Assessment On Zinc Susceptibility And Study On Zinc Nutrition For Different Wheat And Maize Genotypes

The content of available Zn is relatively low in calcareous soil, zinc deficiency symptoms or potential Zn deficiency occur very easily for crops and fruit trees growing on calcareous areas, this leads to decrease of yield and degradation of product quality. Zn deficiency not only affects growth and development of cereal crops, but also decrease zinc content in seeds. Furthermore, zinc intake and nutrition for people, who live on cereal as main food and other kinds of foodstuff consume less. This will influence health of people, and this phenomenon is called as "hidden hunger". The human beings will suffer from many kinds of diseases because of zinc deficiency, such as disease of manikin, sugar diabetes etc, and especially affecting development of children. The studies indicated that the susceptibility degree of zinc deficiency and adaptation is very different among plant species and different genotypes of a given species. Therefore, through studying, the difference of zinc susceptibility, and metabolism mechanism of different wheat or maize genotypes is expected to be clarified, and genotypes with high Zn efficiency will be selected, and using them to make full of resource of zinc nutrient in soil, and this work is very important. In this dissertation, we used water culture and sand culture methods to investigate zinc susceptibility and response to Zn of different wheat and maize genotypes. The main results obtained were as follow:(1) Solution culture experiment with winter wheat seedlings were conducted to study the effect of high HCO₃^- concentration (10 mmol/L) on biomass and metabolism of active oxygen at two pH under different condition of Zn supply. In acid or alkaline solutions, the effects of HCO₃^- on biomass were not obvious at Zn sufficiency. At Zn deficiency, HCO₃^- increased the production rate of active oxygen by 9.9% and 3.9% in root and leaves at pH 6, respectively;and 10.9% and 5.7% at pH 8, respectively. Under normal supply of Zn, although HCO₃^- also increased the production rate of active oxygen in wheat root and leaf, the extent decreased. At Zn deficiency, HCO₃^- decreased the activity of three protective enzymes in roots to large extent, viz. peroxidase (POD), catalase (CAT) and superoxide dismutatase (SOD), however, sufficient Zn supply could mitigate proxidation of HCO₃^- to membrane lipid in root tissues of wheat. Comparing with normal Zn supply and Zn deficiency, the latter increased significantly the autoxidation rate of wheat roots and leaves.(2) A solution culture experiment with winter wheat seedlings were conducted to study the effect of high HCO₃^- concentration (10 mmol/L) on growth and zinc uptake of wheat plants at different pH and two Zn supply levels. The results showed that at initial pH of 6 HCO₃^- could obviously inhibit the growth of roots at Zn deficiency, whereas there was nearly no influence on growth at normal zinc supply. At initial pH of 8, the inhibition of HCO3" was unobvious to growth of the roots and shoots of wheat plants with or without zinc supply, HCO₃^- increased dramatically zinc uptake in the roots and shoots of wheat plants in the acid nutrient solution, especially in the roots, however, HCO3" restricted zinc uptake of the roots and shoots of wheat plants in the alkaline solutions. In addition, HCO₃^- significantly inhibited transport of zinc from the roots to shoots, and thus Zn mainly accumulated in the roots. The addition of HCO₃^- led to production of a small amount of CO₃^2-, and maintained the high level to the pH of the nutrient solution.(3) The two sand experiments were conducted to assess the susceptibility of thirty five maize genotypes to zinc deficiency and the corresponding influencing factors under the conditions of zinc deficiency. According to susceptibility of maize genotypes to zinc deficiency, the tested maize genotypes were classified into three types, namely, susceptible genotypes, medium susceptible genotypes and non-susceptible genotypes, this classification was mainly based on the severity and time of occurrence of zinc deficiency. In addition, the respective five genotypes for susceptible genotypes and non-susceptible genotypes were carefully studied, and it indicated that there was no obvious influence of zinc deficiency in environment on yields of maize seedlings, and zinc deficiency symptoms occurred only on the appearance of susceptible genotypes. Zinc deficiency obviously decreased the chlorophyll content of the leaves for susceptible types to zinc deficiency, whereas, the leaf chlorophyll content still maintained higher level for non-susceptible genotypes. The susceptible genotypes to zinc deficiency had higher root shoot ratios and more than 1, however, the root shoot ratios were less or equal to 1 for non-susceptible genotypes. The P/Zn ratios of seeds for the susceptible genotypes were higher than that of non-susceptible genotypes. The results showed that the P/Zn ratio of maize seeds can be regarded as an important index to justify the susceptibility of young seedlings to zinc deficiency. Finally, under the conditions of zinc deficiency, the transfer rates of zinc from seed to shoot was higher for susceptible genotypes to zinc deficiency compared to non-susceptible genotypes.(4) The sand experiment was conducted to study the responses of maize seedling with different zinc-susceptibility to zinc under the zinc supplying rate. The results showed that, when low rate of Zn (0.5 mg/kg) was provided, the maize plants of zinc-susceptibility type to zinc deficiency were more sensitive than non-susceptibility maize plants. The susceptible genotypes to zinc deficiency had higher root to shoot ratios than non-susceptibility maize plants, because the root of zinc susceptibility genotypes grew better to absorb more nutrient. The decrease of root to shoot ratios promoted correspondingly growth of the shoot. The zinc non-susceptibility genotypes under low zinc supply were damaged and more serious than that under zinc deficiency. Zinc uptake of the maize seedling was accumulated mostly in the root when the amount of zinc supply was 0.5 mg/kg. Zinc content was increased when zinc rate was increased to 1.5 mg/kg. The increase range of zinc content in root, stem and leaf for zinc susceptibility types were 20.5%, 53.6% and 18.5%, respectively;And the increase range of zinc content in root, stem and leaf of non-zinc susceptibility type were 5.5%, 83.4% and 60.7%, respectively. The Zn taken up of zinc-susceptibility types was translocated to shoot, therefore, the zinc content of shoot increased. And the range of zinc content was increased more than the non-zinc susceptibility.(5) The sand experiment was conducted to study the responses of different wheat genotypes on zinc supply. According to the comparison among the root to shoot ratios, based on whether the root to shoot ratios were more then 1 or less than 1, as well as the observation results were considered, the 25 genotypes of wheat were classified into two types, namely, susceptible genotypes and non-susceptible genotypes, this classification was mainly based on the severity and time of occurrence of zinc deficiency. The result showed that zinc supply could accelerate the growth of wheat shoot and root and increase the chlorophyll SPAD value (The increased range of 18 wheat genotypes among 25 genotypes were more than 10% )o The correlationship were not significant between the zinc content of seed and chlorophyll SPAD value, and between the root shoot ratio of seedling and chlorophyll SPAD value. Because zinc content in the shoot was relatively high, the zinc non-susceptibility type can mitigate effectively damage caused by zinc deficiency. The comparison between the zinc non-susceptibility and zinc susceptibility was carried out, the mean of zinc content in the shoot and root increased 96.6% and 28.8% under zinc deficiency, respectively;it increased 47.6% and 10.9 under zinc supply, respectively. Zinc supply could not affect phosphorus content of thenon-zinc susceptibility and zinc susceptibility wheat plant. The P/Zn ratios were affected mainly by the change of zinc content in wheat plant and did not have obvious relation to phosphorus content.