Saturday, 15 July 2006

Improving zinc availability in rice grains: the role of the soil-plant system in the food chain.

Xiaopeng Gao1, Chunqin Zou2, Fusuo Zhang2, Wen Jiang1, Kai Chen1, Sjoerd Van Der Zee3, and Ellis Hoffland3. (1) China Agricultural University, Dept. Plant Nutrition; Wageningen University, Dept. Soil Quality, Netherlands., Yuanmingyuan West Road No 2, Haiding District, Beijing, China, (2) China Agricultural University, Dept. Plant Nutrition, Beijing, PR China, Yuanmingyuan West Road, No2, Haidian District, Beijing, China, (3) Wageningen University, Dept. Soil Quality, PO Box 8005, Wageningen, 6700EC, Netherlands

About 30% of the agricultural soils worldwide are Zn deficient and approximately 2 billion people in developing countries suffer from Zn deficiency. Awareness is growing that Zn density and bioavailability in edible parts of plants need to be increased through both plant biotechnology and nutritional management in soil-plant system. At present, rice production in north China is undergoing an important change from traditional high water-consuming lowland rice cultivation to a promising new cultivation method of “aerobic rice” because of water constraints. New rice genotypes are being bred for this purpose. By field experiments in combination with soil chemistry modeling, we demonstrated and explained that this cultivation change will reduce Zn availability for the crop and Zn allocation to the grain. Both shoot Zn concentration and Zn uptake decreased under aerobic conditions. Preliminary modeling results confirm that Zn speciation may differ largely between flooded and aerobic conditions. The expected higher pH in aerobic fields would explain lower Zn availability and lower Zn uptake compared to flooded fields. Grain Zn concentration reflects the ability of a genotype to take up Zn from the soil, to mobilize Zn within plants and to load it in the grain. Grain Zn concentration under aerobic conditions was significantly lower than that under flooded conditions (Fig.1). This indicated the introduction of aerobic rice system would increase Zn deficiency problems for human body. Zinc fertilization did increase the grain yield but not grain Zn concentration. Nutrient solution experiments confirmed that grain Zn concentration only increased at toxic Zn supply level. As fertilization is shown to be inadequate for overcoming Zn deficiency, exploring genotypic variation seems a more promising strategy to increase grain Zn concentration. We found considerable variation in Zn efficiency among genotypes. Regression analysis showed that Zn uptake is the major factor explaining this variation. We are currently trying to explain this by focusing on rhizosphere processes involved in Zn mobilization from alkaline low Zn soils. For lowland rice, we found higher root citrate exudation at Zn deficiency in more efficient genotypes. Similar experiments with aerobic rice genotypes are ongoing. We plan to evaluate and quantify the effect of rhizosphere modification on plant Zn uptake by using a mechanistic model. Grain Zn availability to the human consumers is highly depended on the grain phytic acid/Zn ratio. Phytic acid is an antinutritional compound that complexates Zn and thereby can reduce Zn bioavailability to less than 3% of total Zn in the grain. Phytic acid/Zn molar ratios < 20 are considered to induce Zn deficiency in humans. A survey was conducted on phytic acid/Zn ratios in brown rice from different growing regions in China. In all grain samples, ratios were higher than 20. We are now testing the hypothesis that high soil P levels contribute to this. Our soil/grain survey on aerobic rice producing soils showed that average soil P-Olsen levels (36 mg/kg) are well above sufficiency level, due to excessive P application rates. Reduction of P fertilization might decrease grain phytic acid concentration and increase grain Zn bioavailability to the human consumers.

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