| Panicle nitrogen fertilizer(PNF)is the main source of nitrogen in the late growth stage of rice,which is related to the grain filling,rice quality formation,and starch biosynthesis.The grains of japonica rice are short-round,with high head milled rice rate and taste value,and is favored by consumers.There are significant differences in grain filling between grains growing at different positions on the ear axis,but the quality,starch characteristics and structural differences of superior and inferior grains,as well as the regulation of gradient PNF,still need further research.In this paper,Nanjing 9108 and Nanjing 0212 were used as experimental materials.Four PNF levels(0,60,120,180 kg N ha-1)under the same basal and tiller fertilizer(150 kg N ha-1)were set,respectively expressed as 0N,60N,120N and 180N.The yield,spikelet structure and the quality,starch physicochemical properties and fine structure of superior and inferior grains were measured,the PNF application rate of high yield and good quality with stable yield were analyzed,and the difference characteristics of the above indexes of superior and inferior grains in japonica rice and the regulation changes of the gradient PNF were explored.The main results are as follows:1.With the increase of PNF application rates,the yield of both japonica rice varieties increased first and then decreased in 2020-2021,reaching the maximum at 120N treatment.Under 180N treatment,the yield of the two varieties decreased slightly compared with 120N treatment,with a reduction of 6.24%-7.32%in two years.The yield increase is mainly related to the increase of panicles and grains per panicle.The number of secondary branches and the number of grains growing on them contribute greatly to the increase of grains per panicle.Through polynomial fitting,it was found that the amount of PNF was 116.54 kg N ha-1 and 113.03 kg N ha-1 respectively when Nanjing 9108 and Nanjing 0212 achieved the highest yield level under certain nitrogen accumulation before jointing stage.In terms of rice quality,compared with inferior grains,superior grains have higher head rice rate and amylose content,and lower length-width ratio,chalky grain rate,chalkiness and protein content.The taste quality of superior grains is better,and the difference of taste value between superior and inferior grains is obvious with the increase of PNF.Topdressing PNF improved the processing quality of superior and inferior grains in japonica rice,deteriorated the appearance quality,decreased the content of amylose,and increased the content of total protein and component protein.The taste quality of superior and inferior grains deteriorated after applying nitrogen panicle fertilizer,and the taste value of inferior grain deteriorated more significantly,and the hardness of texture characteristics changed the most.In general,topdressing 60 kg N ha-1 PNF can achieve better yield and quality performance,which is the nitrogen application amount of high-quality and stable yield panicle fertilizer.2.Compared with the inferior grain,the superior grain has less total starch,amylopectin and crude fat,and more amylose,thus having a higher amylose/amylopectin ratio.The starch swelling power of the superior grain in two japonica rice varieties was 6.24%-12.81%lower than that of the inferior grain.Under the gradient PNF,the pasting characteristics of the superior grain were better,and the peak viscosity,hot viscosity,final viscosity and breakdown were higher than those of the inferior grain.In terms of thermal properties,the superior grain starch has low gelatinization enthalpy,high onset temperature and peak of gelatinization temperature,and obvious retrogradation after cooling.After topdressing with PNF,the total starch,amylose,amylopectin and crude fat content of superior and inferior grains in japonica rice were decrease,and the amylose/amylopectin ratio became lower.The crude fat content of topdressing with PNF was 24.44%-35.83%lower than that of 0N treatment.The solubility and swelling power of starch increased with the increase of PNF application rate.Topdressing PNF deteriorated the pasting properties of superior and inferior grains in japonica rice.The pasting viscosity decreased,the pasting temperature increased,the breakdown increased first and then decreased,and the setback decreased first and then increased.The relaxation time T21 increased after topdressing PNF,indicating that the bound water movement ability was enhanced,and the binding force of rice paste gel network on bound water was weakened.Topdressing PNF significantly increased starch gelatinization enthalpy,significantly reduced retrogradation enthalpy and retrogradation degree,but had little effect on gelatinization temperature(T0,TP and TC).3.The difference of starch grain size between superior and inferior grains is different among varieties.The proportion of small and middle starch grains(0-5μm)in the superior grains of Nanjing 9108 is higher than that in the inferior grains,while the proportion of large starch grains(≥5 μm)and volume mean diameter are lower than that of the inferior grains,while that of Nanjing 0212 is the opposite.The superior and inferior grains of japonica rice are all A-type crystals,but the relative crystallinity of the superior grain starch is lower,the content of single helix and double helix is less,and the amorphous region is more.In addition,the superior and inferior grains of japonica rice have little difference in the ordered structure of starch surface layer and the distribution of amylopectin chain length.Topdressing PNF increased the proportion of small and middle starch granules in superior and inferior grains of japonica rice,and decreased the proportion of large starch granules and the volume mean diameter.The relative crystallinity and double helix content of superior and inferior grains starch in japonica rice increased with the increase of PNF application rate,while the amorphous region and single helix content decreased.After applying PNF,the infrared ratio of 1045/1022 cm-1 of superior and inferior grain starch increased,and the infrared ratio of 1022/995 cm-1 decreased,but the effect was small,and the coefficient of variation of both varieties was only 0.79%-2.49%.In terms of amylopectin chain length distributions,topdressing PNF reduced the A-chain content of amylopectin in superior and inferior grain of japonica rice,increased the B2 and B3 chain content,thus reducing the ratio of amylopectin short chain to long chain(A1+B1/B2+B3),and lengthening the average chain length.Compared with 0N treatment,the content of A-chain in 180N treatment decreased by 3.30%8.18%,and B3 chain increased by 6.46%-17.98%.To sum up,an appropriate amount of PNF(60 kg N ha-1)can significantly increase the yield of japonica rice,improve the processing and nutritional quality of superior and inferior grains,reduce the deterioration of appearance and eating quality.The average chain length of amylopectin is shortened,the crystallinity of starch is low,the average diameter is large,and the physical and chemical properties are good.However,under excessive(180 kg N ha-1)PNF,the taste quality deteriorated,the starch crystallinity was high,the pasting characteristics were poor,and the inferior grains changed significantly.Form this,topdressing with 60 kg N ha-1 PNF can realize excellent quality and stable yield coordination production of japonica rice. |
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