Effects of nitrogen application rates on starch composition and pasting properties of high quality wheat

In field conditions, 3 typical cultivars, YM47 (strong-gluten), SN8355 (medium-gluten) and YM50 (weak-gluten) were used to investigate the starch granule distribution, composition of amylose and amylopection, and pasting characteristics of wheat starch under three nitrogen application rates (N 0, 15 and 30 g/m2). The results indicate that the range of starch granule diameters in wheat endosperm is from 1 to 45 μm. Wheat endosperm has a unimodal or bimodal curve in starch granule number distribution, and both starch granule volume and surface area distribution have a bimodal curve. The nadir of those bimodal curves is on the point of 10 μm, and therefore the starch granules in wheat endosperm could be divided into two groups, A-type granules (1-10 μm) and B-type granules (10-45 μm). The composition of wheat starch granules in high quality wheat cultivars varies significantly with genotypes. The strong-gluten wheat YM47 has a much higher ratio of B-type starch granules, while the weak-gluten wheat YM50 has a higher ratio of A-type starch granules, and the distribution of starch granules in the medium-gluten wheat SN8355 has a central tendency. The N rates influence the distribution of starch granules in high quality wheat endosperm significantly. In the conditions of this study, increasing N fertilization improves the ratio of A-type starch granules and decreases the ratios of B-type starch granules in the endosperm of strong-gluten wheat YM47. The B-type starch granule ratios in the weak-gluten wheat YM50 and medium-gluten wheat SN8355 are increased under the N fertilization, on the contrary, the A-type starch ratios of those two cultivars are declined after using more N, and the amplitude of the variation of the weak-gluten wheat YM50 is much higher than that of the medium-gluten wheat SN8355. Increasing N fertilization appropriately could heighten the starch content in wheat grain, but as the amount of N continued to increase, grain starch content might decrease. The amylose content in wheat grain or in isolated wheat starch is decreased after increasing N applications. The pasting characteristics of isolated starches in high quality wheat cultivars are greatly affected by increasing N amounts, and the effects vary with genotypes and N rates. In strong-gluten wheat cultivar YM47, increasing N properly could increase the through viscosity, final viscosity, setback, pasting temperature and peak time, and decrease peak viscosity and breakdown. As the amount of N increasing to 30 g/m2, pasting temperature and peak time are decreased, and those parameters using RVU as unit are increased compared to the control. The RVA parameters of isolated starches of weak-gluten wheat YM50 are declined after using more N fertilizer, while those of the medium-gluten wheat SN8355 are increased. The correlation analysis between RVA parameters and components of wheat starches indicates that the number, volume and surface area ratios of B-type starch granules are significantly positively correlated with peak viscosity and breakdown, and are significantly negatively correlated with through viscosity, final viscosity and setback. The amylose, amylopectin and starch contents of wheat grains are significantly negatively correlated with peak viscosity and breakdown, and are positively correlated with through viscosity, final viscosity, setback and peak time. The correlation trend between amylose content of isolated wheat starch and parameters of RVA is similar to that of amylose content in wheat grain, but the former does not reach the significant level. From the above results, we conclude that the N fertilization affects the pasting characteristics of wheat starch with the changes in constitute of amylose and amylopectin, and starch granule distribution.