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Plant Signaling & Behavior

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Research Paper

Effects of low doses of UV-B radiation supplementation on tuber quality in purple potato (Solanum tuberosum L.)

Wencai Qia College of Life Science, Henan Normal University, Xinxiang, ChinaView further author information
,
Jingwen Maa College of Life Science, Henan Normal University, Xinxiang, ChinaView further author information
,
Jinguo Zhanga College of Life Science, Henan Normal University, Xinxiang, ChinaView further author information
,
Mengyuan Guia College of Life Science, Henan Normal University, Xinxiang, ChinaView further author information
,
Jingyuan Lia College of Life Science, Henan Normal University, Xinxiang, China;b Engineering Laboratory of Green Medicinal Material Biotechnology, College of Life Science, Henan Normal University, Xinxiang, Henan Province, ChinaView further author information
&
Liang Zhanga College of Life Science, Henan Normal University, Xinxiang, China;b Engineering Laboratory of Green Medicinal Material Biotechnology, College of Life Science, Henan Normal University, Xinxiang, Henan Province, ChinaCorrespondencezhangliang@htu.edu.cn
ORCID Iconhttps://orcid.org/0000-0003-1611-3205View further author information
ORCID Icon
Article: 1783490
Received 27 Apr 2020
Accepted 05 Jun 2020
Published online: 24 Jun 2020

Research Paper

Effects of low doses of UV-B radiation supplementation on tuber quality in purple potato (Solanum tuberosum L.)

ABSTRACT

ABSTRACT

UV-B is an important environmental factor that differentially affects plant growth and secondary metabolites. However, our knowledge regarding the physiological and biochemical changes in under-ground plant organs responded to UV-B treatment remains limited. In this study, we investigated potato plant (Solanum tuberosum L.) and tuber responses to short-term supplemental UV-B exposure performed during tuber development. Our results indicated that the supplemental UV-B radiation with relative low dose had no obvious adverse impact on plant growth or tuber production. Nutritional composition analyses of tubers revealed that the contents of starch, soluble sugars, and proteins were significantly increased under lower UV-B radiation relative to controls. Similarly, low dose of UV-B treatment promoted the health-promoting compounds, including anthocyanin, phenols, and flavonoids in tubers. Moreover, higher activities of antioxidant enzymes were significantly induced in tubers in response to lower UV-B radiation. These findings suggest that short-term UV-B radiation supplementation at relative low doses can improve the tuber quality in potato plants.

Introduction

The Ultraviolet B (UV-B) radiation (315–280 nm), a small portion of the solar spectrum reaching the earth’s surface, has been traditionally considered as an environmental stress factor. The increase of UV-B radiation resulted from ozone layer thinning has pleiotropic effects on developmental processes and morphological architecture of plants, such as growth inhibition, thickening of leaves, increased axillary branching, and reduced pollen fertility.1,2 Nevertheless, many evidences indicate that low levels of UV-B exposure can trigger the expression of a range of genes and initiate regulatory responses.3 Thus, the biological responses of plants to UV-B exposure are undoubtedly associated with UV-B wavelength, duration of exposure, growth condition, and genotypes within a species.4,5

As the consequences of distinct changes in secondary metabolism of plants subjected to UV-B stress, irradiation with UV-B has been recommended as a tool to enhance the nutritional quality in post-harvest fruits and vegetables. Concentrations of phenolic compounds in the post-harvest peaches and nectarines were reportedly increased by UV-B treatment probably via modulation of phenylpropanoid biosynthetic gene expression.6 The contents of ascorbic acid and carotenoids in tomato fruit flesh and peels were promoted with post-harvest UV-B exposure at a lower intensity.7 Recently, the UV-B irradiation on mung bean sprout promoted vitamin C, phenolic, and flavonoid accumulation.8 Furthermore, the application of short-term UV-B radiation (10% greater than ambient UV-B exposure) induced a significant increase in the levels of chlorogenic acid and flavone in the flower of medicinal plants (Qi chrysanthemum).9 Although increasing evidences demonstrate that UV-B treatment can enhance the health-promoting compounds or active ingredients in kinds of plants, how tuber plants respond to increased UV-B irradiation is not well studied.

Potatoes (Solanum tuberosum L.) are the important non-cereal food crop that is a tuber in the world. In addition to being a rich source of starch, potatoes contain proteins, minerals, and several phytochemicals including flavonoids, phenolics, carotenoids, and polyamines.10 Recently, potato varieties with colored flesh, such as purple-flesh potato, are received more attention, because high anthocyanin content of the raw material has beneficial effects on human health and is highly desirable in diet.11 Since potatoes are the highly consumed crops and grown throughout the world, it will be an interesting study that the contents of health-promoting compounds in potatoes can be increased via an environmental-friendly approach.

With respect to how potatoes respond to increased UV-B levels, previous investigations mainly focus on the changes of biomass, photosynthetic pigments, antioxidant enzymes, and ultrastructure in potato leaves.12,14 The aims of this work were to investigate how short-term increased UV-B exposure affected a purple-flesh potato cultivar with respect to tuber quality under our experimental condition. This work extends the knowledge of how UV-B exposure impacts plant growth and development.

Materials and methods

Plant materials and UV-B radiation supplementation

For potato plant growth, Solanum tuberosum seed tubers (cultivar ‘Hei Meiren’ with purple flesh) were planted in pots containing a pasteurized mixture of nutrient soil: vermiculite (3:1, v/v) in early April. Pots were maintained in an open place at the botanic garden of Henan Normal University, Henan, China (N 35°19′, E 113°54′, 70 m above the sea level). During the plant growth, mean temperature ranged from 15.2°C to 32.4°C and photosynthetically active radiation (PAR) averaged approximately 1000 µmol m−2 s−1 at midday.

The potato plants grown during the flower stage (tuber initiation) were subjected to UV-B radiation artificially provided with UV-B lamp tubes (Philips Ultraviolet B, TL 20 W-12RS, 315–280 nm, Philips Electronics, Eindhoven, The Netherlands) outdoors. For one dose of UV-B radiation two UV-B lamps were placed parallelly in two lamp frames and one UV-B lamp stretches over three pots upward to 50 cm away from plants. The intensity of UV-B lamp was 20.5 µW cm−2 calibrated by an ultraviolet radiac (Photo-Electricity Instrument Factory of Beijing Normal University, Beijing, China). The UV-B radiation supplementation was provided simultaneously at the durations (doses) of 0 min (0 kJ m−2), 10 min (0.12 kJ m−2), 20 min (0.24 kJ m−2), and 40 min (0.48 kJ m−2) at about 9 a.m. per day. The experiments with different doses were repeatedly performed three times, respectively, per day. The control plants received only ambient UV-B exposure (9.63 ± 2.14 kJ m−2). The tubers were harvested after 20-day UV-B radiation for further physiological assays.

Plant growth and tuber production parameters

For growth determinations, shoot lengths were quantified from six individual plants for each UV-B treatment. The number of compound leaves of six plants was determined. After the potato plants containing intact roots were carefully dug out from plots upon final tuber harvest from six plants per condition, the number, fresh weight, and dry weight of tubers per plant were assessed for tuber production.13 The net photosynthetic rate was analyzed with a portable photosynthetic system (LI-6400, LI-COR, NE, USA) at 7 and 14 days, respectively, on the leaves selected from the whole plant in each plot after the beginning of UV-B radiation.

Measurement of starch and soluble sugar content

The starch content in potato tubers was assessed via an I2-KI method.15 Fresh tissue (2.5 g) was homogenized in 5 mL 80% ethanol. After centrifugation with 4000 × g for 5 min, an 80% calcium nitrate solution was used to extract the residue, and after which boiling water was used to heat the sample for 5 minutes. Next, 0.5% iodine was added to mediate precipitation, and precipitates were twice washed using a 5% calcium nitrate, 0.008% iodine mixture, after which the precipitate was completely dissolved by adding hot 0.1 N NaOH, and subsequently adding 0.5% I2-KI and 1 N HCl. Absorbance at 590 nm was then assessed, with a standard curve used to calculate starch contents.

The anthrone method16 was used to soluble sugar quantification. A total of 0.5 g of fresh tuber tissue was homogenized in 5 mL 80% ethanol, after which it was spun for 10 minutes at 5,000 xg. Supernatants were then incubated at 80°C for 30 min, and 0.25 mL supernatant was mixed with 1.25 mL of ice-cold anthrone reagent (2 g anthrone in 1 L 72% sulfuric acid). Mixtures were then heated for 10 minutes in a bath of boiling water, following by cooling on ice. Absorbance at 630 nm was then assessed. The soluble sugar content was calculated according to the glucose standard curve.

Measurement of soluble protein content

A Coomassie Brilliant Blue G-250 (CBBG) was used to measure soluble protein contents, as described by Snyder and Desborough (1978).17 The CBBG assay is similar to a Bradford assay (1976), but with a smaller aliquot being used for analyses (0.1 mL rather than 0.4 mL). The protein content was calculated based on the established standard curves with bovine serum albumin (BSA).

Measurement of vitamin C levels

Tuber vitamin C levels were measured via a colorimetric method with molybdenum blue.8 The fresh tuber tissue (0.5 g) was ground sufficiently with 4 mL oxalate-EDTA solutions in a cold mortar in the dark. After centrifugation for 15 minutes at 4°C at 12,000 xg, 2.0 mL of the sample supernatant was combined with 500 uL of 3% (w/v) metaphosphoric acid prepared in 10% (v/v) acetic acid, 2 mL of 5% (w/v) ammonium molybdate, and 1 mL of 5% (v/v) sulfuric acid. Next, samples were diluted appropriately with dH2O, prior to a 15-minute incubation at 30°C. Absorbance at 760 nm was then assessed, with L-Ascorbic acid (0.4 g L−1) serving as a standard. Vitamin C levels were calculated as μg ascorbic acid equivalents per gram of fresh weight (μg AAE g−1 FW).

Measurement of total phenols and flavonoids content

The contents of total phenols in tubers were determined via a Folin–Ciocalteu colorimetric approach.18 Tuber extract (100 uL) and Folin–Ciocalteu reagent (200 uL) were combined. After 5 min, 1 mL of Na2CO3 (20%) and 2 mL dH2O were mixed with the sample, which was then incubated for 20 minutes more in the dark. Absorbance at 765 nm was then measured, with gallic acid serving as a standard for measurement purposes. Final measurements were calculated as grams of gallic acid equivalent (GAE) per kg dry weight (DW).

Total flavonoid content was quantified according to the method described by 19. Initially, 5 mL of 70% ethanol was used for extracting tissue powder (0.5 g), after which 250 µL of this extract was mixed for 6 minutes with 1.25 mL dH2O and 275 uL 5% NaNO2. Next, 10% AlCl3 (150 µL), 1 M NaOH (0.5 mL), and dH2O (275 µL) were added to the sample, after which absorbance at 510 nm was assessed after 5 minutes. Rutin was used as a standard in order to prepare a standard curve. Results are given as g Rutin equivalents (RE) per Kg in dry weight (mg RE Kg−1 DW).

Measurement of anthocyanin content

The anthocyanin content in tubers was measured according to 20. 1 g fresh tissue was homogenized in 10 ml extraction buffer (methanol:H2O:HCl, 79:20:1, v:v:v). Methanol and HCl concentrations were, respectively, 99.8% and 35.4%. The solution was heated at 60°C for 1 h to facilitate anthocyanin extraction. After cooling for 15 minutes at room temperature, the absorbance of the supernatant at 530 nm was measured three times and the average values were represented for the relative anthocyanin content in different treatments.

Measurement of malondialdehyde (MDA) content

Malondialdehyde (MDA) concentration in the tuber tissue was assessed via a trichloroacetic acid (TCA) approach.21 Fresh tuber tissue (0.5 g) was ground with small amounts of SiO2 in a 5 mL volume of 10% TCA. Homogenates were then spun for 10 minutes at 5000 xg at 25°C. 2 mL supernatant was combined with 2 mL 0.6% (w/v) thiobarbituric acid for 15 minutes in a boiling water bath, after which samples were transferred to ice in order to terminate the reaction. Samples were then again spun for 15 minutes at 5000 xg, and absorbance of supernatants was then assessed at 532 nm and at 450 nm.

Antioxidant enzyme activity measurement

Cold PBS (50 mM; pH 7.8; 2 mL) supplemented with 4% PVP and 1 mM EDTA was used to homogenize fresh tuber tissue (0.5 g). Homogenates were then spun for 15 minutes at 4°C at 12,000 xg. Superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities in these supernatants were then assessed at 4°C.

In order to measure SOD activity, a previously described protocol was used in order to measure nitroblue tetrazolium (NBT) photochemical reduction inhibition.22 Briefly, 3 mL reaction mixtures were prepared with PBS, 0.75 mM NBT, 0.1 mM EDTA, 130 mM methionine, 0.02 mM riboflavin, and 0.1 mL enzyme extract. Samples were then incubated for 15 minutes with a 5000 1x light intensity, after which the light source was removed to stop this reaction. As controls, tubes that were not exposed to light or did not contain sample homogenates were also analyzed in parallel. SOD activity was quantified in terms of units, with one unit being the enzyme amount necessary to result in a 50% disruption of NBT reduction assessed at 560 nm.

Guaiacol23 was used for measuring POD activity. Briefly, a 20 µL sample extract was mixed with 20 µL Guaiacol and 10 µL hydrogen peroxide in a 3 mL volume of PBS (pH 7.0). For 5 minutes post-addition of the enzyme-containing sample extracts, absorbance at 470 nm was measured.

Rates of H2O2 elimination were used to measure CAT activity.24 Briefly, a 3 mL mixture was prepared with 50 mM PBS (pH 7.0), 20 mM H2O2, and 100 uL of the enzyme-containing sample homogenate. Reductions in H2O2 levels were assessed at 240 nm for a minimum of 3 minutes.

Statistical analysis

All assays were performed in three independent experiments. Data are means ± standard errors. Data were compared via one-way analyses of variance (ANOVAs) with Tukey’s multiple comparison test in SPSS 14.0 (SPSS Inc., IL, USA). P < .05 was regarded as significant.

Results and discussion

Impacts of short-term increased UV-B exposure on plant growth and tuber production

We first assessed how potato plant growth and tuber production were impacted by increases in short-term UV-B exposure (Table 1). Increased UV-B exposure did not significantly influence shoot length and number of compound leaves compared with the control plants. In addition, the analysis of tuber production, including the tuber numbers, fresh, and dry weight per plant, was not altered upon UV-B treatment compared with the control plants, although a slight increase of tuber weight was observed in the plants exposed to 20 min day−1 of UV-B radiation.

Table 1. The analysis of various growth parameters of potato plants in response to the elevated UV-B radiation. Values represent means ± SD, n = 3. Means with different letters are significantly different from each other (P < .05).

Many evidences indicate that how UV-B radiation affects plants depends upon the contexts, such as radiation dose, temperature, nutritional status, stress acclimation, and plant species. For example, basil growth characteristics including assimilating leaf area and both fresh/dry biomass were markedly enhanced upon short-term increased UV-B radiation at the early growth and flowering stage.25 During different growth stages, 10 days of elevated UV-B radiation had no significant impact on flower biomass accumulation in the medicinal plant Qi chrysanthemum.9 Although the UV-B exposure reduced the plant height of Lithuania-bred potato cultivars, leaf fresh and dry biomass remained unchanged compared to the control.14 Herein, our results showed that the plant growth and tuber production were not affected obviously in response to the short-term and low-dose supplemental UV-B radiation. Thus, the elevated UV-B exposure did not impact economic benefits of tubers under the experimental conditions. As such, we further explored how elevated UV-B radiation on nutritional ingredients and secondary metabolites in potato tubers.

Elevated UV-B radiation impact on nutritional ingredients in tubers

UV-B exposure at the durations of 10 and 40 min day−1 induced obvious increase in starch content of potato tubers compared with the content in controls. Interestingly, the starch content in tubers was maximally promoted after UV-B application of 20 min day−1 (Figure 1a). While the soluble sugar contents of potato tubers were increased by the UV-B treatment. Particularly, the maximal increase of 26% (P < .05) for soluble sugar content was recorded in tubers under the UV-B radiation (20 min day−1) in comparison with the content in control (Figure 1b). The contents of starch and soluble sugars are important for the quality of potato tubers.26 Our results indicated that more starch and soluble sugars were obviously accumulated in tubers of plants irradiated with UV-B radiation (20 min day−1) compared with the controls. Shukla et al. (2002)27 reported that the starch and soluble sugar contents were increased in wheat seedlings exposed to UV-B radiation. Treatment with mineral nutrients promoted the content of reducing sugars in potato tubers by exposure of plants to UV-B radiation,13 indicating that abundant nutrient contributes to alleviating the negative effects of UV-B radiation. In this scenario, the increased sugar content was probably due to the enough nutrient soil supplied in this study. Moreover, in the present study, the net photosynthetic rates were significantly enhanced in response to the elevated UV-B radiation during tuber formation compared with controls (Figure 2). Similarly, the exposure of lettuce seedlings to UV-B radiation during the propagation stage of growth increased net photosynthetic rate and final harvestable yields.28 Several evidences have implicated that supplemental UV-B radiation at low dose induces enhanced photoprotection against other environmental stresses, such as high light and drought.29,31 Thus, in this study, more carbohydrates were accumulated available during tuber production after relative low-dose UV-B treatment.

Figure 1. The analysis of starch and soluble sugar contents in tubers responded to supplemental UV-B radiation. (a) The contents of starch in tubers. (b) The contents of soluble sugar in tubers. Values represent means ± SD, n = 3. Different letters signify significant differences between samples (P < .05).

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Figure 2. The measurement of net photosynthetic rates in potato leaves after UV-B radiation at 7 (a) and 14 (b) days. Values represent means ± SD, n = 3. Means with different letters above bars are significantly different from each other (P < .05).

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In addition, the levels of soluble protein in tubers displayed distinct responses to supplemental UV-B radiation (Figure 3a). The increase of 17% (P < .05) was recorded by the UV-B radiation of 20 min day−1. While the UV-B treatment at relative low (10 min day−1) and high dose (40 min day−1) did not change the contents of soluble protein in tubers obviously. In addition, the analysis of vitamin C content in tubers revealed no obvious changes under the short-term supplemental UV-B radiation (Figure 3b). In the leaves of certain Lithuanian potato cultivars, the concentrations of soluble proteins increased after the UV-B exposure.14 The total protein content in storage organ radish, but not carrot was found to increase when the plants were subjected to enhanced UV-B radiation at the fully developed stage.32 Proteomic studies revealed that distinct protein groups involved in plant responses to UV-B radiation can be up- or down-regulated, which depends on the plant genotypes.33 Thus, the changes in the protein levels could be due to the direct action of UV-B radiation on the protein synthetic machinery or the protein turn over rate.

Figure 3. The measurement of soluble protein and vitamin C contents in potato tubers under supplemental UV-B radiation. The contents of soluble protein in tubers. (b) The contents of vitamin C in tubers. Data are means ± SD, n = 3. Means with different letters above bars are significantly different from each other (P < .05).

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Elevated UV-B radiation impact on secondary metabolites in tubers

Pigmented potato tubers from cultivars with purple or red skin and/or flesh contain higher levels of anthocyanins, compared to those with white or yellow tubers.34,35 Therefore, we analyzed the changes in anthocyanin content in tubers in response to UV-B treatment. The results showed that the UV-B radiation of 10 min day−1 had no obvious effects on anthocyanin accumulation. The doses of UV-B radiation (20 and 40 min day−1) significantly enhanced anthocyanin production, i.e., these were increased by 26% and 19% (P < .05), respectively, in comparison with the control levels (Figure 4a). Moreover, the contents of total phenols and flavonoids were positively affected by UV-B treatment in this experiment, except the content of total phenols under UV-B radiation with shorter duration. In more detail, the contents of total phenols in control tubers were 3.58 ± 0.06 g GAE Kg−1 DW. While treatment with UV-B radiation of 20 and 40 min day−1 promoted the total phenol accumulation to the levels of 4.16 ± 0.24 and 4.25 ± 0.44 g GAE Kg−1 DW significantly compared with the control (Figure 4b). Similarly, high levels of flavonoids were also observed with elevated UV-B radiation. Compared with the control, the flavonoid content increased obviously from 31% (10 min day−1) and 81% (20 min day−1) to the maximum of 115% (40 min day−1) along with the increasing UV-B radiation (Figure 4c).

Figure 4. The effects of supplemental UV-B radiation on accumulation of anthocyanin, total phenols and flavonoids in potato tubers. (a) The relative contents of anthocyanin in tubers. (b) The contents of total phenols in tubers. (c) The contents of total flavonoids in tubers. Data are means ± SD, n = 3. Different letters signify significant differences between samples (P < .05).

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As the consequences of response to UV-B treatment, the accumulation of anthocyanin, phenols, and flavonoids is triggered by plants, thus providing a defense mechanism against UV-B stress.36 The induction of anthocyanin accumulation had been reported in medicinal plants and post-harvested fruits exposed to UV-B radiation.37,38 In potato leaves, the exposure to UV-B radiation stimulated the production of UV-B absorbing flavonoid compounds and activities of antioxidant enzymes.12 Recently, the higher increase in UV-absorbing phenolic compounds (UVAC) was observed in wild potato leaves collected from lower altitude when exposed to UV-B treatment.5 Moreover, the increased flavonoid contents in underground organ radish were observed after the exposure of radish plants to enhanced UV-B radiation.32 In the present study, our results showed that the contents of anthocyanin in tubers were increased by below ambient doses of UV-B radiation on potato plants during tuber formation (Figure 4). Moreover, the contents of total phenols and flavonoids were significantly promoted by low-dose UV-B treatment (Figure 4). Phenylalanine ammonia-lyase (PAL) is the key enzyme of the phenylpropanoid pathway involving phenolic biosynthesis in plants, including phenolics, flavonoids, and anthocyanins, and also serves as a biochemical marker of environmental stress.39 The increased PAL activity or related gene expression had been documented in different plant organs subjected to supplemental UV-B radiation.9,40 Therefore, as the defensive responses to UV-B treatment the potato plants including tuber tissues had a trend to produce more UV-B absorbing compounds likely through modulating the PAL activities.

Effects of elevated UV-B radiation on antioxidant capacity in tubers

The contents of MDA in tubers were reduced significantly (P < .05) by the supplemental UV-B radiation compared with the control (Figure 5a). While the analysis of antioxidant enzymes showed that increases of enzyme activities were recorded in response to UV-B treatment. The SOD activity was enhanced significantly only by the UV-B radiation of 20 min day−1 (Figure 5b). Similarly, the analysis of POD and CAT activities revealed obvious increases of 30% and 34% (P < .05), respectively, under the UV-B radiation of 20 min day−1 (Figure 5c, d). UV-B radiation usually triggers the production of reactive oxygen species that result in oxidative stress and activate the antioxidant enzymes, such as POD, SOD, and CAT.41 As responses to UV-B radiation, the activity of the antioxidant enzymes CAT, APX, and POD increased associated with the induction of new isoforms of CAT and POD in potato leaves.12 It was reported that the leaves of potato plants grown at recommended mineral nutrients exhibited higher activities of SOD, POD, and CAT in response to UV-B radiation.42 In the present study, our results showed that the purple potato cultivar subjected to low-dose UV-B radiation displayed increased antioxidant enzyme activities, including SOD, POD, and CAT, in tuber tissues, thus reducing the levels of lipid peroxidation as indicated with MDA contents (Figure 5). The higher activities of the antioxidant enzyme may account for the unchanged vitamin C content following UV-B treatment in this study (Figure 3b), since vitamin C can be used as substrates for the enzyme to scavenge active oxygen, protecting plant from the oxidative damage.43 Although increasing evidences prove that plants respond to UV-B radiation through activation of the antioxidant defense system, the information about the response of underground organs to UV-B treatment is still limited. Based on the previous and our results, we propose that the aerial and underground parts of the whole plant likely respond to UV-B radiation coordinately and systematically.

Figure 5. The analysis of antioxidant capacity in tubers responded to supplemental UV-B radiation. (a) The contents of MDA in tubers. (b) The SOD activities in tubers. (c) The POD activities in tubers. (d) The CAT activities in tubers. Data are means ± SD, n = 3. Different letters signify significant differences between samples (P < .05).

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Conclusions

In conclusion, the current study mainly investigated the biological effects of elevated UV-B radiation on potato tuber quality in a purple potato cultivar. We found that UV-B radiation with relative low dose during tuber development significantly promoted tuber quality without interfering with the plant growth and tuber production. The contents of functional compounds such as anthocyanin, phenols, and flavonoids, and nutritional compositions in terms of starch and soluble sugars in tubers were obviously accumulated. In addition, the UV-B treatment enhanced the antioxidant capacity in tubers. Our results lay foundation for the further investigation to uncover the mechanisms for the responses of underground plant organs to UV-B radiation.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Data availability statement

Research data are not shared.

Additional information

Funding

This work was funded by the National Natural Science Foundation of China [31770197, 31300163, 31700162, and 31270225] and Research Start-up Funds [2016PL12, 5102099179107, and 5101049170808].

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