| Currently, the soil copper pollution is worsening, due to the rapid development of industry and agriculture. It not only caused heavy metal contamination of horticultural products, but also had an impact on animals, microorganisms and soil enzymes. Melon (Cucumis melo L.) is the major type of fruit and vegetable in the world. In recent years, with the rapid development of ecological agriculture, fruit quality and safety requirements are becoming more stringent in the market. Grafting as an important agronomic measure, has been reported that it would increase copper, cadmium and other heavy metals’ tolerances of cucumbers, grapes and other horticultural crops. But whether it will improve the melon resistance by grafting under copper stress is still unclear. Thus, this experiment studied physiological mechanisms of resistance improved by grafting in two kinds of melon seedlings (’IVF097’,’VF117’) under copper stress. In the experiment, pumpkin ’JingXinZhen No.31 as rootstock, seedings were cultivated in compound substrate composed of perlite and vermiculite (2:1 by volume). Grafted and ungrafted seedlings were studied to analysis the physiological mechanisms of resistance improved by grafting. The main results were presented as follows:1. Copper stress inhibited the accumulation of biomass in two varieties of melon seedlings, but under the same stress conditions, the height, stem diameter, leaf area and fresh and dry weight of leaves, stems, roots in the grafted seedings were greater than that of ungrafted seedings. The results showed that grafting reduced copper stress on growth inhibition of melon seedling, and promoted melon seedling growth.2. Under copper stress condition, photosynthetic pigment contents of two melon varieties of seedlings decreased, which caused photosynthetic parameters change. Under the same stress, photosynthetic pigment contents, Pn, Tr, Gs and Ci in leaves of grafted seedings were greater than that of ungrafted seedings. However, water use efficiency and Ls were smaller than that of ungrafted seedings. These results suggested that copper stress caused a decline in photosynthetic characteristics mainly in consequence of stomata limiting. Copper weakened stress on photosynthetic pigments of grafted seedlings and improved its photosynthesis.3. Copper stress weakened the ability of sugars synthesize in ’IVF09’ seedlings and changed total activity of glucose metabolism enzymes in leaves. Under the same stress conditions, the contents of glucose and fructose in grafted seedlings were higher than that of ungrafted ones, but the opposite results indicated in the contents of raffinose and stachyose. The SS total activity in leaves of grafted seedlings was lower than that of ungrafted ones, but the SPS, NI, AI total activity were higher than that of ungrafted ones. Grafting changed glucose metabolism enzymes activity and promoted sucrose, stachyose, raffinose converting into small molecules of glucose and fructose, which involved in osmotic adjustment. So, grafting increased the resistance of melon seedlings under copper stress.4. Under copper stress conditions, membrane permeability and osmolyte content of two varieties increased. The relative electrolyte leakage in leaves of grafted seedings was significantly smaller than that of the ungrafted seedlings. The soluble sugar and soluble protein contents were all increased under copper stress condition, at the same time, those were also higher than that in ungrafted seedimgs. The results showed that grafting weakened cell membrane permeability, promoted the synthesis of melon seedlings osmotic adjustment substance, and enhanced ability of seedlings’ osmotic adjustment.5. MDA content significantly increased in two varieties and POD, CAT activity of melon seedlings increased under copper stress, but at the same stress treatment, the POD, SOD, CAT activity of grafted seedlings were always greater than the ungrafted ones and MDA showed the opposite result. Grafted seedlings reduced the effect of copper stress on membrane lipid peroxidation, increased the activity of antioxidant enzymes, and reduced oxidative damage of membrane lipids.6. Under copper stress condition, Cu content in leaves and roots of two varieties seedlings increaded. Cu content in leaves was lower than that in roots, and grafting could reduce the Cu absorption of melon seedlings. Copper stress also affected the balance of mineral elements absorption in melon seedlings. Grafting promoted K, P absorption of seedlings, but inhibited Na, Mg absorption. While the K, P, Na content were all higher in leaves and roots and Mg content were only higher in roots.7. Endogenous hormone content and endogenous hormone balance in leaves of ’IVF09’ seedings were changed in copper stress condition. The IAA, ABA content in levels were increased and GA3 content decreased. ZR content decreased in leaves of grafted seedings, but ZR content of ungrafted seedings increased. Under the same stress condition, IAA content in grafted seedlings was significantly higher than that of ungrafted one, but ABA, GA3, ZR content in leaves of grafted seedings were significantly lower than that of ungrafted ones. In addition, ABA/IAA, ABA/GA3, ABA/ZR values of grafted leaves were lower than that in ungrafted ones and ABA/IAA, ABA/GA3 value changed more less in grafted seedings. So, grafting changed endogenous hormone levels and enhanced resistance of copper stress, and effectively regulateed the balance of endogenous hormones in plant, which promoted normal growth of plants. |
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