| Copper is an essential nutrient for the growth and development of plants. It is a constituent micronutrient of protein components of several enzymes, mainly of those participating in electron flow, catalyzing the redox reaction in mitochondria and chloroplasts in the cytoplasm of plant cells. When absorbed in excess amounts, copper can cause damage leading to total inhibition of plant growth. Recently, the increasing application of fertilizer and agrochemical, the mining and smelting of metal and the irrigation of wastewater result in the progressive accumulation of toxic copper in the soil. Copper, one of main toxic metals in the soil, has attracted considerable attention. However, the mechanisms of copper toxicity and tolerance need for the further investigation. In this paper, solution culture experiments were conducted using maize and rice as experimental materials. The effects of different copper levels on morphology and structure of maize roots were investigated. Transmission electron microscopy was used to observe effects of different copper concentrations on cell ultrastructure in maize roots and leaves as well as distribution of copper. Xylem sap was collected to study the effects of different copper concentrations on translocation of nutrients and copper in maize. The differential centrifugation technique and sequential chemical extraction method were used to study the subcellular distribution and the chemical forms of copper in the roots, leaves and stems of maize. Growth, copper accumulation and nutrient uptake of maize were studied. The main results were as follows:(1) Under the condition of 1μmol·L-1Cu treatment, growth, root length, root surface area, root volume and average diameter of maize seedlings were not inhibited. With elevation of copper concentration, growth of maize was inhibited. All the parameters of root morphology markedly decreased under high copper concentrations. According to classification, the growth of 0.6mm-1Cu level.(2) For root tip of maize, cell structure, cell number unit length and cell length in zone of elongation were no obvious difference between the 1μmol·L-1Cu level and the control treatment. With the increase of copper concentration, calyptrogens became obviously short, cells of which began to destroy and shell; in root cortex the destruction of parenchymatous cells and vessels was more serious, and parenchymatous cells collapsed and vessels broke at 80μmol·L-1Cu level; cell length increased, and cell number unit length decreased in zone of elongation. Excess copper may cause reduction in cell division instead of reduction in cell elongation.(3) As for the ultrastructure of maize root cells, dense cytoplasm, plasmalemma sticking cell walls, abundance of cell organelles, small nuclei and nucleolus, homogenous nucleoplasm and folded nuclei membrane were observed in cortical cells of 1μmol·L-1Cu-treated roots; plasmolysis and folded plasmalemma were found in vascular bundle cells. With the increase of copper concentration, in cells of root cortex plasmolysis and the destruction of plasmalemma became more serious, cell organelles disintegrated completely, nucleus with disintegrated nucleolus became smaller, nucleus membrane bursted, chromatin agglomerated, nucleus membrane finally disappeared leading to nucleolus and nucleoplasm distributing in cytoplasm, and dense and compact materials deposited in vacuole at 20μmol·L-1 Cu level, in nuclei membrane at 50μmol·L-1 Cu level and in plasmalemma at 80μmol·L-1 Cu level; in vascular bundle cells plasmalemma and structure of cell organells were badly destroyed, and dark sediment distributed in cytoplasm, plasmalemma and vessel walls.(4) Compared with the control treatment, blurred plasmalemma was found in epidermal cells of maize leaves while no obvious significance was appeared in mesophyllic cells cells under 1μmol·L-1 Cu treatment. With the increase of copper concentration, the damage of plasmalemma and plasmolysis became more serious in epidermal cells of maize leaves, and dark granular materials deposited in cytoplasm and cell walls; damaged plasmalemma and disintegrated membrane and disorderly lamella of chloroplasts were detected in mesophyllic cells; dark materials deposited in plasmalemma under 80μmol·L-1 Cu-treated condition; under 20μmol·L-1 Cu and 50μmol·L-1 Cu treatments large lipids and many variform vesicles were formed in chloroplasts; the decrease number of cristae and disintegrated membrane were found in mitochondria, and finally cristae and membrane of mitochondria disappeared.(5) The content and accumulation of copper in maize shoots had no obvious difference between 1μmol·L-1 Cu treatment and the control. With the elevation of copper supply, compared with the control, the copper content in shoots markedly increased. The content and accumulation of copper in maize roots increased, which in the 20μmol·L-1Cu-treated roots were the largest among the five Cu levels. Excess copper had little influence on the uptake of P by maize. Both N uptake by maize roots and N allotment in shoots were inhibited under the 80μmol·L-1Cu treatment. K uptake by maize roots and K distribution in shoots were obviously retarded. All 4 copper levels inhibited Ca allotment in maize shoots, whereas 1μmol·L-1Cu treatmentimproved Ca uptake by roots. Mg uptake by roots and Mg allotment in maize shoots were limited under 50μmol·L-1Cu treatment. Zn uptake by roots and Zn allotment in maize shoots were limited under 80μmol·L-1Cu treatment. Fe distribution in maize shoots was accelerated at 1μmol·L-1Cu level, while uptake and allotment of Fe and Mn in maize plants were obviously inhibited with the elevation of copper supply.(6) The effects of different copper levels on translocation rates of maize xylem sap were different. Compared with the control, transport rates of xylem sap increased in 1 and 20μmol·L-1Cu-treated maize whereas just the contrary in 50 and 80μmol·L-1 Cu-treated plants. Under 20μmol·L-1Cu treatment, copper concentration in maize xylem sap was higher, and copper transport rates increased. For different forms N in xylem sap, NH4+-N was sensitive to copper stress, translocation of which was retarded simply under the low copper concentration. Excess copper had no inhibition on transport of organic nitrogen. Its translocation rates and organic N/inorganic N increased with the elevation of copper supply. Among 5 copper levels, 1μmol·L-1Cu treatment stimulated the translocation of NO3--N, organic N, P and K synchronously.(7) Copper was mainly bound to cell walls and soluble fraction, and little was found in the cell organelle fraction in maize cells. Copper content in cell walls was more than in the soluble fraction in the control and 1μmol·L-1Cu-treatment roots. With the elevation of copper concentration in solution, distribution of copper was reduced in cell wall fraction and increased in the soluble fraction. Copper content in soluble fraction was highest in subcellular parts of maize stems, whereas that in cell wall fraction came next under different copper concentrations. On the contrary, copper mainly distributed in cell wall fraction in leaves under different copper concentrations. Chemical forms of copper in roots, stems and leaves of maize were significantly different at different copper levels. HCl extractable copper was the main form in roots while several copper forms were observed in shoots under the control condition. HCl extractable copper was still superior to other forms in roots and NaCl extractable copper was advantage in shoots under 1μmol·L-1Cu concentration condition. Ethanol extractable copper became dominant in roots and leaves with the increase of copper supply. In stems, NaCl extractable copper was mainly existed in the 20 and 50μmol·L-1 Cu treatments while water and NaCl extractable copper most in 80μmol·L-1 Cu treatment.
|
PDF Full Text Request