| More than half of the world’s population depends on rice as food, increasing rice zinc level is very important for solving the problem of human zinc deficiency. A series of field experiments were conducted in 2012 and 2013 to study the effects of foliar zinc application during grain filling stage on zinc nutrition in rice grains under the different environment (CO2) and cultivation conditions (N application rate, N application proportions, planting density and different rice cultivars), and how the impacts were influenced by different grain components or different parts of a panicle. The objective is to provide new information in helping solve human zinc nutrition problem. The five experiments were: ① By using FACE (Free Air CO2 Enrichment) platform, a hybrid Indica rice Ⅱ-you084 was grown under two levels of CO2 concentration (ambient and 50% higher than ambient), N application (15 and 25 g m-2), planting density (16 and 24 hills m-2) and foliar Zn application (with or without 0.2%Zn2+); ② By using FACE platform, a conventional japonica rice Wuyunjing 23 was grown under two levels of CO2 concentrations and foliar Zn application; ③ Two japonica rice cultivars Wuyunjing 23 and Wuyunjing 29 were grown under three levels of N application (14, and 20 and 26 kg/667 m2) and foliar Zn application; ④ Two japonica rice cultivars Wuyunjing 23 and Wuyunjing 29 were grown under three levels of N application proportions (The proportion of N fertilizer allocated to early growth stage relative to late growth stage is 4:6,5:5, or 6:4) and foliar Zn application; ⑤ Nine japonica rice cultivars Wuyunjing 29, Nanjing 1, Wuyunjing 23, Ningjing 3, Zhendao16, Yangjing 4227, Changnongjing 7, Tongjing 981 and Nanjing 5055 were grown under two levels of foliar Zn application. Foliar Zn applications (ZnSO4,0.2% Zn2+) started right after flowering in 5-day intervals, sprayed four times at the rate of 80 ml m-2 in experiment ① and three times in other experiments. At maturity, rice seeds were dehulled, milled rice, brown rice and bran were obtained to analyze the concentrations of Zn and phytate, and molar ratio of phytate to Zn were calculated. The results were as follows:1. The FACE study of Super rice Ⅱ-you084 showed that the Zn concentration, phytate concentration, and the molar ratio of phytate to Zn in different components of rice grain shared the same pattern as milled rice< brown rice< bran. Zn concentration in different parts of panicle showed UP> MP> LP in milled rice, brown rice and bran, but the molar ratios of phytate to Zn showed an opposite trend. The variation of the main indexes among different components or different parts of panicle in other trials displayed similar pattern as in trial ①. Elevated CO2 significantly decreased Zn concentration of milled, brown rice and bran by 4%**,5%**and 3%, respectively; Foliar Zn application significantly increased Zn concentration of milled rice, brown rice and bran by 41%**,61%** and 107%**, respectively. Increasing N application and planting density showed a trend of increase on rice Zn concentration. Zn treatment reduced Zn distributed in milled rice by 13%**, increased Zn distributed in bran by 27%**. But the CO2, nitrogen, or density treatment had no significant effect on Zn distribution. The CO2 or Zn treatments had little effect on phytate concentration of each component, but higher levels of N application and planting density decreased phytate concentration in brown rice significantly (6%). The molar ratios of phytate to Zn in milled, brown rice and bran were significantly increased by elevated CO2 in average of 9%**,6%** and 5%*, but decreased by foliar Zn application of 28%**,39%, and 51%**, respectively. The high levels of N application and planting density decreased the molar ratio of phytate to Zn in different components of rice, and the significant effects were observed on brown rice and bran.2. The FACE study of conventional japonica Wuyunjing 23 showed that elevated CO2 had no significant effects on Zn concentrations of different components of rice grain; Foliar Zn application significantly increased Zn concentration of milled rice, brown rice and bran by 10%**, 18%** and 25%**, respectively. Elevated CO2 reduced Zn distributed in milled rice by 5%+, increased Zn distributed in bran by 9%+, but Foliar Zn application had no significant effect on zinc distribution. The CO2 or Zn treatments had little effect on phytate concentration of each component. The molar ratios of phytate to Zn in milled and brown rice were not affected by elevated CO2, but decreased by 12%** in bran. Foliar Zn application significantly decreased the molar ratios of phytate to Zn of milled rice, brown rice and bran by 10%**,14%** and 18%**, respectively.3. The N application test showed, the Zn concentration of Wuyunjing 23 was significantly higher than Wuyunjing 29 in milled rice, brown rice and bran by 25%**,24%** and 17%**, respectively, but significantly lower than Wuyunjing 29 in milled rice, brown rice and bran mostly. The Zn concentration of grains from plants received 26 kg N application was less than those of 14 kg and 20 kg N in milled rice, brown rice and bran. Foliar Zn application significantly increased Zn concentration of milled rice, brown rice and bran by 25%**,36%** and 63%**, respectively. Zn treatment reduced Zn distributed in milled rice by 10%**, increased Zn distributed in bran by 17%**. Zn content distribution ratio in milled rice of 20 kg of N treatment was higher than 26 kg and 14 kg N application, but lower in bran. Phytic acid concentrations of different N application was in the order of 20 kg>14 kg>26 kg in milled rice, brown rice and bran, Zn application significantly decreased phytate concentration of brown rice by 8%** and bran by 2%*, respectively. The molar ratios of phytate to Zn in different components of rice grain were decreased by 18%** in milled rice,32%** in brown rice and 39%** in bran through foliar Zn application.4. The N application proportion test showed that the Zn concentration of grains from plants grown under 6:4 nitrogen ratios was less than that of 6:4 and 4:6 N applications in milled rice, brown rice and bran. Foliar Zn application significantly increased Zn concentration of milled rice, brown rice and bran by 25%**,41%** and 71%**, respectively. Zn content distribution ratio in milled rice of plants received 6:4 of N proportion treatment was higher than 5:5 and 4:6 N application proportion, Zn treatment reduced Zn distributed in milled rice by 12%**, but increased Zn distributed in bran by 20%**. Phytate concentrations of milled rice, brown rice and bran were not affected by either N application proportion or Zn treatments. The molar ratios of phytate to Zn of different N application was in the order of 6:4>4:6>5:5 N application proportion in different components of rice grain, and the molar ratios of phytate to Zn were decreased by 17%** in milled rice,31%** in brown rice and 41%** in bran through foliar Zn applications.5. The cultivars tests showed that the range of variation in Zn concentration of milled rice, brown rice and bran was 17 to 25,26 to 36, and 85 to 118 mg kg-1, and that of phytic acid concentration was 1 to 2,6 to 9, and 50 to 60 mg g-1, and that of the molar ratio of phytate to Zn was 5 to 8,19 to 32, and 48 to 68, respectively, and the cultivars difference was highly significant. On an average of all cultivars, Zn treatment showed no significant effect on phytic acid concentration in milled rice, brown rice and bran, but significantly increased Zn concentration of milled rice, brown rice and bran by 29%**,40%** and 81%**, respectively, and significantly decreased the molar ratios of phytate to Zn by 23%**,31%** and 45%**, respectively. Zn treatment also altered the distribution of Zn content in rice:it reduced Zn distributed in milled rice by 12%**, and increased Zn distributed in bran by 26%**.6. The analysis of variance showed that although the interactions of Zn application during grain filling and other factors during rice growth on phytic acid concentration were small, its effects on zinc concentration were rather large. The effects of Zn application on zinc concentration of rice varied greatly between cultivars; The effects of Zn application on zinc concentration of grains in upper part of panicle were greater than that of the other parts of panicle; Increasing total nitrogen fertilizer supply or the amount of nitrogen fertilizer in the later growth stage may enhance the effects of Zn application; The effects of foliar Zn application was decreased by high CO2 concentration for hybrid rice, but decreased for japonica rice.In conclusion, our results indicated that foliar Zn application during grain filling stage can improve Zn concentration and bioavailability of rice grains under different cultivation measures, the increasing range varied with different cultivars, different parts of panicle, and different components of rice. Environment and cultivation conditions and their interactions with foliar Zn application could change, to some extent, the levels of rice zinc nutrition. These results can provide important basis for zinc biofortification in rice production practices. |
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