Effect of light and carbon dioxide on photosynthesis,chlorophyll fluorescence,and fruit yield in strawberry ( Fragaria × ananassa Duch.) plants

Abstract

BACKGROUND:

The use of two-floor bench bed systems to improve the strawberry productivity in Korea is increasing.

OBJECTIVE:

This study aimed to investigate the effect of different combinations of light intensity and carbon dioxide (CO₂) concentration on photosynthesis, chlorophyll fluorescence, and fruit yield in strawberry (Fragaria × ananassa).

METHODS:

A two-floor bench bed system was used for light intensity treatments. CO₂ fertilizer was supplied, from December 5 to January 30, when the concentration in a greenhouse fell below 700μmolmol^–¹, while the control received only ambient CO₂.

RESULTS:

Strawberry plants that were grown under high light intensity in the upper bed had a higher photosynthesis rate than plants grown under low light intensity in the bottom bed; however, light intensity had a negligible effect on the stomatal conductance (Sc) and transpiration rate (Tr) of strawberry leaves. In contrast, CO₂ fertilization lowered Sc and Tr values in strawberry leaves. Plants grown under low light intensity without CO₂ fertilization had the highest chlorophyll fluorescence decrease ratio. Fruit yield was highest in plants grown under high light intensity, with CO₂ fertilization having relatively little effect; however, CO₂ fertilization stimulated the accumulation of anthocyanins.

CONCLUSIONS:

CO₂ fertilization is effective in a two-floor bench bed system.

Keywords

Anthocyanin bed fertilization greenhouse light intensity

1 Introduction

Parameters related to photosynthesis and chlorophyll fluorescence are widely used to assess the growth of and physiological changes in the horticultural crops grown under various environmental conditions. In particular, the photosynthesis rate (Pr), stomatal conductance (Sc), and transpiration rate (Tr) have been used to determine the cultivation status of the horticultural crops and to predict their productivity [1, 2]; whereas, the maximum potential quantum efficiency of photosystem II (Qy), nonphotochemical quenching (NPQ), and variable chlorophyll fluorescence decline ratio (R_fd) have been used to indicate the degree of stress and recovery of the photosystem in plants grown under harsh environmental conditions [3 –5]. Furthermore, chlorophyll fluorescence imaging is being widely used as an alternative method to accurately quantify the tolerance and acclimation of leaves to the environmental stresses [2, 6].

Strawberry (Fragaria × ananassa Duch.) is one of the majorly consumed horticultural crops worldwide, eaten either as a fresh fruit or used in processed products. Moreover, strawberry is economically essential, particularly in Korea, where its production revenue in 2016 was $1.3 bn, representing the highest revenue among the horticultural crops [7]. Strawberry also contains large amounts of nutrients such as soluble sugars, organic acids, phenolic compounds, and anthocyanins [8, 9] and its consumption is known to reduce the oxidative stress related to several human diseases [10]. Due to the value of strawberry in the agricultural industry, several studies have investigated new methods to increase the quality and yield by growing plants under various environmental conditions, including different light intensities [11, 12], temperatures [2 , 14], and CO₂ concentrations [15].

The use of a two-floor bench bed system for strawberry cultivation in Korea is increasing as it reduces production costs; however, this system results in strawberry plants grown in the bottom bed receiving less light owing to shading by the upper bed structure, resulting in reduced production in the bottom bed. In the low-temperature growth period, it is reported that in low light intensity conditions, the productivity of strawberry fruit is increased to a certain extent by increasing the night-time temperature [12]. CO₂ concentration is also an important environmental factor in increasing the crop productivity. It has been reported that CO₂ concentration markedly affects the growth and yield of crops [16]. In the present study, we tested the effect of CO₂ fertilization during the low-temperature growth period on the productivity of strawberry plants grown in the bottom bed.

In this study, we assessed the effects of light intensity and CO₂ concentration on the leaf physiology, fruit quality, and yield of strawberry plants grown in a two-floor bench bed system by examining the photosynthesis and chlorophyll fluorescence in leaves and by phytochemical analysis.

2 Materials and methods

2.1 Plant materials and cultivation

Strawberry runners (Fragaria × ananassa Duch.cv. Solhyang) obtained from a single mother plant were grown as seedlings from May to September 2015. These seedlings were then planted in a two-floor bench bed system (Fig. 1) filled with a commercial medium (Tosille Medium, Shinan Grow Co., Jinju, Korea) in greenhouses at the Protected Horticulture Research Institute, Haman, Korea (35° 19^′ N, 129° 22^′ E), where they were cultivated from October 2015 to January 2016. The plants were supplied with water and a nutrient solution [macro-elements (N: P: K: Ca: Mg: S = 12.5:3.0:5.5:6.5:2.5:3.0 me·L^–1), micro-elements (Fe: B: Mn: Zn: Cu: Mo = 1.12:0.27:0.55:0.46:0.05:0.05 mg·L^–1), electrical conductivity (EC) = 1.0–1.2 dS·m^–1, and hydrogen ion concentration (pH) = 5.5–6.5, using strawberry nutrient solution formulated by the Research Station for Floriculture and Glasshouse Vegetables (PBG)] via a drip irrigation system at 2-min intervals up to five times per day. The air temperature of the greenhouse was controlled by heating when the temperature fell below 8°C and by the opening of side ventilators when the temperature rose above 25°C.

Fig. 1.

Strawberry (Fragaria × ananassa) plants growing on a two-floor bench bed system.

Photosynthetically active radiation (PAR) levels on each level of the bed system were recorded at 1-h intervals using LI-190 quantum sensors (Licor, NE, USA) installed 20 cm above the two bed floors in two greenhouses and located at the midpoint along a 5-m bed floor containing 100 strawberry plants. Two identical greenhouses were used to expose the plants to different CO₂ concentrations from December 5, 2015 to January 30, 2016. In one greenhouse, on clear days when the side ventilator was closed, if the CO₂ concentration fell below 500μmolmol^–1, then CO₂ fertilizer was supplied in the form of liquefied CO₂ gas, and the supply was stopped when the CO₂ concentration rose above 700μmolmol^–1. In the other greenhouse, the plants were only exposed to ambient CO₂ concentrations. Thus, four combinations of environmental conditions in the two identical greenhouses were created in this experiment: upper bed with high light intensity and CO₂ fertilization (UC), bottom bed with low light intensity and CO₂ fertilization (BC), upper bed with high light intensity and no CO₂ fertilization (UN), and bottom bed with low light intensity and no CO₂ fertilization (BN).

2.2 Analysis of photosynthesis and chlorophyll fluorescence

Photosynthetic and chlorophyll fluorescence parameters were measured in 10 different 20-day-old strawberry leaves per treatment. The Pr, Sc, and Tr of the leaves were measured in the greenhouse using a portable photosynthesis system (LI-6400, Licor, NE, USA) over 5 days on clear mornings in January (under 1000 μmol·m^–2·s^–1 by LED panel light).

For analysis of chlorophyll fluorescence induction kinetics, the strawberry leaves were harvested at 7 am on a clear day in January, sealed in a dark bottle, swiftly moved to the laboratory to minimize water stress and were then subjected to measurements with a chlorophyll fluorescence instrument (FluorCam FC 800, Photon Systems Instruments, Drásov, Czech Republic). Chlorophyll fluorescence induction kinetics of strawberry leaves, measured with pulse amplitude modulated fluorometry after 20 min dark adaptation, is provided in Fig. 2. Chlorophyll fluorescence parameters were calculated from the kinetics and were used to determine the NPQ coefficients, Qy and R_fd.

Fig. 2.

Chlorophyll fluorescence induction kinetics of leaves with pulse amplitude modulated fluorometry. The explanation of chlorophyll fluorescence parameter are as below, F_o: the minimum chlorophyll fluorescence in dark-adapted state; F_m: the maximum chlorophyll fluorescence in dark-adapted state; F_v: the variable fluorescence in dark-adapted state; F_p: the peak fluorescence during the initial phase of the Kautsky effect; F_t: the instantaneous fluorescence during light adaptation; F’_m: the maximum fluorescence during light adaptation; Q_y: the maximum PSII quantum yield; NPQ: non-photochemical quenching; R_fd: the fluorescence decline ratio in light.

2.3 Analysis of sugars, organic acids, and phytochemicals

On January 26, 2016, we assessed the strawberry plants to be fully mature and the fruit as fully ripe, and hence harvested the strawberries. The sugar, organic acid, and phytochemical content in the fruit from 20 plants (1 kg) were then analyzed in triplicate. The fruit samples were homogenized, and the extracts were centrifuged (64R Centrifuge, Beckman Coulter Inc., CA, USA) at 16,000× g for 30 min at 4°C. The supernatants were then filtered through a filter paper (Whatman No. 2, Sigma-Aldrich Co., MO, USA) and stored at –70°C as previously described [12]. Before measuring the sugar, organic acid, and phytochemical content in the fruit samples, the frozen extracts were thawed and filtered through 0.45 μm syringe filters following which the filtrates were diluted with distilled water. The sugar content in the fruit extracts was analyzed using a high performance liquid chromatography system (YL9100, Younglin Co., Anyang, Korea) equipped with a Sugar-Pak column (4.6 mm × 250 mm, Supelco, PA, USA) and a refractive index detector (YL9170 RI, Younglin Co., Anyang, Korea) as previously described [12]. The organic acid content in the fruit extracts was analyzed with an ion chromatography system (ICS 5000, Dionex, CA, USA) equipped with an Ion-Pac column (9 mm × 250 mmICE-AS6, Dionex, NY, USA) and a suppressor (AMMS ICE300, Dionex, NY, USA) as previously described [12]. The anthocyanin content in the fruit extracts was measured at 530 nm using a UV-visible spectrophotometer (Evolution 300, Thermo Co., CA, USA) calibrated against pelargonidin-3-glucoside as the standard following the method previously described [12]. The phenolic content in the fruit extracts was determined using a UV-visible spectrophotometer (Evolution 300) as previously described [17], with gallic acid equivalents as the standard. Folin–Ciocalteu reagent (50%) and Na₂CO₃ (20%) were added to the aliquots of the extracts sequentially, and the mixture was incubated at 37°C for 45 min. When blue color was developed in each tube, absorbance of the test solutions was measured at 750 nm with the reagent used as a blank.

2.4 Experimental design and statistical analysis

A randomized block design was used with two identical greenhouses. Each treatment was separated into four blocks (two light intensities × two CO₂ concentrations), each block comprising 100 plants, respectively. Fruit samples harvested from each block were used to analyze the average yield and phytochemicals, and photosynthesis and chlorophyll fluorescence were measured by randomly selecting 10 plants from each block. The effects of light intensity and CO₂ concentration were analyzed by two-way analysis of variance. Data were analyzed to determine the significant main effects or interactions for relevant comparisons. To confirm differences between the experimental groups, the data were analyzed using one-way analysis of variance with Duncan’s multiple range test at p = 0.05 in SAS (SAS Institute Inc., NC, USA).

3 Results and discussion

3.1 Cultivation environment

Changes in the ambient light intensity and CO₂ concentration on a clear day in January, as a result of the structure of the bench system and CO₂ fertilization, are presented in Fig. 3. No difference was observed in the ambient light intensity between the upper and bottom beds until 11 am; however, from 11 am to 4 pm the ambient light intensity in the bottom bed was significantly lower than that in the upper bed resulting in a 70% reduction in the total ambient light intensity in the bottom bed caused by the overshadowing (Fig. 3A and C). In the greenhouse supplied with liquefied carbon dioxide fertilizer, the CO₂ concentration was maintained 200–300 μmolmol^–1, higher than the ambient CO₂ concentration (Fig. 3B). The air temperatures in the two greenhouses were lowest (13.1±0.5°C) in January (Fig. 3D).

Fig. 3.

Changes in the environmental conditions in the two identical greenhouses. The variation of ambient light intensity (A) and CO₂ concentration (B) during a clear day in January; monthly mean values of light intensity (C) and temperature (D).

3.2 Photosynthesis parameters

The Pr, Sc, and Tr of the strawberry leaves under the four treatments BC, UC, BN, and UN are listed in Table 1. Strawberry plants grown in the upper bed (UC and UN), which received a higher light intensity, had higher Pr values than plants grown in the bottom bed (BC and BN). Sc and Tr values were significantly higher in plants grown under ambient CO₂ (BN and UN) than in those supplied with CO₂ fertilizer (BC and UC). Thus, the Pr of plant leaves is more affected by light intensity than by the CO₂ concentration, while the reverse is true for Sc and Tr. These results support the previous findings [18], which revealed that an increased CO₂ concentration decreased the Sc and Tr of plants in a forest. It is considered that Sc and Tr are increased in the leaves of plants cultivated in the ambient CO₂ greenhouse because their stoma open further to absorb CO₂ than those in leaves of plants in a high CO₂ atmosphere.

Table 1
Photosynthetic parameters in strawberry (Fragaria × ananassa) plants grown under four different combinations of light intensity and CO₂ concentration. BC: bottom bed with low light intensity and CO₂ fertilization; UC: upper bed with high light intensity and CO₂ fertilization; BN: bottom bed with low light intensity and no CO₂ fertilization; UN: upper bed with high light intensity and no CO₂ fertilization

Treatment Photosynthetic rate Stomatal conductance Transpiration rate

(μmol CO₂ m^–2s^–1) (mol H₂O m^–²s^–¹) (mmol H₂O m^–²s^–¹)

BC 16.09 b z 0.21 b 3.86 bc

UC 18.32 a 0.25 b 3.71 c

BN 16.15 b 0.32 a 4.70 ab

UN 18.07 ab 0.37 a 5.11 a

Effect (P value) *

Light 0.002 0.120 0.323

CO₂ 0.725 0.001 0.001

Interaction 0.640 0.547 0.301

Treatment	Photosynthetic rate	Stomatal conductance	Transpiration rate
BC	16.09 b z	0.21 b	3.86 bc
UC	18.32 a	0.25 b	3.71 c
BN	16.15 b	0.32 a	4.70 ab
UN	18.07 ab	0.37 a	5.11 a
Effect (P value) *
Light	0.002	0.120	0.323
CO₂	0.725	0.001	0.001
Interaction	0.640	0.547	0.301

^*p values were determined by two-way ANOVA.

^z Values that are followed by different lower-case letters within a column are significantly different (Duncan’s multiple range test, p < 0.05, n = 10).

3.3 Chlorophyll fluorescence parameters

The values of the associated chlorophyll fluorescence parameters of the leaves of plants grown under the four combinations of light intensity and CO₂ concentration are provided in Table 2. It was observed that the leaves of strawberry plants grown in the bottom bed had more efficient chlorophyll fluorescence induction kinetics than the leaves of plants grown in the upper bed, with Q_y and R_fd being higher in plants grown under low light intensity (BC and BN) than in plants grown under high light intensity (UC and UN). Plants grown under a combination of low light intensity and ambient CO₂ (BN) had the highest values of F_o and R_fd, (p < 0.05), and the interaction of light and CO₂ was found to affect F_o and R_fd (Table 2 and Fig. 4). In this experiment, the values of the chlorophyll fluorescence parameters of leaves were reported to be significantly affected by the light intensity but not by CO₂ concentration. Furthermore, there was no significant difference in NPQ among the treatments and the interaction of light and CO₂. It is confirmed that the chlorophyll fluorescence of leaves was not significantly affected by the concentrations of CO₂ from ambient up to 700 μmolmol^–1. Thus, it can be considered that light intensity had a significant effect on the chlorophyll fluorescence reaction in leaves, whereas CO₂ had no significant effect (Table 2). The Q_y value is considered to be an indicator of stress chlorophyll fluorescence in leaves and has similarly been known to be affected by two different light intensity conditions in barley (Hordeum vulgare) plants [19]. It has been reported [20] that Q_y represents the maximum efficiency of photosystem II, with values of 0.79–0.84 being optimal for several plants. The strawberry plants in this experiment were in normal growth as the Q_y value ranged from 0.79 to 0.81; however, by varying the light intensity and CO₂ concentration, we confirmed that the actual Pr efficiency of leaves can depend significantly on environmental conditions while the Q_y value for each treatment remains normal.

Table 2
Chlorophyll fluorescence parameters in the leaves of strawberry (Fragaria × ananassa) plants grown under four different combinations of light intensity and CO₂ concentration. BC: bottom bed with low light intensity and CO₂ fertilization; UC: upper bed with high light intensity and CO₂ fertilization; BN: bottom bed with low light intensity and no CO₂ fertilization; UN: upper bed with high light intensity and no CO₂ fertilization

Treatment F_o F_m Q_y NPQ R_fd

BC 130.71 b z 684.41 a 0.81 a 0.030 a 1.39 ab

UC 126.39 b 589.68 b 0.79 b 0.030 a 1.30 b

BN 139.74 a 689.71 a 0.80 ab 0.024 a 1.68 a

UN 129.43 b 618.62 b 0.79 b 0.038 a 1.18 b

Effect (P value) *

Light 0.001 0.001 0.001 0.201 0.004

CO₂ 0.066 0.278 0.235 0.851 0.356

Interaction 0.028 0.449 0.083 0.201 0.034

Treatment	F_o	F_m	Q_y	NPQ	R_fd
BC	130.71 b z	684.41 a	0.81 a	0.030 a	1.39 ab
UC	126.39 b	589.68 b	0.79 b	0.030 a	1.30 b
BN	139.74 a	689.71 a	0.80 ab	0.024 a	1.68 a
UN	129.43 b	618.62 b	0.79 b	0.038 a	1.18 b
Effect (P value) *
Light	0.001	0.001	0.001	0.201	0.004
CO₂	0.066	0.278	0.235	0.851	0.356
Interaction	0.028	0.449	0.083	0.201	0.034

^*p values were determined by two-way ANOVA. Chlorophyll fluorescence parameters see Fig. 2.

^z Values that are followed by different lower-case letters within a column are significantly different (Duncan’s multiple range test, p < 0.05, n = 10).

Fig. 4.

Chlorophyll fluorescence imaging in leaves of strawberry (Fragaria × ananassa) plants grown under four different combinations of light and CO₂ concentration. BC: bottom bed with low light intensity and CO₂ fertilization; UC: upper bed with high light intensity and CO₂ fertilization; BN: bottom bed with low light intensity and no CO₂ fertilization; UN: upper bed with high light intensity and no CO₂ fertilization. F_o is the minimum chlorophyll fluorescence in dark-adapted state. F_m is the maximum chlorophyll fluorescence in dark-adapted state. Q_y is the maximum PSII quantum yield. R_fd is the fluorescence decline ratio in light.

3.4 Fruit yield

The yields of strawberry fruit are presented in Fig. 5. Light intensity and CO₂ concentration both had a significant effect on the fruit yield. Strawberry plants grown under a combination of high light intensity and CO₂ fertilizer (UC) had the highest yield, followed by plants grown under high light intensity with only ambient CO₂ (UN). A combination of low light intensity and ambient CO₂ (BN) resulted in the lowest yield. In this experiment, when CO₂ fertilizer was supplied, there was a 12% increase in the strawberry production in the bottom bed and a 19% increase in the upper bed, compared to the strawberry plants under ambient CO₂ conditions. Our results confirm what is generally known, that a higher light intensity increases CO₂ fertilization efficiency. The variations in yield under the four different treatments reflect the variation in the photosynthetic parameter Pr (Table 1 and Fig. 5). Previous reports [2] have stated that light intensity and temperature have equal effects on the yield of strawberry fruit during winter cultivation, whereas we found that light intensity had a greater effect than CO₂ concentration on the production. Therefore we propose that light intensity and temperature are of primary importance for the cultivation of strawberry during winter, with CO₂ concentration being secondary.

Fig. 5.

Fruit yield of strawberry (Fragaria × ananassa) plants harvested at the end of the cultivation period under four different combinations of light intensity and CO₂ concentration. BC: bottom bed with low light intensity and CO₂ fertilization; UC: upper bed with high light intensity and CO₂ fertilization; BN: bottom bed with low light intensity and no CO₂ fertilization; UN: upper bed with high light intensity and no CO₂ fertilization. ^*p values were determined by two-way ANOVA on total yield. Values marked with different lowercase letters are significantly different by Duncan’s multiple range test (p < 0.05, n = 100).

3.5 Accumulation of phytochemicals

Changes in the accumulation of soluble sugars and organic acids in the fruit of plants grown under the four combinations of light intensity and CO₂ concentration are listed in Table 3. Strawberry plants grown under ambient CO₂ conditions (BN and UN) accumulated more fructose and sucrose in their fruit than plants grown under CO₂ fertilization (BC and UC), whereas fruit from plants grown under BC and UC had higher citric and oxalic acid content than plants grown under BN and UN. In contrast, fruits from plants grown under high light intensity (UC and UN) had greater malic acid content than from plants grown under low light intensity (BC and BN). Thus, CO₂ fertilization is associated with a decrease in the sugar content and an increase in the organic acid content of the fruit. These results differ from the previous research [21], which found that the sugar content in birch (Betula pendula) leaves increased at a CO₂ concentration of 700 μmolmol^–1. The effects of different combinations of light intensity and CO₂ concentration on the content of phenolic compounds and anthocyanins in strawberry are presented in Fig. 6. No significant difference was found in the accumulation of phenolic compounds in the fruit among the four treatments.; however, CO₂ fertilization (BC and UC) strongly enhanced the anthocyanin content in the fruit; light intensity also increased the anthocyanin content, but the effect was not as strong. These findings are in general accordance with the previous research [22]. It is expected that CO₂ fertilization would help with the reddening of strawberry fruit skin.

Table 3
Soluble sugar and organic acid content in strawberry (Fragaria × ananassa) plants grown under four different combinations of light intensity and CO₂ concentration. BC: bottom bed with low light intensity and CO₂ fertilization; UC: upper bed with high light intensity and CO₂ fertilization; BN: bottom bed with low light intensity and no CO₂ fertilization; UN: upper bed with high light intensity and no CO₂ fertilization

Treatment Soluble sugars (g·100g^–1, FW) Organic acids (mg·kg^–1, FW)

Fructose Glucose Sucrose Citric acid Malic acid Oxalic acid

BC 2.58 bc z 2.78 a 0.72 c 478 ab 267 b 9.3 a

UC 2.52 c 2.76 a 1.00 bc 514 a 290 a 8.5 ab

BN 2.96 ab 2.96 a 1.19 b 447 b 268 b 7.0 b

UN 3.17 a 3.17 a 1.59 a 449 b 280 ab 8.1 ab

Effect (P value) *

Light 0.598 0.422 0.014 0.148 0.018 0.571

CO₂ 0.002 0.068 0.001 0.001 0.476 0.001

Interaction 0.337 0.513 0.662 0.170 0.424 0.003

Treatment	Soluble sugars (g·100g^–1, FW)	Organic acids (mg·kg^–1, FW)
BC	2.58 bc z	2.78 a	0.72 c	478 ab	267 b	9.3 a
UC	2.52 c	2.76 a	1.00 bc	514 a	290 a	8.5 ab
BN	2.96 ab	2.96 a	1.19 b	447 b	268 b	7.0 b
UN	3.17 a	3.17 a	1.59 a	449 b	280 ab	8.1 ab
Effect (P value) *
Light	0.598	0.422	0.014	0.148	0.018	0.571
CO₂	0.002	0.068	0.001	0.001	0.476	0.001
Interaction	0.337	0.513	0.662	0.170	0.424	0.003

^*p values were determined by two-way ANOVA. FW is fresh weight.

^z Values that are followed by different lower-case letters within a column are significantly different (Duncan’s multiple range test, p < 0.05).

Fig. 6.

Phytochemical content in the fruit of strawberry (Fragaria × ananassa) plants grown under four different combinations of light intensity and CO₂ concentration. BC: bottom bed with low light intensity and CO₂ fertilization; UC: upper bed with high light intensity and CO₂ fertilization; BN: bottom bed with low light intensity and no CO₂ fertilization; UN: upper bed with high light intensity and no CO₂ fertilization. ^*p values were determined by two-way ANOVA. Values marked with different lowercase letters are significantly different by Duncan’s multiple range test (p < 0.05).

4 Conclusion

Based on these findings, the following general conclusions can be drawn. Although CO₂ fertilization during winter has a slight effect on the Pr of leaves and the yield of fruit in strawberry, light intensity has a major effect on these parameters. In terms of chlorophyll fluorescence reaction as an index of stress response in plants, the difference in the response according to light intensity was evident; however, no significant difference was observed in the response to CO₂ fertilization at the concentrations applied in this experiment. Moreover, although the increase in efficiency (productivity and photosynthetic rate) with CO₂ fertilization was only moderate, it is considered that as CO₂ fertilization improves the accumulation of anthocyanins, it is helpful for maturation and redness of the fruit. Therefore, when comparing the light intensity and carbon dioxide as environmental factors affecting the productivity and fruit quality in strawberry, we propose that light intensity should take precedence over carbon dioxide fertilization for the improvement of strawberry plants grown in the low temperature conditions such as during the winter season.

Conflict of interest

The authors have no conflict of interest to report.

Footnotes

Acknowledgments

This study received financial assistance from the Technology Development Program for Agriculture and Forestry, Ministry for Food, Agriculture, Forestry and Fisheries, Republic of Korea (Project No. 315004-5).

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