外源乙烯对镉处理下玉米幼苗生理代谢的影响

为探讨外源乙烯缓解玉米（Zea mays）幼苗镉（Cd）毒害的生理机制，通过水培试验研究了Cd处理下，外源乙烯对玉米幼苗相关生理指标与Cd的亚细胞分布的影响，以不做任何处理为空白对照，以Cd处理和（NH₄）₂SO₄处理为试验对照。结果显示，相对Cd处理，乙烯和（NH₄）₂SO₄处理可显著降低Cd胁迫下玉米幼苗H₂O₂和丙二醛（MDA）含量，使净光合速率分别提升1.23倍和1.22倍；显著降低抗氧化物酶超氧化物歧化酶（SOD）、过氧化氢酶（CAT）]活性，抗氧化物质抗坏血酸（AsA）、谷胱甘肽（GSH）]含量则显著上升。另外，相对于Cd处理，乙烯+Cd处理可使玉米幼苗ATP硫酸化酶活性、谷胱甘肽还原酶（GR）活性、半胱氨酸和还原型谷胱甘肽（GSH）含量分别上升54.43%、27.93%、50.77%和49.85%，而对非蛋白硫醇（NPT）和植物螯合素（PCs）含量无显著性影响。在乙烯+Cd处理的基础上添加GSH合成抑制剂BSO（buthionine sulfoximine）可导致玉米叶片GSH含量显著降低，H₂O₂含量上升，光合速率下降。外源乙烯可显著降低Cd胁迫下玉米叶片Cd含量，而显著提升根部细胞壁和液泡中Cd含量。因此，外源乙烯一方面通过提升玉米叶片GSH和AsA含量，增强叶片非酶促抗氧化能力，而非通过抗氧化酶促反应和NPT、PCs的螯合作用；另一方面则通过根细胞壁的固定作用和液泡区室化作用，减少Cd向玉米叶片中的转移，从而缓解Cd毒害。研究结果可为乙烯作为潜在的作物重金属拮抗剂提供理论依据。

Effect of exogenous ethylene on physiological metabolism of Zea mays seedlings under cadmium stress

There has been increasing heavy metalsespecially cadmium (Cd)] pollution in farmlands in China. Studies have identified the crucial role of exogenous ethylene in the reversal of Cd stress in plants such as Arabidopsis thaliana mustard. However, few studies have been done on maize (Zea mays), which is the second largest staple crop in China. To investigate the potential process by which exogenous ethylene alleviates Cd stress in maize, hydroponic experiments were conducted. The experiments included a treatment that served as a blank control and others that were Cd and (NH₄)₂SO₄ treatments. Changes in physiological indexes of maize seedling leaf along with subcellular distribution of Cd in leaves and roots of the plant were determined under Cd treatment, exogenous ethylene treatment and exogenous sulphur treatment. The results suggested that H₂O₂ and malondialdehyde (MDA) contents of maize seedling leaf decreased under exogenous ethylene and exogenous (NH₄)₂SO₄ treatments, comparing with Cd treatment alone. Also, the rate of net photosynthesis was promoted by 1.23 times and 1.22 times respectively under exogenous ethylene and exogenous (NH₄)₂SO₄ treatments. The activity of antioxidant enzymessuperoxide diamutase (SOD), catalase (CAT)] significantly decreased, while the contents of antioxidantsascorbic acid (AsA) and glutathione (GSH)] significantly increased under exogenous ethylene or exogenous (NH₄)₂SO₄ treatments with Cd stress. The results suggested that exogenous ethylene reduced Cd-induced oxidative stress and the degree of lipid peroxidation by enhancing non-enzymatic antioxidant reaction. However, it did not affect enzymatic antioxidant reaction, but then promoted photosynthetic processes. Compared with Cd treatment alone, the activities of ATP sulfurylase and glutathione reductase (GR), and the contents of cysteine and GSH in maize seedlings increased respectively by 54.43%, 27.93%, 50.77%, and 49.85% with exogenous ethylene treatment. However, there was no significant change in non-protein thio (NPT) and phytochelatins (PCs) contents. The results showed that ethylene potentiated GSH biosynthesis to resist Cd conditions. To show this that was the case, a GSH biosynthetic inhibitor-buthionine sulfoximine (BSO) - was applied on maize seedlings under Cd and exogenous ethylene conditions. Compared with Cd plus exogenous ethylene treatment, BSO significantly decreased GSH content, increased H₂O₂ content and reduced net photosynthesis rate. Furthermore, Cd content in roots significantly increased while it decreased in leaves after treatment with exogenous ethylene under Cd stress. Further analysis showed that Cd content in cell wall and vacuole of roots was enhanced with exogenous ethylene treatment. Totally, exogenous ethylene reversal of the effect of Cd stress on maize was a complex process involving the promotion of GSH and AsA contents and Cd distribution in roots. On the one hand, exogenous ethylene treatment enhanced non-enzymatic antioxidant capacity by increasing the contents of GSH and AsA, and not by improving the activities of antioxidant enzymes nor chelating NPT and PC in maize leaf. On the other hand, translocation of Cd from maize root to leaf was reduced by enhancing Cd sequestration in cell walls and vacuoles of maize root. The results provided the fundamental information for the application of ethylene in the reversal of heavy metal stress.