The Effects Of Nitrogen On Poplar Growth And Chlorophyll Synthesis Under Cadmium Stress

Cadmium is a major heavy metal pollution of soil and water bodies, and great harm to human health and ecosystems. The mechanism of cadmium poisoning to the plant and the phytoremediation of cadmium contaminated soil is the world’s hot issues of the heavy metal in biological research. There is a lot of research on this, but the real reason and mechanism of cadmium poisoning to the plant are unclear. The author discovered that the addition of nitrogen to the poplar plants under the cadmium stress could promote the growth of poplar and alleviate the toxic effect of heavy metals in the past experiments. In order to explain the phenomena and verify that whether nitrogen play a key role in the alleviation of cadmium toxicity in poplar, the change law of the intermediates product of chlorophyll synthesis is analyzed in the study."2025poplar" is used as material to study the effect of nitrogen on the poplar growth, chlorophyll content, chlorophyll and ultrastructure of chloroplasts under cadmium stress. The results in the study is help to provide theoretical basis for studying the poplar’s mechanism resistance to cadmium, to solve the pollution of cadmium and landscape plant ecological restoration of soil under cadmium pollution. The main findings are as follows:(1)The poplar leaf changed lesions such as the leaf color from green to yellow under cadmium stress. Under cadmium stress, leaf veins turned red first, and then changed russet spots, at last the leaf turned yellow obviously and began to fall. After added nitrogen, poplar morphology was same with control group basically, without distinct differences, which showed that nitrogen could alleviate the toxic effects of poplar under cadmium stress.(2)Under cadmium stress, plant height and ground diameter of poplar had extremely significant difference compared with control group, plant height and ground diameter were significantly decreased, which showed that the high cadmium concentration had inhibition of poplar growth. After added nitrogen, plant height and ground diameter of poplar significantly increased compared to control group and cadmium stress group, which explained that added nitrogen could alleviate the toxic effects of poplar under cadmium stress and promote the plant growth. (3)High concentrations of cadmium exert toxic effects to root, stem and leaf of poplar. Cadmium was enriched in various plant organs, and the order of cadmium content of the poplar different organs is root>stem>leaf. After added nitrogen, cadmium content in soil was less than it under cadmium stress, the cadmium content decreased by6.3percent, while cadmium content of leaf was greater than it under cadmium stress significantly, the cadmium content increased by77.6percent, which showed that after adding nitrogen, poplar strengthened the enrichment of cadmium.(4)Cadmium inhibited poplar chlorophyll a, chlorophyll b, carotenoid, chlorophyll synthesis and photosynthetic pigments contents. While after added nitrogen, chlorophyll a, chlorophyll b, carotenoid and chlorophyll content were increased, photosynthetic pigments metabolism to speed up, and leaf color returned to normal.(5)Under cadmium stress, the minimum Chlorophyll fluorescence (Fo) was greater than treatment, and the maximum fluorescence (Fm) and maximum quantum efficiency (PS II) were less than treatment. After added nitrogen, F0was less than treatment, Fm and PS II were greater than treatment. The net photosynthetic rate, transpiration rate, stomatal conductance and the intercellular CO2concentration of poplar under cadmium stress had extremely significant difference with control treatment. After added nitrogen, the net photosynthetic rate, transpiration rate, stomatal conductance and the intercellular CO2concentration were higher than control treatment. Nitrogen promoted the growth of poplar.(6)MDA content fluctuates during the treatment periods. Under cadmium stress, MDA content went higher significantly compared with control. After added nitrogen, MDA content was lower than cadmium stress treatment, which explained that nitrogen alleviated the poplar toxic effects of cadmium.(7)In the process of chlorophyll biosynthesis metabolism, the relative content δ-aminolevulinic acid, ALA, porphobilinogen and uroporphyrinogenⅢ were higher than control treatment under cadmium stress, and then coproporphyrinogenⅢ, protoporphyrin Ⅸ, Mg-protoporphyrin Ⅸ and pchlide were lower than control treatment. After added nitrogen,δ-aminolevulinic acid,ALA, porphobilinogen, uroporphyrinogen Ⅲ, coproporphyrinogenⅢ, protoporphyrinIX, Mg-protoporphyrinⅨ and pchlide were the same as control treatment. The above results showed that in the process of chlorophyll biosynthesis metabolism, there was hindrance synthesis from uroporphyrinogenⅢ to coproporphyrinogen Ⅲ, nitrogen alleviated the inhibition of the process of poplar chlorophyll synthesis under cadmium stress, and chlorophyll synthesis returned to normal.(8)Under high concentrations of cadmium stress, there was obvious change of poplar leaf chloroplast ultrastructure, chloroplast ruptured and was out of shape, the internal structure was destroyed, even appeared that chloroplast broke up. It would be the reason that poplar leaf color turned to yellow under cadmium stress. After added nitrogen, poplar leaf chloroplast ultrastructure had not changed obviously, only a little chloroplast was out of shape, the internal structure was not changed yet, which showed that nitrogen alleviated the toxic effects of poplar chloroplast under cadmium stress.