Effect of Nutrient Management During the Nursery Period on the Growth, Tissue Nutrient Content, and Flowering Characteristics of Hydroponic Strawberry in 2022

Abstract

The objective of a strawberry nursery is to produce numerous runners and improve the quality of the runner plants, ensuring their uniformity and health. About 80% of the strawberry nurseries in Wanju-gun, Republic of Korea, are cultivated by the growers themselves, which results in variations in the cultivation techniques. Different methods are employed to reduce the nitrogen levels to promote floral bud initiation in the later nursery stage, and these methods vary between farms. This study was conducted on the ‘Sulhyang’ cultivar (Fragaria × ananassa Dutch. cv. Sulhyang) to measure the nutrient content in runner plants obtained from eight growers using various cultivation methods, supply EC levels, nutrient solution termination times, etc., and to correlate the nutrient levels with floral bud initiation during the nursery period. Four investigations were conducted at 15-day intervals from Julian day 214 (2 August) to Julian day 259 (16 September) in 2022, focusing on nutrient management during the nursery period, runner plant growth, petiole nitrate nitrogen content (NO₃-N), and soluble solid content (SSC). The NO₃-N content decreased, and the SSC/NO₃-N ratio increased near the transplanting period, as assessed using a rapid inorganic nutrient diagnostic device. The proportion of runner plants that had advanced to stage 3 or above in floral bud initiation was investigated using a stereomicroscope. As a result, differences in the percentage of floral bud initiation were confirmed based on the varying nutrient management among the farms. In this study, on Julian day 243 (31 August 2022), there was a strong negative correlation (r = −0.86, r² = 0.73) between the NO₃-N content in the runner plants and the percentage of floral bud initiation. These results emphasize the importance of nutrient management during the nursery period, especially for strawberry self-propagating growers, and demonstrate a strong correlation between nutrient content and floral bud initiation in strawberries.

1. Introduction

Strawberries propagate through vegetative growth, requiring many nursery processes of approximately 10 months [1]. This process involves planting a mother plant to produce daughter plants, then rooting these runners and grow them into full-sized plants, which are required for fruit harvest [2,3,4]. Initially, it is crucial to maximize the nutrient uptake and promote a rapid runner production from the mother plant [3]. After the runner plants are layered, inducing floral bud initiation through proper nutrient management becomes essential [5]. The three methods for inducing floral bud initiation involve the exposure to low temperature, short-day conditions, or low nitrogen levels [6,7]. While low temperature and short-day conditions often require high-cost facilities such as shade screens and air conditioning [8,9,10], low-nitrogen conditions can be achieved through defoliation and nutrient management, which makes it the preferred method among farmers [11].

In Korea, most farmers individually produce their own daughter plants rather than relying on specialized nurseries. This approach results in variations in root nutrient management and nitrogen uptake, leading to differences in flowering rates and speeds [12]. The nitrogen level of runner plants greatly impacts the timing of floral bud initiation in strawberries [13]. This timing influences decisions regarding cultivation methods and yield. While the exact mechanism of the nitrogen effect on floral bud initiation remains unclear, it may be related to plant hormones such as gibberellin and ABA [14], as well as to the plant sensitivity to floral induction factors such as temperature and photoperiod.

Particularly, the nutrient management during the late nursery period significantly impacts the flowering characteristics after transplantation. To better understand the relationship between the growth of strawberry runner plants, the content of inorganic elements during the late nursery phase, and the flowering characteristics, it is essential to collect and analyze nutrient management practices and nursery growth data from individual farmers. This analysis aims to provide insights and feedback to achieve the desired flowering stage based on late nursery growth data.

Analyzing the sap from crop leaves and petioles is a practical method for determining the concentrations of mineral nutrients [15]. However, the effectiveness of sap analysis and foliar nutrient assessment can vary across cultivars [16], environmental conditions [17], and the specific nutrient requirements of different strawberry cultivars [18]. These factors collectively underscore the complexity of nutrient management for strawberries [19].

The objectives of this study were the following: (1) to measure the nutrient content of strawberry runner plants collected from eight growers practicing different nutrient management techniques using sap diagnosis; (2) to determine the correlation between nitrate nitrogen content and flowering characteristics of strawberries. In conclusion, this research indicates that leaf sap measurement devices have potential as accessible, on-site tools for diagnosing the impact of different nutrient management approaches on strawberry growth and flowering at the farm level.

2. Materials and Methods

2.1. Plant Materials and Cultivation Condition

This experiment was conducted across eight self-propagating strawberry farms growing the plants in single-span plastic film greenhouses, located at Wanju-gun, Jeonbuk-do, Republic of Korea. The samples were collected from each farm and analyzed at each stage of propagation (Figure 1 and Figure 2). The average maximum outside temperature during the experiment period was 29.8 °C, with a minimum of 21.6 °C, a daily average temperature of 25.4 °C, and a daily integrated irradiance of 15.1 MJ (Figure 3). The strawberry cultivar used in this study was ‘Sulhyang’ (Fragaria × ananassa Duch.), which accounts for over 81% of the strawberry market share in Korea and thus being a representative cultivar in Korean strawberry production. The investigation was conducted at 15-day intervals from Julian day 214 (2 August) to Julian day 259 (16 September), 2022. A survey on the fertilization management during nursery cultivation was completed. The survey included information on the type of nutrient substrates used for mother plant cultivation, the start date of the nursery, the date of runner cutting, the runner plant substrates, the date of completion of the nursery period, and the timing of termination of the nutrient supply for each farm near the end of the nursery period (

Table 1. Cultivation data for the 8 farms located in Wanju-gun, Korea.

Farm	Mother Plants			Daughter Plants
	Substrate z	Planting Date	Cutting Date	Substrate	End of Layering Date	Cutting Date
No. 1	PM	5 March	10 July	PM	20 June	15 August
No. 2	PM	12 March	4 August	PM	22 June	10 August
No. 3	MIX	14 March	13 July	MIX	5 July	16 August
No. 4	PM	27 March	10 July	MIX	30 June	30 August
No. 5	MIX	23 April	20 August	MIX	18 August	20 August
No. 6	PM	30 March	30 July	MIX	30 June	5 August
No. 7	MIX	3 April	10 July	MIX	5 July	5 August
No. 8	MIX	14 March	17 July	MIX	30 June	20 July

^z PM: peatmoss; MIX: mixture of coir, peat moss, and pearlite.

). Each farm chose different nutrient solutions, supply EC levels, and nutrient supply termination dates (

Table 2. Rhizosphere data for the 8 farms located in Wanju-gun, Korea.

Farm	Mother Plants	Daughter Plants
	Supply EC Level (dS∙m−1)	Supply EC Level (dS∙m−1)	Nutrient Supply Termination Date
No. 1	1.0	1.0	13 August
No. 2	0.8	0.8	5 August
No. 3	0.8	0.8	5 August
No. 4	1.0	0.7	Not terminated
No. 5	1.1	Raw water	-
No. 6	1.0	0.8	30 July
No. 7	0.8	0.8	2 August
No. 8	K2O-MgO fertilizer 4 g∙L−1		-

).

2.2. Growth Characteristics of the Runner Plants

According to the vegetable research data standard manual of the Rural Development Administration [20], the growth characteristics of the runner plants were measured. The survey items were plant height, petiole length, leaf length, leaf width, crown diameter (measured as the maximum diameter of the crown), number of primary roots, and fresh and dry weight of the aboveground and underground parts. The fresh weight of the plant was measured immediately after sampling; then, the sampled runner plants were dried in a dryer (KNS-520S; Kyeongnong, Yeoncheon, Republic of Korea) at 75 °C for 72 h. To achieve a simple determination of the content of inorganic elements in the body of the plants, the leaf sap was extracted from the petioles using a handheld juicer, while an inorganic nutrient measuring device (LAQUAtwin, Ion Meter; Horiba Ltd., Kyoto, Japan) was used to measure the nitrate nitrogen (NO₃-N) and calcium (Ca) content. A portable digital refractometer (PAL-1; ATAGO CO. Ltd., Tokyo, Japan) was used to measure the soluble solid content (SSC) of the petiole juice and calculate the ratio of SSC to NO₃-N. Leaf greenness was measured using a chlorophyll measuring device (SPAD-502; KONICA MINOLTA, Tokyo, Japan), and the leaf area was measured using a foliar area measuring device (Li-3100C; LICOR, Inc., Lincoln, NE, USA).

2.3. Nutrient Content Analysis

To analyze the mineral content of the runner plants during the nursery period, plants from each farm were collected before planting. A total of four surveys were conducted for each farm on Julian days 214, 229, 243, 259 (2, 17, 31 August, and 16 September). Five runner plants were collected per farm at each measurement time, resulting in a total of 15 runner plants being collected and analyzed per farm.

The aboveground and underground parts of the plant were separated and dried in a dryer at 75 °C for 72 h and then ground and used as samples for nutrient analysis. A quantitative analysis of the plant nutrients was performed according to the soil and plant analysis method by the National Institute of Agricultural Science and Technology [21]. After the degraded sample was naturally cooled, it was placed in a 100 mL flask with filter paper (No. 6; Advantec, Tokyo, Japan). Total nitrogen (T-N) was analyzed using a nitrogen and total carbon analyzer (Primacs SNC; Skalar Analytical B.V., Breda, The Netherland), while K, Ca, Mg, and Na were analyzed using inductively coupled plasma spectroscopy (Integra XL; GBC Scientific Equipment Pty. Ltd., Melbourne, Australia). P was analyzed using the vanadate method and a spectrophotometer (AA3; Seal Analytical Ltd., Southampton, UK) to measure sample absorbance at a wavelength of 660 nm.

2.4. Detection of the Floral Bud Initiation Stage

The flowering characteristics after transplanting were observed based on the standards described in the Rural Development Administration’s Vegetable Research Data Standard Manual [20]. Starting on Julian day 248 (5 September), the budding and flowering status were assessed at 5–7-day intervals. The onset of budding was defined as the time when the flower bud had grown to approximately 1 cm in about 40% of the assessed plants, while full bloom was defined as the time when the first flower had fully opened in about 40% of the assessed plants. To analyze the flowering characteristics of the studied strawberries, five plants per farm were periodically observed with a microscope for floral bud differentiation. Floral bud differentiation was categorized into six stages: Stage 1. from an undifferentiated state to early differentiation; Stage 2. floral bud differentiation; Stage 3. flower cluster differentiation; Stage 4. sepal formation; Stage 5. carpel formation; Stage 6. stamen formation [22]. The floral bud differentiation ratio was determined by counting only the plants that had progressed beyond the early differentiation of Stage 1.

2.5. Statistical Analysis

The data were analyzed using a two-way analysis of variance (ANOVA). A statistical analysis was conducted using Sigmaplot (SigmaPlot 12.0; Systat Software, Inc., Chicago, IL, USA) and R (R 4.3.1; R Foundation). Multiple comparisons of the means were performed using Duncan’s multiple range test (p ≤ 0.05).

3. Results

3.1. Growth Characteristics of the Runner Plants in the Eight Farm Sites

When the runner plants from the eight farmers were compared throughout the nursery period, the average number of leaves was 3.7 in the early nursery stage, which increased to 5.2 in the transplantation stage (Figure 4 and

Table 3. Growth characteristics of the runner plants from the eight farms in Wanju-gun, Korea, with diverse nutrient management practices.

Farm (A)	Sampling Date (Julian Day) (B)	No. of Leaves	Plant Height (cm)	Petiole Length (cm)	Leaf Length (cm)	Leaf Width (cm)	Crown Diameter (mm)	No. of Primary Roots
No. 1	214	3.8 a z	36.7 a	23.1 a	7.6 a	6.1 a	8.8 a	11.8 b
	229	3.0 a	42.0 a	25.6 a	9.5 a	7.0 a	9.41 a	14.4 ab
	243	4.6 a	40.4 a	22.9 a	7.5 a	5.3 a	8.13 a	16.4 a
	259	5.4 a	37.3 a	21.8 a	8.1 a	5.8 a	7.95 a	17.4 a
No. 2	214	5.0 a	33.0 a	17.2 a	8.8 a	5.9 a	11.40 a	17.6 a
	229	3.8 a	31.3 a	18.4 a	9.1 a	6.7 a	9.63 a	15.6 a
	243	5.2 a	34.0 a	17.7 a	8.6 a	6.5 a	10.04 a	19.0 a
	259	5.6 a	34.9 a	13.2 a	7.2 a	5.1 a	8.81 a	13.6 a
No. 3	214	3.2 a	37.0 a	25.6 a	7.5 a	6.1 a	9.01 a	15.4 a
	229	2.8 a	36.1 a	21.9 a	8.1 a	6.2 a	9.48 a	17.4 a
	243	4.4 a	37.4 a	23.0 a	7.2 a	5.6 a	8.85 a	17.0 a
	259	4.8 a	37.7 a	19.4 a	7.4 a	5.7 a	7.9 a	16.8 a
No. 4	214	3.6 a	25.9 a	15.2 a	6.7 a	4.9 a	8.07 a	13.6 a
	229	4.4 a	26.3 a	15.7 a	7.1 a	5.2 a	8.76 a	17.0 a
	243	5.6 a	30.0 a	15.5 a	5.7 a	4.1 a	8.81 a	15.8 a
	259	5.4 a	29.7 a	15.6 a	5.9 a	4.3 a	7.72 a	15.8 a
No. 5	214	5.0 a	23.9 a	11.1 a	8.1 a	5.9 a	9.36 a	18.2 a
	229	5.4 a	23.2 a	10.0 a	7.3 a	5.4 ab	9.99 a	22.0 a
	243	5.2 a z	25.5 a	12.7 a	7.4 a	5.1 ab	9.94 a	26.8 a
	259	5.8 a	16.1 a	7.9 a	5.9 a	4.1 b	8.34 a	17.8 a
No. 6	214	3.0 b	24.1 a	14.6 a	6.9 a	5.3 a	9.09 a	16.4 a
	229	3.4 b	25.5 a	14.3 a	7.4 a	5.8 a	8.29 a	16.0 a
	243	4.2 ab	25.2 a	12.1 a	6.8 a	4.8 a	8.00 a	18.2 a
	259	5.2 a	36.3 a	17.8 a	6.7 a	5.0 a	8.85 a	13.4 a
No. 7	214	3.0 a	32.0 a	16.9 a	8.5 a	5.9 a	10.70 a	14.6 a
	229	2.6 a	35.7 a	21.2 a	10.3 a	7.9 a	8.83 a	18.2 a
	243	3.4 a	32.6 a	20.5 a	8.9 a	6.9 a	9.22 a	17.6 a
	259	4.2 a	31.0 a	15.5 a	6.9 a	5.4 a	8.44 a	18.6 a
No. 8	214	2.4 d	19.7 a	12.3 a	6.3 a	5.1 a	7.65 a	17.2 a
	229	3.2 c	21.3 a	10.7 a	6.9 a	4.8 a	7.96 a	18.0 a
	243	4.0 b	19.8 a	10.6 a	6.4 a	4.4 a	9.55 a	21.8 a
	259	4.8 a	19.5 a	7.4 a	5.5 a	4.2 a	7.92 a	18.6 a
Significance
Farm (A)		*** (0.0001)	*** (<0.0001)	*** (<0.0001)	** (0.0032)	** (0.0073)	NS (0.1092)	** (0.0034)
Sampling date (B)		*** (<0.0001)	NS (0.3190)	NS (0.0550)	** (0.0015)	*** (0.0003)	*** (0.0002)	NS (0.1228)
A × B		NS (0.5283)	NS (0.8820)	NS (0.7090)	NS (0.5318)	NS (0.8465)	NS (0.0569)	NS (0.9304)

^z Mean separation within columns by DMRT at a 5% level. Different lowercase letters indicate significant differences (p < 0.05) between treatments (n = 5). NS, **, *** non-significant or significant at p ≤ 0.01 and 0.001, respectively.

). This increase might be due to the adjustment of the defoliation extent during the nursery period. The leaf length and leaf width decreased by 15.4%, 13.1%, and 14.0% from the early to the late nursery periods, likely due to the shortening of the leaves caused by the continuous defoliation during the planting period and the increased shade caused by the higher number of leaves in the late nursery period. This led to a reduction in the length and width of the third leaf, which was under investigation.

The comparison of the aboveground portion and root fresh weights and dry weights showed that the average aboveground portion fresh weight for farm No. 8 increased by 24.3%, likely due to the increase in the aboveground biomass, by controlling the amount of defoliation late in the nursery season to increase the number of leaves (Figure 5). However, farm No. 8 reported the lowest average aboveground portion weight of 5.7 g/plant in the total survey. This is likely because the farmer applied less nutrients during the nursery period, which resulted in an average of 3.6 leaves due to extensive defoliation.

On the other hand, farm No. 1 supplied high EC for a prolonged period; farm No. 8 reported a 30% lower average aboveground portion weight and a 70% higher average root weight compared to farm No. 1. It is supposed that the higher amount of nutrients caused the aboveground part to become more robust and the root part to be reduced, resulting in more developed root growth for the farmer who supplied less nutrients.

3.2. Evaluation of the Nutrient Level Using Petiole Sap Analysis

Using strawberry petiole sap analysis, the average nitrate nitrogen content across the eight farms was 81% lower in the late nursery season compared to that in the early nursery season (Figure 6). Although the eight growers used nursery management strategies, their common practice of reducing or eliminating nutrient applications in the later stages of the nursery season may have contributed to the decrease in nitrate nitrogen content in the strawberry plants.

The laboratory analysis of total nitrogen showed similar results as those obtained by the simple analysis method. The average total nitrogen content for farm No. 8 decreased by 29%, from 2.42% on Julian day 214 (2 August) to 1.73% on Julian day 259 (16 September).

The average soluble solid content (SSC) for the eight farms was 5.3%, and the total carbon content was 56.84%. The SSC/NO₃-N value calculated by the simple method increased by 2.1 times in the late nursery period compared to that in the early nursery period, and the C/N ratio analyzed in the laboratory increased by 21%. This simple method can be effectively utilized in the field, where laboratory analysis is difficult, to easily monitor the nitrate nitrogen content.

The phosphorus, potassium, calcium, magnesium, and sodium levels decreased during the late nursery season (Figure 7). Similar to the decrease in plant nitrogen content, the reduction in plant inorganic element content was likely due to all eight farmers using a strategy of reducing the nutrient supply to induce flowering during the late nursery season. Specifically, the phosphorus, potassium, calcium, and magnesium levels were 41%, 22%, 25%, and 18% lower, respectively, on Julian day 259 (16 September) compared to Julian day 214 (2 August). Sodium was undetectable, indicating that strawberry plants nearing the transplanting date contain very little sodium.

3.3. Correlation Between Inflorescence and Nutrient Concentration in the Sap

The analysis revealed that a lower NO₃-N content in the runner plants corresponded to a higher percentage of flower differentiation. The correlation and regression analysis conducted between the nitrate nitrogen content measured on Julian day 243 (31 August) and the percentage of floral bud initiation (Figure 8) showed a highly negative correlation coefficient of −0.86 and a regression analysis determination coefficient of 0.73. However, the correlation and regression analyses conducted between the nitrate nitrogen content measured on Julian day 259 (16 September) and the percentage of flower differentiation showed a relatively lower negative correlation coefficient of −0.45 and a regression analysis determination coefficient of 0.20.

4. Discussion

The nitrate nitrogen content varied by nutrient management strategy among the farms (Figure 6). For instance, farm No. 1 provided the plants with a high EC of 1.0 dS∙m⁻¹, resulting in a nitrate nitrogen content of 6000 mg∙L⁻¹ on Julian day 214 (2 August), which was 22 times higher than that of farm No. 8, which had the lowest nitrate nitrogen content at the same measuring time. These results are consistent with previous studies on lettuce and tomato, where nutrient solution deprivation before harvest led to a significant reduction in the leaf nitrate levels [23,24]. Nitrate nitrogen (NO₃-N) is known to be an important regulator of flowering [12,25]. Analyzing the sap of crop leaves and petioles is convenient for determining mineral nutrient concentrations, which is essential for developing precise fertilization strategies to enhance plant nutrition management [15]. Through leaf and petiole sap analysis, the nutritional status of strawberries can be effectively monitored, enabling the optimization of fertilizer applications tailored to the specific requirements of a crop. The evaluation of nutrient concentration in tomato petiole sap demonstrated the feasibility of sap analysis in identifying the nutritional status of horticultural crops [26]. While the simple leaf sap method may not be as accurate, it is a quick and inexpensive way to check the nitrate nitrogen content in the field [15,27].

Nutrients are crucial for plant growth, as all physiological processes depend on them [23]. A nutrient strategy for promoting strawberry flowering could involve reducing nitrogen and increasing phosphorus and potassium in the nutrient solution [11]. This approach would balance the plant nutrient uptake while promoting flowering. Since the eight growers did not change the nutrient composition of the nutrient solution but rather stopped supplying nutrients altogether, the results showed a reduction in the levels of all elements. It is expected that the nutrient uptake characteristics would be different if the nutrient composition were adjusted. This hypothesis needs to be confirmed in future studies.

The correlation and regression analyses (Figure 8) showed a highly negative correlation coefficient of -0.86 and a regression analysis determination coefficient of 0.73, indicating a strong predictive capability of the percentage of flower differentiation based on the nitrate nitrogen content at the end of August. However, the analyses of the nitrate nitrogen content measured on Julian day 259 (16 September) and the percentage of flower differentiation indicated a lower predictive capability of the percentage of flower differentiation based on the NO₃-N content in mid-September.

These research findings can be utilized as valuable data to understand the growth characteristics of strawberries based on different nutrient management strategies in individual farms and to understand the relationship between tissue inorganic element content and flower differentiation characteristics. Further research would be needed to analyze the nutrient absorption characteristics in other cultivars.

5. Conclusions

This study investigated runner plant nutrient content using the data collected from eight strawberry farms practicing different nutrient management strategies and aimed to identify the correlation between tissue nutrient content and flower differentiation characteristics. The strawberry cultivar used was ‘Sulhyang’, and the study targeted strawberry self-nursery farms in Wanju-gun, Republic of Korea. Four surveys were conducted at 15-day intervals from Julian day 214 (2 August) to Julian day 259 (16 September) in 2022, We found that the NO₃-N content decreased, while the SSC/NO₃-N ratio increased, as the planting time approached. The correlation analysis and regression analysis between the runner plant NO₃-N content on Julian day 243 (31 August) and the percentage of floral bud initiation revealed a highly negative correlation coefficient of −0.80 and a regression analysis determination coefficient of 0.73, indicating a strong explanatory power. The findings suggest that leaf sap measurement devices can be used as simple on-site diagnostic tools to evaluate the growth and flowering characteristics of strawberries cultured using different nutrient supply methods in individual farms.