Biogeochemical Behaviour Of Iodine In Soil-vegetable System And The Mechanism Of Exogenous Iodine Uptake By Vegetables

It has been well known that iodine deficiency can result in goiter,and this has become a global sanitation problem.In order to reduce the incidence of iodine deficiency disorders(IDD),the agricultural biofortification was used to increase iodine content in vegetable of human food chain to improve human nutrition.In our studies,the vegetable was cultivated as target crops and different forms of iodine (inorganic and organic) including seaweed were chosen in hydroponic,pot and field experiment,which were carried out to investigate the biogeochemical characteristics of iodine and mechanism of iodine enriched by vegetable.The objectives of the studies are to provide theoretic basic for the breeding of iodine-enriched vegetable and popularizing of iodine biofortification in practice,and ultimately to seek an effective,safe and scientific approach to the prevention and cure of iodine deficiency disorders(IDD).The main conclusions obtained from our studies are as follows:1.In the hydroponic experiment,the low levels of iodine(＜0.5 mg/L) can promote the biomass of vegetable efficiently;however,high iodine levels can inhibit the growth of vegetables.The toxicity effects of different iodine forms are in the following order:â… ^-＞CH₂ICOO^-＞IO₃^- except for water spinach(the toxicity of iodine is:CH₂ICOO^-＞I^-＞IO₃^-).Iodine uptake by vegetables was enhanced with the iodine addition,and the respective effects of iodine uptake by vegetables are: I^-＞CH₂ICOO^-＞IO₃^-.The capacity of iodine-enrichment and endurance of iodine toxicity varies in different genotypes of pakchoi,of which Suzhouqing and JiaxingAijiaoqing etc.possessed greater capacity and stronger endurance and could be recommended as iodine-enrichment vegetables.2.The hydroponics experiments showed that the uptake rate of iodine increased sharply in a short time(＜60 min),then decreased slowly with the time.Under low iodine concentration(0-0.5 mg/L),the relation of uptake rate of iodate and iodate concentration was exactly suited to the characteristics of Michaelis Equation,which indicated that the iodate may be active absorbed by vegetable under low concentration.The uptake rate of iodine was linearly related with the iodine concentration when the concentration is higher than 0.5 mg/L,which indicated that iodine is passive absorbed by vegetable.The content of iodine in edible parts of vegetable increased in the first week then decreased with the incubation time when the exogenous iodine was added once only,and the content of iodine in vegetable increased steadily when the iodine concentration was preserved constantly.The cholrine added in solution decreased the uptake rate of iodine at low concentration of iodine(＜0.5 mg/L),but the effect of chorine became weak with iodine concentration increasing.3.The isothermal curves of iodine adsorption on three garden soils could be described by either Langrnuir or Freundlich equation(r＞0.97^**) for each equation.As compared with the iodide,the iodate could be easily adsorbed by soils;and the REQ (red soil developed on Quaternary red earths(clayey,kaolintic thermic plinthite Aquult)) adsorbed more I^- and IO₃^- than the IS(Interceptisol soil) and the AS (alluvial soil).The distribution coefficient(k_d) of iodine in the soils decreased exponentially with increasing iodine loading.4.In the pot soils,the iodide was easily adsorbed by pakchoi,and caused more toxic to vegetable,the rate of iodine loss in soil was higher as compared with the iodate. The iodine bioavaibility in AS was the strongest but the soil iodine persistent was the shortest among three tested soils,and the REQ showed the polar trend to the AS.¹²⁵I introduced into the soil inclined with the soil depth,but most of the applied ¹²⁵I remained in the surface layer and lower of ¹²⁵I moved downwards and outwards.The ¹²⁵I,adsorption in the soil and absorption in the plant,could be translate into gaseous state and released into the air.5.In the pot experiment,the toxicity of iodine and iodine concentration in tissues were much greater for plants grown with KI than with iodine fertilizer composed of seaweed in the first cutting,and the contrary trend was showed in the second cutting. The content of iodine in edible part of vegetable was as following order:Chinese cabbage＞lettuce＞carrot＞tomato,and the endurance to iodine toxicity was showed as:carrot＞Chinese cabbage＞lettuce＞tomato.The distribution of iodine in different parts of vegetables(Chinese cabbage,leetuce,tomato) was root＞leaf＞stem＞fruit, but the iodine content of carrot shoot was much higher than the earthnut.The residual iodine in soil decreased with the increasing in growth time.The content of iodine applied with KI decreased by 41.5%-58.0%,whereas,iodine fertilizer composed of seaweed decreased to 56.0%-69.0%.As compared with the pot without vegetable,the pot with vegetables has much greater loss rate of soil iodine.6.In the field experiments,the iodine in the edible parts of vegetable increased with the increasing iodine addition in soil.Among these tested vegetable,the leaf vegetable was the type of strongest capability in iodine enrichment,then was stem vegetable,the fruit vegetable was the most weak to the iodine accumulation.Except the soybean,the iodine in edible parts of vegetable decreased with the grow time. Under two different application technique(root application and ex-root application), soil iodine root application was more favourable to iodine accumulation in vegetable, of which the distribution of iodine in leaf,stem and fruit vegetable was showed as: root＞leaf＞stem＞fruit,but the iodine distribution in root vegetable was showed as: earthnut＜shoot.Whreas with iodine ex-root application(spraying to the leaf),the iodine content of leaf was the highest,then was stem,and the iodine accumulation in fruit and root was the minimum in all these tested vegetables.7.Water soluble iodine of vegetable mainly consists of I^-,which was 66.7%of total water soluble iodine.Iodine associated with protein was the main form of organic iodine,with the amounts to 22.43%of total iodine of vegetable.The distribution of iodine in the subcell ofpakchoi showed that iodine was mainly present in soluble part of cell accounting for 54.8%-63.9%,with the iodine addition increasing,the proportion of cell wall had a upward tendency.In root cell,iodine combins with fibrous tissue,in stem and leaf cell,iodine consists in chloroplast.