Inhibition of high Mg concentration on seedling growth, free radical metabolism of reactive oxygen and zinc nutrition of maize in saline fluvo-aquic soils

【Objectives】 Zinc deficiency is very often happened in maize in saline fluvo-aquic soils in Hexi Oasis Area of Gansu Province, the high soil magnesium levels have been regarded as a major contributor to this deficiency. The inhibition effect of magnesium to zinc in saline fluvo-aquic soils in Hexi Oasis Area were discussed in this paper. 【Methods】 A pot experiment was conducted with maize as materials. Exchangeable magnesium contents of 287.3, 349.2, 411.6, 487.9 and 755.2 mg/kg were obtained by irrigating different amount of MgSO4 solution. The maize plants were harvested at 46 days, the biomass of above and under ground parts were dried and weighed, the Zn contents were measured. The second leaf under the interior leaf was used for the determination of active oxygen and some enzyme activities. 【Results】 1)More high and extremely high exchangeable Mg level in soil inhibits the growth of maize seedlings. When magnesia applied levels is 221 mg/kg, i.e. the exchangeable Mg content in soil is over 487.9 mg/kg, the growth of maize is stressed, when the Mg2+ applied level is 515 mg/kg, i.e. the exchangeable Mg content in soil is over 755.2 mg/kg, the plant height is markedly decreased by 14.5%, showing symptoms of magnesia poison and zinc deficiency, such as withered leaf margins and partial white stripes on the leaves (P＜0.01). 2)With the increase ofor exchangeable Mg levels, the dry weight in the ofMg2+ addition maize are reduced by 11.9%-38.3% and 4.6%-11.9% respectively. The reduction in stem weight is significantly higher than in roots. 3)Maize leaf chlorophyll contents are reduced by 9.4%-45.9% with the increase of the Mg2+ addition . The Mg level of 515 mg/kg would lead to withered and yellow leaves. The blade superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities are increased at first and then reduced quickly, the peak value appears at the exchangeable Mg levels of 147, 74 and 147 mg/kg respectively. When the dose reaches 515 mg/kg, the values are reduced by 49.75%, 48.06% and 32.21% respectively. The malondialdehyde (MDA) contents are increased consistently in the range of 20.39%-183.58%. 4)The exchangeable Mg levels significantly inhibit the uptake of Zn. Compared with the CK, the zinc contents in above ground parts are reduced by 4.05%-57.09%, the uptake reduced by 15.41%-73.55%; the root zinc contents are reduced by 7.55%-18.99%, and the uptake reduced by 11.62%-37.40%. The zinc content and absorption in stems/leaves are reduced more than in roots. High exchangeable Mg level would also lead to lower upward transfer of zinc. 5)Soil available zinc contents are significantly reduced with the increase of exchangeable Mg levels. When the Mg level reaches 515 mg/kg, i.e. the exchangeable Mg content in soil is over 755.2 mg/kg, the available zinc content falls to 0.47 mg/kg, which is below the critical value of zinc deficiency (DTPA-Zn＜0.5 mg/kg).【Conclusions】 Zinc deficiency of maize in saline fluvo-aquic soil in Hexi Oasis Area is closely related to soil magnesium content. The high levels of exchangeable Mg contents show significant restraining effects on the maize seedling growth and biomass, the leaf chlorophyll content and the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT). Over high exchangeable Mg levels decrease the zinc absorption by maize seedlings, and the inhibition in stem/leaf is significantly higher than in root system, which inhibites the upward transfer of Zn from root to shoot. The available zinc content in soil will also be reduced, when the Mg level reaches 515 mg/kg, the available zinc content is below the critical value of zinc deficiency.