| Zinc (Zn) is an essential micronutrient for humans, while phytic acid (PA) is a chelating agent which binds minerals leading to a low zinc bioavailability in many cereal grains because of a high molar ratio of phytic acid to zinc, which is considered as an important parameter for evaluating zinc bioavailability in the human body. Zinc deficiency is a widespread problem in the world, especially in the developing regions where staple foods derived from cereals are the main source of daily micronutrients intake. Therefore, the increase of Zn content in the endosperm, which is consumed as wheat flour by human being, is the main challenge in the research area of biofortification.In order to improve grain Zn concentration and its bioavailability of local modern wheat, an experiment using foliar fertilizer application was conducted in Yangzhou, China. Winter wheat cultivar Yangmai16, Yangmai13and Yangfumai2were treated by foliar Zn application at3rates of0.05%,0.1%and0.2%ZnSO4at grain filling stage, the plants in control plots received clean water. The first Zn treatment was applied at1week after flowering (WAF), the second and third application were at2and3WAF, respectively. The results showed that foliar Zn application of0.05%,0.1%and0.2%ZnSO4increased grain Zn concentration by15%,30%,39%and decreased grain PA/Zn (Zinc and Phytic Acid molar ratio) by12%,21%,25%respectively, but had no significant effect on grain PA concentration. Wheat grains were further separated by Sedimat Laboratory Mill into bran, shorts and flour, and Zn concentration in each grain fraction were analyzed by ICP-AES. Zn and PA concentration in different milling fractions followed the order of bran>shorts>flour, Zn concentration in bran was8times higher than in flour and PA showed the same trend among the fractions. Compared with control,0.1%and0.2%ZnSO4foliar application increased Zn concentration in bran, shorts and flour by29%,32%,22%and43%,32%,33%and decreased PA/Zn by21%,19%,19%and29%,23%,26%respectively. As the case of Zn concentration in grain fractions, Zn contents in bran, shorts and flour showed same trend in the response to foliar Zn application at grain filling stage. However, foliar Zn application at grain filling stage did not affect Zn distribution in different milling fractions of mature grains.An experiment with the same cultivars as above was designed to investigate how the Zn concentration in the culture medium affects Zn uptake and distribution in detached wheat ears in different growth stage. Treatments represented normal to high Zn concentrations of30,60,120and240μM in the form of natural isotopes of ZnSO4. The ear culture started from one week after flowering (1WAF) to4WAF and the treatment duration was7days. All the samples were separated into seven different portions from the bottom to top:node, stem, sheath, leaf, rachis, glume and grain. The results showed that Zn treatment significantly increased zinc concentration in detached ears but differed by Zn treatment and its time, cultivars and various parts of detached ears. Zinc concentration improvement in different wheat organs became more visible in response to high Zn treatments but less by later stage Zn treatments. Compared within cultivars, Zn treatments had greater effect on Yangfumai2than other two varieties and the increase in Zn concentration in node, stem and rachis turned out to be higher than in other tissues while the glume and grain showed the weakest response to Zn treatments. Zn distribution in node, stem, sheath, leaf, rachis and glume showed the same trend as Zn concentration in the response to Zn treatment at different stage, however grain showed the opposite trend.Another experiment with Yangmai15was designed to investigate Zn uptake and distribution in detached wheat ears at different growth stage. Zn treatment was60μM in the form of enriched stable isotope70Zn. The ear culture started from one week after flowering (1WAF) to4WAF and the treatment duration was7days. All the samples were separated into seven different portions from the bottom to top:node, stem, sheath, leaf, rachis, glume and grain. The results showed that70Zn concentration in these portions were respectively72,50,37,98,80,16and47μg g-1at one week after flowering, but they significantly decreased especially for grain in response to later stage70Zn treatment. Compared with1WAF,70Zn treatments significantly increased70Zn distribution in node, stem, sheath, leaf and rachis while decreased70Zn distribution in grain after a firstly short rise.In order to detect the distinction of zinc distribution among different organs between zinc-rich and normal wheat cultivars, another experiment with Qingzi1and Yangmai17was designed to investigate Zn uptake and distribution in detached wheat ears at two weeks after flowering (2WAF). Zn treatment was60μM in the form of enriched stable isotope70Zn and the treatment duration was7days. All the samples were separated into seven different portions from the bottom to top:node, stem, sheath, leaf, rachis, glume and grain. The results showed that70Zn concentration in these portions of Qingzi1and Yangmai17were respectively82,20,41,68,75,16,40μg g-1and42,18,30,50,92,12,24μg g-1.Compared with Yangmai17, Zn concentration in glume and grain of Qingzi1had respectively increased by37%and68%, while a significant decline in rachis by19%and no difference in stem.Foliar Zn application after flower improved Zn concentration and its bioavailability in different grain fractions of modern wheat cultivars, which is a fast and effective biofortification approach to improve Zn nutrition in humans as well as animals. However, the barrier of Zn transport into wheat grains may exist between the stem tissue rachis and the grain, but we fortunately discovered that zinc-rich wheat cultivar Qingzi1weakened its influence in contrast to common wheat cultivar Yangmai17which may be an important inspiration for the breeders. |
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