| Human zinc deficiency is a prominent public health problem,which is mainly related to low Zn concentration and low Zn bioavailability in grains of staple food crops including rice.Rice is one of the most important food crops.Previous studies mainly focused on the effects of foliar Zn application on Zn concentration in rice grains,with few reports on Zn bioavailability.Previous experiments on a small number of cultivars showed that foliar Zn application could increase Zn concentration in grains,but the factors affecting the efficiency of foliar Zn application,especially in modern cultivars,are still unclear.In this study,rice cultivars with different seed Zn content were grown under soil culture conditions,the effects of foliar Zn application during early grain-filling stage on Zn concentration and Zn bioavailability in the rice grains were studied,and the responses of different cultivars were compared.Meanwhile,the relationship between the efficancy of foliar zinc absorption and leaf structure characteristics were studied in order to provide scientific basics for the formulation of effective Zn biofortification strategies in cereals production.Soil culture experiments were conducted in three consecutive rice growing seasons.In 2016,eleven rice cultivars with different seed zinc content were used as plant materials.Foliar Zn spray(0.2%ZnSO4)were applied at heading,6 days and 12 days after heading(DAH),and the same amount of water was sprayed as the control.In 2017,the tested cultivars were the same as those in 2016,but additional two treatments,0.2%ZnSO4+1%nicotinamide(NA,for all cultivars)and 1%NA(for three cultivars)were included in the experiment.In 2018,twelve rice cultivars were tested,zinc treatments were same as in 2016 except adding the treatment of 0.2%ZnSO4+1%Urea.Stomatal conductance and leaf surface structure of flag leaves were measured at heading and early grain filling stages(2017-18).The yield traits,the concentrations of nutritional elements and anti-nutrient factor phytic acid(PA)in brown rice(2016-18),and Zn concentration in flag leaves(2018)were determined at maturity.The main results are as follows:The results in 2016 showed that,compared with the control,Zn application increased Zn concentration in brown rice by 24.9%(P<0.01).Zn application had no effect on the PA concentration of brown rice,but the mole ratio of phytic acid to Zn decreased by 20.2%(P<0.01)in average.Variance analysis showed that the above indicators were significantly different among cultivars,and significant Zn by variety interactions were detected for phytic acid,molar ratio of phytic acid to zinc of brown rice.The experiment in 2017 showed that foliar application of ZnSO4 and Zn-NA increased Zn concentration in brown rice by 34.3%(P<0.01)and 32.0%(P<0.01),respectively.Foliar application of ZnSO4 and Zn-NA had no significant effect on the phytic acid concentration of brown rice,but the mole ratio of phytic acid to Zn decreased by 27.6%(P<0.01)and 26.4%(P<0.01),respectively.Variance analysis showed that the above indicators were significantly different among cultivars,and significant Zn by variety interactions were only detected for the mole ratio of phytic acid to Zn in brown rice.The effect of 1%NA application alone and its interaction with cultivars on these parameters were not significant.Net assimilation rate and stomatal conductance of flag leaf at heading stage and 20 DAH,as well as stomatal length and density at heading stage were significantly different among cultivars.The results of electron microscopic observation of flag leaf epidermis showed that there was no obvious regularity in the degree of sparsity of waxy crystals among cultivars.But for each cultivar,the distribution of waxy crystals on the leaf surface,including papillae,was dense,while the distribution around stomata was sparse.The papillae distribution on leaves of indica rice was significantly different from thatof Japonica rice,and the papillae on stomata of indica rice were generally less than that of japonica rice.The correlation analysis showed that the increment of Zn concentration in the brown rice due to foliar Zn application was not related to the leaf stomatal length and density at heading stage,but positively correlated with the stomatal conductance at the early grain-filling stage,with the correlation at heading stage being significant.In addition,the increment of zinc concentration in brown rice was negatively correlated with zinc concentration in brown rice.The 2018 experiment showed that there were significant difrerences in Zn concentration,phytic acid concentration and the molar ratio of phytic acid to zinc in both brown rice and polished rice.Compared with the polished rice,Zn concentration,phytic acid concentration and the molar ratio of phytic acid to zinc in brown rice increased by 29.6%(P<0.01),222.1%(P<0.01)and 150.7%(P<0.01),respectively.Compared with the control,foliar application of ZnSO4 and Zn-Urea increased Zn concentration in brown rice by 27.5%(P<0.01)and 30.4%(P<0.01),and that in polished rice by 22.1%and 26.9%,respectively.Foliar application of ZnSO4 and Zn-Urea had no significant effect on the phytic acid concentration of brown rice and polished rice,but the mole ratio of phytic acid to Zn decreased by 24.3%(P<0.01)and 24.2%(P<0.01)in brown rice,17.9%(P<0.01)and 20.3%(P<0.01)in polished rice,respectively.Zn by variety and Zn by portion interactions were significant for Zn concentration in brown rice,and Zn by portion interactionsthe molar ratio of phytic acid to zinc were also significant.The results from net assimilation rate,stomatal conductance,stomatal length and stomatal density of flag leaves after anthesis,as well as the leaf surface structure observed by electron microscopy,were similar to those in 2017.The correlation analysis showed that the increment of Zn concentration in the brown rice and polished rice due to foliar Zn application were not related to the stomatal length or density at heading Stage,but positively correlated with the stomatal conductance in the early grain-filling stage,and the correlation was significant at heading stage.In addition,the increament of grain Zn concentration was negatively correlated with Zn concentration in the same portion from the control plants.The results of yield and element concetrations(N,P,K,Ca,Mg,S,Cu,Fe,Mn)from three years experiments showed that all treatments had no significant effect on rice yield traits.In most cases,the treatments had no significant effect on the concentration of macroelements or icroelements in brown rice and polished rice(except S),and the trend of different cultivars was consistent.In conclusion,foliar Zn application during the early grain-filling stage had no effect on rice yield,but significantly increased Zn concentration and Zn bioavailability.In general,the effect of Zn application on indica rice was greater than that on japonica rice,and brown rice greater than polished rice.In addition,the addition of urea in zinc foliar spray could further increase the grain Zn concentration.The lower Zn level of rice plants and the larger stomatal conductance of leaves might associate with the greater absorption of Zn fertilizer into rice leaves. |
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