Introduction

Jeju is located approximately 100 kilometers south of the Korean Peninsula. Its mean annual temperature and precipitation is approximately 15°C and 1,800 mm respectively. Citrus cultivation in Korea is limited to Jeju Island and the southern coast of the Korean Peninsula (Kim et al., 2008). Therefore, Korean farmers cultivate the most popular early-maturing ‘Miyagawa’ Satsuma mandarin in open fields due to its strong cold tolerance compare to other mandarin varieties (Jeju Citrus Cooperative, 2000). The average soluble solids content (SSC) of the fruit produced in Jeju Island is less than 10°Brix, and the acidity is lower than 1% (Jeju Special Self-governing Province Citrus Marketing &Shipping Association, 2015). Citrus industry is facing a difficult situation as the price continues to drop due to the deterioration in the quality of Satsuma mandarin in the open field, frequent rainfall during fruit maturation caused by climate change, and increase in imported of foreign fruits with high sugar content for multilateral FTA negotiations (Ko, 2015; Moon et al., 2018).

The porous sheet mulching cultivation is a relatively simple cultivation method among quality improvement measures currently being carried out and has been widely practiced promote SSC and coloring (Hayashi, 2006). However, weather conditions such as heavy rain and high-temperature drying, which made it difficult to show the effect of sheet mulching cultivation and the same time, strong drying stress on the trees occurred over a long period of time, resulting in delay of harvest time due to delays reducing organic acids in the fruit juice. Those problems with the sheet mulching cultivation, such as delay in the harvest season and decrease in tree vigor, lowered the producer’s motivation for the cultivation (Maotani and Machida, 1980; Nakazato and Kishino, 1999). In addition, there is a drawback of reducing the yield or increasing alternate bearing the tree by the water stress to the fruit setting (Kihara and Konakahara, 2000). As high-quality fruit production is becoming an important factor influencing prices, the importance of quality control through irrigation management has been raised, and irrigation facilities such as sprinklers have been actively introduced in citrus orchards. There has been a demand for a new management system available at production site or a water-saving irrigation system which can be used even on slopes in Japan where it is difficult to secure a water source. For this reason, a new water-saving irrigation fertilization management, which was rarely introduced in fruit trees, was developed, a type of nutrient solution soil cultivation, ‘Anniversary mulching drip irrigation simultaneous fertilization method’. In response to meteorological changes such as rainfall and drought, it is disseminated for the purpose of stable production of high-quality fruits in an elimination of labor way by blocking rainfall or preforming irrigation and fertilization (Morinaga et al., 2010).

Jeju Special Self-governing Province has been continuously expanding and promoting the transplanting of adult phase trees, ‘weed stop’ mulching and installing the drip irrigation system to control the weed growth and tree vigor’s recovery for 2 years, then the porous sheet mulching as part of planting distance maintenance from dense planting in the age large–scale production of project to improve the SSC. However, fruit setting ratio is unattainable situation resulting of withering trees occurrence and unstable fruit setting due to the tree vigor weakening by over fruiting and water stress as the porous sheet mulching after the transplanting. Jeju agricultural research & extension services is to distribute the annual stable fruiting technology with the partial fruiting of sub-main branches (Yang et al., 2022).

Drought stress significantly increased sucrose synthase activity in fruits, helping to achieve osmotic adjustment (Hockema and Etxeberria, 2001). Citrus flowering in response to stress conditions is 7 variety dependent and active compounds that increase in response to stress proline, are positively correlated with flowing intensity. Overall, the intensity of oxidative damage in Citrus leaves during stress could act as an indicator of flowering intensity (Mantías et al., 2012). Phytohormones are central players in sensing and signaling numerous environmental conditions, such as drought stress. De Ollas et al. (2013) reported that transient accumulation of jasmonic acid (JA) in the root of ‘Citrumelo CPB 4475’, a Citrus rootstock, was required for ABA increase under dry stress conditions. It was inferred that the internal causes of sugar content distribution in apple fruit are closely related to differences in sucrose synthesis by sucrose phosphate synthase, pulp cell size, and ABA biosynthesis depending on the site (Ohar, 2017). Tipburn occurrence according to soil moisture, calcium deficiency, and day/night relative humidity regime in highland Kimchi cabbage (Kim et al., 2021a). ‘King’ mandarin (Citrus nobilis Lour) × Poncirus trifoliata ((L.) Raf.) hybrids screened as salt stress-tolerant citrus rootstocks (Martínez-Cuenca et al., 2021). Response of citrus to freezing tolerance differs depending on genotypes and growing conditions (Kim et al., 2021b).

In this study, in order to find an indicator of the degree of damage according to trees water stress in the porous sheet mulching cultivation for high-quality stable production of Satsuma mandarin, the sugar accumulation in fruit was interpreted by osmotic regulation and simultaneously JA and ABA hormone concentration changes related to trees oxidative damage and drying stress were investigated.

Material and Method

Experimental Trees, Microclimate and Soil Moisture Content

In this experiment, approximately 20 years old ‘Miyaga Wase’ trees were used in a densely planted orchard with a planting distance of 4 × 3 m between a row and trees in a citrus orchard composed of silty loam mother material belonging to more conservative Haengwon soil series of volcanic ash soil located in Namwon, Seogipo-si. On June 23th 2022, a porous sheet (Tyvex, DuPon, USA) was mulched over the entire orchard to set up a mulching area, and through an already installed drip lake (Inline trip, 50 cm × 400 m, Jain irrigation, Korea). At intervals of about 2 weeks, 2–4 tons of water was irrigated through a water tank and an agricultural pump (PU-1700M, single-phase 2HP, Willow, Korea) for all treatments.

On August 15th 2022, the porous mulching sheet of 5 trees in a row was peeled off to set up a non-sheet mulching (control), restoring the porous sheet (mulching) throughout this experiment period, and the drip irrigation (Mul. + Drip) group watered from approximately June 20th at an interval of 2 weeks until the end of the experiment period. For orchard management, standard fertilizing of 28, 40, and 28 kg of nitrogen, phosphoric acid and potassium fertilizers were applied, respectively per 10a for a year. On September 1st, wireless repeater (ZF-10F, USEM, Korea) and senor unit (Senor Unit, ZF-8S1/8S2, USEM, Korea) were installed in the middle of the orchard. Quantum sensor (ZF-150, LI-COR, USA), temperature and relation humidity sensor (ZF-100H, USEM, Korea) were also installed in the in the middle of the orchard. The soil moisture sensors (HA-M05, Decagon, USA) were well kneaded in excavated soil at a soil depth of approximately 25 cm around the canopy and buried by treatments on September 1st, and mention the year. The micro-weather information and soil moisture content were measured at intervals of 1h. Measurements were recorded from September 10th to November 30th.

Leaf and Vesicle Tissue Moisture Status

In order to measure the moisture status of leaf and vesicle tissue samples were collected between 4:30 and 6:00 AM six times from September 12 to November 17 at intervals of about two weeks. Samples were put in a zipper pack, sealed, and transported to the Jeju National University, Citrus & Flowers Science Technology Center, subtropical pomological laboratory. Those samples were inserted in a psychrometer sample chamber (C-52, Wescor, USA) in the laboratory, parallelized for 3 hours, and measured with a dew point microvolt meter (HR-33T, USA). Water and osmotic potential were measured and turgor pressure was calculated using the method of (Han et al., 2014).

Changes in Fruit Quality

The fruits of upper, middle and lower fruiting positions of three trees for each treatment were sampled, and juice extracted at intervals of approximately 2 weeks. SSC of the juices were measured using a pocket refractometer (ATAGO PAL-1, Japan). The 5 mL juices were also diluted with 20 mL of distilled water, three drops of the phenolphtalein solution were added, and the acidity was titrated using an automatic burett (Digitrate & Digitrate Pro, VWR interation Ltd, Hunter, England) using a neutralization reaction of 0.1 N sodium hydroxide (NaOH).

Measurement of Chlorophyll Fluorescence and Photosynthetic Rate

The photosynthetic rate was measured on September 22, the fruit enlargement period, by using a photosynthetic instrument (Potable photosynthesis system, LI-64000, LI-COR, USA), 6 sheet leaves attached to bearing and vegetative branch were measured per tree in each treatment. Measurements were taken between 10am and 11:30am. Simultaneously, the chlorophyll fluorescence measurement was performed using an OS5-FL1 portable fluorescence meter (Opti-Science, Inc., Hudson, NH, UK) after dark treatment for approximately 15 minutes with dark cream to determine PSII, Fo (initial fluorescence value) and Fv/Fm (maximum quantum efficiency)

Concentration Measurements of Plant Hormones ABA and JA

The samples used to measure the moisture status of leaf and vesicle tissues were instantly frozen in liquid nitrogen, stored at ‒80°C in cryogenic freezers (Deep freezer, Refrigerant, Nihon freezer, Japan), and then sequentially freeze-dried (Lyophilizer, Clean Vac 8, Hanill scientific industry, Korea). Freeze-dried leaves, pericarp and flesh samples were extracted with 80% methanol overnight, and then the acid plant hormone layer separated using the method of Jikumaru et al. (2007) was fractionated, and then purified with HLB and WAX cartridge. These purified samples were qualitative and quantitative analyzed using UHPLC (Nexera X2 Shimadzu, Japan) and MS/MS (LCMS-8050, Shimadzu, Japan) equipment of the Jeju National Human Interface Media Center.

Statistical Processing

All experiments other than the fruit quality other than the chlorophyll fluorescence parameters and photosynthesis rate were conducted using four replicates. Fruit quality, chlorophyll fluorescence and photosynthesis rate were performed 9 and 6 replicates. Statistical significance was analyzed at the p < 0.05 level using the Duncan’s multiple range test after performing a minimum significance (LSD) test on the average value. Analyses were conducted using the statistical program Sigma Plot 15 (USA).

Results and Discussion

In this experiment, phycrometer measurement method was used to set the control, mulching, and Mul. + Drip group to the farmhouses, porous sheet mulching cultivation of Satsuma mandarin in 2022. Change values in moisture status for leaves and vesicle tissue are listed in Table 1 and Table 2. The moisture status fluctuations in the leaves and vesicle tissue for each treatment of the variations in the soil moisture contents were also showed in Fig. 1, according to the microclimate; temperature, humidity, and quantum yield from September to November 2022.

Fig. 1.

Changes of soil moisture as control, mulching and Mul. + Drip) at Satsuma mandarin porous sheet mulching cultivation orchard.

In September, the rainfall continued for 15 days due to the influence of a typhoon, showing high relative humidity and low light intensity, with approximately 2,500 µmol on a sunny days. Due to the influence of the weather, the moisture content of the soil of the control and Mul.+drip irrigation group compared to the mulching group maintained an average of 30–50%, whereas the mulching treatment showed a low content 30–40%. Although there was intermittent rainfall in October, most of the weather was clear, but unlike September, the light intensity maintained an average of 1,500 µmol. Unlike September, the soil moisture content was approximately 55% at the beginning of the untreated period due to rainfall, and gradually decreased as the sunny days continued, dropping to a minimum of 30%. However, the mulching group maintained an average dry state of 30% due to blocking rainwater, and the Mul. + Drip, unlike the control group, increased its content due to the effect of irrigation at 2-week intervals and then remained and then remained dry again. The average water content of approximately 35%, which is an intermediate state of the mulching, was maintained. In November, based on the changes in relative humidity and excess, rainy weather was judged in the 10th and 12th and the 19th and 20th. The light intensity recorded an average of 1,200 µmol, lower than in October. Due to frequent rainfall, control soil showed a minimum of approximately 35–40% after 10 days, whereas the mulching group maintained an average of 25% in the dry state, and an irrigation point due to the effect of irrigation at 2-week intervals in the early stage, and maintained an average of 36% effect.

Looking at the leaf water potential change, after the typhoon in early September, on the 12th of September on consecutive sunny days, the control group recovered from ‒2.48 MPa to ‒0.88 MPa with rainfall over several days after the 12th, and then in early October. It decreased to ‒2.48 MPa due to continuous dry weather after temporary rainfall. Due to the influence of dry weather in October, it decreased to approximately ‒4.0 MPa from November 4 until November 11, and then recovered to ‒1.98 MPa with rain rainfall for 2–3 days after November 11. Conversely, mulching and Mul. + Drip group showed potential of ‒2.89 MPa, which was lower than the control group on 12th of September, in the 24th, despite the temporary rainfall, potential of ‒2.83 and ‒2.17 MPa was obtained, respectively, which was lower than the control groups no significant difference between the treatments (p > 0.05) were found. After October, no significant difference between treatments were found due to continuous dry weather, however Mul. + Drip group potential tended to be higher than the other treatments at ‒1.96 to ‒2.64 MPa, whereas the average value of mulching was ‒3.29 to ‒4.70 MPa, being the lowest of all treatments. Due to the continuous dry weather of November, control and mulching groups potential were ‒4.00 and ‒4.60 MPa, whereas the Mul. + Drip groups was ‒1.98 and ‒2.08 MPa, Mul. + Drip groups presented the highest value at ‒1.13 MPa due to the effect of drip irrigation. There was temporary rainfall in October, however, no change in soil moisture content despite the drip irrigation of 2 tons of water for 2 weeks, no significant difference between treatments even when irrigated with 4 tons of water were found.

Table 2 shows summarizes the change in water status in the fruit vesical tissue. The water potential summarized in Table 2 shows a similar trend to the leaf water potential. In September 12, control was ‒0.67, mulching ‒2.53, and Mul. + Drip was ‒1.95 MPa, showing a significant difference between control and mulching group.

Table 3 summarizes the results of SSC and acidity of fruits according to the porous sheet mulching. A significant difference was recognized between the control group and the mulching group, and the SSC measured on November 26 was 11.05°Brix and 14.55°Brix in the control group and the mulching group. There was no significant difference between the mulching and Mul. + Drip groups. Since the fruits of the Satsuma mandarin are produced by the development of the ovarian wall, which constitutes the organ of the flower, the ovarian wall protrudes to form a special tissue called vesicle (Kadoya, 2000). This specification is caused by cell division of the endocarp during the flowering period, and with the growth of the fruit, it becomes larger by cell division and hypertrophy. During the maturation period, the specification of the succulent occupies the inside of the juice sac. The inside of the vesicle tissues consists of parenchyma rich in large vacuoles, where succulents accumulate (Iwahori and Kadoya, 1999).

Drying stress increases the sugar content of fruit, but at same time suppresses hypertrophy or lowers photosynthetic activity. When plants are subjected to water stress, growth inhibition is first observed, followed by reduced photosynthetic and transpiration rates (Boyer, 1970; Hsiao, 1973). The photosynthetic rate of Satsuma mandarin decreased by half when the soil pF was lower than 2.7 (Ono, 1985). In addition, when the leaf water content ratio is less than ‒1.30 ~ ‒1.50 MPa, photosynthetic rate starts to decrease, and at about ‒2.50 MPa, it is reduced by about half (Morinaga, 1993). Looking at the daily changes in the photosynthetic rate and moisture characteristics of the leaves in hydroponics, the daily leaf water content percentage of the high concentration hydroponics was ‒2.5 MPa, which was significantly (p > 0.05) lower than that of low-concentration nutrient solution (Yakushiji, 2000). The taste of the fruit of the Satsuma mandarin is determined by the concentration of sugar and organic acid in the juice. If such strong drying stress is not applied in summer and autumn, it is difficult to produce fruit with high sugar content. In response to frequent rainfall during fruit maturation due to recent climate change, citrus farms in Japan and Jeju produce high-quality fruits through porous sheet mulching cultivation and house cultivation (Yakushiji et al., 1996; Han et al., 2014). There are few reports on the mechanism of sugar accumulation in the Satsuma mandarin (Kadoya, 1972, 1973; Sugai and Torikata, 1979). However, the physiological mechanism of water stress in terms of the water relationship to trees has not been elucidated. Yakushiji et al. (1996) measured soil, fine roots, pericarp, and vesicle tissue of porous sheet mulching using an isobaric phychrometer measurement method. The water potential and osmotic potential of the peel considerably decreased. The turgor pressure of roots, peel, and vesicle tissue of trees subjected to water stress did not decrease. To maintain turgor pressure, osmotic regulation occurred in the Sastuma mandarin trees in response to water stress. The osmotic potential gradually decreased in the Satsuma mandarin vesicle tissues of fruits subjected to water stress, simultaneously, sugar was accumulated in the vesicle tissue. These results suggest that sugar accumulation in Satsuma mandarin is not due to dehydration caused by water stress, but rather by active osmotic control in response to water stress.

Chlorophyll fluorescence measurement values summarized in Table 4, according to the porous sheet mulching measurements of September 22nd, the fruit enlargement period, and October 18, the fruit maturation period. The initial fluorescence value (Fo) of vegetative branch on September 22nd was 111.6 in the mulching group, which was significantly higher than other treatments, however, no significant differences between the control groups and Mul. + Drip groups were found. Maximum quantum efficiency (Fv/Fm) significance 0.6 in the control group, 0.5 in the mulching group, and 0.6 in the Mul. + Drip group, and there was no statistical significance among the treatments. The Fo value of the bearing branch was 109.8 in the control group and 107.3 in the mulching group, with no difference between the treatments, but the Mul + Drip group Fo was 90.0, the lowest among the treatments. In addition, the Fv/Fm of the bearing branch was 0.6 in the Control group, 0.4 in the Mulching group, and 0.5 in the Mul. + Drip group. The initial fluorescence value of vegetative branch measured on October 18 was 100.2 in the control group and 108.0 in the mulching group, showing no difference between the treatments, however, the Mul. + Drip group was lowest at 92.3. The Fv/Fm was 0.6 in the control group, 0.2 in the mulching group, and 0.4 in the Mul. + Drip group. Conversely, the Fo value of bearing branch was 109.8 in the control group and 107.3 in the mulching group, with no statistical difference between the treatments, and 89.7 in the Mul. + Drip group, which was lower than the other treatment groups. The Fv/Fm ratio among the treatment groups was 0.6 in the control group, 0.4 in the mulching group and 0.5 in the Mul. + Drip group. No statistical differences were found between the groups. On September 22th, Fo value of the mulching group was 116.6, which was significantly different, whereas, no significant difference between the control group and Mul. + Drip group was found.

The photosynthetic light reaction is a process in which plants use light energy to reduce electron acceptors and produce O₂, and it is a photochemical reaction in which photosynthetic pigments convert light energy into chemical energy. The light reaction is composed of Photosystem I, which mainly uses wavelengths around 700 nm, Photosystem II which mainly uses wavelengths around 680 nm. Absorption of light energy occurs mainly in photosystem II. Photosynthetic pigments that absorb light energy become excited and transfer energy to the next electron transporter, plastoquinone (PQ). The light generated is called florescence of plant and is highly related to chlorophyll florescence of photosystem II (Scheiber et al., 1994). Chlorophyll florescence provides meaningful information on leaf environment stress (Long et al., 1994), and chlorophyll florescence under natural conditions (Somersalo and Krause, 1990; Lee et al., 1995). Research has been actively conducted. Frequently used variables include Fo, Fm, and Fv/Fm. Fo means florescence emitted when PQ is completely oxidized, and Fm means florescence emitted when PQ is completely reduced. It is the value obtained by dividing Fv, which is the difference between Fv, by Fm, and means the efficiency with which photon absorbed by the chloroplast is used for the reaction (Kitajima and Butler, 1975). Fv/Fm usually exists in the range of 0.80 to 0.83 in most plants, and this variable is known as one of the scales representing the maximum light efficiency of photosystem II (Demming and Björkman, 1987). In other words, since this value means the potential of the plant leaf to perform photosynthesis, it is used as an indicator of the health of the photosynthetic machinery, the degree of damage caused by stress (Lee et al., 2011).

Mantías et al. (2012) placed two years old ‘Valencia’ orange and ‘Ellendaie’ tangor seedlings in a 15 L pot in a severe water stress environment with a leaf water potential (Ψw) of ‒4.00 MPa or less for 70 days, and PSII (Fv/Fm, maximum quantum efficiency value) and stomatal conductance (gs) were investigated. After 10 days of stress, the stressed seedlings showed the lowest potential for the first time. The stomatal conductance of plants in this group also decreased simultaneously. After 20 days for both cultivars, considerably lower values were reached compared to plants not subjected to water stress. Additionally, Fo (initial fluorescence value) showed the highest values of about 250 in ‘Valencia’ orange at 50 days after treatment. The peak quantum efficiency value also decreased at 37 and 50 days after treatment in both strains under stress, with stressed ‘Valencia’ oranges having a maximum quantum efficiency value of about 0.67. In the cultivation of Citrus fruits, which are perennial woody crops, it is very important to secure annual stable fruit production while maintaining tree growth (Nakamura et al., 2010). Nagatani et al. (2011) reported that the number of fine roots decreased due to moisture stress in summer due to excessive fruit set in Satsuma mandarin. The untreated plot in this experiment was an adult tree, and despite the temporary rainfall in September due to dry weather for about 2weeks from August 15, the summer, the number of fine roots decreased due to moisture stress in summer due to the burden of fruiting compared to the drip irrigation treated plot. As a result, Fo value was judged to be low. The sheet mulching treatment group was the lowest in the vegetative branch due to the continuous dry weather in October, and no difference between the treatment groups was found.

Table 5 lists the results of the photosynthetic rate of the leaf obtained by dividing it into the vegetative branch and bearing branch in September 22, the fruit enlargement period, and October 18th, the maturity period. On September 22th there was no significant difference between the treatments in October 18th, the control and Mul. + Drip groups were significantly (p > 0.05) higher than the mulching group in the vegetative and bearing branches. The fruit of Satsuma mandarin is always a large sink organ of photosynthetic assimilation nutrients in its growth and maturation process. Although drying stress inhibits polysaccharide synthesis and increases the sugar content of fruit, it is thought that the translocation and increase of photosynthetic products in the fruit are more important for the increase in sugar content of the fruit (Yakushiji, 2000). Cell hypertrophy is very sensitive to drying stress and is related to changes in the water potential gradient associated with growth (Molz and Boyer, 1978; Nonami and Boyer, 1987, 1990, 1993; Nonami et al., 1997). It is known that when the osmotic function works under water stress, the cell volume and turgor pressure are maintained by the active accumulation of solutes in the cell (Morgan, 1984). Yakushiji (2000) thought that fruit growth could be maintained even in Satsuma mandarin subjected to drying stress by the accumulation of dissolved substances used for osmotic pressure regulation. Kadoya (1973) found that when ¹⁴C-labeled carbon dioxide was applied to the Satsuma mandarin trees, the ¹⁴C activity of ethanol-soluble fraction was higher than that of the ethanol-insoluble fraction in soil-drying fruits. It was thought that the increase in the amount of ¹⁴C activity in this this ethanol-soluble fraction was the result of photosynthetic assimilation products transported to the fruit and metabolized and accumulated as alcohol-soluble sugars, amino acids, and organic acids. In a ¹⁴C assimilation test using grapefruit, most of the current substances of photosynthetic assimilation products in the flesh were ethanol-soluble fractions (Yen and Koch, 1990). As a result, it was possible to explain the increase in the current from the source of photosynthetic assimilation products with high sink activity for sugar accumulation in the fruits of the mild drying stress zone.

Fig. 2 and Fig. 3 shows the change in the concentration of endogenous ABA and JA according to the time and treatment using the leaf and vesicle tissue water potential measurement samples before dawn to determine the degree of damage from drying stress to the tree. In the September 12th, the control group showed a lower concentration of endogenous ABA and JA than the mulching group, similarly in October 23th, when the dry weather continued the control group showed the highest value of 13.4 ng/g F.W. This value was about three times higher than that of other treatments, and it showed a low value in the pre-treatment due to rainfall several times in November, however, no significant difference between treatment groups was found. Under water stress conditions most ABA accumulates in plant tissues (Rodrigo et al., 2006). Plant responses to desiccation, including stomatal closure and organ detachment, play an important role in regulating ABA physiological strength. De Ollas et al. (2013) found that the transient accumulation of JA in roots was required for ABA increase under drying stress conditions in Citrus rootstock ‘Citrumelo CPB 4475’. It was reported that the content of resistant ABA increased with the increase in sugar content in peach fruit that were severely subjected to moisture stress (Kobashi et al., 1997) Okuda et al. (1995) reported the effects of Satsuma mandarin fruit on photosynthetic rate, free ABA concentration in leaf and flowering in the following year, and the ABA concentration in leaf was 0–4 nmol/g FW. The analysis of the free ABA concentration in the leaf in this report was measured using GC-ECD. In this experiment, the concentration was 0–13.4 ng/g FW. The molecular weight of ABA was converted 264.32, and 1 nmol was about 2.64 ng and 4 nmol was about 9 ng. In the experiment, up to 13 ng was shown underwater stress. Gómez-Cadenas et al. (2002) reported that ‘Salustiance’ [Citrus sinensis (L.) Osbeck] orange trees subjected to salinity stress (100 mM NaCl) accumulated large amounts of chloride, increased ethylene production and become defoliated. In addition, the stomatal conductance and photosynthetic rate also rapidly decreased. However, treatment with 10 mM ABA as a nutrient solution 10 days before exposure to salt stress reduced ethylene production and leaf fall. Trees not subjected to salt stress showed that ABA reduced stomatal conductance and CO₂ assimilation; however, plants subjected to salinity stress slightly increased the above parameters, suggesting that ABA plays a protective role against Citrus fruits under salinity stress. The sharp increase in the concentration of ABA in the leaves untreated group compared to the other treatments is thought to be due to the activation of ABA as a plant control function to cope with the environment of rapid dry weather in October.

Fig. 2.

Changes of leaf (A), fruit peel (B) and flesh (C) endogenous ABA concentration before down in Satsuma mandarin porous sheet mulching cultivation as control, mulching and Mul. + Drip during the fruit growth season. Different letters indicated significant difference at p < 0.05, Duncan’s multiple range test (n = 4).

Fig. 3.

Changes of leaf (A) and fruit peel (B) endogenous JA concentration before down in Satsuma mandarin porous sheet mulching cultivation as control, mulching and Mul. + Drip during the fruit growth season. Different letters indicated significant difference at p < 0.05, Duncan’s multiple range test (n = 4).

Kobashi et al. (1997) investigated the effects of water stress on the quality and ABA content of peach fruits, and the effect of severe soil moisture stress treatment (‒0.06 MPa) in July, the fruit ripening period, in the increase of the fruit sugar content, and the leaf water potential was with an increase of sugar content from 7.60°Brix to 8.27°Brix in the untreated group at 2.0 MPa, endogenous ABA content also increased from 209.12 ng to 321.61 ng / F.W g on July 10, and from 800 to 1,350 ng/g F.W in july 20th, the harvest season. The results show a maximum of about 500 ng/g F.W in the porous sheet mulching treatment group, and the 1,283 ng/g F.W of the peel was added, it was about 1,700 ng/g F.W, which was higher than that of peach fruit, and in September 22th the photosynthetic rate of bearing branch was reduced to 3.62 µg/m²/s in the mulching group compared to 5.05 in the control group (Table 5). The SSC of fruit was 11.05°Brix in the control group, 14.55°Brix in the mulching fruit, and 13.96°Brix in the Mul. + Drip group on November 26th, the harvest season (Table 3). Water stress at fruit maturity reduced dry matter production, as was the case for peach fruits with 254.6 ng/g F.W in the mulching and 108.6 ng/g F.W in the Mul. + Drip; however it promoted the partitioning of assimilated into the fruit by increasing the ABA concentration in the fruit. It was thought that the sugar content of the fruit was increased by doing so. In particular, in this experiment, the ABA concentration in the fruit peel was also investigated. In November 17, the ABA concentration was 169.67 ng in the control group, 1,283ng in the mulching group, and 706 ng/g F.W in the Mul. + Drip group, the concentrations were higher than those in the control group.

JA and its metabolites, collectively known as jasmonates, are lipid-derived prostaglandins found in animals (Agrawal et al., 2004). The signal of jasmonate is a response to biological stress factors such as insects and pathogens, including defense responses, biological environmental factors such as ozone (Sasaki et al., 2005), and stress wounds (Lorenzo et al., 2004). Citrus fruit have been proposed as a biological stress model in woody plants (Gómez-Cadenas et al., 2003). Plant hormones detect numerous environmental conditions such as drying stress and play a central role in signal transduction. De Ollas et al. (2013) found that the transient accumulation of JA in the roots of ‘Citrumelo CPS 4475’, a Citrus rootstock for ABA increase under drying stress conditions. In this experiment, JA activity according to the degree of water stress was shown only in the leaves and fruit peel, with very low concentrations found in the control group in the flesh. In general, when the ABA concentration rises, the JA concentration tends to increase, implying that the temporary accumulation demonstrated by De Ollas et al. (2013) is necessary for the increase in ABA.

Yakushiji (2000) found that the application of drying stress is essential for high sugar fruit production in the current Satsuma mandarin cultivars. However, improper control of drying stress not only damages the quality of the fruits, but also considerably reduces the supply of nutrients to vegetative growth organs such as thin roots and shoots. Because of this, nutrients necessary for the growth in the following spring are in a state of extreme shortage, and it is easy to cause a decrease in tree vigor. However, it was thought that it was effective in increasing fruit sugar content and the burden on the tree was small under the condition of dry stress at an acceptable level of osmotic regulation function in fruit. In addition, in order to minimize the negative factors accompanying the drying stress treatment, it was said that it is required to establish a simple moisture characteristic evaluation method, a water-retaining technology based on biometric information, and a subtraction technology.

Jeju Special Self-governing Province has been continuously expanding and promoting the transplanting of adult phase trees, with the porous sheet mulching as part of planting distance maintenance from dense planting in the age large–scale production of project to improve the SSC. However, fruit setting ratio is unattainable situation resulting of withering trees occurrence and unstable fruit setting due to the tree vigor weakening by over fruiting and water stress as the sheet film mulching after the transplanting. In this experiment, in order to find indications of the degree of damage according to the level of water stress and the increase in fruit sugar content in the cultivation of Satsuma mandarin porous sheet, the change in fruit quality was physiologically interpreted, and at the same time, JA and ABA concentrations related to oxidative damage and drying stress conducted for research purposes.

As a result of the above, sugar accumulation in fruit was interpreted by osmotic regulation according to trees water stress in the porous sheet mulching cultivation. The maximum quantum efficiency value (Fv/Fm) of chlorophyll PSII related to photosynthetic oxidative damage, the initial value (Fo), and JA and ABA, which are hormones related to drying stress, were found as indicators of the degree of damage according to the degree of water stress.