Influence of Pre-Harvest Gibberellic Acid and Post-Harvest 1-methyl Cyclopropane Treatments on Phenolic Compounds, Vitamin C and Organic Acid Contents during the Shelf Life of Strawberry Fruits

In recent years, significant portions of the fresh fruits and vegetables produced worldwide have been decaying before reaching the consumer because of insufficient preservation after harvest. In this direction, we carried the study out to investigate the effect of gibberellic acid (GA3) and 1-methyl cyclopropane (1-MCP) applications on phenolic compounds and organic acid contents of the strawberry fruits (cv. Albion) during shelf-life. Gibberellic acid treatments, which prepared in two different concentrations (50 and 100 ppm), were performed by spraying the leaves before harvest. 1-methyl cyclopropane applied after harvest. The results of the study showed a greater decrease in organic acids (except oxalic and succinic acid) in Gibberellic acid-applied fruits during shelf-life. Citric acid was recorded as the most abundant organic acid in the control group. In phenolic compounds, gallic acid (15.22 mg 100 g−1) and ellagic acid (9.38 mg 100 g−1) were recorded as the highest phenolic compounds on the third day. 1-MCP and GA3 (50 ppm) + 1-MCP treatment reduced the breakdown of vitamin C during the shelf-life of strawberry fruits compared to the control group. As a result, phenolic compounds, vitamin C, and organic acids decreased during the shelf-life, and 1-MCP applications slowed down the breakdown of these compounds.


Introduction
Strawberry (Fragaria x ananassa Duch.) is a fruit, in the berry fruits, widely used in the food and cosmetics industry and for fresh consumption. Turkey ranks first in Europe with a production of 486,705 tonnes [1]. It ranks fifth in the world after China, the USA, Mexico, and Egypt [2]. Strawberry, which is important for human health, is a fruit rich in ascorbic acid, phenolic compounds, folates, sugars, and acids. Strawberry fruits are important to protect phytochemicals such as phenolic compounds and organic acids in strawberry fruit after harvest. It is important to keep these secondary metabolites and nutritional values in healthy conditions during their shelf-life during the post-harvest period. In literature researches, research on the change in phenolic compounds and organic acid contents during the shelf-life of strawberry fruits observed limited. In this study, the effects of GA 3 and 1-MCP treatments on the phenolic compound and organic acid contents of strawberry fruits were investigated during shelf-life.

Organic Acids and Vitamin C
In this study, the effects of GA 3 and 1-MCP treatments on the organic acid composition of strawberry fruits during shelf-life examined. Results showed the treatments greatly affected the fruit's organic acid properties. Similarly, increasing shelf-life had a significant effect on organic acid contents (p ≤ 0.05). Organic acid values decreased dramatically with the increase in shelf-life.
Citric acid was the pre-dominant organic acid in fruits and followed by malic, succinic, oxalic, and fumaric acid, respectively. The highest amount of oxalic acid in the shelf-life periods of the third (598.92 mg 100 g −1 ) and the fifth days (571.99 mg 100 g −1 ) observed in the 1-MCP application. The lowest amount of oxalic acid determined on the 5th day of the 100 ppm GA 3 treatment (470.35 mg 100 g −1 ). In the study, citric acid content was determined as 14.95 mg 100 g −1 on the third day and 1425 mg 100 g −1 on the fifth day in the application of 1-MCP. In GA3 (50 ppm) treatment, it was recorded as 1346.96 mg 100 g −1 on the third day and 872.78 mg 100 g −1 on the fifth day. The minimum citric acid loss during the shelf-life was in the 1-MCP and the most occurred in the 100 ppm GA 3 application. GA 3 applications negatively affected malic acid content. The changes in tartaric acid content were like malic acid (Table 1). In comparison with the control group, 1-MCP mostly prevented the breakdown of organic acids except for fumaric and succinic acid. However, GA 3 + 1-MCP prevented fumaric and succinic acid contents from decreasing more than the control group. In comparison with the control group, 1-MCP mostly prevented the breakdown of organic acids except for fumaric and succinic acid. However, GA 3 + 1-MCP prevented fumaric and succinic acid contents from decreasing more than the control group. In the measurements made during the shelf-life of strawberry fruits, the lowest vitamin C content measured in 100 ppm GA 3 treatment on the fifth day. When shelf-life periods compared, the decrease rate in the vitamin C content of the 5th-day fruits measured by approximately 50% (Table 2). In the study, 1-MCP application preserved the vitamin C content and prevents breakdown according to the control group. Compared with the control group, fruits treated with 1-MCP were found to reduce respiratory rate [32], reduce biochemical changes, and preserve organic acid and vitamin C content more. The positive effect of 100 ppm applications on organic acid change of fruits in GA 3 applications has not been determined. When the fruits finish growing, ripening begins. It is a complex process in which maturation, softening and taste change and various metabolic activities take place [33]. The metabolic stages that take place in the maturation process require high energy, and this energy sourced from carbon compounds (sugars, amino acids, and organic acids) [34]. These metabolic changes a stage of accelerated ripening normally associated with increasing respiration [34]. The carbon source needed for the formation of soluble sugars in fruits; provided from starch, organic acids, and the cell wall. Since the amount of starch in strawberry fruit is low, organic acids and cell walls mostly used as a source and softening occurs in fruit flesh [35]. In this study, a significant decrease observed in organic acid levels in all treatments, supporting physiological notes about ripening. On the other hand, differences in reduction levels between treatments were not ruled out. Since strawberries do not continue to ripen after harvesting due to non-climacteric fruit formation, the fruits are harvested when they have a variety-specific size and color. Delay in harvest is understood by the softening of the fruit, losing its normal color and turning it darker. For distant markets, the harvest is done when three quarters of the fruit is browned. Well colored and shiny fruits are generally preferred in the market [36]. Exogenous application of plant growth regulators can promote fruit development [37]. Therefore, GA 3 application, as a plant growth promoter, accelerates fruit development, and ripening. In the light of this information from the literature, the lowest organic acid in all applications of GA 3 levels expected to detect, and it is believed that GA 3 can contribute to the climacteric properties of the fruits as well as accelerating the harvest time. Unlike GA 3 , the 1-MCP application provided more stability in organic acid composition, supporting previous reports on the effects of 1-MCP [35,38]. This reveals that 1-MCP reduces breathing rate and ethylene synthesis, prevents fruit spoilage, and reduces the breakdown of organic acids [32]. In this study, it was reported that malic and citric acid contents of the "Dorit" and "Selva" strawberry cultivars decreased continuously during the storage similar to ours. Rahman et al. [3] reported that the highest titratable acidity is observed in 1/3 ripe fruits and decreases gradually as the maturity stage and storage period increase in all genotypes. In addition, the same researchers noted that the ascorbic acid content of strawberries gradually decreased during the storage period. It was determined that citric and malic acid levels was as 7.4 and 1.9 g L −1 in Benihoppe cv., respectively [39]. The researchers' levels slightly higher than ours (14.95 g L −1 citric, and 0.80 g L −1 malic at most). The difference might occur because of cultivar differential. Table 1. Effects of 1-MCP and GA 3 on oxalic, citric, tartaric, and malic acid contents during shelf-life of strawberry fruits (mg 100 g −1 ).

5
Oxalic acid Control 535.99 ± 5.00 c * 521.45 ± 0.50 b 1-MCP 598.92 ± 1.00 a 571.99 ± 1.00 a GA 3 (50) 543.83 ± 2.00 c 522.20 ± 1.50 b 1-MCP + GA 3 (50) 578.27 ± 0.50 b 520.61 ± 2.00 b GA 3 (50) 1346.96 ± 1.50 c 872.78 ± 1.00 c 1-MCP + GA 3 (50) 1390. 47 (50) 660.60 ± 0.50 e 284.15 ± 1.50 e 1-MCP + GA 3 (50) 692.55 ± 1.00 c 392.31 ± 1.00 c GA 3 (100) 638.04 ± 0.50 f 254.10 ± 1.00 f 1-MCP + GA 3 (100) 673.45 ± 1.00 d 397.13 ± 1.50 b *: Values are mean value ± standard deviation. n = 3. The difference among the means indicated with the same lower-case letter in columns was not significant (p ≤ 0.05).  (50) 672.62 ± 0.50 b 640.91 ± 0.50 a 1-MCP + GA 3 (50) 681.91 ± 1.00 a 620.04 ± 1.00 b GA 3  The PCA analysis performed to determine relations between treatments, times, and organic acids. As a result, the first two components described almost 90% of the data ( Figure 1). As a result of the PCA analysis, it was determined that the effective treatment was 1-MCP and it was located in the same PCA plane with the combination treatment with a 50 ppm dose of gibberellic acid. It was observed that the control group and GA 3 treatments were in the same PCA plane and in the negative region compared to other treatments. Factor analysis also showed the first two components are enough for the explanation (p < 0.001). Citric, tartaric, malic, and succinic acid had almost the same amount of effect on PC1 (0.44 to 0.46) and described by them. PC2 described by fumaric acid (0.75) and vitamin C (0.42) contradicting PC1. The results also showed close relations between malic, tartaric, and oxalic acids. Furthermore, the 1-MCP and 1-MCP + GA 3 (50) separated with only the amounts of these organic acids, although they distinguished with these from others. Another interesting detail PCA showed was between 100 ppm doses of GA 3 , which was similar to 1-MCP and 1-MCP + GA 3 (50) but in the reverse direction. Overall, the control group clearly separated from all treatments (Figure 1).

Phenolic Compounds
In the study, the effects of treatments on changes in the specific phenolic compounds of fruits examined during shelf-life. The treatments caused statistically significant differences in the number of phenolic compounds (p ≤ 0.05). The decrease in phenolic compounds on the fifth day found to be approximately 50% in comparison with the third day. However, the decrease in ferulic acid ratio recorded less. In this study, the breakdown of phenolic compounds in fruits treated with 1-MCP found less than in other applications. Considering the changes of phenolic compounds during their shelf-life, on the third day, the highest gallic acid (11.43 mg 100 −1 ), catechin (13.29 mg 100 −1 ), and ellagic acid (9.38 mg 100 −1 ) values were recorded in 1-MCP treatment. Protocatechuic acid found at the lowest rate compared to other phenolics (Tables 3-5). Among treatments, 1-MCP was more effective during the shelf-life (on the third day: 9.38 mg 100 −1 and the fifth day: 7.80 mg 100 −1 ) in preventing the degradation of ellagic acid, which has an anti-carcinogenic effect [39]. Although the content of ellagic acid varies from plant to plant, the species in the genus including strawberries known as the fruits with the highest ellagic acid content [40]. A report on ellagic acid content of raspberries showed ellagic levels vary between 270 mg 100 g −1 (wild raspberries) to 1900 mg 100 g −1 (yellow raspberries) [41]. In our study, ellagic acid levels varied between 5.14 (GA 3 (100)) and 9.38 mg 100 g −1 (1-MCP) that was significantly lower than in raspberries. It was noted that 1-MCP treatment during the shelf life prevented phenolic compounds loss of strawberry fruits more than the control group. In the study, the same changes recorded in other phenolic compounds. Pre-harvest gibberellic acid applications generally caused a decrease in gallic acid, protocatechuic acid, chlorogenic acid, caffeic acid, vanillic acid, ellagic acid, p-coumaric acid, and phloridzin contents during shelf-life. However, according to the control group, the disintegration of catechin and routine contents was found less during shelf-life. The application of 50 ppm GA 3 had a positive effect on protecting ferulic acid and quercetin content, but the treatment of 100 ppm GA 3 decreased the same compounds. The anthocyanin and phenolic compounds in berry fruits differ mostly depending on the species, variety, ripening stage, region, cultivation, and ecological conditions. Among these compounds, quercetin is the most common in flavonoids [42]. Caffeic and ferulic acids are the most common ones among many phenolic acids [43]. Both quercetin and ferulic acid exhibit crucial biological activities such as anti-inflammation and antioxidant, but one of the most crucial effects is the antioxidant capacity by scavenging reactive oxygen species (ROS) and enhancement of lipid oxidation related free radical [44]. Table 3. Effect of 1-MCP and GA 3 treatments on the gallic, protocatechuic, catechin, and chlorogenic acid contents during shelf-life of strawberry fruits (mg 100 g −1 ).
Allende et al. [45] noted that UV-C application caused a decrease in the amount of phenolic substance the increasing during storage time [45]. In some studies, the distribution and amount of some phenolic compounds in plants affected by factors such as maturity stage, cultivar, cultural applications, origin, growing season, storage conditions after harvest, processing, and plant growth regulators [46][47][48][49][50][51]. Besides, pre-and post-harvest applications of plant growth regulators affect fruit quality [52]. In another study, the effects of GA 3 applications before harvest on the chemical and biochemical composition of "Obilnaja" Japanese plum fruits were limited [53].

Plant Material and Trial Pattern
In this study, treatment parcels with frigo seedlings of Albion cv. established in the Bolu (Turkey) province. Seedlings planted in rows covered with black mulch and a drip irrigation system. Experiments conducted in randomized parcels design with 3 replications with 15 plants in each replication. Seedlings planted over seedling beds at 30 × 30 cm planting distance in a triangular sowing pattern. Seedling beds covered with black plastic mulching and irrigations performed through drip lines (Figure 3). Foliar GA 3 spray treatments performed two times at 50 and 100 ppm doses. Kumar et al. [54] reported that 50, 100 and 150 ppm GA 3 dozes were applied and the most effective results were obtained from 50 ppm treatment. Therefore, GA 3 doses were used in this study by Kumar et al. [54]. The first application of GA 3 was starting in bloom, and the second was 2 weeks after the first application. No application implemented in the control group. The fruit samples that came to harvest maturity collected by hand and carried to the laboratory in cloth bags. Smartfresh ® tablets used in the 1-MCP (0.625 ppb) application. The fruits brought to the laboratory placed in perforated plastic boxes. Then, these boxes placed in a gas-tight styrofoam box and 1-MCP applied at 20 • C for 24 h (Figure 4). 1-MCP application and the control groups' fruits preserved on 22 • C for 3 and 5 days, and organic acids, vitamin C, and phenolic compounds contents of fruits measured on these days.

Determination of Organic Acids
Organic acids identified by the technique reported by Bevilacqua and Califano [56]. Organic acid standards used in research; oxalic acid, tartaric acid, malic acid, citric acid and fumaric acid standards were prepared at the levels of 100, 200, 300, 400, 500, 600, 700 and 800 ppm and formed curves. Each sample (50 g) mixed with 80 mL of 0.009 N H 2 SO 4 (Heidolph Silent Crusher M, Berlin, Germany), then homogenized for 1 h with a shaker (Heidolph Unimax 1010, Berlin, Germany). The mixture centrifuged for 15 min at 15,000 rpm, and supernatants filtrated twice with 0.45 µm membrane filter following filtration with coarse filter (Millipore Millex-HV Hydrophilic PVDF, Millipore, Burlington, MA, USA) and run through a SEP-PAK C18 cartridge. Organic acid readings performed with HPLC using Aminex column (HPX-87 H, 300 mm × 7.8 mm, Bio-Rad Laboratories, Richmond, CA, USA) at 214 and 280 nm wavelengths, on Agilent package program (Agilent, Santa Clara, CA, USA).

Statistical Analysis
The introductory statistics belonging to analysis and measurement results presented as average ± standard deviation. Experiments conducted in randomized parcels design with 3 replications with 15 plants in each replication. In this study, 3 different parallel analyzes were made except for replications. Analysis of variance was used to determine the differences between treatments' averages in terms of phenolic compounds, organic acids, and vitamin C. The Duncan test was used as a multiple comparison test to express the differences between the averages. In the statistical evaluations, Windows SPSS 20 used, and the differences between the means evaluated by subjecting to ANOVA variance analysis and determined with Duncan multiple comparison tests (p < 0.05). The principal component analysis (PCA) was used to determine the correlation between treatments, traits, and shelf-life. Principal Component Analysis (PCA) biplot graph performed with The Comprehensive R Archive Network [58].

Conclusions
One of the goals of storage is to provide optimum quality criteria by minimizing changes in the color, aroma, physical, and biochemical structure of fruits and vegetables. In this study, the effect of GA 3 and 1-MCP treatments on organic acid, vitamin C, and phenolic compound contents of Albion fruits found statistically significant. The effect of the treatments varied according to the organic acid and phenolic substances. Besides, the shelf-life was effective on organic acid, vitamin C, and phenolic compound amounts. In this study, it was determined that 1-MCP applications, which cause slowing down of physiological changes by decreasing the respiratory rate, prevent the degradation of organic acids and phenolic compounds in strawberry fruits. Generally, a decrease in the organic acid contents detected in fruits treated with gibberellic acid. The organic acid contents of fruits with 1-MCP applied determined to higher than the control group. Many researchers reported that 1-MCP slows down fruit ripening after harvest and therefore the conversion of organic acids into sugars. In the present study, a decrease in phenolic compounds determined as shelf-life increases. Additionally, 1-MCP treatment, which slows down the ripening of the fruit after harvest, prevented the breakdown and reduction of phenolic compounds more than other treatments. Studies on phytochemical changes that occur during the shelf-life of fruits limited. Therefore, this study thought to be an important guide and milestone for studies on strawberries shelf-life.