Effects of phosphorus management on physicochemical properties of wheat starch granules under different water conditions

  Objectives  The physical and chemical properties of wheat starch are not only controlled by genetype of varieties, but also affected by environmental ecological factors and cultivation measures. Studying the effects of different water conditions and application of phosphate fertilizer on the physicochemical properties of wheat grain starch can provide a theoretical basis for scientific application of phosphate fertilizer.

  Methods  Field experiments were carried out using a high yield winter wheat cultivar of Xindong20 in Xinjiang. The two water conditions were drought stress (DT, total irrigation amount of 5625 m³/hm²) and suitable water irrigation (WT, total irrigation amount of 9000 m³/hm²). Under each water condition, three phosphate fertilizer managements were set: P1 (applying all P fertilizer at wheat regreening stage of winter wheat), P2 (applying 50% P fertilizer at regreening stage and 50% at jointing stage), and P3 (applying 40% at regreening stage, 30% at jointing stage and 30% at grain filling stage). The grain starch granules were extracted for the determination of average diameter, ratio of amylose to amylopectin, crystal structure, enzymatic hydrolysis, thermal properties and gelatinization properties.

  Results  Compared with WTP1 treatment, the average diameter of starch granules in DTP1 treatment was reduced by 5.92%. In DTP2 treatment was 7.61% lower (P<0.05) than that in WTP2 treatment. While, in DTP3 treatment was 13.29% higher (P<0.05) than that in WTP3 treatment. The average diameter of starch grains was P3>P2>P1 under DT condition, and P2>P1>P3 under WT condition. Under WT condition, the ratio of amylose to amylopectin was P1>P2>P3. Different moisture conditions did not significantly change the basic morphology of starch granules. The number of surface damaged starch granules under DT conditions was more than that of WT, among which DTP3 treatment had the largest number of surface damaged starches. WTP3 treated starch granules had more starch microchannels. Under DT conditions, the number of channel proteins treated with P1 and P2 was lower than that of P3 treatment; under WT conditions, the number of channel proteins treated with P2 was the least, and the amount of channel proteins treated with P3 was the most. The crystallinity of DTP1、DTP2 and DTP3 treatment increased by 66.50%, 0.55% and 46.20%, respectively, compared with the corresponding WT treatments. The results showed that the onset temperature of WTP3 treatment was the highest, and WTP1 treatment was the lowest. DTP3 significantly (P<0.05) reduces peak temperature compared to WTP3 treatment. For the conclusion temperature, the difference among the treatments under WT condition was significant, P3 was the highest and P1 was the lowest. For the gelatinization enthalpy, DTP2 treatment and DTP3 treatment were both higher (P<0.05) than the corresponding WT treatments. In this study, five items of gelatinization parameters in the DTP3/WTP3 group reached significant (P<0.05) differences. Three items of gelatinization parameters in the DTP2/WTP2 group reached significant differences, only one gelatinization parameter reached significant difference in DTP1/WTP1 group. Correlation analysis showed that there was a significant (P<0.05) positive correlation between the average diameter and the breakdown (BD) value. The total amount of starch was significantly positively correlated with the crystallinity. The ratio of amylose/amylopectin was only significantly positively correlated with final viscosity (FV). There was a significant negative correlation between 1000 grains weight and gelatinization temperature. Grain phosphorus content had a significant negative effect on starch gelatinization characteristics. The crystallinity was significantly positively correlated with the trough viscosity (TV) and FV of starch.

  Conclusions  Under drought stress, split application of phosphorus fertilizer in three times (applying 40% at regreening stage, 30% at jointing stage and 30% at grain filling stage), compared with other phosphorus application methods, can significantly increase the average diameter of endosperm starch grains, and the number of micropores and internal channel proteins on the surface of endosperm starch grains. Compared with other phosphorus application methods, applying all the phosphorus fertilizer at regreening stage can significantly improve the crystallinity of starch grains in the endosperm. Under normal irrigation, applying all the phosphorus fertilizer at regreening stage can significantly increase ratio of amylose to amylopectin of starch grains in endosperm. There is a significant correlation between phosphorus content in grains and gelatinization characteristics. Therefore, different processing qualities of wheat can be obtained in production by adjusting water condition and phosphorus fertilizer proportions in different growth periods.