Potato growth, photosynthesis, yield, and quality response to regulated deficit drip irrigation under film mulching in a cold and arid environment

The effects of the amount and timing of regulated deficit drip irrigation under plastic film on potato (‘Qingshu 168’) growth, photosynthesis, yield, water use efficiency, and quality were examined from 2017 to 2019 in cold and arid northwestern China. In the four stages of potato growth (seedling, tuber initiation, tuber bulking, starch accumulation), eight treatments were designed, with a mild deficit was in treatments WD1 (seedling), WD2 (tuber initiation), WD3 (tuber bulking), and WD4 (starch accumulation); and a moderate deficit in WD5 (seedling), WD6 (tuber initiation), WD7 (tuber bulking), and WD8 (starch accumulation). The net photosynthetic rate, stomatal conductance, and transpiration rate decreased significantly under water deficit in the tuber formation and starch accumulation stages. Although water deficit reduced potato yields, a mild deficit in the seedling stage resulted in the highest yield and water use efficiency at 43,961.91 kg ha−1 and 8.67 kg m−3, respectively. The highest overall quality was in potatoes subjected to mild and moderate water deficit in the seedling stage. Principal component analysis identified mild water stress in the seedling stage as the optimum regulated deficit irrigation regime. The results of this study provide theoretical and technical references for efficient water-saving cultivation and industrialization of potato in northwestern China.

Potato leaf photosynthetic characteristics. Net photosynthetic rate. The net photosynthetic rate (Pn) of potato leaf from seedling to starch accumulation stage increased first and then decreased, showing a singlepeak in CK and all treatments (Fig. 1). Mild regulated deficit treatments did not significantly affect the rate in the seedling stage. In the moderate deficit treatment WD5, the Pn decreased by 13.14% compared that with in CK, although the decrease was not significant. In the tuber initiation stage, the rate decreased significantly to 0.410 μmol m −2 s −1 in WD2 (mild deficit) and to 0.337 μmol m −2 s −1 in WD6 (moderate deficit), decreases of 19.28% (WD2) and 33.70% (WD6) compared with that in CK. The Pn was not significantly affected in the other treatments in this stage.
In the tuber bulking stage, the Pn decreased by 8.31% in WD3 (mild deficit) and by 29.37% in WD7 (moderate deficit), compared with that in CK. The compensation effect in WD2 after rehydration was greater than that in WD6, but neither rate was significantly different from that in CK. The Pn did not decrease from the tuber initiation stage to the tuber expansion stage, and the mean value in the bulking stage was between 0.323 and 0.463 μmol m −2 s −1 . In the starch accumulation stage, the absolute value of the Pn was lower than that at tuber initiation and expansion stages, with a mean value between 0.227 and 0.348 μmol m −2 s −1 . However, the Pn was significantly higher than that in the seedling stage. The Pn decreased by 15.12% in WD4 (mild deficit) and by 33.46% in WD8 (moderate deficit), compared with that in CK, indicating that a regulated deficit at the starch accumulation stage negatively affected the net photosynthetic rate.
Stomatal conductance. Throughout the growth period, the stomatal conductance (Gs) increased successively from the seedling stage to the tuber initiation stage and then tuber bulking stage; however, the Gs decreased at the starch accumulation stage. There were differences in the range of decrease among the treatments (Fig. 2). The lowest Gs was in the seedling stage, ranging from 5.96 to 6.71 mol m −2 s −1 . In WD1 (mild deficit) and WD5 (moderate deficit), Gs decreased significantly by 7.29% and 11.09%, respectively, compared with that in CK. The Gs in the seedling stage was not affected in the other treatments. The Gs increased in the tuber initiation stage in all treatments except WD2 and WD6, with values increasing to between 12.68 and 14.70 mol m −2 s −1 . The Gs in WD2 decreased by 13.70% and that in WD6 decreased by 14.83%. In the tuber bulking stage, Gs increased to the highest levels of the entire growth period, ranging from 14.73 to 17.00 mmol m −2 s −1 , although it was lower in regulated deficit treatments than in CK. Compared with that in CK, the Gs decreased significantly in WD3 www.nature.com/scientificreports/ by 8.96% and in WD7 by 13.34%, which showed that regulated deficit in the tuber initiation and expansion stages did not favor stomatal opening. The Gs decreased significantly from the tuber bulking stage to the starch accumulation stage. With sufficient irrigation in this period, Gs was not significantly different from that in CK, whereas in WD4 (mild deficit) and WD8 (moderate deficit), it decreased significantly by 17.22% and 25.05%, respectively. This result demonstrated that regulated deficit during starch accumulation affected stomatal conductance. Table 1. Indices of potato growth in conventional irrigation (CK) and regulated deficit drip irrigation treatments in 3 years and averaged across years. Diameters are those of tubers. Different lowercase letters within a column for a year or the average indicate significant differences among treatments (P < 0.05). The irrigation treatments were conventional irrigation (CK) and mild or moderate water deficit during each of four stages of potato growth. Mild deficit was in treatments WD1 (seedling), WD2 (tuber initiation), WD3 (tuber bulking), and WD4 (starch accumulation); moderate deficit was in treatments WD5 (seedling), WD6 (tuber initiation), WD7 (tuber bulking), and WD8 (starch accumulation). Values followed by the same lowercase letters within each year are not significantly different at the P < 0.05 level. *, **, and *** are significant at the P < 0.05, 0.01, and 0.001 levels, respectively. ns not significant. www.nature.com/scientificreports/ Transpiration rate. In all treatments, the transpiration rate (Tr) first increased as growth progressed and then decreased, with a peak at the tuber expansion stage (Fig. 3). In the seedling stage, the Tr decreased by 13.01% in WD1 (mild deficit) and by 22.29% in WD5 (moderate deficit), compared with that in CK. In the tuber initiation stage, the Tr was 5.45 μmol mol −1 s −1 in WD2 (mild deficit) and 4.84 μmol mol −1 s −1 in WD6 (moderate deficit), decreasing by 10.68% in WD2 and by 20.63% in WD6 compared with that in CK.
In the tuber bulking stage, the Tr in WD1, WD2, WD5, and WD6 was not significantly different from that in CK after rehydration, showing evidence of a compensation effect. Compared with the tuber initiation stage, the Tr showed a large increase in all treatments. The Tr in WD7 (moderate deficit) decreased significantly by 10.28% compared with that in CK, whereas in WD3 (mild deficit), the Tr decreased by 5.25%, but it was not significantly different from that in CK. In the tuber initiation and tuber bulking stages, the Tr was higher than that in the seedling stage. In the starch accumulation stage, the Tr was 4.59 μmol mol −1 s −1 in WD4 (mild deficit) and 4.53 μmol mol −1 s −1 in WD8 (moderate deficit), decreasing by 15.58% in WD4 and by 16.64% in WD8 compared with that in CK, although the decreases were not significant. After rehydration, the Tr in WD3 and WD7 decreased, but it was not significantly different from that in CK.
Tuber yield and its components. Regulated deficit in all growth stages significantly affected water consumption, yield, water use efficiency, and irrigation water use efficiency ( Table 2). Water deficit in all growth stages significantly decreased water consumption by potato. Compared with 5837 m 3 ha −1 in CK, water consumption decreased to 4977 m 3 ha −1 in WD3: 4952 m 3 ha −1 in WD5: 4769 m 3 ha −1 in WD6, and 4724 m 3 ha −1 in WD8, which were decreases of 14.73% (WD3), 15.16% (WD5), 18.30% (WD6), and 19.08% (WD8) compared with that in CK. In WD2 (mild deficit) and WD7 (moderate deficit), the decrease was slight. The regulated deficit treatments caused decreases in potato yield to different degrees. The water deficit in the seedling stage in WD1 caused the smallest decrease in yield to 43,962 kg ha −1 , a decrease of only 4.50% compared with that in CK. Water deficit in the tuber bulking stage in WD7 (moderate deficit) caused the greatest decrease in yield to 33,835 kg ha −1 , a decrease of 26.50% compared with that in CK. The decreases in yield under moderate regulated deficit treatment were greater than those under mild treatment, indicating that the larger decrease in irrigation had a greater negative effect on potato yield. Affected by yield, the output values showed similar trends by treatment. Output value per cubic water was the lowest in WD7 (moderate deficit), decreasing by 15.56% compared with that in CK. By contrast, the values increased in the other treatments by 0.07-50.93%, compared with that in CK. of potato leaf. The irrigation treatments were conventional irrigation (CK) and mild or moderate water deficit during each of four stages of potato growth. Mild deficit was in treatments WD1 (seedling), WD2 (tuber initiation), WD3 (tuber bulking), and WD4 (starch accumulation); moderate deficit was in treatments WD5 (seedling), WD6 (tuber initiation), WD7 (tuber bulking), and WD8 (starch accumulation). www.nature.com/scientificreports/ Effect of regulated deficit drip irrigation under film on potato quality. Regulated deficit treatment had different effects on the characteristics of potato quality (Table 3). Compared with that in CK, deficient irrigation in the seedling stage did not significantly affect total sugar content in potato. However, in the other growth stages, regulated deficit reduced sugar content by 6.55-44.64%, and the greatest reduction was in WD8 with the deficit at the starch accumulation stage. With the deficit in the seedling stage, the protein content in WD5 (moderate deficit) was 2.18 mg g −1 , an increase of 0.77% compared with that in CK. The protein contents decreased by 4. 46-32.46% in the other treatments. The greatest reductions compared with the protein content in CK were in WD3 (18.31%), WD7 (21.85%), and WD8 (32.46%). With the deficit in the seedling stage, the starch content reached 36.06% in WD1 (mild deficit), a 3.34% increase compared with that in CK. In the other mild deficit treatments, although the starch content decreased, it was not significantly different from that in CK. In the moderate deficit treatments, the starch content decreased significantly by 10.66% in WD6, by 20.41% in WD7, and by 27.55% in WD8. With the deficit in the seedling stage, vitamin C and calcium contents decreased significantly by 9.21% in WD1 (mild deficit), compared with the contents in CK. However, the vitamin C content increased in the other mild and moderate deficit treatments, with the values increasing by 11.55-55.35% compared with that in CK. In addition, the vitamin C content tended to increase with the delay in deficit treatment, and the greatest increase was in WD8, with the content increasing significantly by 55.35% compared with that in CK.
Water use efficiency and irrigation water use efficiency. With water deficit in the starch accumulation stage, water use efficiency increased most significantly in WD4 by 13 Comprehensive evaluation of different irrigation deficit methods. We calculated the correlation matrix of 11 beneficial evaluation indices of the regulated deficit irrigation methods in the Hexi Oasis (Tables 4, 5, 6). Feature analysis of the matrix showed that the first five major components (comprehensive indices) had an accumulated contribution of 99.57% to the evaluation equation. Thus, we established the comprehensive formulas based on the first five indices: yield, WUE, IWUE, output value per cubic water, and output value: www.nature.com/scientificreports/ According to the principal component analysis, the principal component values of deficit irrigation at different growth stages were ranked as WD1 > CK > WD5 > WD4 > WD2 > WD3 > WD6 > WD8 > WD7. In conclusion, WD1 was optimal in the valuation.

Discussion
Regulated deficit irrigation abides by crop water-demand laws in each growth stage to induce water stress to different degrees, causing crop growth conditions to change so as to stabilize yield, save water, and adjust quality 25-28 . Deng et al. 29 found that regulated deficit drip irrigation under film changes the hydrothermal environment of farmland soil, thereby affecting crop growth. In this study, the amount of water irrigated in drip irrigation affected the indices of plant height, stem diameter, tuber transverse diameter, tuber longitudinal diameter, and leaf area of potato. These indices all tended to decrease, and regulated deficit irrigation had a greater effect in the late growth stage than in the early growth stage. The explanation might be that the root system in the seedling stage was small and required less water. In addition, slight regulated deficit had a mild influence on potato growth. By contrast, the late growth stage is essential for nutrient production in potato and requires more water. Water stress inhibited the natural growth and development of potato, hindering tuber formation.
Photosynthesis is highly sensitive to water stress. Water deficit may hinder CO 2 from entering leaves or affect the CO 2 carboxylation ability of mesophyll cells, thereby inhibiting photosynthesis [30][31][32] . According to Reddy et al. 33 , water deficit can close the stomata in crop leaves; further reducing Gs and then Pn. In this study, in WD2 and WD6 and WD4 and WD8, the Pn decreased significantly in the tuber formation and starch accumulation www.nature.com/scientificreports/ stages. The decrease increased as the regulated deficit amount increased. Chai et al. 34 found that as the regulated deficit degree gradually increases, the Gs decreases significantly, thereby lowering the Pn. This result is consistent with the conclusion of this study. A similar conclusion was also reached in a study of regulated deficit drip irrigation under film with Isatis indigotica in an oasis environment 35 . The result in this study might be explained by water deficit inhibiting the aboveground growth of potato, and as a result, both leaf area index and the transpiration rate decreased. In addition, with a water deficit in the soil, the partial or complete closure of stomata decreases the transpiration rate, thus leading to an overall decrease in net photosynthetic rate [36][37][38] . In this study, moderate water deficit led to a greater decrease in Gs, likely because the increase in water stress increased stomatal closure, which further reduced the transpiration rate. In this study, regulated deficit irrigation decreased potato yield and output values to different extents, ranging from 4.50 to 26.50%, which reduced production benefits. Enciso et al. 39 found that a moderate regulated deficit in the seedling and the mature stages can increase crop yields and economic benefits. The result is in contrast Table 2. Potato yield, output value, and water use in conventional irrigation (CK) and regulated water deficit drip irrigation treatments in 3 years and averaged across years. The irrigation treatments were conventional irrigation (CK) and mild or moderate water deficit during each of four stages of potato growth. Mild deficit was in treatments WD1 (seedling), WD2 (tuber initiation), WD3 (tuber bulking), and WD4 (starch accumulation); moderate deficit was in treatments WD5 (seedling), WD6 (tuber initiation), WD7 (tuber bulking), and WD8 (starch accumulation). www.nature.com/scientificreports/ to those in this study, which may because of differences in regulated deficit amount, test conditions, and potato varieties. In this experiment, compared with the output in CK, mild (WD3) and moderate (WD7) regulated deficits in the tuber enlargement stage significantly decreased the output by 20.86% and 26.50%, respectively. Consistent with this study, Mustafa Ünlü et al. 40 , Hassan et al. 41 , Nagaz et al. 42 , and Im et al. 43 also found that water deficit at the tuber enlargement stage reduces production by approximately 20% compared with that with a sufficient water supply. As tubers begin to divide and expand in the tuber enlargement stage, potatoes transition from the reproductive stage to the vegetative growth stage. Water deficit decreases potato transpiration Table 3. Indices of potato quality at harvest in conventional irrigation (CK) and regulated deficit drip irrigation treatments in 3 years and averaged across years. Different lowercase letters within a column for a year or the average indicate significant differences among treatments (P < 0.05). The irrigation treatments were conventional irrigation (CK) and mild or moderate water deficit during each of four stages of potato growth. Mild deficit was in treatments WD1 (seedling), WD2 (tuber initiation), WD3 (tuber bulking), and WD4 (starch accumulation); moderate deficit was in treatments WD5 (seedling), WD6 (tuber initiation), WD7 (tuber bulking), and WD8 (starch accumulation).Values followed by the same lowercase letters within each year are not significantly different at the P < 0.05 level. *, **, and ***are significant at the P < 0.05, 0.01, and 0.001 levels, respectively. ns not significant. www.nature.com/scientificreports/  www.nature.com/scientificreports/ and photosynthesis, and compensation and recovery with rehydration are difficult. As a result, yield is severely reduced, decreasing economic benefits. Compared with the output of potatoes in CK, the output a with mild regulated deficit in the seedling stage (WD1) decreased by only 4.50%, likely because root activity and absorption were low in the early growth stage. During the seedling stage, the water deficit likely promoted deep root penetration. With the irrigation deficit in the seedling stage, potato had a longer time to recover following the subsequent rehydration, and thus, water deficit had little effect on yield and economic benefits. The highest WUE was in WD1, followed by that in WD5, WD4, and WD8, which increased by 10.87% (WD1), 5.84% (WD5), 4.60% (WD4), and 4.99% (WD8) compared with that in CK. The lowest WUE was in WD7 at only 6.81 kg m −3 , which was a decrease of 12.92% compared with that in CK. The highest IWUE was in WD5, reaching 23.54 kg m −3 , followed by that in WD8 (22.79 kg m −3 ), which increased by 30.13% and 25.98%, respectively, compared with that in CK. The lowest IWUE was in WD7 (15.66 kg m −3 ), which was a decrease of 13.45% compared with that in CK. The IWUE of the other treatments increased to different degrees, with increases ranging from 2.52 to 30.13%. Therefore, moderate water deficit at the seedling and starch accumulation stages helped to improve the WUE and IWUE of potato, consistent with the findings of Li et al. 44 and Liuyang et al. 45 . However, an unreasonable water deficit can cause significant yield reduction and thus reduce output and benefits.
While maintaining yield, timely and moderate water deficit can increase WUE and the quality of products [46][47][48] . In this study, a moderate regulated deficit increased protein, starch, vitamin C, potassium, and calcium contents in potato. Mild regulated deficit in the seedling stage increased starch and potassium contents without reducing total sugar content. By comparison, moderate regulated deficit increased protein, vitamin C, and calcium contents. Regulated deficit irrigation in the other growth stages did not lead to the accumulation of total sugar, protein, starch, and potassium. Guizani et al. 49 found that water deficit in the seedling stage can improve potato quality, which is consistent with the conclusion in this study. However, Zhang 50 found that a regulated deficit had no significant effect on starch content during potato growth stages. In contrast, the results of in this study showed that a regulated deficit significantly reduced potato starch content. The inconsistency between studies might be due to differences in factors such as soil type and potato variety. Because there are few comprehensive reports on how the amount of regulated deficit and the stages in which the deficit occurs affect potato yield and quality, additional experiments are needed to explore the effects of regulated deficit irrigation on yield and quality with different varieties and in different regions. . The area has a typical continental arid climate, with abundant sunshine and large differences between day and night temperatures, which are conditions conducive to photosynthesis, nutrient accumulation, and yield formation in crops. The average annual temperature is 6.0 °C. The thermal integral exceeding 0 °C is 3,500 °C day, whereas the thermal integral over 10 °C is 2985 °C day". The average annual sunshine time is 3000 h, and the average frost-free period is 136 days. According to meteorological data from 2000 to 2018, the average annual rainfall in the area is 328 mm, and the evaporation is 1900 mm. Soil in the experimental site is light loam with medium fertility and a pH of 7.22. The field capacity of tilled soil is 24.0%, the wilting point of the soil is 8.2%, and the soil bulk density is 1.48 t/m 3 . The 0 to 20 cm of topsoil contained 12.8 g/kg organic matter, 63.5 mg/kg alkali-hydrolyzable nitrogen, 13.1 mg/kg available phosphorus, and 192.7 mg/kg available potassium. The salinization effect was mild because of the deep source of groundwater. The rainfall and temperature for the three potato seasons are shown in Fig. 4 (Fig. 5). A white plastic film (140 cm wide, 0.01 mm-thick; China Dongguan Shuotai Industrial Co., Ltd.) covered two rows of potatoes with a planting density of 77,000 plants/ ha. Drip irrigation was applied under the film with the irrigation pipe placed between two rows. Each treatment and control were repeated three times, and 140 potato plants were sown in each test plot. Each section covered 33.6 m 2 (7 m × 4.8 m). There were two levels of water deficit: mild with soil moisture at 55-65% of field capacity and moderate with soil moisture at 45-55% of field capacity. The soil moisture with conventional irrigation (CK) was 65-75% of field capacity. Each level of deficit was applied in each of four growth stages of potato: seedling, tuber initiation, tuber bulking, and starch accumulation stages. Thus, there were eight total treatments: WD1: mild, seedling; WD2: mild, tuber initiation; WD3: mild, tuber bulking; WD4: mild, starch accumulation; WD5: moderate, seedling; WD6: moderate, tuber initiation; WD7: tuber bulking; WD8: starch accumulation (Table 7).

Agronomic practices.
To ensure crop yield, the experimental section was tilled to 30 cm 10 days before sowing. Weeds were cleared manually. Diammonium phosphate (18% nitrogen and 46% P 2 O 5 ) at 400 kg/ha and western compound fertilizer (15% nitrogen, 15% P 2 O 5 , and 15% K 2 O) at 750 kg/ha were applied as base fertilizers one time at sowing. Physiological indices. Photosynthetic indicators were measured starting on the 5th day after the regulated deficit treatment in each reproductive period. Three separate measurements were taken on 3 days during each reproductive period when the weather was clear, and the results were the mean values of three measurements. Each measurement was conducted using a LI-6400 portable photosynthesis system (LI-COR, USA) from 9:30 to 11:00 a.m. Photosynthetic indicators measured included net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr).
Yield and tuber morphological properties. After potatoes ripened, the potatoes in each section were harvested and measured. There were three plots in each experiment, and the average value of the three plots was taken for analysis in each experiment. In other words, the average value of three replicate plots was the output of each treatment. An electronic scale (accuracy to 0.01 g) was used to weigh the potatoes, and the yield was converted to kg/ha. In addition, 20 potato plants were harvested separately in each section, and the tubers were cut off and washed. The transverse and longitudinal diameters of the tubers were measured with vernier calipers that were accurate to 0.01 mm. The average value of each treatment was determined.
Water use efficiency and irrigation water use efficiency. Water use efficiency (WUE) and irrigation water use efficiency (IWUE) were calculated using the following formulas 50 : where WUE (kg/m 3 ) and IWUE (kg/m 3 ) are the water use efficiency and the irrigated water use efficiency, respectively, in all growth stages; Y (kg/ha) is the yield per unit area; ETa (m 3 /ha) is the actual water consumption per unit area in all growth stages; and I (m 3 /ha) is the irrigation amount per unit area in all growth stages.
Irrigation volume. The experiment used PVC pipe water delivery and drip irrigation under the film to irrigate water. All gate valves and water meters were installed in each treatment area, and the corresponding WUE = Y/ETa, IWUE = Y/I,  www.nature.com/scientificreports/  Table 7. Soil water content (% field capacity) in conventional irrigation and regulated deficit drip irrigation treatments during potato growth. The treatments were two levels of water deficit that occurred in each of four growth stages. The data in the table show the percentage of soil mass water content in field water holdup under different experimental treatments. "Slight" means that the soil water deficit level was controlled at a "Mild" level with soil water content maintained at 55-65% of field capacity during certain plant growth stage in the treatment while it was maintained at 65-75% of field capacity at the other plant growth stages in the same treatment. Similarly, "Medium" means that the soil water deficit level was controlled at a "Moderate" level with soil water content maintained at 45-55% of field capacity during certain plant growth stages in the treatment while it was maintained at 65-75% of field capacity at the other plant growth stages in the same treatment. www.nature.com/scientificreports/ irrigation volume was quantitatively controlled by the water meter. When the soil water content dropped to the lower limit of the design value, irrigation was should be carried out in time, and the required irrigation amount was calculated by the irrigation quota formula: 51 where M (mm) is the irrigation volume, ρb(g/cm 3 )is the soil bulk density of the planned wet layer. H (cm) is the depth of the soil plan wet layer, β i is the target moisture content (field water holding capacity multiplied by the upper limit of the design target relative moisture content), and β j is the soil moisture content before irrigation.
Leaf area index. In the four growth periods of potatoes, for each treatment plot, three uniformly growing potatoes, were randomly selected. All the leaves, were cut from each potato in order to measure the maximum length and maximum width of the leaves and calculate the area of all the leaves. The total leaf area of a single plant was derived from the sum of all leaf areas, the leaf area was corrected with a coefficient of 0.76, and the average leaf area was finally taken 52 .
Soil moisture content. The test mainly followed the traditional soil drilling and soil drying weighing method to determine the soil moisture in each treatment plot. According to previous research results, the root activity range of potato is mainly concentrated in the 0-40 cm of the soil layer. Therefore, the soil was sampled every 7 days during the whole growth period of the potato, and the sampled soil depth was 80 cm, which was divided into five parts: 0-10 cm, 10-20 cm, 20-40 cm, 40-60 cm, and 60-80 cm. The soil water content (SWC) is calculated as follows: 51 where SWC (%) is the soil moisture content; W1(g) is the total mass of the fresh soil sample and the aluminum box; W2(g) is the total mass of the dried soil sample and the aluminum box; and W3(g) is the mass of the empty aluminum box. The detailed soil moisture data of the 3-year field experiment are shown in supplementary Table S1, Table S2 and Table S3.

Calculation of evapotranspiration.
Evapotranspiration was calculated using the water balance method: 53 where ET I-II (mm) is the water consumption in the potato stage; i is the serial number of the soil layer; n is the total number of soil layers; r i (g·cm −3 ) is the dry bulk density of the ith layer of soil; H i (cm) is the thickness of the ith layer of soil; Wi I , and Wi II respectively are the mass water content of the ith soil at the beginning and end of the growth period, respectively, %; M (mm) is the amount of irrigation water during the growth period; P (mm) is the rainfall during the growth period; and K (mm) is the amount of groundwater replenishment in the growth period. The groundwater depth in the test area is greater than 20 m, so K = 0. D (mm) is the drainage volume within the stage, and there is no runoff drainage in the test area, so D = 0.
Quality. The Coomassie Brilliant Blue G-250 method was used to determine protein content, and anthrone colorimetry was used to determine total sugar content. The content of starch was determined by enzymatic hydrolysis, vitamin C by 2, 6-dichloroindophenol titration, potassium by the potassium tetraphenylborate gravimetric method, and calcium by the gravimetric titration method.

Statistical analyses.
Excel 2017 (Microsoft 365) was used to perform calculations, and Duncan's multiple comparison method in SPASS 19.0 (Stanford University) software was used to determine significant differences between means. Origin 8.0 (Origin lab) was used prepare the diagrams of average values. All analyses were performed using 3-year (2017, 2018 and 2019) averages.

Conclusions
In the Hexi Corridor Oasis Irrigation Area, due to low rainfall, high evaporation intensity, water shortage, serious irrigation water waste and other factors, the development of local agriculture has been severely restricted.
To minimize these negative impacts, we tested a novel planting method: regulated deficit drip irrigation under film mulching. The experiment found that different levels of water deficit in different growth periods of potatoes will affect the growth and development of their potatoes, and the degree of impact will continue to increase as the growth period advances. The analysis showed that the best water regulation scheme was mild water stress in the seedling stage. However, there was no significant difference in plant water content among potato treatments after harvest, the values of the water status of the plant is 74.10-79.70%. Therefore, the relative water content www.nature.com/scientificreports/ in soil should be maintained between 55 and 65% in the seedling stage, whereas in the other growth stages, it should range from 65 to 75%. The irrigation method proposed in this study can simultaneously stabilize output and maintain good quality, which is significant for yield improvement, water conservation, quality adjustment, and industrialization of potatoes cultivated in the Hexi Corridor Oasis Irrigation Area. www.nature.com/scientificreports/