response to ABA application. The ABA-decreased Cd uptake was found to correlate with the ABA-inhibited IRT1 expression in the roots of Col-0 plants fed two different levels of iron. Furthermore, the Cd uptake of irt1 mutants was barely affected by ABA application. These results indicated that inhibition of IRTl expression is involved in the decrease of Cd uptake in response to exogenous ABA application. Interestingly, ABA application increased the iron level in both Col-0 plants and irtl mutants, suggesting that ABA-increased Fe acquisition does not depend on the IRT1 function, but on the contrary, the ABA-mediated inhibition of IRT1 expression may be due to the elevation of iron level in plants. From our results, we concluded that ABA application might increase iron acquisition, followed by the decrease in Cd uptake by inhibition of IRT1 activity. Thus, for crop production in Cd contaminated soils, developing techniques based on ABA application potentially is a promising approach for reducing Cd accumulation in edible organs in plants.(2) Theoretically, split application of ammonium and amide fertilizers could increase the duration period of a certain proportion of ammonium in soil, thereby increasing the N uptake by roots in form of ammonium, which could minimize pH decrease resulted from ammonium nitrification, thus alleviating the acidification-increased Cd solubility. Therefore, we compare the difference between the effects of single and split fertilizations on Cd accumulation in plants. A pot soil experiment was conducted with Chinese cabbage using different forms of fertilization. In Cd-contaminated soils, compared to single application treatments, split application of either ammonium or amide fertilizers significantly increased biomass, concentration of total phenolics and DPPH scavenging activity but reduced Cd level in the edible parts. Split application of nitrate fertilizer also slightly increased the biomass and slightly reduced the Cd level in edible part of Chinese cabbage, but there was no statistical difference. However, the concentrations of soluble protein, total phenolics and flavonoids were significantly increased under split application of nitrate fertilizer. Meanwhile, split application of ammonium fertilizer also obviously increased the concentration of soluble sugar in the edible parts. These results showed that split application of either of N fertilizers is a suitable option for vegetable cultivation in Cd-contaminated soil.(3) A pot experiment was conducted to study the role of soil pH-buffering capacity in affecting the effect of N form on Cd accumulation and nutrition quality in Chinese cabbage. We gained high pH-buffering capacity soil by adding calcium carbonate (CaCO3) to a soil (Control) with low pH-buffering capacity. We found that, under Cd exposure, in Control, compared to nitrate fertilizer, ammonium fertilizer significantly increased the biomass and the soluble protein level, but reduced the levels of Cd and soluble sugar in the edible parts of Chinese cabbage, In addition, little difference was observed in DPPH scavenging activity of edible parts between these two N treatments. However, in the soil with higher pH-buffering, compared to ammonium fertilizer, nitrate fertilizer significantly decreased both the levels of Cd and soluble protein in the edible parts. In addition, compared to ammonium fertilizer, nitrate fertilizer only slightly but not statistically significantly reduced the biomass, soluble sugar concentration and DPPH scavenging activity. Considering of the above findings, for safe crop production, it is better to apply nitrate fertilizer in low pH-buffering soil, whereas ammonium fertilizer is in high pH-buffered soil.