| Vegetables are indispensable and important diet in the human daily life. With the improvement of living condition in recent years, there is an increasing concern about the nutritive and safety quality as well as sensory quality of vegetables. High content oxalate in vegetables is regarded as one of toxins and antinutritional factors. High accumulation of oxalate in vegetable usually leads to calculus such as kidney stones, bladder stones and urinary stones. Therefore, it is of great importance to reduce oxalate accumulation in vegetables.Presently, imbalanced fertilization, typically excessive application of nitrogen over P and K, is a great problem in nutrient management for vegetables. This has been causing not only environmental problems but also accumulation of toxins such as oxalate to a toxic level. So it is urgently required that nitrogen and potassium be applied in a proper proportion. However, little information is available on the effects of nitrogen and its combination with potassium on oxalate content in vegetables. Therefore, the objective of this study is to gain some information, by using hydroponic and soil culture techniques, on oxalate accumulation in spinach as affected by forms, levels, and types of nitrogen fertilizers under different K supply levels. The effect of nitrification inhibitor is also studied. Special attention is paid to the physiological and biochemical mechanisms of oxalate accumulation manipulated through N and K supply. The results are summarized as follows:1. The data from solution culture shows that 8mmol/L N with 4mmol/L K distinctly increases plant fresh weight, the content of chlorophyll (Chi), Vc and soluble sugar, and the activities of superoxide dismutase (SOD) and peroxidase (POD), with a significant reduction in MDA content. Highest content of oxalate and activity of glycolate oxidase (GO) were observed either in K2 treatment regardless of N levels or in N2 treatment irrespective of K concentration in solution. Oxalate contents in spinach treated with different N and K levels are positively correlated (y=23.293x+686.87,r=0. 3438*) with GO activities but negatively correlated(y=-7.5117x+1294.3,r=-0.3413*) with Vc contents, which evitadently indicates that oxalate in spinach is synthesized chiefly through glycolic acid path in photorespiration rather than through ascorbic acid path. Proper N and K level, 8mmol/L N and 4mmoI/L K in this case, is strongly recommended to maximally decrease oxalate accumulation in spinach.2. In solution culture experiment, fresh weight and dry weight are gradually decreased with the lowering of the ratio of NO3"-N to NH/-N; the highest biomass and content of Chi, activities of SODand POD were obtained in the treatment of N70/3owith 4 mmol/L K. MDA content remains the lowest in K2 treatment, and is increased with the lowering of the ratio of NO3~-N to NH/-N. The contents of Vc and soluble sugar in spinach were increased with K levels, and significantly enhanced with the lowering of the ratio of NO3"-N to NH4+-N. The content of oxalate and activity of GO are lowest in K2 treatment under all N form ratioes and are substantially decreased with the lowering of the ratio of NO3"-N to NH4+-N under all K levels. In this experiment, the significant positive correlation of oxalate content with glycolate oxidase(GO)activity (y = 182.6x - 8.425, r = 0.5621**), and the significant negative correlation with Vc content (y = -7.6693x + 1001, r=-0.6943**) further confirm the hypothesis that oxalate in spinach is synthesized mainly through glycolic acid path in. photorespiration rather than through ascorbic acid path. Above results demonstrates the necessity of a proper N form ratio and K level (N7o/3oK.2 or N5o/5o K2) to ultimately decrease oxalate accumulation in spinach.3. The results from soil culture experiment with two types of N fertilizers combined with different rates of DCD show that NO3" -N concentration in soil is effectively decreased by DCD and thus higher content of NH/-N in soil can be maintained. This effect can... |
PDF Full Text Request