Sodium Alginate Composite Coating Inhibited Postharvest Greening and Improved Nutritional Quality of Potato Tubers by Regulating Chlorophyll Biosynthesis

Abstract

Potato tuber (Solanum tuberosum L.) was prone to greening and quality deterioration during postharvest storage due to various factors, affecting the regulation of chlorophyll biosynthesis. In the present study, potato tubers were placed at 600 lux and 25 °C after sodium alginate—xanthan gum—glycerin composite coating. During storage, the apparent color changes and a* value of the surface were observed and determined, meanwhile the contents of nutrients, chlorophyll, and its intermediates in photosynthetic metabolism were analyzed. The results showed that after 9 d, compared to the control group, the sodium alginate coating treatment significantly inhibited greening, delayed the decline of appearance quality and nutrients including dry matter, starch, reducing sugar, soluble protein, and ascorbic acid. Furthermore, the sodium alginate coating promoted the contents of 5-aminolevulinic acid (ALA) (1.33 fold), porphobilinogen (PBG) (1.06 fold), and uroporphyrinogen III (Uro III) (1.07 fold), meanwhile, inhibited the production of protoporphyrin IX (Proto IX) (13.86%), Mg-protoporphyrin IX (Mg-Proto IX) (14.15%) and protochlorophyllide (Pchlide) (25.97%), which were key intermediates in the chlorophyll synthesis, indicating that the sodium alginate coating delay the greening by blocking the conversion of Uro III to Proto IX. These results provided valuable insights for the postharvest preservation of potato tuber.

1. Introduction

Potato ranks as the fourth largest food crop globally, and potato tuber is recognized as the third most significant source of phenolic compounds in the human diet [1], which is rich in dietary fiber, vitamins, high-quality proteins, and other nutrients, and contains carotene, ascorbic acid that does not exist in cereal grains. In the consumption process, the absence of blemishes and the presence of shiny skin are important criteria for consumers when selecting potato tubers [2]. However, potato tubers are highly susceptible to greening by light, temperature, and other factors in the process of harvesting, transportation, storage, and marketing [3]. After greening, potato tubers have a bitter taste, and their appearance and commercial value are significantly reduced. The greening of potato tuber has been reported to be caused by the accumulation of chlorophyll, a key production of photosynthesis, in the cells of the epidermal and subcutaneous parts of the tissue [4].

Studies have shown that plant chlorophyll content is regulated by the optimal balance between chlorophyll synthesis and catabolism [5]. Chlorophyll in potato tubers primarily consists of chlorophyll a and chlorophyll b, which can be converted into each other under certain conditions. The biosynthesis process of chlorophyll in plants has been relatively well-studied [6]. Firstly, glutamate is converted to protoporphyrinogen IX (Proto IX) under the catalysis of enzymes such as glutamyl-tRNA reductase, glutamate-1-semialdehyde transaminase, 5-aminolevulinic acid dehydratase bilinogen deaminase, uroporphyrinogen III synthetase and decarboxylase. 5-aminolevulinic acid (ALA), porphobilinogen (PBG), Uroporphyrinogen III (Uro III) are the primary intermediates in this process. Then, Proto IX combines with Mg²⁺ under the catalysis of Mg-chelatase, etc., to form magnesium–protoporphyrin IX (Mg-Proto IX), which is converted into the prochlorophyllide (Pchlide) in turn. Finally, Pchlide is reduced to chlorophyllide a, which subsequently combines with phytol to form chlorophyll a under the catalysis of chlorophyll synthetase. Chlorophyll a can be further converted to chlorophyll b by chlorophyll b synthase; meanwhile, chlorophyll b is converted to chlorophyll a by reducing the aldehyde group on the pyrrole ring B of chlorophyll b to a methyl group with the action of reductase. ALA, PBG, Uro III, Proto IX, Mg-Proto IX, and Pchlide are key intermediates in the chlorophyll synthesis pathway, which can be used to determine the location of the photosynthesis blocking site by comparing the content of them [7]. It was shown that the accumulation of chlorophyll induced by low light via increasing the Proto IX content in the tea plants subjected to shading treatment [8]. Under salt stress conditions, the conversion of PBG to Uro III in spinach leaves was blocked, resulting in a decrease in chlorophyll synthesis [9]. In addition, in peach leaves, dialkylaminoethanol carboxylate spraying promoted chlorophyll synthesis through improving the conversion efficiency of ALA to PBG [10]. Therefore, different substances or environmental conditions have effects on chlorophyll biosynthesis, but with different mechanisms of action.

Edible coatings are thin layers of packaging material formed on the surface of food to enhance its optical appearance or to protect it from environmental influences [11]. Studies have shown that it could be used for postharvest storage of agricultural products to improve their appearance quality and nutritional value, thereby extending the shelf life [12,13]. Sodium alginate, a natural polysaccharide, has been widely studied as a film-forming matrix material for edible coatings due to its unusual colloidal properties and excellent film-forming properties, as well as its low cost, non-toxicity, tastelessness and degradability [14]. Coating walnut kernels with films that formed by the mixture of sodium alginate and tocopherol could maintain the color, inhibit the oxidation process of unsaturated fatty acids, and effectively preserve the quality them [15]. Iñiguez-Moreno et al. Ref. [16] reported that combing sodium alginate with yeast (Mycobacterium caribeanum) for avocado preservation could delay the weight loss, increase pH and soluble solids, and maintain the firmness of the whole fruit, thus extending its shelf life from 12 to 17 days. Furthermore, relevant studies have shown that the sodium alginate composite coating also has a significant effect on maintaining the quality of postharvest potato tubers [17]. However, current research on postharvest quality control of potato tubers primarily focuses on inhibiting sprouting of potato tubers and browning of fresh-cut potatoes, with limited impact on the post-harvest preservation of intact potato tubers. Additionally, further research is still needed on the control of the potatoes post-harvest greening, especially the systematic research on the regulatory mechanism of chlorophyll synthesis in potato tuber epidermis by sodium alginate composite coating and its role in maintaining the nutritional quality of potato tubers. In the present study, the effect of the composite coating formed by the mixture of sodium alginate, xanthan gum, and glycerol on the apparent color, nutritional quality changes and chlorophyll biosynthesis of potato tubers were analyzed, aiming to provide new insights for the postharvest storage and preservation of potato tubers.

2. Materials and Methods

2.1. Experimental Materials, Treatment, and Storage

About 200 potato tubers (Solanum tuberosum L. cv Atlantic) were selected and purchased from the local market, with similar size, intact epithelium, free from sprouting, discoloration and mechanical damage, and transported to the laboratory immediately.

The potato tubers were washed and then sterilized with sodium hypochlorite (200 μL/L) for 3–5 min, and afterwards rinsed with deionized water, the surface was dried at room temperature in the dark, and it was randomly divided into two groups, with 90 in each group. One group was used as the control group, and the other group was treated with the coating film referred to the method of Amir et al. [15] with slight modifications. Briefly, a precisely measured 9.00 g of food-grade sodium alginate (Henan Wanbang Industrial Co., Ltd.) was homogenously dispersed in distilled water, resulting in sol–gel formation, then 2.50 g xanthan gum (Zhejiang Nuoyi Biotechnology Co., Ltd., Quzhou, China) and 2.50 g glycerin (Henan Wanbang Industrial Co., Ltd., Zhengzhou, China) were added in turn as the plasticizer. A homogeneous and transparent sodium alginate solution with a mass fraction of 1.8% was prepared at 45 °C for 40 min by magnetic stirrer (SZCL-2, Gongyi Yuhua Instrument Co., Ltd., Gongyi, China) at 800 rpm based on the preliminary experimental results and sample quantity, approximately 3 L of composite coating solution was ultimately prepared. Subsequently, it was degassed with 500 × 300 × 150 mm ultrasonic cleaner (KQ-600KDE, Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China) for 30 min for subsequent use. The prepared coating solution was evenly applied to the surface of potato tubers using a brush and dried naturally in the dark. Afterwards, the samples were immersed in a 2.0% CaCl₂ solution for 2 min to cross-link and dried in the dark. Both the control and the treated groups of potato tubers were stored at 25 °C, 85% RH, and 600 lux in an artificial climate chamber (RDN-400C-4, Ningbo Southeast Instruments Co., Ltd., Ningbo, China). The samples were collected at 0, 1, 3, 5, 7, and 9 d, respectively, and then stored at −80 °C for the subsequent determine of the relevant indexes.

2.2. Measurement of Total Chlorophyll Content

The content of total chlorophyll was determined referring to Tilahun et al. [18], with modifications. A total of 2.00 g of finely ground potato tuber epidermis sample powder was mixed with 5 mL of 95% ethanol and placed at 25 °C for 5 min in the dark. The mixture was centrifuged at 10,000 rpm for 10 min at 4 °C using a centrifuge (Velocity 14, Miaosheng Technology Co., Ltd., Shanghai, China). The absorbance of the supernate was measured using an ultraviolet-visible spectrophotometer (TU-1810, Beijing Puxi General Instrument Co., Ltd., Beijing, China) at 665 nm and 649 nm with 95% ethanol as the blank reference. The total chlorophyll content was calculated according to the formula:

Total chlorophyll (mg/kg) = (18.08 × A₆₄₉ + 6.63 × A₆₆₅) × V/M

(1)

where V (mL) is the volume of the extract, and M (g) is the weight of the sample, A₆₄₉, A₆₆₅, are the two different absorbances at 649 nm and 665 nm, respectively.

2.3. Color Measurement

The epidermis was peeled off from the stem end, sprout end and equatorial part of the potato tubers, afterwards cut into round slices of about 2 mm thick and φ16 mm in diameter with a hole punch. The a* value was measured and recorded with a color difference meter (CR-400, Konica Minolta Instruments, Inc., Tokyo, Japan) calibrated against a standard white board to indicate the color change in the potato tuber epidermis.

2.4. Measurement of Dry Matter Content

The dry matter content of potato tuber tissue was measured with the drying method. A dry, clean, constant weight aluminum box was taken and the mass of it was recorded as m₁, then 2.00 g of finely ground peeled potato tuber powder of that obtained at 2.00 cm away from the bud eye, as referred to by Das et al. [19], was weighed and recorded as m₂. Subsequently, it was blanched at 105 °C for 10 min, baked at 75 °C until reaching a constant weight, then cooled and the mass was recorded as m₃. The dry matter content was calculated using the following formula and presented as percentage-based fresh weight.

Dry matter (%) = (m₃ − m₁)/m₂ × 100%.

(2)

2.5. Measurement of Starch Content

The starch content of potato tuber was measured according to the method referring to Zhang et al. [20]. Briefly, an accurate 0.15 g of finely ground peeled potato tuber powder was homogenized in 10 mL of 80% hot ethanol, then centrifuged at 4000 rpm for 10 min. The sediment was blended with 10 mL of 30% perchloric acid and centrifuged again. Afterwards, the supernate was adjusted to 50.00 mL with distilled water. Subsequently, 4.00 mL of starch extract solution was mixed with 5.00 mL of anthrone reagent (0.2 g anthrone dissolved in 100 mL of 76% sulfuric acid), and the mixture was incubated at 90 °C for 15 min, and the absorbance of it was measured at 620 nm. The result was calculated by constructing the standard curve standard, and it was presented as percentage based fresh weight.

y = 4.9517x − 0.0756 (R² = 0.9996)

(3)

where y is the absorbance value at 620 nm, x (mg/mL) is the concentration of glucose standard.

Starch sugar (%) = (C × V × N)/(1.11 × W × 1000) × 100%

(4)

where C (mg) is the mass of glucose from the standard curve, V (mL) is the volume of extraction solution, N is the dilution factor of extraction solution, W (g) is the weight of the fresh sample, 1.11 is the constant used to convert the glucose content measured by this method into starch content, and 1000 is the unit converted to g from mg.

2.6. Measurement of Reducing Sugar Content

The reducing sugar content was determined by the 3,5-dinitrosalicylic acid colorimetry method [21]. Briefly, 3.00 g of finely ground peeled potato tuber powder was fully mixed with 40 mL of distilled water, in a 50.00 mL centrifuge tube. After leaching in a constant temperature water bath at 50 °C for 20 min, the mixture was centrifuged at 10,000 rpm for 10 min and the supernate was collected. Then 2.00 mL of the supernate was mixed with 1.5 mL of 3,5-dinitrosalicylic acid reagent (6.3 g/L 3,5-dinitrosalicylic acid, 0.524 mol/L sodium hydroxide, and 185 g/L potassium sodium tartrate) and reacted in a boiling water bath for 5 min. Afterwards, the solution was cooled to 25 °C instantly, diluted to 25.00 mL with distilled water and mixed thoroughly. The absorbance of the obtained mixture was measured at 540 nm, and the reducing sugar content of the sample was calculated according to the following standard curve and formula, and it was presented as percentage-based fresh weight.

y = 0.9064x + 0.0528 (R² = 0.9951)

(5)

where y is the absorbance value at 540 nm, x (mg/mL) is the concentration of glucose standard.

Reducing sugar (%) = (C × V)/(V_S × W × 1000) × 100%

(6)

where C (mg) is the mass of glucose from the standard curve, V (mL) is the volume of extraction solution, vs. (mL) is the volume of the solution to be measured, W (g) is the fresh weight of the sample, and 1000 is the unit converted to g from mg.

2.7. Measurement of Soluble Protein Content

The content of soluble protein was determined using the Coomassie Brilliant Blue method (CBB) [22]. Briefly, 4.00 g of finely ground peeled potato tuber powder was evenly mixed with 30.00 mL of 0.9% NaCl solution, centrifuged at 10,000 rpm at 4 °C for 10 min, and the supernate was collected. Afterwards, 0.20 mL of the supernate was mixed with 0.80 mL NaCl solution and 5.00 mL of Coomassie Brilliant Blue G-250 (100 mg/L Coomassie Brilliant Blue G-250, 4.7% v/v of 95% ethanol, and 8.5% v/v of 85% phosphoric acid) staining solution in turn and placed for 3 min. The absorbance of the mixture was measured at 595 nm and the content of the soluble protein was calculated according to the standard curve and the formula and the results were expressed as mg/g based on fresh weight.

y = 0.6131x + 0.0089 (R² = 0.9877)

(7)

where y is the absorbance value at 595 nm, x (mg/100 g) is the concentration of soluble protein standard.

Soluble protein (mg/g) = (C × V)/(V_S × W × 1000)

(8)

where C (μg) is the mass of soluble protein from the standard curve, V (mL) is the volume of extraction solution, vs. (mL) is the volume of the solution to be measured, W (g) is the fresh weight of the sample, 1000 is the unit converted to mg from μg.

2.8. Measurement of Ascorbic Acid Content

Ascorbic acid content was determined based on the method referred to by Tao et al. [23] with slight modifications. Briefly, 5.00 g of finely ground peeled potato tuber frozen-powder was weighed, homogenized with 20.00 mL of a buffer solution (1 g/L oxalic acid and 4 g/L anhydrous sodium acetate) and extracted for 10 min at 4 °C. Afterwards, the homogenate was centrifuged at 9000 rpm for 15 min at 4 °C, and the supernatant was collected. A total of 10.00 mL of the supernatant was titrated against a solution containing 295 mg/L 2,6-dichlorophenol-indophenol solution and 100 mg/L sodium bicarbonate. The ascorbic acid content in the samples was quantified using standard solutions of ascorbic acid at different concentrations (0.1 g/L, 0.01 g/L, 0.005 g/L, 0.0025 g/L, and 0.00125 g/L). The results were expressed as mg/100 g based on fresh weight.

2.9. Measurement of ALA Content

The ALA content was determined as described by Wang et al. [6]. A total of 2.00 g of finely ground potato tuber epidermis sample was fully mixed with 6.00 mL of 0.8 moI/L sodium acetate buffer (pH 4.6), centrifuged at 5000 rpm for 15 min at 4 °C, then 4.00 mL of the supernate was mixed with 100 μL ethyl acetoacetate and incubated at 100 °C for 10 min. Afterwards, the mixture was mixed with an equal volume of fresh Ehrlish reagent (2% 4-Dimethylaminobenzaldehyde in 22% hydrochloric acid) and incubated for another 10 min, and the absorbance was measured at 554 nm. The ALA content was calculated using a standard curve prepared by the method of Harel and Klein [24], and the results were expressed as ng/g based on fresh weight.

y = 0.904x + 0.1031 (R² = 0.9981)

(9)

where y is the absorbances at 554 nm, and x (mg/L) is the concentration of the ALA standard.

2.10. Measurement of PBG and Uro III Content

The contents of PBG and Uro III were determined by Plant PBG ELISA Kit (JLC11664) and Plant Uro III ELISA Kit (JLC11343) provided by Shanghai Jingkang Bioengineering Co., Ltd., Shanghai, China. The sample pretreatment procedure was consistent with that in 2.9. A total of 50.00 μL of standard solutions of different contents were dispensed into the respective standard wells of the ELISA plate. Then, 40.00 μL of sample diluent and 10.00 μL of the test sample were added into the sample wells in turn, mixed well, and sealed with a sealing film. Subsequently, the plate was incubated at 37 °C for 30 min and then washed. Afterwards, 50.00 μL of enzyme-labeled reagent was added to each well, incubated again and washed. A total of 50.00 μL of chromogenic solution A and chromogenic solution B was added to each well, thoroughly mixed, and incubated at 37 °C in the dark for 10 min, followed by the addition of 50 μL of stop solution. The optical density values were measured at 450 nm, and the contents of PBG and Uro III were determined based on the standard curve. The results were expressed as ng/g based on fresh weight.

y = 0.0061x − 0.0907 (R² = 0.9975)

(10)

where y is the absorbance value at 450 nm, x (ng/L) is the concentration of PBG.

y = 0.023x − 0.1649 (R² = 0.9952)

(11)

where y is the absorbance value at 450 nm, x (ng/L) is the concentration of Uro III.

2.11. Measurement of Proto IX, Mg-Proto IX and Pchlide Content

Proto IX, Mg-Proto IX, and Pchlide levels were determined according to Hodgins et al. [25]. Briefly, 0.30 g of finely ground potato tuber epidermis powder was evenly mixed with 25 mL of 80% alkaline acetone (80% v/v of acetone, 20% v/v of 0.1 mol/L NH₃·H₂O). After filtering the impurities, the absorbance of the obtained mixture was measured at 575 nm, 590 nm, and 628 nm and the content was calculated according to the following formulas, and the results were expressed as ng/100 g based on fresh weight.

Proto IX = (0.1802 × A₅₇₅ − 0.0404 × A₆₂₈ − 0.0452 × A₅₉₀) × V/m

(12)

Mg-Proto IX = (0.0608 × A₅₉₀ − 0.0194 × A₅₇₅ − 0.0034 × A₆₂₈) × V/m

(13)

Pchlide = (0.0356 × A₆₂₈ + 0.0072 × A₅₉₀ − 0.0296 × A₅₇₅) × V/m

(14)

where A₅₇₅, A₅₉₀, A₆₈₂ are the three different absorbances at 575 nm, 590 nm, and 682 nm, respectively, V (mL) is the volume of the extract, m (g) is the fresh weight of the sample.

2.12. Statistical Analysis

The experiment was conducted using three duplicate samples. IBM SPSS Statistics 25.0 software was used for significance analysis. Mean comparisons were performed using t-test (p < 0.05). Origin 2023 was generated for data visualization and graphical representation. All data in the graphs are presented as mean ± standard error.

3. Results

3.1. Effect of Sodium Alginate Coating Treatment on Total Chlorophyll Content, a* Value and Apparent Color of Potato Tuber Epidermis

The color of potato tuber epidermal is one of the critical quality indicators for potato tuber as commodities [26]. As shown in Figure 1A, the rate and degree of greening in the control group were significantly higher than that in the sodium alginate-treated group. The epidermis of the control group turned green on 3 d, and it was 2 d later in the sodium alginate-treated group. Moreover, a* values of potato tuber epidermis in both groups decreased along with the storage time, indicating that both groups of potato tubers gradually turned green but at different rates. The sodium alginate-treated group showed a higher a* value compared with the control group with significant differences observed on 1 d (p < 0.05). Finally, the a* values of the control and the sodium alginate-treated groups were −0.68 and −6.89 on 9 d, respectively, indicating that sodium alginate coating effectively inhibited the greening of potato tubers. The total chlorophyll content increased gradually during the whole storage period (Figure 1C). Compared with the sodium alginate-treated group, the total chlorophyll content in control group was consistently higher and demonstrated a faster growth rate on 1 d, with significant differences (p < 0.05). After the 9 d of storage, the total chlorophyll contents of the sodium alginate-treated group was 4.53 mg/kg, a decrease of 69.06% as compared with the control group (14.64 mg/kg), indicating that sodium alginate treatment significantly inhibits chlorophyll synthesis in the epidermis of potato tuber epidermis.

3.2. Effect of Sodium Alginate Coating Treatment on the Nutritional Quality of Potato Tubers

As shown in Figure 2, the contents of dry matter, starch, reducing sugar and ascorbic acid in both sodium alginate-treated and the control groups both exhibited a continuous downward trend during the storage period. However, the rate of decrease in the treated group was slower compared with the control group, and the content of those in the sodium alginate-treated group were 1.08, 1.26, 1.26, and 1.07-fold of that in the control group, respectively (Figure 2A–C,E). In contrast, the soluble protein content in both groups initially increased and then decreased (Figure 2D), which peaked at 3 d (10.78 mg/g) in the control group and 5 d (9.40 mg/g) in the sodium alginate-treated group, respectively. Furthermore, the soluble protein content in the treated group at the end of storage was 1.17 fold of that in the control group.

3.3. Effect of Sodium Alginate Coating Treatment on ALA, PBG and Uro III Contents of Potato Tuber Epidermis

In order to determine the blocking site of the chlorophyll biosynthesis pathway in the epidermis of potato tubers caused by the sodium alginate coating treatment, the contents of several important intermediates were analyzed. The contents of ALA, PBG, and Uro III in both the sodium alginate-treated group and the control group increased gradually throughout the storage period, and the Uro III content of the treated group remained consistently higher than the control group during the whole storage period (Figure 3A,B). After 3 d of storage, the ALA and Uro III contents in the treated group increased rapidly and were significantly higher than those in the control group (p < 0.05). On 5 d, the PBG content in the treated group was significantly higher than that in the control group (p < 0.05). Finally, the ALA content in the sodium alginate-treated group (2.97 ng/g) was about 1.33-fold of that in the control group (2.24 ng/g). Furthermore, compared with the control group, the contents of PBG and Uro III of the sodium alginate-treated group were increased by 1.06 and 1.07-fold, respectively.

3.4. Effect of Sodium Alginate Coating Treatment on Proto IX, Mg-Proto IX and Pchlide Contents of Potato Tuber Epidermis

As shown in Figure 4, the contents of Proto IX and Mg-Proto IX in the potato tuber epidermis treated with sodium alginate coating initially decreased slightly and then gradually increased, but the contents in the control group increased continuously with a significantly higher accumulation rate (Figure 4A,B). The Pchlide content in both sodium alginate-treated and the control groups exhibited a continuous upward trend, but the magnitude and rate of increase in the treated group were lower than those in the control group (Figure 4C). Moreover, compared with the treated group, the contents of Proto IX, Mg-Proto IX, and Pchlide in the control group were consistently and significantly higher during storage (p < 0.05). On the 9 d of storage, the Proto IX content in the sodium alginate-treated group (4.66 μg/100 g) was 13.86% lower than that in the control group (5.41 μg/100 g). Meanwhile, the contents of Mg-Proto IX and Pchlide in the treated group were 14.15% and 25.97% lower, respectively, compared to those in the control group.

4. Discussion

Compared with other polysaccharide-based composite coatings—such as chitin coatings may trigger allergic reactions in certain populations [27], and shellac coatings exhibit low breathability and are prone to accumulating odor-causing substances [28]—sodium alginate coatings demonstrate significant advantages in terms of application performance. The weak gelling and film-forming properties of sodium alginate could effectively mitigate the adverse effects of gas and moisture, retard the oxidation process, and consequently enhance the sensory and nutritional quality of fresh agricultural products [29]. Furthermore, the incorporation of xanthan gum and glycerol generates a synergistic effect within the composite membrane system, thereby improving both structural stability and preservation performance.

The greening of potato tubers is influenced by multiple factors, including cultivar characteristics, physiological maturity, harvesting period, storage duration, and environmental conditions, particularly light exposure during the storage and selling period. In the present study, sodium alginate coating treatment was found to effectively maintain the color of potato tuber skin, delaying the increase in a* value and total chlorophyll content, indicating its inhibitory effect on the greening of potato tubers.

Dry matter content is an important indicator to evaluate the storage quality of agricultural products, and continuous biochemical reactions in potato tubers consume the dry matter [17]. The dry matter content in the sodium alginate-treated group was lower than that in the control group, indicating that the sodium alginate coating alleviated the loss of dry matter (Figure 2A). This is because the coating treatment could inhibit respiration and chlorophyll synthesis, thus reducing the consumption of organic matter as well as the decomposition of macromolecules into CO₂ and H₂O [13]. Starch is the main energy storage substance in potato tubers. In the later stage of storage, a higher loss rate of starch was exhibited in the control group compared with the treated group. (Figure 2B). This may be due to the conversion of amyloplasts into chloroplasts under the light induction. However, the synthesis of chlorophyll was inhibited by sodium alginate treatment, thereby suppressing the conversion of amyloplasts to chloroplasts and reducing starch consumption compared to the control group [30]. Meanwhile, starch is typically converted into sugars or other substances to prepare for germination during storage [31], while the sodium alginate coating could prevent these processes. Reducing sugars were consumed during the vital activities in potato tubers. At the end of storage, the reducing sugar content in the sodium alginate-treated group was significantly higher compared with the control group (Figure 2C). As a living organism, potato tubers actively maintained physiological metabolism during the storage period. However, due to the lack of external nutrition and energy supply, the carbohydrates or other components in potato tubers were required to utilize for oxidative decomposition, resulting in a decrease in reducing sugar content. After sodium alginate treatment, the respiration and chlorophyll synthesis of potato tubers were inhibited, reducing internal energy demand and preventing the decomposition of carbohydrates such as reducing sugars [32]. Soluble proteins are synthesized and degraded in response to specific physiological processes in plant cells. The soluble protein content in the control group was significantly higher than that in the treated group during first 3 days (Figure 2D). The initial upward trend is primarily attributed to physiological stress caused by mechanical damage during the harvesting and transportation of potato tubers, resulting in an increase in soluble protein content [33]. A downward trend of soluble protein content in both groups was observed during the later storage period, attributed to degradation of proteins into free amino acids for germination-related nutrient provision [34]. Under the inhibition of chlorophyll synthesis in the sodium alginate treatment group, the sprouting of potato tubers was also inhibited, alleviating fluctuations in soluble proteins. Ascorbic acid, an antioxidant, could be serve as an indicator of anti-aging, and its enzymatic oxidative decomposition is accelerated under sufficient oxygen supply [35]. Compared with the control group, the ascorbic acid content in the treated group was consistently higher (Figure 2E). A film formed by sodium alginate on the surface of potato tubers could create a microenvironment with low oxygen and high carbon dioxide levels, reducing the decrease in ascorbic acid content [36]. In the present study, the appearance quality of potato tubers was effectively maintained, while component degradation and nutrient loss were significantly delayed by the sodium alginate coating treatment. Similar findings were reported by Nkede et al. [37] that sodium alginate composite coatings could effectively maintain the post-harvest quality of citrus fruits, significantly reducing changes in weight, appearance, color, and total soluble solids, thereby extending shelf life. Yang et al. [38] found that, compared with the control group, chestnuts treated with sodium alginate composite film showed a significant decrease in respiratory intensity, which slowed the degradation in starch and ascorbic acid content and maintained the nutritional quality of the chestnuts. Meanwhile, Chen et al. [39] also reported similar results in cherry tomatoes, indicating that polysaccharide edible coatings could significantly delay the decrease in quality of cherry tomatoes, such as weight loss, firmness, soluble solids and ascorbic acid.

Multiple intermediates are involved in chlorophyll synthesis, and their content is decisive for the content of end-products. If the conversion efficiency of intermediate products is inefficient, the content of substances upstream the blocking site will increase, but those downstream will decrease, ultimately leading to a reduction in chlorophyll accumulation [40]. The results indicated that the greening was significantly inhibited by the sodium alginate coating treatment, and the chlorophyll content was constrained by the accumulation levels of its intermediate products (Figure 5). The contents of ALA, PBG, and Uro III in the sodium alginate coating treated group were significantly increased compared with the control group at the early stage of chlorophyll synthesis. However, the contents of Proto IX, Mg-Proto IX, and Pchlide downstream of Uro III were significantly reduced. The above indicated that the conversion of Uro III to Proto IX during chlorophyll synthesis could be inhibited by sodium alginate coating. This may be due to the fact that Coprogen III, a photosensitive compound, is generated during the conversion of Uro III to Proto IX. Potato tubers treated with sodium alginate coating may receive less light, thereby delaying the decomposition of Coprogen III [41]. Meanwhile, in the oxidation reaction of Uro III to Proto IX, key enzymes such as protoporphyrinogen III oxidase and protoporphyrinogen oxidase require oxygen as an electron acceptor to exert their catalytic activity. The coating treatment inhibited gas exchange in potato tubers, reduced surface oxygen concentration and inhibited the reaction, ultimately resulting in a decrease in Proto IX content [42]. Furthermore, Proto IX is converted to Mg-Proto IX by Mg-chelatase, which is recognized as a critical reaction in the chlorophyll biosynthesis [43]. The activity of Mg-chelatase is significantly positively correlated with light intensity. However, potato tubers coated with sodium alginate receive less light, which inhibits their activity and consequently affects the synthesis of downstream intermediates [44]. Therefore, a reduction in Proto IX content also leads to a corresponding decrease in chlorophyll content of potato tubers. Consistent with the findings that the blocking site of chlorophyll synthesis step in tomato seedlings under salinity stress occurs at the conversion of Uro III to Proto IX [45]. Moreover, chlorophyll deficiency in mustard-type oilseed rape yellowing mutants is due to impaired chlorophyll synthesis, mediated by the conversion of fecal porphyrinogen III to protoporphyrinogen III, and is unrelated to chlorophyll degradation [46]. The reason for this difference may be attributed to the variations in the types of test crops and environmental conditions [47].

5. Conclusions

In this study, the sodium alginate coating treatment was found to effectively preserve the appearance color of potato tubers, while significantly reducing the loss of dry matter, starch, reducing sugars, soluble proteins, and ascorbic acid contents. Briefly, the sodium alginate coating was able to maintain the nutritional quality of potato tubers better. During storage, the total chlorophyll content in the treated group was significantly lower compared with the control group, indicating that the sodium alginate coating treatment inhibited the greening process of potato tubers. Meanwhile, the contents of chlorophyll synthesized intermediates in both groups increased with prolonged storage time. However, the contents of ALA, PBG, and Uro III in the treated group were significantly higher, and the contents of Proto IX, Mg-Proto IX, and Pchlide were significantly lower than those in the control group. It was shown that the regulation of chlorophyll biosynthesis in potato tuber epidermis by sodium alginate coating treatment, with the blocking site located at the conversion stage from Uro III to Proto IX. The results of the present study provide new insights into extending the shelf life and improving the quality of potato tubers, with significant implications for postharvest storage and quality preservation of agricultural products. However, the specific mechanism by which sodium alginate coating inhibits chlorophyll synthesis remains unclear. Future research should focus on this area to enhance understanding of post-harvest preservation methods for potato tubers.