Objectives We studied the availability of zinc (Zn) biofortification of brown rice and the bioavailability of Zn in brown rice.

Methods Zn biofortification experiment was carried out in Liyang County, Jiangsu Province. The tested Zn fertilizer was alditol chelated Zn (Zn 170 g/L), and the tested rice cultivar was mid-maturing late japonica rice ‘Nanjing 46’. The treatments included one foliar application of Zn fertilizer at rate of 2.55 (Zn1), and 5.10 kg/hm² (Zn2), two foliar application at total rate of 5.10 (Zn3) and 10.20 kg/hm² (Zn4), and spraying water was used as control (CK). At maturing stage, brown rice samples were collected for measurement of the content of total Zn, four Zn fractions, and phytic acid (PA), and the molar ratio of phytic acid to Zn (PA/Zn) was calculated. The dissolution rate of brown rice Zn in the gastric and gastrointestinal stage was detected by in vitro gastrointestinal simulation method.

Results Compared with CK, Zn biofortification did not significantly change the phytic acid content, but increased the total Zn content of brown rice. Zn1, Zn2, Zn3, and Zn4 treatment increased the total Zn content by 23.93%, 37.51%, 82.38% and 87.81%, respectively. Zn3 and Zn4 treatment had similar but significantly higher promotion effect than Zn1 and Zn2. Zn1 and Zn2 treatment did not change the PA/Zn ratio of brown rice significantly, while Zn3 and Zn4 treatment reduced the ratio. Zn treatments affected the contents of the four Zn fractions in brown rice to varying degrees. Zn2 increased the salt-soluble and alkali-soluble Zn content, Zn3 treatment increased all the four Zn fraction contents, and Zn4 treatment increased all the Zn fraction contents except water soluble Zn. Compared with CK, Zn1 had no significant impact on the proportion of Zn fractions; Zn2 reduced the proportion of complex Zn; Zn3 reduced the proportion of water-soluble Zn, Zn4 reduced the proportion of water-soluble Zn but increased that of salt-soluble Zn. Zn biofortification enhanced the dissolution amount of brown rice Zn at the gastrointestinal stage, with the Zn dissolution amount of 19.52, 24.15, 23.14, 30.62 and 32.55 mg/kg under CK, Zn1, Zn2, Zn3, and Zn4 treatments, respectively. Zn3 and Zn4 treatment had similar but significantly higher Zn dissolution amount than Zn1 and Zn2. According to the correlation analysis, the gastrointestinal dissolution amount of Zn was positively correlated with total Zn content, negatively correlated with water-soluble Zn proportion, not correlated with complex Zn proportion. The bioavailability of Zn in brown rice was significantly correlated with PA/Zn ratio.

Conclusions Zn biofortification is effective in increasing the total Zn content of brown rice and reduce the PA/Zn ratio, so increasing the bioavailability of Zn. Low Zn application rate does not affect the proportion of various Zn existing fractions in brown rice, while high Zn application rate significantly reduces the water-soluble Zn proportion and increases the salt-soluble Zn proportion, resulting in high gastrointestinal Zn dissolution amount. Aiming the Zn biofortification of brown rice, the effective method is applying foliar application alditol chelated Zn fertilizer in two times at a total rate of 5.10 kg/hm².