Take a 2~3 g mature potato tuber, break it off with tweezers to obtain a cross section, cut it with a blade to obtain another flat cross section, and fix it on the sample with conductive adhesive. After gold spraying, the sample was directly observed by a scanning electron microscope (S4800II, Hitachi). A small amount of purified starch was picked up with tweezers and dispersed on the conductive adhesive. After gold spraying, it was directly observed by a scanning electron microscope. In order to compare the starch particle size distribution, the LS13320 laser diffraction particle size analyzer of Beckman Coulter Company analyzed starch particles. Put 50 mg of starch into a centrifuge tube, add 5 mL of distilled water for suspension, swirl and mix for 1 hour at 4°C, oscillate once every 10 minutes, then transfer to the dispersion box of a laser diffraction particle size analyzer, and measure its distribution.

For an examination of starch chain length distribution, Isoamylase (E-ISAMY, Megazyme, Ireland) was used to remove the branches of starch, and then the relative molecular weight distribution of starch was determined by high temperature gel exclusion chromatography (GPC, PL-GPC 220, USA). High performance liquid chromatography (HPAEC, Thermo ICS-5000, USA) was also used to look at the distribution of chain lengths in amylopectin (Zhang et al., 2017).

The relative crystallinity of starch was analyzed by a polycrystalline X-ray diffractometer (D8-ADVANCE, Germany). A Fourier transform infrared spectrometer (Varian 7000 FTIR, USA) was used to analyze the short-range ordered structure of starch. The crystal lamellar strength and thickness of starch were analyzed by a small-angle X-ray scatterometer (Li et al., 2016).

Measure the thermal characteristics of DSC6. Weigh 4 mg (dry basis) of starch or rice flour samples in an aluminum crucible (PE0219-0062), add deionized water at a mass ratio of 1:2, cover and seal, place at room temperature and balance overnight, and then measure. The empty crucible was used as the reference. The scanning temperature range was 20-140°C, and the scanning rate was 10°C/min. The temperature was 30 -120°C, and the thermal characteristic parameters and enthalpy (H) values were calculated. Two parallel samples were taken from each sample, and the results were averaged (Singh et al., 2003).

The data were processed using statistical analysis software such as Microsoft Excel 2010 and SPSS 27, and a variance analysis was performed.

As shown in the data in Table 2, the content (%) of amylose and amylopectin and the ratio of amylose/amylopectin in potato tubers reached extremely significant levels among different varieties, different altitudes and the interaction between varieties and altitudes. At different altitudes, the starch content of dry sample of Jiusen No.1 and Qingshu No.9 was about 80%~86%, the amylose content of dry sample of Jiusen No.1 potato starch was about 26% ~ 31%, the amylopectin content was about 69% ~ 74%, and the amylose content of dry sample of Qingshu No.9 potato starch was about 25% ~ 31%, and the amylopectin content was about 69% ~ 75%. The starch content of both increased with the elevation. The amylose content of Jiusen No.1 was higher than that of Qingshu No.9. As the altitude increases, the straight-chain starch of Qingshu NO.9 shows a trend of first decreasing and then increasing.

Figure 2 showed an electron microscope scanning picture of potato starch granules with magnification. It was observed that potato starch granules were mostly oval and round and the starch granules of Jiusen No.1 and Qingshu No.9 were roughly the same in shape. At low altitude, some starch granules of Jiusen No.1 showed irregular shapes, but at high altitude, the shapes were more uniform. At low altitude, the starch granule surface of Qingshu 9 was very rough, with many irregular bulges, while at high altitude, the surface was smoother. In general, potato starch granules extracted from Gaohai were more regular in shape and smoother in surface.

According to Figure 3 and Table 3, it can be seen that the potato granule volume was roughly unimodal, and there was a big difference in starch granule volume among different potato varieties. Qingshu No.9 was significantly larger than Jiusen No.1 was. With the increase of altitude, the starch granule volume changes of the two varieties were basically consistent, while Jiusen No.1 and Qingshu No.9 both decreased with the increase of altitude. According to Figure 4, at the same altitude, the number of starch granules of Jiusen No.1 was more than Qingshu No.9 in quantity distribution, and the difference of potato starch granules in low altitude was large, but the difference of potato starch granules in high altitude was small, which indicates that potato starch granules in high altitude were evenly distributed. According to Figure 5, in terms of surface area distribution, at the same altitude, the surface area of starch granules of Qingshu No.9 was larger than that of Jiusen No.1 with the increase of altitude, the surface area of starch granules of two varieties decreased significantly. Overall, the starch granules of the two potato varieties decreased with the elevation, and the average volume and surface area of the starch granules of Jiusen at all elevations were smaller than those of Junqingshu No.9.

In order to clarify the influence of starch fine structure on potato quality, the relative molecular weight distribution of these two potato starches was analyzed by high temperature gel permeation exclusion chromatography (GPC). As shown in Figure 6, the starches of the two samples were divided into three peaks. Based on the previous studies, it could be known that the peak 1 and the peak 2 were the short chain (AP1) and the long chain (AP2) of amylopectin respectively (Zhang et al., 2017). Among them, the tendency of AP1 components of Jiusen No.1 and Qingshu No.9 starch was the same, with the increase of altitude, AP1 first increases and then decreases. Studies has shown that AP1/AP2 reflects the branching degree of amylopectin, and the larger the ratio, the higher the branching degree of starch. By calculating the branching degree of two samples, it was found that the ratio of AP1/AP2 of Jiusen No.1 to starch (2.89 0.03) was significantly higher than that of Qingshu No.9 (3.09 0.02) at low altitude A1. The ratio of starch AP1/AP2 of Jiusen No.1 was lower than that of Qingshu No.9 at A2 and A3 altitudes. The third peak showed the amylose component (AM), from which it could be seen that the amylose component of Qingshu 9 was significantly more than that of Jiusen 1, especially the short chain component of amylose was more obvious.

In order to further clarify the chain length distribution characteristics of amylopectin, information was obtained by means of high performance liquid anion chromatograph (HPAEC). According to previous studies, the chain length distribution of amylopectin could be divided into four types: one short chain (A, DP6-12) and three long chains (B1, DP13-24;; B2, DP25-36; B3, DP > 36) Figure 7 showed the chain length distribution diagram. The chain length distribution diagram of Jiusen No.1 and Qingshu No.9 was similar, with the highest peak at DP12-13 and double peaks. As shown in Table 4, at the altitudes A1, A2 and A3, the chain length ratio of Jiusen No.1 and Qingshu No.9 varieties A, B1 and A/B increased with the increase of altitude, while the chain length ratio of B2 and B3 decreased with the increase of altitude, showing an opposite trend. However, the average polymerization degree was highest in Xichang, which increases at first and then decreases with the increase of altitude. The starch chain length ratio of Jiusen No.1 and Qingshu No.9 was mainly concentrated in chain A and B1, with the ratio of 18.21%~24.28% and 44.68%~53.49% respectively. However, the proportion of B2 and B3 chains was small, and the short chain of Qingshu 9 was significantly higher than that of Jiusen 1 at different altitudes.

Potato granules are mainly semi-crystalline complexes with double helix crystal structure formed by amylopectin molecules and amorphous structure formed by some amylopectin and amylose, so the molecular structure of starch has an important influence on the crystal type and crystallinity of starch (Gan et al., 2009). Firstly, the crystal diffraction of two starch samples were analyzed by X- ray diffraction (XRD). Figure 8 showed the X-ray diffraction patterns of two potato starches at different altitudes. From Figure 8, it can be observed that there was no obvious difference in the peak types of the two potato starches, with strong diffraction peaks around 5.5 ~ 5.6, 15.0, 19.7, 22.2 and 24.0 at 2θ angles. Therefore, it was be judged that this potato starch might be a B-type crystalline structure. This was the same as the research results of other varieties of potato starch reported in the literature (Chen et al., 2013). However, after further comparison and analysis of the long-range ordered structure (relative crystallinity) of the two starches, it was found that there were significant differences between the relative crystallinity of the two potato starches at different altitudes, and the relative crystallinity of Qingshu No.9 (21.42%, 48.30%, 24.84%) was significantly higher than that of Jiusen No.1 (15.05%, 39.75%) With the increase of altitude, the relative crystallinity of different potato varieties first increased and then decreased, and the relative crystallinity ranges from 15.05% to 39.75% and from 21.42% to 48.3% respectively. This result was in good agreement with the difference of fine structure of starch. Because the starch crystal is mainly composed of double helix structure formed by medium and long chains of amylopectin, it can be inferred that the relatively large number of long chains of amylopectin in Qingshu No.9 starch was the main reason for its high crystallinity. In addition, this also explains the high enthalpy value of Qingshu No.9 starch, because the higher the crystallinity of starch, the more heat it needs in the gelatinization process.

Fourier transform infrared spectroscopy can accurately analyze the short-range ordered structure of starch crystal structure. As shown in Figure 9 and Table 5, the infrared absorption curves of Jiusen No.1 and Qingshu No.9 starch were consistent. Usually, the infrared absorption peaks of 1045/cm and 1022/cm in the infrared spectrum were the structural characteristics of starch crystallization region and amorphous region, and the ratio of them (1045/1022) can reflect the short-range order degree of starch molecules (Jay-lin, 2009). Through the calculation of characteristic values, it was found that the short-range order degree of Jiusen No.1 and Qingshu No.9 starch was significantly different at different altitudes, and the difference trend was consistent with the crystallinity, with the short-range order degree ranging from 0.81 to 0.94. At low altitude (Chongzhou), the short-range order degree of Jiusen No.1 starch was lower than that of Qingshu No.9 starch, and that of Jiusen No.1 starch was higher than that of Qingshu No.9 starch at other altitudes.

According To the comparative analysis of Table 6, the thermodynamic properties of starch were determined as TO (initial temperature), Tp (peak temperature), Tc (conclusion temperature) and ΔH (enthalpy of crystal melting). All the results were shown in Table. These parameters reflect the proportion of crystals related to molecular order. Amylose content was considered as the main factor to control the thermal properties of starch, but it is not the only factor. Other factors affecting thermal properties include molecular structure of amylopectin/amylose and amylose lipid complex (Liu et al., 2006).

At different altitudes of Chongzhou, Xichang and Litang, the initial temperature, peak temperature, final gelatinization temperature and enthalpy change of Jiusen No.1 and Qingshu No.9 potato varieties all showed the same trend of first increasing and then decreasing. At all altitudes, the initial temperature, peak temperature, final gelatinization temperature and enthalpy change of Qingshu No.9 starch were slightly higher than those of Jiusen No.1 variety, and the changes were 58.17~63.70°C, 61.20~67.70°C and 67.29 ~74.60°C, respectively. In addition, because Qingshu No.9 starch has high enthalpy, it needs to absorbed more heat. Figure 10 (DSC endothermic curve) of the thermal gelatinization characteristics of two potato starch varieties showed that in Xichang, the temperature span of Jiusen No.1 and Qingshu No.9 for gelatinization is longer, that is to say, the time required for the thermal gelatinization process is also longer.

Further dimension reduction analysis of potato starch indexes showed in (Figure 11) that the contribution rates of principal component 1(PC1) and principal component 2(PC2) were 55.0% and 33.9% respectively. The total variance contribution rate of them was 88. 9%, which means that PCA1 and PCA2 jointly explain 88.9% of the differences in the physicochemical properties and structure of potato starch at different altitudes. It could be seen from the figure that the spatial distribution of different altitudes was obviously different, indicating that the differences of starch indexes in altitude areas were significant due to meteorological factors, and there were also significant differences among varieties. Accumulated temperature, accumulated effective accumulated temperature were positively correlated with starch thermodynamic properties (To, Tp, Tc, H), polymerization degree, crystallinity, weighted average diameter, medium and long chain B2 (DP25-36), B3(DP≥37), 1022/995cm^-1. Accumulated radiation and accumulated precipitation have significant positive correlation with short chain A. Jiusen No.1 and Qingshu No.9 were mainly affected by accumulated radiation and accumulated rainfall in Litang at high altitude, and by accumulated temperature and accumulated effective accumulated temperature in Xichang at middle and high altitude.

Starch is the main component of potatoes, and the starch content in different varieties of potatoes was significantly different. The starch content in fresh potatoes was 9%-23%, and that in dry potatoes was 66%~80%. It was divided into amylose and amylopectin. From the morphology of starch grains, Jiusen No.1 and Qingshu No.9 in this study have no obvious difference in starch shapes, both of which were round or oval. However, with the increase in altitude, the surface of starch was smoother and the number of small granular starches increases. Generally, starch granules were divided into ≤2μm (small starch granules), 2~5μm (medium starch granules) and ≥5μm (large starch granules) according to their particle size. In the particle size distribution of potato starch granules, different varieties of potato starch granules account for the largest proportion (Chung et al., 2014). However, this study believed that planting potatoes at low altitude and at high altitude was beneficial to increase the proportion of starch granules and reduce the average weighted body area number and surface area diameter of starch granules. The results of this study were consistent with those of its predecessors, and so is this study. The influence of temperature during grain filling on the particle size distribution of potato starch by predecessors was reported by (Liu et al., 2017). It also showed that the increase in the proportion of starch granules in rice grains was not conducive to the improvement of the cooking and eating quality of rice. Therefore, potatoes planted in high-altitude areas taste better.

In general, high temperatures tend to raise the proportion of amylopectin with a long chain A chain and decrease the proportion with a short chain B chain, whereas low temperatures have the reverse effect. (Tao et al., 2020) reported how the environment affects the distribution of rice’s starch chain length; it was thought that the proportion of long-chain amylopectin was positively connected with nighttime temperatures while the proportion of short-chain amylopectin was negatively correlated. (Chung et al., 2019) reported that amylopectin with a shorter or less long chain lowers the gelatinization temperature (To and Tp). Additionally, (Du, 2019) demonstrated that the amount of amylopectin’s chain length has an impact on how easily starch gelatinizes. The gelatinization temperature of rice starch increases with the number of long amylopectin chains present and becomes more challenging to gelatinize. The retrogradation degree and retrogradation enthalpy rise concurrently, the rice hardens after cooling, and this is not advantageous for the enhancement of rice flavor quality. According to the findings, potatoes grown at low and medium altitudes considerably raised the proportion of long chain amylopectin and decreased the fraction of short chain amylopectin. The amount of long chain amylopectin significantly dropped while the amount of short chain amylopectin increased when potatoes were cultivated at a high altitude.

The crystalline structure of starch from different sources was different. According to the different X-ray diffraction patterns, the crystalline structure of starch could be divided into three types, namely, type A, type B, and type C. The difference was that the rotation angle range of the strong diffraction peak was different (Zhong et al., 2017). The results of this study showed that potato starch has a B-type crystalline structure at different altitudes, which was consistent with previous studies. Some studies have shown that the order of the surface structure of starch could be judged by the ratio of crystallinity of 1045/1022 cm^-1 and 1022/995 cm^-1 (Zhou et al., 2020). Compared with low altitude, the crystallinity and the ratio of 1045/1022 cm^-1 in the high altitude area were lower. Previous studies had shown that the high crystallinity of starch improved the stability of starch, lead to an increase in gelatinization temperature, and reduced the eating quality of rice (Lu et al., 2011; Zhang et al., 2011). Therefore, potatoes planted in high altitude areas had better tasting quality.

The thermodynamic properties of starch were an important index to reflect the quality of starch, which was not only affected by the content of amylose and amylopectin but also by the chain length distribution of amylose and amylopectin (Chung et al., 2011). On the influence of ambient temperature on starch thermodynamic (DSC) characteristics, (Liu et al., 2017) considered that high temperature stress increases the gelatinization temperature of starch, and the main reason was that high temperature increases the proportion of large-sized starch and the size of starch granules. (Chun et al., 2015) considered that high temperatures during grain filling reduced the short chain ratio of amylose and amylopectin and increased the medium chain ratio of amylopectin, which leads to the increase in the gelatinization temperature of starch. (Neoh et al., 2020) reported that cold-treated wheat grains significantly affect the starch structure, and the gelatinization temperature of starch becomes lower, which affects the function of flour and the quality of final products. (He et al., 2010; Zhu, 2018) showed in their correlation analysis that the distribution ratio of small starch granules were negatively correlated with gelatinization temperature, while the proportion of large starch granules and the diameter of starch surface area were positively correlated with gelatinization temperature. The relative crystallinity and peak strength of starch were positively correlated with gelatinization temperature, enthalpy, and retrogradation enthalpy. This study showed that compared with potatoes in high altitude areas, potatoes plucked from low and middle altitude areas have more large starch granules and larger starch granule diameter. The higher the gelatinization temperature, the higher the relative crystallinity and peak strength of starch, and the higher the gelatinization heat. It was consistent with previous studies. Therefore, the amount of starch and starch granules in high altitude areas was higher, the gelatinization temperature was lower, and the amount of heat needed for gelatinization was less. This makes high altitude areas a better place to plant potatoes to make them taste better.