HOU Fu-yun, DONG Shun-xu, XlE Bei-tao, ZHANG Hai-yan, Ll Ai-xian, WANG Qing-mei

Crop Research Institute, Shandong Academy of Agricultural Sciences/Scientif ic Observation and Experimental Station of Tuber and Root Crops in Huang-Huai-Hai Region, Ministry of Agriculture and Rural Affairs, Jinan 250100, P.R.China

Abstract The root quality of sweet potato cultivated during the summer season is poor in northern China; thus, this study was conducted to determine whether root quality could be improved through mulching with plastic f ilm (MPF). The effect of MPF on root starch and its composition, the activity of starch synthesis enzymes, and other quality-related parameters were investigated in two purple f lesh sweet potato cultivars, Jishu 18 and Ayamurasaki (Aya). The results indicated that root dry matter, anthocyanin content, adenosine triphosphate (ATP), and starch content were higher in both cultivars under the MPF treatment than those under the control treatment. The root adenosine diphosphate glucose pyrophosphorylase/uridine diphosphate glucose pyrophosphorylase (ADPGPPase/UDPGPPase) activity and adenosine triphosphatease (ATPase) activity were increased using MPF. However, under the MPF treatment, the amylose content, soluble sugar content, and granule-bound synthase (GBSS) activity increased in Jishu 18 but decreased in Aya, and the amylopectin content, protein content, and soluble starch synthase (SSS) activity decreased in Jishu 18 but increased in Aya. Therefore, MPF seems benif it to improve the quality of sweet potato, but the effects of this treatment condition may be dependent on the cultivar.

Keywords: m ulching with plastic f ilm, summer-sown sweet potato, root quality, starch

1. lntroduction

Sweet potato is an important crop in China, where it is mainly used as food, feed, and industrial materials. China is the world's biggest producer of sweet potato; approximately 70% of the global supply is from China, according to FAO statistics (FAO 2016). The storage root is mainly used as a whole root for human consumption and manufactures products such as starch, regional domestic products, where consumers prefer roots with sweet f lesh that is orange or purple. Multiple parameters are generally used to evaluate sweet potato root quality, including starch composition, soluble sugar, and protein content. The eating quality of sweet potato depends on sugar composition, its texture and the quantity and quality of carbonhydrates (Kitahara et al. 2017). Existing research has shown that starch content has a positive correlation with dry matter content in sweetpotato, but negatively correlated with sugars and proteins. However, these parameters can vary greatly depending on the cultivar, harvest season, production area, and climatic conditions (Bryan et al. 2003; Chattopadhyay et al. 2006). In northern China, sweet potato is customarily cultivated in raised beds during summer after the wheat harvest, but its root production and quality are lower than that cultivated in spring (Azevedo et al. 2014).

Mulching with plastic f ilm (MPF) has been used as a water-saving measure in sweet potato cultivation in China. Previous studies have found that MPF may be detrimental to sweet potato yield because it improves soil water moisture, soil bulk density, and soil porosity compared with conventional cultivation by reducing runoff and soil evaporation, as well as increasing inf iltration and water storage (Kuranouchi et al. 2010; Gajanayake et al. 2014; Hou et al. 2015; Vial et al. 2015). Mulching can improve juice and jaggery quality of sugarcane (Mathew and Varughese 2005). Najafabadi (2012) reported that plastic mulch in low rainfall and warm season could increase garlic quality by affecting vitamin C and f lavonoids content as the second crop in rice f ield. Other studies have reported mulching obtained better color in the “Mondial Gala” apple (Iglesias and Alegre 2009) and higher amounts of soluble solids in peach and plum (Layne et al. 2001; Kim et al. 2008). However, whether mulching can be used to improve sweet potato quality remains unclear.

Beyond the nutritional qualities of the common sweet potato, purple f lesh sweet potato (PFSP) is rich in anthocyanin content. However, insuff icient light has been found to suppress the accumulation of starch and anthocyanin in the roots of PFSP (Wang Q M et al. 2011). The root quality of PFSP depends on the cultivar, climatic conditions, and agronomic management. Recently, Jiang et al. (2014) showed that MPF can signif icantly improve the rate of dry matter accumulation in sweet potato roots. Using of MPF in summer sweet potato production achieved a higher yield (Hou et al. 2015). However, the root quality characteristics of sweet potato, such as starch, anthocyanin, amylopectin, and amylose content, under MPF have not yet been investigated. The aim of this study was to evaluate the effect of MPF on the root quality of PFSP.

2. Materials and methods

2.1. Plant materials

The experiments were conducted during the summer sweet potato production at the experimental station (36°42′N, 117°4′E; altitude of 48 m) of the Shandong Academy of Agricultural Sciences, China, during the 2014 and 2015 cultivation seasons. The experimental soil contained 1.56% organic matter, 92.5 mg kg-1of hydrolytic nitrogen, 39.5 mg kg-1of available phosphorus, and 103.2 mg kg-1of rapidly available potassium. The sweet potato cultivars Jishu 18 and Ayamurasaki (Aya), which differ in their anthocyanin contents, were selected for study. Sweet potato seedlings of the two cultivars were transplanted on 15 June and harvested on 20 October during 2014 and 2015. After transplanting, 33% pendimethalin EC herbicide (JiangSu Longdeng Chemical Company, Kunshan C ity, China) was sprayed onto the soil surface according to the manufacturer's protocol before the surface was covered with plastic f ilm. The experimental treatments consisted of cultivation with MPF and without MPF (CK), respectively. For the MPF, a high-density black polyethylene f ilm (0.02-mm thick, 1.2-m wide) was used as the mulching plastic. Treatments were applied in triplicate in a split-plot, randomized complete block design using 6 m×9 m plots consisting of eight rows with 9 m in length and 0.75 m in width. A total of 45 sweet potato seedlings with 20 cm length and similar weight were transplanted at 25 cm plant spacing per row. Weeds were removed several times from the f ield without MPF, but no weed management measure was applied in the f ield with MPF. No additional management practice (e.g., irrigation, pest management) was applied during the entire growing season in either f ield.

A 1-cm slice of 5-8 medium sized roots were collected from 35 days after transplanting (DAT). At least 10 plants were sampled at each time, and each plant possessed approximately three roots. The fresh weight of all roots was measured. The samples were frozen immediately in liquid nitrogen. Subsequently, we collected samples at 50, 75, 90, and 120 DAT.

2.2. Determination of soil water content and soil macronutrient content

During the growing stage, soil samples were collected from the 0-10 and 10-20 cm layers of each plot at 0, 15, 35, 50, 75, 90, and 120 DAT with a ring sampler. To determine the soil water content, soil samples were weighed after collection and weighed again after being dried out in a fan-aided oven at 105°C for 48 h (Song et al. 2012).

Soil samples from the 0-20 and 20-40 cm layers were also taken from individual plots at 35 and 90 DAT to determine water-hydrolyzable nitrogen (WHN), rapidly available potassium (RAK), and rapidly available phosphorus (RAP). The WHN, RAP, and RAK contents were determined using the alkali solution diffusion method, NH4OAc extraction f lame photometry and 0.5 mol L-1NaHCO3extraction colorimetry, respectively (Ou et al. 2016). All measurements were replicated three times in independent experiments.

2.3. Measurement of biomass productivity

At least 10 plants were sampled at 35, 50, 75, 90, and 120 DAT for each cultivar. The top parts were separated from the roots, washed in fresh water, and dried in an oven to determine fresh and dry weights.

2.4. Root adenosine triphosphatease (ATPase) activity and adenosine triphosphate (ATP) content

Frozen root was crushed in liquid nitrogen in a microcentrifuge tube and suspended in ice-cold extraction buffer containing 100 m mol L-1Tris-HCl (pH 6.5). After thawing, the sample was thoroughly mixed and then centrifuged at 10 000 r min-1for 15 min at 4°C. The supernatant was used to determine the activity of ATPase following the method described by Shi et al. (2002), by which the sample was added to a solution with 100 m mol L-1Tris-HCl (p H 6.3) and 30 mmol L-1ATP and then stopped in 5% TCA after 30 min. The ATP was extracted and detected using an ATP Assay Kit purchased from Promega (Madison, WI, USA) according to the manufacturer's instructions.

2.5. Root anthocyanin, soluble sugar, and protein content

Root samples (2 g) were ground and suspended in Mcllvaine's buffer (p H 3.0) containing 80 mmol L-1citric acid and 40 mmol L-1disodium hydrogen phosphate. Following incubation for 30 min and centrifugation for 10 min at 12 000 r min-1, the absorbance of the supernatant was measured at 530 nm with a SPD6AV spectrophotometer (Shimadzu, Japan). The soluble sugar and protein contents were measured using the method described by Hou et al. (2015).

2.6. Root dry matter, starch, amylose, amylopectin and rapid visco analyzer (RVA) assays

Root samples were directly sliced with a food processor into approximately 4-mm chips. To measure the root dry matter content, a freshly mixed 20-g sample of root slices was oven-dried to a constant weight at 80°C for 48 h. The dried root was ground to a powder, and the starch content was determined as previously described (Li et al. 2011). The amylose and amylopectin contents were determined using the dual wavelength method (620/480 nm) described by Hovenkamp-Hermelink et al. (1988). RVA measurements were conducted using RVASuper4 (Newport Scientif ic Pty., Ltd., Australia) as reported by Noda et al. (1996). Each sample was measured three times.

2.7. Root ADPGPPase, UDPGPPase, SSS, and GBSS enzyme assays

Root samples (1 g) were homogenized in 10 mL of icecold Hepes-NaOH buffer with 50 mmol L-1hydroxyethyl piperazine ethanesulfonic acid (HEPES), 5 mmol L-1ethylene diamine tetraacetic acid (EDTA), 1 mmol L-1dithiothreitol, 2 m mol L-1KCl, and 1% polyvinyl pyrrolidone (PVP) (Douglas et al. 1988). Following the addition of 30 μL homogenate to 1.8 mL of the above buffer and centrifugation for 3 min (34 000 r min-1at 4°C), the deposit was suspended with the Hepes-NaOH buffer to detect granulebound synthase (GBSS) activity. The other homogenate was centrifuged for 10 min at 20 000 r min-1, and the supernatant was used to adenosine diphosphate glucose pyrophosphorylase (ADPGPPase), uridine diphosphate glucose pyrophosphorylase (UDPGPPase), and soluble starch synthase (SSS) enzyme activities.

To determine the ADPGPPase and UDPGPPase activities, 50 μL of extraction was added to a reaction mixture containing 100 μL of the above buffer, 50 μL of 50 mmol L-1MgCl2, and 10 μL of 5 mmol L-1ADPG/UDPG. The reaction was initiated by adding 100 μL of 20 mmol L-1pyrophosphate (PPi) for 15 min and then stopped by heating for 1 min. The cooling reaction was preincubated for 10 min and added to a solution consisting of 100 μL of 6 m mol L-1nicotinamid adenin dinucleotid phosphat (NADP), 1.5 U of phosphoglucomutase, 0.25 U of glucose-6-phosphate dehydrogenase, and 0.3 mL of Hepes-NaOH buffer. The base-rate activity was measured at 340 nm before the reaction was initiated by the addition of 10 μL of 5 mmol L-1ADPG/UDPG, and because the reaction rate remained linear, the production of NADH was determined at 340 nm for 10 min (Nakamura et al. 1989).

To determine the SSS and GBSS activities, the reaction was started by adding 50 μL of extraction to the reaction mixture, whose volume consisted of 0.35 mL of buffer (p H 7.5) with 50 mmol L-1Hepes-NaOH, 1.6 mmol L-1ADPG, 15 mmol L-1DTT, and 0.7 mg of amylopectin. Then, the reaction was incubated for 20 min and terminated with boiling water for 1 min. The cooled reaction mixture was added to 0.35 mL of ADP-ATP buffer (50 mmol L-1Hepes-NaOH, 4 mmol L-1PEP, 200 mmol L-1KCl, 100 mmol L-1MgCI2, and 1.2 U pyruvate kinase with a p H of 7.5) for 30 min at 30°C. The production of ATP was measured following the addition of f luorescein-f luorescent reagent (Liang et al. 1994).

2.8. Statistical analysis

Statistical analysis was performed using Excel 2007, differences between means were analyzed through ANOVA using the DPS statistical software (v. 7.55, Ref ine Information Tech. Co., Ltd., Hangzhou, Zhejiang, China). Mean values are reported with the standard error. Due to no signif icant differences in years or year×treatment interactions, data were pooled across years.

3. Results

3.1. Soil water content

Throughout the growing period, sweetpotato development relied on natural rainfall without artif icial irrigation. Fig. 1 shows that the soil water content in control was apparently higher than that of MPF at 35 and 90 DAT, meanwhile the change amplitude of them under MPF was smaller than that of the control.

3.2. Soil available nutrient content

Soil WHN, RAP, and RAK in the 0-20 and 20-40 cm soil layers under the MPF treatment were signif icantly higher than that of the control (Table 1). At the depth of 0-20 and 20-40 cm, there was a signif icant difference in soil WHN, RAP, and RAK between the MPF treatment and the control at 35 and 90 DAT. Under MPF treatment, there was little difference in the soil WHN, RAP, and RAK between 35 and 90 DAT, while the difference in these parameters on bare land was apparent.

Fig. 1 Effects of mulching with plastic f ilm (MPF) on soil water content. Values followed by the small letter in a column are signif icantly different at 5% level. Bars are SE.

3.3. Sweet potato biomass productivity

Top/Root (T/R) ratio was detected by the sweet potato biomass productivity. In the two cultivars, T/R ratio under the MPF treatment were signif icantly lower than those of the control from 35-90 DAT (Table 2). At harvest (120 DAT), there was no signif icant difference in T/R ratio between MPF treatment and the control.

3.4. Root ATP content and ATPase activity

In the two cultivars, root ATP content and ATPase activity were higher under the MPF treatment than those under the control treatment (Fig. 2). Throughout the growing stage, root ATP content of MPF signif icantly increased by 20.3 and 14.9% (P＜0.05) compared with the controls in Jishu 18 and Aya, respectively, while root ATPase activity was signif icantly enhanced by 21.5 and 32.0% in Jishu 18 and Aya, respectively, compared with the control (P＜0.05).

3.5. Root dry matter content

Under the MPF treatment, the root dry matter content was higher than that under the control treatment in both cultivars (Fig. 3). Throughout the development stage, the root dry matter content under the MPF treatment was 24.61 and 34.01% in cultivars Jishu 18 and Aya, respectively, which was signif icantly higher than that under the control treatment (2.72 and 1.87% higher, respectively).

3.6. Root anthocyanin content, root soluble sugar, and protein content

In the Jishu 18 and Aya cultivars, the root anthocyanin content under the MPF treatment was signif icantly increased by 19.5 and 15.0%, respectively, compared with the control (P＜0.05) (Fig. 3). In Jishu 18, the amount of root anthocyanin accumulation before 75 DAT was higher than that in the latter growing stage. The root anthocyanin content in Aya was signif icantly higher than that in Jishu 18.

Table 1 Dynamics of soil available nutrient content in different treatments

Table 2 Effect of MPF on sweetpotato top/root (T/R) ratio

As shown in Fig. 3, the root soluble sugar content in Jishu 18 under the MPF treatment was higher than that under the control treatment, while the root soluble sugar content of Aya was the opposite. Throughout the growing stage, the root soluble sugar content increased by 16.2% in Jishu 18 (P＜0.05) but decreased by 24.1% (P＜0.05) in Aya under the MPF treatment compared to their respective controls. Additionally, the root protein content decreased by 9.1% (P＜0.05) in Jishu 18 but increased by 13.0% in Aya compared with the control (P＜0.05) (Fig. 3).

3.7. Root starch content

As shown in Fig. 4, there was a signif icant difference in the root starch content of the two cultivars under the MPF treatment. In Jishu 18, the root starch content under the MPF treatment was 2.45% higher than that under the control treatment throughout the development stage (P＜0.05). In Aya, the root starch content was 0.39% higher under MPF treatment than that under the control treatment throughout the development stage.

3.8. Root amylose and amylopectin content

Under the MPF treatment, the starch component differentially varied in the two cultivars (Fig. 4). In Jishu 18, the amylose content was signif icantly increased by 18.5% under the MPF treatment, while the amylopectin content did not change signif icantly under the MPF treatment when compared with the controls. As a result, the Jishu 18 amylopectin/amylose ratio under the MPF treatment was lower than that under the control treatment. In Aya, the amylose content was decreased by 7.6%, and the amylopectin content was increased by 2.1% compared to the controls. Therefore, the amylopectin/amylose ratio under the MPF treatment was 10.4% higher than that under the control treatment (P＜0.05).

3.9. Starch accumulation rate

The starch accumulation rates of both cultivars under the MPF treatment were higher than those of the controls throughout the growing stage (Fig. 4). At 35-50 and 105-120 DAT, no signif icant difference was observed in the starch accumulation rate of the two cultivars between the treatments and controls. The starch accumulation rate in both cultivars peaked at 50-105 DAT, while that of Jishu 18 was higher than that of Aya under both the control and MPF treatment conditions.

Fig. 2 Effects of mulching with plastic f ilm (MPF) on root adenosine triphosphatease (ATPase) activity and adenosine triphosphate (ATP) content. Aya, Ayamurasaki. Values followed by the small letter are signif icantly different at 5% level. Bars are SE.

Fig. 3 Effects of mulching with plastic f ilm (MPF) on root dry matter content, root anthocyanin content, root soluble sugar, and protein content. Aya, Ayamurasaki. Values followed by the small letter are signif icantly different at 5% level. Bars are SE.

3.10. Root RVA parameters

Under the MPF treatment, the RVA values changed signif icantly in the two cultivars (Table 3). In Jishu 18, peak viscosity (Pv), hold-through (Ht), set back (Sb), f inal viscosity (Fv), peak time (Pt), and pasting temperature (PT) under the MPF treatment were all lower than those under the control treatment, while break down (Bd) was signif icantly higher. In Aya, Pv, Ht, Bd, and Fv were all signif icantly higher under the MPF treatment than those under the control treatment, but there were no signif icant differences in Sb, Pt, or PT between the control and MPF treatments.

3.11. Root ADPGPPase and UDPGPPase activities

In Jishu 18, the ADPGPPase activity increased by 19.5% under the MPF treatment, which was signif icantly higher than that under the control treatment throughout the growing stage (P＜0.05). The ADPGPPase activity in Aya under the MPF treatment was higher than that under the control treatment before 75 DAT but was subsequently lower than that under the control treatment (Fig. 5).

The root UDPGPPase activity of the two cultivars was affected by the MPF treatment (Fig. 5). Throughout the growing stage, the root UDPGPPase activity in Jishu 18 and Aya under MPF treatment increased by 7.8 and 0.6%, respectively (P＜0.05), compared with their controls, but not signif icant.

3.12. Root SSS and GBSS activitites

Fig. 4 Effects of mulching with plastic f ilm (MPF) on root starch content, root amylose content, root amylopection content, and root starch accumulation rate. Aya, Ayamurasaki. DAT, days after transplanting. Values followed by the small letter are signif icantly different at 5% level. Bars are SE.

Table 3 Effect of mulching with plastic f ilm (MPF) on characteristics of rapid visco analyzer (RVA)1)

In Aya, the root SSS activity under the MPF treatment was increased by 6.4% compared with that under the control treatment (P＜0.05), but the root SSS activity in Jishu 18 decreased by 0.6%. The change in GBSS under the MPF treatment differed from that in SSS for the two cultivars; in Jishu 18, the root GBSS activity signif icantly increased by 9.5% under the MPF treatment compared with its control (P＜0.05), while it signif icantly decreased by 8.2% in Aya (P＜0.05).

4. Discussion

4.1. Relationship between soil characteristics and sweet potato root quality under MPF

Fig. 5 Effects of mulching with plastic f ilm (MPF) on root adenosine diphosphate glucose pyrophosphorylase (ADPGPPase), uridine diphosphate glucose pyrophosphorylase (UDPGPPase), soluble starch synthase (SSS), and granule-bound synthase (GBSS) enzyme activity. Values followed by the small letter are signif icantly different at 5% level. Bars are SE.

Plastic mulch was found to inf luence the dry matter, protein, and carbohydrate content in potato and cassava roots (Cadavid et al. 1998; Wang F X et al. 2011). In our study, we found that MPF treatment in sweet potato production resulted in a relatively stable soil water content and a higher soil available nutrient content, which could create favourable conditions. These results were consistent with Gajanayake et al. (2014). Elevated RAK occurred in MPF, and could supply K sources for sweet potato growth and root enlargement. Similarly, George et al. (2002) reported that root dry matter, carotene content, and anthocyanin content increased with K application in sweet potato.

4.2. Effect of MPF on root starch properties

The use of MPF in summer sweet potato cultivation could elevate the root ATP content and ATPase activity, which not only supplies energy for rapid root expansion but also increases H+/sucrose transport, promoting the transport of carbon assimilates to the roots, where these molecules are transformed into starch. So, MPF in summer sweet potato might improve root quality. In the present study, the root dry matter and starch content under MPF were increased in the two cultivars. In this study, the anthocyanin content, root dry matter, and starch content increased in the two cultivars under the MPF treatment. These results are consistent with the change in ADPGPPase and UDPGPPase activities under the MPF treatment, conf irming that ADPGPPase and UDPGPPase directly inf luence the total starch content of sweet potato roots (Tsubone et al. 2000; Kwak et al. 2006). Previous studies have also shown that ADPGPPase isoforms might be negatively affected by the endogenous sucrose content of storage roots (Kwak et al. 2006).

There are two forms of starch synthase, SSS, and GBSS, which control amylopectin and amylose synthesis, respectively, and affect root processing. In wheat (Triticum aestivum), the amylose, amylopectin, and total starch accumulation rates were found to be signif icantly related to SSS and GBSS activities (Wang et al. 2014). Wang et al. (2012) reported that shading could lead to lower amylopectin content and decreased processing quality in the two sweet potato cultivars examined. As a result, MPF had different effects on the storage root quality in different sweet potato cultivars: the amylose content, soluble sugar content, and GBSS activity increased in Jishu 18 but decreased in Aya, and the amylopectin content, protein content and SSS activity decreased in Jishu 18 but increased in Aya under the MPF treatment. These results indicate that the amylose and amylopectin contents response to MPF might be related to the genotype of the cultivar. The storage root of Jishu 18 is processed to produce potato chips due to its high amylopectin content, and Aya is mainly used in pigment extraction and the production of root powder due to the high amylose content in the root. In our study, SSS activity was increased in both Aya and Jishu 18, while GBSS activity was enhanced in Jishu 18 and decreased in Aya, which is consistent with the observation of Wang et al. (2014) who conf irmed that the activities of SSS and GBSS were signif icantly correlated with the amylose and amylopectin content.

4.3. Effect of MPF on root other quality traits

Under MPF treatment, the root RVA parameters of Pv, Ht, Fv, Sb, and PT were signif icantly lower than those under the control treatment in Jishu 18, while Pv, Ht, and Bd were higher in Aya. With MPF, the amylose content was higher in Jishu 18, and the amylopectin content was enhanced in Aya. These results indicate that root RVA parameters might be correlated with starch content. A comparison of the two cultivars has shown that there are similar trends in ATP content, ATPase activity, root dry matter content, starch content, UDPGPPase activity, and ADPGPPase activity, but opposing trends in root soluble sugar content, protein content, amylose content, RVA, SSS activity, and GBSS activity under MPF treatment. Therefore, plastic mulching had different effects on storage root quality in the different sweet potato cultivars.

5. Conclusion

Mulching with plastic f ilm seems benif it to improve the root quality of sweet potato produced during the summer season, but the characteristics of different cultivars might be differentially altered by MPF treatment. In northern C hina, the investment of sweetpotato production with MPF is more 1 200-1 500 CNY than that of control per ha, the sold price of good quality sweetpotato is about 1.4-2.0 CNY kg-1and is 50-100% higher than that of control, and the root yield under MPF could be 15 000-30 000 kg ha-1and enhanced by 20-30% compared with the control. So, the income of sweetpotato root production with MPF was 21 000-60 000 CNY and was far more than its investment. Therefore, MPF cultivation should be selected according to its actual demand.

Acknowledgements

This research was supported by the Natural Science Foundation of Shandong Province, China (ZR2014YL015), the Agricultural Seed of Shandong Province, China (2016LZGC005), the earmarked fund for China Agriculture Research System (CARS-10-B7 and CARS-10-B8), and the Youth Foundation of Shandong Academy of Agricultural Sciences, China (2014QNM31).