Identification of Appropriate Light Intensity and Daytime Temperature for Cucumber (Cucumis sativus L.) Seedlings in a Plant Factory with Artificial Lighting for Use as Grafting Material

Abstract

In this study, an experiment was conducted to suggest optimal daytime temperature and light intensity for cucumber scion and rootstock production in a plant factory with artificial light. Plant growth of cucumber scions and rootstocks at a day/night temperature of 26/18 °C and photosynthetic photon fluxes of 50, 120, 190, 260, 330, and 400 μmol·m⁻²·s⁻¹ was investigated. Plant growth under daytime temperatures of 25/18, 26/18, 27/18, and 28/18 °C at a photosynthetic photon flux of 260 μmol·m⁻²·s⁻¹ was investigated. As the photosynthetic photon flux increased, hypocotyl length was shortened in cucumber scions and rootstocks, but Dickson quality index, compactness, and Seedling Health Index were improved. As the daytime temperature increased, the hypocotyl length of cucumber scions increased, but the quality of seedlings decreased. The root growth of scions decreased as the daytime temperature increased. As for the correlation between the major growth indicators, Dickson quality index, compactness, and Seedling Health Index showed a high correlation of more than 0.8 in stem diameter, leaf, and root weight. Therefore, it is judged that it is most efficient to maintain the photosynthetic photon flux at 260 μmol·m⁻²·s⁻¹ throughout the year and adjust the daytime temperature to 25 to 28 °C according to the season for farms and commercial nurseries that produce cucumber seedlings by installing plant factories with artificial light.

1. Introduction

Several attempts at stable vegetable production have recently been made due to the escalation of climatic instability, such as very high temperatures, extreme cold temperatures, droughts, and heavy rains. One of them is utilizing high-quality seedlings rather than direct sowing to cultivate vegetables, which is gaining popularity. Research on the advantages of raising seedlings has already demonstrated advantages such as productivity improvement, harvest period extension, insect resistance, high salt tolerance, and moisture resistance through various studies [1].

In particular, interest in grafted seedlings, which are advantageous for stable initial graft-take and maintaining sufficient strong plant vigor, is increasing even in unstable climates. It is crucial to generate scions and rootstocks of uniform size with corresponding cutting and bonding surfaces to produce high-quality grafted seedlings [2]. That is why commercial nurseries grow seedlings in greenhouses equipped with various technologies to overcome unstable external climates. However, in recent years, due to excessive climate change, it is difficult to produce uniform scions and rootstocks throughout the year even in facilities such as glass and plastic film greenhouses [3].

To solve this problem, introducing a plant factory with artificial lighting (PFAL) is being attempted [4]. The PFAL is not affected by the external environment and can produce stable scions and rootstocks all year round because it provides the optimal environment for plant growth through the control of environmental factors. In addition, it is introduced and continues to spread in many commercial nurseries due to the effects of constant energy use throughout the year [5], the reduction in farmer manpower and resources [6], quality improvement, and the shortening production period [7,8].

However, since PFAL is in the initial stage of introduction, various studies are being conducted, but commercially applicable technology is still insufficient [4]. Since PFAL completely controls various environmental factors, the technology related to seedling cultivation such as scions and rootstocks is complicated. Therefore, to use it for scion and rootstock production, it is necessary to study the response of plants to environmental factors to confirm the appropriate growth environment. The most important environmental factors in PFAL are temperature and photosynthetic photon flux (PPF) [9]. Temperature has a significant impact on transpiration, which, in turn, influences the shape of the plant and the effectiveness of its photosynthetic process [10,11,12]. In addition, PPF influences carbohydrate production as a major factor in photosynthesis, determining the growth rate and vitality of the seedlings [13,14,15]. Therefore, to produce scions and rootstocks in PFAL, it is necessary to confirm how the plant responds to changes in major environmental factors.

Various studies on the effects of many different environmental conditions in PFAL on the growth of seedlings have been conducted: the relationship between cucumber and tomato evapotranspiration according to scion and rootstock light intensity [16]; effects of seedling period and light intensity on the growth of cucumber grafted seedlings from seedling to planting [17]; growth response of cucumber varieties according to various light environments [18]; conditions for efficient energy utilization through the effects of temperature, photoperiod, and light intensity on seedling quality during cucumber scion and rootstock raising [19]; comparison of the effects of changes in light intensity and temperature during the production of watermelon grafted seedlings [20]; effects of photoperiod, light intensity, and temperature on seedling quality and flower truss of tomato and pepper seedlings [9]; and the effect of light quality and light intensity on tomato seedling period and stress [21].

However, since most of the studies conducted so far are academic studies to the extent that they review the response of growth according to the environment, practical research results that can be used in actual farms and commercial nurseries are insufficient. Therefore, this study was conducted to present the optimal daytime temperature and optimal light intensity for PFAL cucumber scions and rootstocks that are practically applicable within the optimum growth temperature [22]. It is expected that farms and commercial nurseries will apply as needed by comparing the effects of various light and temperature conditions on cucumber scion and rootstock growth and that it will be used as a practical standard for PFAL through correlation comparison between non-destructive and destructive growth indicators.

2. Materials and Methods

2.1. Plant Materials and Growth Conditions

‘Hangangmat’ cucumber scions (Cucumis sativus L.; Farm Hannong Co., Ltd., Seoul, Republic of Korea) and ‘Heukjong’ rootstocks (Cucurbita ficifolia Bouché.; Sakata Korea Co., Ltd., Seoul, Republic of Korea) were used. For sowing, horticultural soil (EC 0.47 dS·m⁻¹, pH 6.18, Pindstrup, Denmark) was used to fill a 162-hole tray (W 280 × L 540 × H 45 mm, Bumnong, Co., Ltd., Jeongeup, Republic of Korea) to receive the cucumber scion seeds, and a 128-hole tray (W 280 × L 540 × H 48 mm, Bumnong, Co., Ltd., Jeongeup, Korea) for the cucumber rootstock seeds, and then the seeds were sown. The seeded trays were sufficiently hydrated through head irrigation and then covered with vermiculite for moisture retention. Germination was carried out for 48 h in a germination room under dark conditions, maintained at a temperature of 25~28 °C and relative humidity of 90% or more. To match the growth for grafting in the same manner as in the conventional commercial nurseries, fast-growing rootstocks were sown two days later than the scions. After germination, cucumber scions and rootstocks were grown in PFAL for 8 and 10 days (Figure 1). Therefore, the total number of cultivation days is 10 days after sowing (DAS) for scions and 12 DAS for rootstocks. The light cycle was adjusted to 16/8 h (day/night) and the relative humidity to 60/70% (day/night). A white light-emitting diode (LED) was used as the light source. During the cultivation period, Technigro 13-2-13 Plus fertilizer (Sun-Gro Horticulture, Bellevue, WA, USA) nutrient solution was used, and, once every 3 days, with pH 5.5 and EC (electrical conductivity) 1.4 dS·m⁻¹ was used for bottom irrigation. The composition of the nutrients was 13 mg·L⁻¹ N, 2 mg·L⁻¹ P₂O₅, 13 mg·L⁻¹ K₂O, 6 mg·L⁻¹ Ca, 3 mg·L⁻¹ Mg, 0.007 mg·L⁻¹ B, 0.003 mg·L⁻¹ Cu, 0.05 mg·L⁻¹ Fe, 0.03 mg·L⁻¹ Mn, 0.001 mg·L⁻¹ Mo, and 0.0026 mg·L⁻¹ Zn.

2.2. Plant Factory with Artificial Lighting (PFAL) Specifications

The PFAL was located in the Hoban agriculture corporation plastic greenhouse (37°55′29′′ N, 127°47′04′′ E, 85 m above sea level). The outside was composed of urethane foam insulation (70 mm) to protect it from the environment. The interior consisted of an air conditioning system, a seedling module, a nutrient solution supply system, and an environment control program (GMP Co., Ltd., Hwaseong, Republic of Korea). A total of six seedling modules were placed and five Styrofoam beds were installed in each seedling module. Each seedling module had five 28 W white LEDs placed on the top wall of the seedbed. The design allowed control of the amount of light intensity, photoperiod, temperature, humidity, irrigation, etc., with the control program. An air conditioning system was used for temperature control and internal air circulation by installing a unit cooler on the central upper wall inside the PFAL.

2.3. Light Treatments

Photosynthetic photon flux (PPF) 50, 120, 190, 260, 330, and 400 μmol·m⁻²·s⁻¹ light intensities were classified to confirm the growth response of cucumber scions and rootstocks according to the light environment. The photoperiod was 16/8 hours, and each treatment group’s daily light integral (DLI) was established at 2.88, 6.91, 10.94, 14.98, 19.01, and 23.04 mol·m⁻²·d⁻¹. Day and night temperature was maintained at 26/18 °C and humidity at 60/70%. As growth progressed, to compare and investigate the growth rate of the fresh weight of the above-ground part, it was investigated every 2 days from the 4 DAS.

2.4. Temperature Treatments

Since the growth rate is fast when the daytime temperature is high, there is an advantage in increasing production in commercial nurseries, but there is also a risk of overgrowth. Therefore, the growth response was confirmed at 25, 26, 27, and 28, which are the high-temperature zones among the optimum temperatures for cucumber growth (Figure 2). The night temperature was maintained at 18 °C and luminous intensity at 260, with a photoperiod of 16/8h and a humidity of 60/70%. To compare and investigate the growth rate at different temperatures, the first growth survey for the scions was conducted on the 7 DAS and the second growth irradiation on the 12 DAS. Rootstocks were examined for the first growth survey on the 5 DAS and the second irradiation on the 10 DAS.

2.5. Growth Investigation

For the growth survey of scions and rootstocks, the fresh and dry weights of leaves, stems, and roots, as well as hypocotyl length, stem diameter, leaf chlorophyll (SPAD) (SPAD-502 Plus, Minolta Inc., Tokyo, Japan) and leaf area (LI-3100, LI-COR Inc., Lincoln, NE, USA) were measured. The dry weight was measured by drying samples in a hot air dryer (convection oven, SANYO Inc., Osaka, Japan) at 80 °C for 72 h after measuring the fresh weight. The root zone growth was measured using WinRHIZO (WinRHIZO PRO 09, REGENT Instruments Inc., Quebec, QC, Canada) to measure the root length and average diameter, as well as the surface area of the root. In the case of the root average diameter, the diameter was classified into four stages at 0.5 mm intervals and analyzed. For comparison of the relative growth rate of shoot fresh weight (RGR_W), hypocotyl length (RGR_H) and leaf area (RGR_L) were calculated using the following formulas.

Relative growth rate of shoot fresh weight [RGR_W] (g∙g∙day⁻¹) = $\frac{ln{H}_1- ln{H}_0}{t_1-t_0}$

• H₀ and H₁: initial and final shoot fresh weight
• t₁ − t₀: growing period (days)

Relative growth rate of hypocotyl length [RGR_H] (cm∙cm∙day⁻¹) = $\frac{ln{H}_1- ln{H}_0}{t_1-t_0}$

• H₀ and H₁: initial and final hypocotyl length
• t₁ − t₀: growing period (days)

Relative growth rate of leaf area [RGR_L] (cm²∙cm²∙day⁻¹) = $\frac{ln{H}_1- ln{H}_0}{t_1-t_0}$

• H₀ and H₁: initial and final leaf area
• t₁ − t₀: growing period (days)

In addition, to compare seedling quality according to the environment in the plant factory, the Dickson quality index (DQI) [23], compactness (CP), and Seedling Health Index (SHI) [24] were obtained by the following formulas.

$$\begin{array}{c}\mathrm{Dickson} \mathrm{quality} \mathrm{index} [\mathrm{DQI}]=\frac{Total dry weight \left(mg\right)}{\frac{Height \left(cm\right)}{Stem diameter \left(mm\right) }+\frac{Top dry weight \left(mg\right)}{Root dry weight \left(mg\right)}}\\ \mathrm{Compactness} [\mathrm{CP}] (\mathrm{mg}·{\mathrm{cm}}^{-1})=\frac{Shoot dry weight \left(mg\right)}{Plant height \left(cm\right)}\\ \mathrm{Seedling} \mathrm{Health} \mathrm{Index} \left[\mathrm{SHI}\right] \left(\mathrm{g}\right)=\\ \left(\frac{Stem diameter \left(mm\right)}{Plant height \left(cm\right)}\times \frac{Root dry weight \left(mg\right)}{Shoot dry weight \left(mg\right)}\right)\times Total dry weight \left(mg\right)\end{array}$$

2.6. Statistical Analysis

Duncan’s Multiple Range Test (multiple test) was performed using the SPSS version 26 (IBM, New York, USA) program for cucumber scion and rootstock growth data with 3 repetitions of randomized block design for 5 plants in the tray, p < 0.05 level In, the significance between each treatment was tested. In addition, the significant differences between the various treatments were compared, and the expressed significant differences were analyzed by two-way analysis of variance (two-way ANOVA), and the expressed significant differences were shown as p < 0.05, 0.01, and 0.001. A boxplot for comparing growth rates and a heatmap for analyzing the correlation between growth indicators were used in the Python Seaborn library. This has the advantage of easily and quickly grasping the overall characteristics of the object and is mainly used to extract data using correlation coefficients that indicate the correlation between different indicators [25].

3. Results and Discussion

3.1. Cucumber Scions and Rootstocks in a PFAL: The Effect of Light Intensity Conditions

The light intensity in PFAL divides into six levels, and the morphological characteristics of cucumber scions and rootstocks were comparatively investigated (Figure 3). As a result of confirming the above-ground part growth according to the light intensity (

Table 1. Growth of cucumber scions and rootstocks as affected by the different light intensity conditions at 12 DAS and 10 DAS in a PFAL.

Seedling	PPF (μmol·m−2·s−1)	Hypocotyl Length (cm)	Stem Diameter (mm)	Leaf Chlorophyll (SPAD)	Leaf Area (cm2)	Leaf Length (cm)	Leaf Width (cm)	DQI z	CP (mg·cm−1)	SHI (g)
Scion	50	10.6 ± 1.1 a y	1.2 ± 0.2 c	37.6 ± 2.6 c	15.3 ± 2.5 c	2.4 ± 0.4 c	2.0 ± 0.3 ab	2.3 ± 0.6 e	3.9 ± 0.6 d	9.6 ± 2.5 e
	120	9.2 ± 1.1 b	1.5 ± 0.1 b	37.9 ± 1.8 c	17.8 ± 2.1 b	2.7 ± 0.4 bc	2.2 ± 0.2 a	3.4 ± 0.7 d	5.3 ± 1.0 c	14.1 ± 2.8 d
	190	6.9 ± 0.7 cd	1.5 ± 0.1 b	44.5 ± 2.6 b	18.4 ± 1.5 b	2.7 ± 0.3 abc	2.3 ± 0.2 a	5.2 ± 0.9 c	8.4 ± 0.8 b	24.6 ± 3.4 c
	260	7.7 ± 1.6 c	1.5 ± 0.2 b	45.7 ± 2.5 b	20.6 ± 2.0 a	3.2 ± 0.4 a	2.2 ± 0.2 a	4.9 ± 0.7 c	8.5 ± 1.4 b	20.9 ± 3.5 c
	330	6.0 ± 1.1 d	1.7 ± 0.1 a	50.1 ± 2.8 a	17.2 ± 1.0 b	2.9 ± 0.3 ab	1.9 ± 0.3 b	7.2 ± 1.4 b	12.2 ± 1.9 a	32.3 ± 6.4 b
	400	6.4 ± 0.9 d	1.8 ± 0.1 a	48.6 ± 3.3 a	17.3 ± 2.3 b	3.1 ± 0.5 ab	1.8 ± 0.1 b	8.8 ± 1.5 a	12.2 ± 2.1 a	36.5 ± 6.1 a
Rootstock	50	11.9 ± 1.2 a	2.7 ± 0.3 a	59.7 ± 3.2 a	43.9 ± 5.3 c	4.8 ± 0.8 c	3.1 ± 0.3 ab	13.3 ± 2.3 e	16.1 ± 3.1 d	64.3 ± 14.5 c
	120	9.5 ± 1.3 b	2.2 ± 0.4 b	58.5 ± 5.0 a	51.8 ± 7.7 ab	5.4 ± 0.6 b	3.2 ± 0.3 a	15.6 ± 3.4 de	22.0 ± 4.5 c	74.5 ± 9.7 c
	190	7.7 ± 1.1 c	2.6 ± 0.2 a	52.1 ± 4.8 b	54.7 ± 7.5 a	5.9 ± 0.7 ab	3.1 ± 0.3 ab	19.6 ± 3.7 bc	27.8 ± 4.8 b	100.1 ± 16.2 b
	260	7.7 ± 1.2 c	2.6 ± 0.2 a	56.6 ± 7.3 ab	48.3 ± 2.4 bc	5.3 ± 0.3 bc	3.0 ± 0.1 ab	17.5 ± 2.5 cd	30.3 ± 4.5 b	100.7 ± 12.6 b
	330	5.9 ± 1.0 d	2.7 ± 0.2 a	59.3 ± 6.0 a	50.1 ± 4.0 ab	5.5 ± 0.4 b	2.9 ± 1.8 ab	23.6 ± 2.8 a	37.9 ± 6.6 a	131.9 ± 20.1 a
	400	6.8 ± 1.0 cd	2.7 ± 0.2 a	57.1 ± 4.2 a	54.0 ± 4.2 a	6.3 ± 0.6 a	2.8 ± 1.7 b	21.3 ± 2.5 ab	35.6 ± 5.3 a	122.1 ± 11.1 a
Significance x
Seedling (A)		*	***	***	***	***	***	***	***	***
Light intensity (B)		***	***	***	***	***	**	***	***	***
Interaction (A × B)		NS	***	***	**	**	NS	**	***	***

^z DQI: Dickson quality index; CP: compactness, SHI: Seedling Health Index. ^y Means for 3 replicates with 5 samples within each column followed by the same letters are not significantly different according to Duncan’s multiple range test at p < 0.05. ^x NS: non-significant, *, **, and *** of significance at p < 0.05, 0.01, and 0.001.

), the hypocotyl length of 400 μmol·m⁻²·s⁻¹ compared to 50 μmol·m⁻²·s⁻¹ was 60 and 57% for scions and rootstocks, respectively, and showed a tendency to shorten as the light intensity increased. Hwang et al. [9] showed that the hypocotyl length of tomatoes and red peppers decreased when the light intensity was over 200 μmol·m⁻²·s⁻¹, and the findings of Park et al. [16] were consistent with the result that the hypocotyl length decreased by 1.3~4.2 cm as the light intensity increased from 50 to 150 μmol·m⁻²·s⁻¹ in cucumber scions and rootstocks. In the case of seedlings, shorter hypocotyl length is better due to the seedling process and management during planting [26]. However, in the case of grafted seedlings, the longer the hypocotyl length, the higher the grafting success rate and grafting union, so there is an advantage in reducing the occurrence of adventitious roots of scions after planting [27]. As a result of examining the stem diameter of the scions, 50 μmol·m⁻²·s⁻¹ was 1.2 mm and 400 μmol·m⁻²·s⁻¹ was 1.8 mm, and the stem diameter showed a tendency 0.1 mm thicker as the light intensity increased by 58.3 μmol·m⁻²·s⁻¹. This was consistent with the results of Garcia and Lopez [28] who showed that the stem diameter of cucumber and pepper seedlings increased as DLI increased.

The SPAD of the scions was proportional to the light intensity, such as the stem diameter, but the rootstocks were not affected by the light intensity. As a result of the leaf area survey, the leaf area of the rootstocks increased by about 10.1 cm² (23%) from 43.9 to 54 cm² as the light intensity increased from 50 to 400 μmol·m⁻²·s⁻¹. Interestingly, the leaf area of the scions at 50 and 400 μmol·m⁻²·s⁻¹ is 15.3 cm² and 17.3 cm², respectively, and 260 μmol·m⁻²·s⁻¹ is 20.6 cm². This means that as the light intensity increases by 39.8 μmol·m⁻²·s⁻¹, it widens by about 1 cm², and when it exceeds 260 μmol·m⁻²·s⁻¹, it becomes smaller by about 1 cm² as it increases by 42.4 μmol·m⁻²·s⁻¹. This is similar to the results of Hwang et al. [9] who demonstrated that the leaf area of tomatoes and red peppers was the widest in the 250 μmol·m⁻²·s⁻¹ when the light intensity increased from 100 to 300 μmol·m⁻²·s⁻¹. The shape of the leaves showed a tendency to become thinner and longer as the light intensity increased in both scions and rootstocks, which was similar to the leaf area.

Since the quality of seedlings should be judged by comprehensively considering various growth factors, indicators such as DQI, CP, and SHI were used. DQI, CP, and SHI of scions all showed a tendency to improve as the light intensity increased, but rather decreased when the light intensity exceeded 330 μmol·m⁻²·s⁻¹ in rootstocks. This is similar to the results of Kwack and An [20] in which the compactness of watermelon rootstocks at 26/18 °C decreased as the light intensity increased from 200 to 250 μmol·m⁻²·s⁻¹.

Table 2. Fresh weight and dry weight of leaves, stems, and roots of cucumber scions and rootstocks under different light conditions at 12 DAS and 10 DAS in a PFAL.

Seedling	PPF (μmol·m−2·s−1)	Fresh Weight (g)			Dry Weight (g)
		Leaf	Stem	Root	Leaf	Stem	Root
Scion	50	0.42 ± 0.06 d z	0.43 ± 0.07 a	0.11 ± 0.02 b	0.025 ± 0.005 c	0.011 ± 0.004 b	0.004 ± 0.002 d
	120	0.49 ± 0.05 c	0.36 ± 0.12 abc	0.13 ± 0.03 b	0.029 ± 0.006 c	0.013 ± 0.004 ab	0.006 ± 0.001 d
	190	0.57 ± 0.07 b	0.34 ± 0.05 bc	0.22 ± 0.03 a	0.039 ± 0.006 b	0.011 ± 0.003 b	0.008 ± 0.002 c
	260	0.67 ± 0.07 a	0.40 ± 0.11 ab	0.22 ± 0.05 a	0.044 ± 0.007 ab	0.013 ± 0.003 ab	0.007 ± 0.001 c
	330	0.63 ± 0.06 a	0.31 ± 0.06 c	0.23 ± 0.04 a	0.049 ± 0.005 a	0.013 ± 0.003 ab	0.010 ± 0.002 b
	400	0.65 ± 0.05 a	0.34 ± 0.05 bc	0.25 ± 0.05 a	0.049 ± 0.010 a	0.015 ± 0.002 a	0.013 ± 0.002 a
Rootstock	50	2.31 ± 0.49 b	1.32 ± 0.19 a	0.58 ± 0.07 a	0.128 ± 0.026 c	0.043 ± 0.003 a	0.018 ± 0.003 c
	120	2.57 ± 0.24 ab	1.12 ± 0.13 b	0.57 ± 0.12 a	0.143 ± 0.022 bc	0.041 ± 0.006 ab	0.021 ± 0.004 bc
	190	2.68 ± 0.39 a	0.95 ± 0.23 c	0.62 ± 0.17 a	0.146 ± 0.024 bc	0.040 ± 0.011 ab	0.024 ± 0.005 ab
	260	2.45 ± 0.18 ab	0.89 ± 0.17 cd	0.56 ± 0.11 a	0.168 ± 0.008 a	0.039 ± 0.006 ab	0.021 ± 0.004 bc
	330	2.57 ± 0.30 ab	0.68 ± 0.16 e	0.67 ± 0.01 a	0.158 ± 0.020 ab	0.035 ± 0.006 b	0.027 ± 0.004 a
	400	2.58 ± 0.26 ab	0.77 ± 0.13 de	0.60 ± 0.09 a	0.176 ± 0.013 a	0.036 ± 0.005 b	0.025 ± 0.004 a
Significance y
Seedling (A)		***	***	***	***	***	***
Light intensity (B)		**	***	**	***	NS	***
Interaction (A × B)		NS	***	NS	NS	*	NS

^z Means for 3 replicates with 5 samples within each column followed by the same letters are not significantly different according to Duncan’s multiple range test at p < 0.05. ^y NS: non−significant, *, **, and *** of significance at p < 0.05, 0.01, and 0.001.

shows the results of comparing fresh weight and dry weight for each part of cucumber scions and rootstocks according to light intensity. The leaf fresh weight of the scions was 0.65 g on average at 260~400 μmol·m⁻²·s⁻¹, about 0.23 g (55%) heavier than that at 50 μmol·m⁻²·s⁻¹. The leaf fresh weight of rootstocks was the heaviest at 190 μmol·m⁻²·s⁻¹ at 2.68 g and increased by 0.1 g as the light intensity increased to 84 μmol·m⁻²·s⁻¹. However, when the light intensity exceeded 190 μmol·m⁻²·s⁻¹, the fresh weight tended to decrease, which was similar to the leaf dry weight. Stem fresh weight was 0.43 g for scions and 1.32 g for rootstocks, the heaviest at 50 μmol·m⁻²·s⁻¹. This is thought to be due to the increase in hypocotyl length as the light intensity decreased, but rather, 400 μmol·m⁻²·s⁻¹ was about 36% heavier than 50 μmol·m⁻²·s⁻¹ among the stem dry weights of the scions. This was consistent with the results of An et al. [19] in which the hypocotyl length shortened and dry weight increased as PPF increased during cucumber scion and rootstock raising in PFAL. In the case of scions, as the light intensity increased from 120 to 190 μmol·m⁻²·s⁻¹, the root fresh weight increased by about 70% from 0.13 to 0.22 g, and the root fresh weight increased as the light intensity increased in rootstocks. There was no statistically significant difference in the root fresh weight of rootstocks, but the root dry weight showed significance according to the light intensity. As the light intensity increased by 31.1 μmol·m⁻²·s⁻¹ from 0.018 g of 50 μmol·m⁻²·s⁻¹ to 0.027 g of 300 μmol·m⁻²·s⁻¹, the dry weight of the roots increased by 0.001 g.

To compare the growth rates of scions and rootstocks according to environmental conditions, the relative growth rate was used to plot the RGR_W of scions and rootstocks at each light intensity as a boxplot (Figure 4). In the case of the scions, the RGR_W increased up to 260 μmol·m⁻²·s⁻¹ but decreased when it exceeded 260 μmol·m⁻²·s⁻¹. The group sizes of 260 and 330 μmol·m⁻²·s⁻¹ were similar, but the area of 75% (3rd quartile) or more of 260 μmol·m⁻²·s⁻¹ was wide, and the median value was higher than that of 330 μmol·m⁻²·s⁻¹. This is thought to be caused by the effect of the growth rate slowing down due to the sufficient light intensity of 330 μmol·m⁻²·s⁻¹ [20]. RGR_W of rootstocks was the highest at about 0.36 g·g·day⁻¹ at 50 and 120 μmol·m⁻²·s⁻¹, and RGR_W tended to decrease as the light intensity increased. In addition, the range of the maximum and minimum values was about 44% larger than that of other treatments, which means that there was a large variation in growth between irradiated individuals. Reducing the PPF can save the light source’s electrical energy, which accounts for 80% of the total energy required for production in PFAL [29], and reduce the initial installation cost of the LEDs [30]. Rootstocks were particularly prone to growth imbalance and succulent growth when the light intensity was poor. Based on these results, it is determined that the light intensity required to cultivate ‘Heukjong’ cucumber rootstocks in PFAL should be at least 190 μmol·m⁻²·s⁻¹ or a DLI of 10.94 mol·m⁻²·d⁻¹ or higher.

3.2. Cucumber Scions and Rootstocks in a PFAL: The Effect of Daytime Temperature Conditions

The temperature in PFAL was divided into four levels, and the morphological characteristics of cucumber scions and rootstocks were comparatively investigated (Figure 5). When checking the above-ground growth of cucumber scions and rootstocks (

Table 3. Growth of cucumber scions and rootstocks as affected by the different daytime temperature conditions at 12 DAS and 10 DAS in a PFAL.

Seedling	Day/Night Temperature (°C)	Hypocotyl Length (cm)	Stem Diameter (mm)	Leaf Chlorophyll (SPAD)	Leaf Area (cm2)	Leaf Length (cm)	Leaf Width (cm)	DQI z	CP (mg·cm−1)	SHI (g)
Scion	25/18	6.5 ± 0.8 c y	1.6 ± 0.3 a	55.3 ± 4.5 a	20.0 ± 1.6 b	3.0 ± 0.4 a	1.8 ± 0.3 a	5.0 ± 1.4 ab	8.1 ± 1.2 a	21.7 ± 5.3 ab
	26/18	7.9 ± 1.3 b	1.6 ± 0.4 a	48.1 ± 3.1 c	21.9 ± 1.4 a	3.2 ± 0.2 a	1.9 ± 0.2 a	5.3 ± 1.5 a	8.1 ± 1.8 a	23.6 ± 5.7 a
	27/18	8.2 ± 1.8 ab	1.6 ± 0.3 a	50.8 ± 4.8 bc	21.1 ± 1.4 ab	3.0 ± 0.3 a	2.0 ± 0.2 a	3.7 ± 2.4 ab	6.5 ± 1.9 b	16.6 ± 8.7 b
	28/18	9.3 ± 1.3 a	1.7 ± 0.2 a	52.3 ± 4.8 ab	21.0 ± 1.8 ab	3.0 ± 0.3a	1.9 ± 0.3a	3.4 ± 2.1 b	5.2 ± 1.1 b	15.7 ± 8.3 b
Rootstock	25/18	8.1 ± 1.8 a	3.6 ± 0.5 a	69.2 ± 2.8 a	33.7 ± 4.5 a	5.9 ± 1.8 a	2.9 ± 0.5 a	22.3 ± 5.5 a	23.9 ± 5.9 a	116.8 ± 30.6 ab
	26/18	7.2 ± 1.1 a	3.8 ± 0.4 a	64.2 ± 4.0 b	34.5 ± 3.1 a	5.8 ± 1.1 a	2.9 ± 0.2 a	21.5 ± 2.4 a	27.4 ± 5.3 a	131.3 ± 23.8 a
	27/18	8.1 ± 1.2 a	3.7 ± 0.3 a	64.2 ± 4.5 b	33.8 ± 6.6 a	6.2 ± 1.4 a	2.7 ± 0.4 a	19.2 ± 5.0 a	25.5 ± 8.8 a	120.1 ± 37.5 ab
	28/18	8.4 ± 1.2 a	3.3 ± 0.4 a	68.6 ± 6.5 ab	25.5 ± 4.7 b	5.1 ± 1.8 a	2.5 ± 0.4 a	19.1 ± 3.8 a	21.8 ± 5.3 a	99.6 ± 24.8 b
Significance x
Seedling (A)		NS	***	***	***	***	***	***	***	***
Day/Night temperature (B)		**	NS	***	***	*	NS	*	*	*
Interaction (A × B)		*	NS	NS	**	*	NS	NS	NS	NS

^z DQI: Dickson quality index; CP: compactness, SHI: Seedling Health Index. ^y Means for 3 replicates with 5 samples within each column followed by the same letters are not significantly different according to Duncan’s multiple range test at p < 0.05. ^x NS: non-significant, *, **, and *** of significance at p < 0.05, 0.01, and 0.001.

), the hypocotyl length of scions was 6.5 and 9.3 cm when the daytime temperature increased from 25 to 28 °C. When the daytime temperature increased by 1 °C, the hypocotyl length increased by about 0.9 cm, and the hypocotyl length tended to lengthen as the temperature increased. This was consistent with Hwang et al.’s [9] results of lengthening the hypocotyl length as the temperature increased to 27 °C in red pepper seedlings. SPAD and leaf area, which are indicators of leaf growth characteristics, did not show a constant trend according to the change in daytime temperature for both scions and rootstocks. Interestingly, however, in scions, the SPAD at 26 °C was the lowest at 48.1 and the leaf area was the widest at 21.9 cm². These characteristics were consistent with the best result of 26 °C in DQI, CP, and SHI, which are indicators for evaluating the quality of seedlings, with 5.3, 8.1 mg·cm⁻², and 23.6 g, respectively. In the rootstocks, there was no significance for DQI and CP, but similar results were shown for SHI. In a confined space, such as raising seedlings in a tray, the leaf area increases to a degree favorable for photosynthesis and does not widen beyond that [31]. Through this, the temperature at which the optimum leaf area at a light intensity of 260 μmol·m⁻²·s⁻¹ can secure in cucumber scions and rootstocks is determined to be about 26 °C, at which point the leaf area does not increase or decrease anymore.

As a result of the comparative examination of DQI, CP, and SHI among seedling quality confirmation indicators, DQI and CP were not statistically significant in the rootstocks, but SHI was the best at 26 °C with 131.3 g. On the other hand, DQI, CP, and SHI of scions tended to decrease as the temperature increased. This was consistent with Hwang et al.’s [9] result that the compactness of tomatoes and red peppers decreased as the daytime temperature increased from 23 to 27 °C. If the daytime temperature is excessively high, the growth is too fast, the hypocotyl lengthens, and the quality of seedlings deteriorates, resulting in a tendency to overgrow. This is a disadvantage in terms of the quality of seedlings, but it is considered that it can be applied as an advantage that can increase the production rate by increasing the growth rate through a short-term high-temperature treatment if there is a case where it is necessary to meet the shipment schedule in commercial nurseries.

Changes in growth characteristics of cucumber scions and rootstocks in the underground part according to temperature changes were confirmed (

Table 4. Root length (four types), root fresh weight, root surface, average root diameter, and number of root tips of cucumber scions and rootstocks at different daytime temperatures.

Seedling	Day/Night Temperature (℃)	Root Length (cm)					Root Fresh Weight (g)	Root Surface (cm2)	Root Average Diameter (mm)	No. of Root Tips
		>0.5 mm	0.5~1.0 mm	1.0~1.5 mm	<1.5 mm	Total
Scion	25/18	80.6 ± 14.5 a z	12.7 ± 1.5 a	2.6 ± 0.5 a	0.9 ± 0.5 a	96.9 ± 15.3 a	0.29 ± 0.11 a	3.5 ± 0.4 a	0.10 ± 0.01 a	1037.7 ± 304.6 a
	26/18	58.2 ± 16.4 b	12.4 ± 2.4 a	3.0 ± 0.6 a	1.2 ± 0.2 a	74.9 ± 18.9 b	0.23 ± 0.08 ab	3.1 ± 0.6 a	0.10 ± 0.02 a	625.6 ± 287.8 bc
	27/18	52.7 ± 13.2 b	9.8 ± 1.8 b	1.9 ± 0.5 b	1.0 ± 0.6 a	65.6 ± 14.3 bc	0.17 ± 0.12 b	2.5 ± 0.4 b	0.08 ± 0.01 b	722.6 ± 241.6 b
	28/18	44.9 ± 15.3 b	8.3 ± 2.0 b	1.9 ± 0.5 b	0.8 ± 0.5 a	56.1 ± 16.3 c	0.18 ± 0.15 ab	2.2 ± 0.5 b	0.07 ± 0.01 b	414.8 ± 157.5 c
Rootstock	25/18	117.2 ± 28.8 a	28.1 ± 9.2 a	6.9 ± 4.0 a	3.7 ± 2.7 a	156.0 ± 41.2 a	0.68 ± 0.23 b	6.7 ± 2.3 a	0.23 ± 0.04 a	1676.0 ± 437.4 a
	26/18	127.0 ± 18.5 a	24.8 ± 6.8 a	6.9 ± 3.8 a	4.7 ± 2.8 a	163.5 ± 27.2 a	0.88 ± 0.12 a	7.0 ± 2.0 a	0.24 ± 0.03 a	1746.6 ± 299.2 a
	27/18	99.0 ± 39.9 a	28.8 ± 7.4 a	6.1 ± 2.3 a	7.9 ± 2.5 a	138.1 ± 46.4 a	0.73 ± 0.15 b	6.6 ± 2.6 a	0.25 ± 0.04 a	1133.2 ± 450.8 b
	28/18	102.8 ± 31.4 a	31.6 ± 6.4 a	7.5 ± 3.4 a	9.6 ± 3.8 a	146.3 ± 36.1 a	0.71 ± 0.13 b	8.3 ± 4.6 a	0.40 ± 0.12 a	1158.6 ± 357.4 b
Significance y
Seedling (A)		***	***	***	***	***	***	***	***	***
Day/Night temperature (B)		**	NS	NS	NS	*	**	NS	NS	***
Interaction (A × B)		NS	*	NS	NS	NS	NS	NS	NS	**

^z Means for 3 replicates with 5 samples within each column followed by the same letters are not significantly different according to Duncan’s multiple range test at p < 0.05. ^y NS: non-significant, *, **, and *** of significance at p < 0.05, 0.01, and 0.001.

). The total root length of the scions decreased from 96.9 cm at 25 °C to 56.1 cm at 28 °C, about 40.8 cm (42%) shorter as the temperature increased. Among them, the length of the feeder root, which plays an important role in the growth of seedlings, was 80.6 cm at 25 °C, which was about 35.7 cm (44%) longer than that at 28 °C. This showed the same trend in average root diameter and the number of root tips. Since root growth was generally excellent at 25 °C, root fresh weight was also the heaviest at 25 °C. In contrast, the total root length of rootstocks was 163.5 cm at 26 °C and 138.1 cm at 27 °C, showing a difference of about 25.4 cm between treatments, but no statistically significant result. However, since the total root length of rootstocks was the highest at 26 °C, root fresh weight and the number of root tips were the highest among the treatments.

RGR_H of scions and rootstocks at daytime temperatures was shown as a boxplot (Figure 6a). The RGR_H of the scions increased when the daytime temperature increased from 25 to 26 °C, but it tended to decrease when the temperature exceeded 26 °C. This was similar to Kwack et al.’s [32] result in which the relative growth rate (RGR) decreased when the temperature exceeded 20 °C, with RGR of 0.113, 0.127, and 0.109 g·g·d⁻¹ as the daily average temperature increased to 10, 20, and 30 °C in the grafted pepper seedlings. This demonstrates that when the temperature is high, the initial development after germination is quick, but the growth slows down in the anaphase stage of growth for grafting. As a result, the quality of seedlings was reduced by 33~35% at the temperature of 27 °C or higher, which showed excessive growth, compared to the temperature of 26 °C or lower (Table 3). Rootstocks showed a tendency to decrease the RGR_H when the temperature exceeded 27 °C, unlike the scions. Compared to scions, more outliers were found in rootstocks, and it was confirmed that growth variance was large. In addition, since there was no significant difference according to temperature in the hypocotyl length and seedling quality index, it is judged that pumpkin, which is often used as rootstock for grafted cucumber seedlings, is insensitive to temperature compared to cucumber (Table 3). A similar trend was observed in RGR_L (Figure 6b). As the temperature increased from 25 to 26 °C, the RGR_L of the scions also increased, and showed a tendency to decrease when the temperature exceeded 26 °C. As the temperature increased from 25 to 26 °C, the RGR_L of the rootstocks increased significantly. Afterward, when the temperature exceeded 26 °C, the decrease was the same as that of scions.

3.3. Correlation Matrix with Heatmap

To visually and easily determine the correlation between light intensity and temperature growth indicators collected in this study, the array of growth indicators was displayed in color using a heatmap (Figure 7). The x-axis and y-axis are the main growth indicators of seedlings, and the closer to green, the more positive, and the closer to white, the more negative. The hypocotyl length showed negative correlations with most growth indices. Among them, DQI, CP, and SHI, which are indicators of seedling quality, showed a negative correlation of more than −0.21, confirming that the shorter the hypocotyl length, the higher the quality of seedlings. Leaf area, leaf fresh weight, and dry weight, which are related to leaf growth indicators, showed high correlations at 0.96 and 0.91. Stem diameter showed a high significant correlation with DQI and SHI at 0.83 and 0.86. Both leaf fresh weight and dry weight showed a highly significant correlation of 0.85 or higher in all three seedling quality indicators. In addition, root fresh weight and dry weight showed a high significant correlation of 0.79 or higher with the three seedling quality indicators. To confirm the seedling quality, a destructive investigation must be carried out. However, since commercial nurseries cannot damage plants, the quality of seedlings is judged based on the experiences of growers. If using this heatmap, it is expected that seedling quality can be predicted based on stem diameter, leaf area, rooting, etc., which can be measured non-destructively, and can be used as an index for seedling cultivation management.

4. Conclusions

To provide useful growth data for the utilization of PFALs in farms and commercial nurseries, we compared cucumber scion and rootstock growth by subdividing temperature and light intensity. As the light intensity increased, the hypocotyl length of the scions decreased from 10.6 cm to 6.4 cm and that of rootstocks decreased from 11.9 to 6.8 cm, and DQI, CP, and SHI were improved. However, from 50 to 260 μmol·m⁻²·s⁻¹ in SHI, 54% of scions and 36% of rootstocks increased, and from 260 to 400 μmol·m⁻²·s⁻¹, 43% of scions and 21% of rootstocks increased. When light energy of 260 μmol·m⁻²·s⁻¹ or higher was input, the degree of improvement in seedling growth slowed down. By lowering the PPF, the light source electrical energy, which accounts for 80% of the total energy required for PFAL production, can be saved, and since fewer LEDs can be installed, the initial installation cost can also be saved. Setting the light intensity to 260 μmol·m⁻²·s⁻¹ is judged to be a way to ensure adequate seedling quality while using energy efficiently when using PFAL for cucumber scion and rootstock production in both farms and commercial nurseries. In cucumber scions, as the daytime temperature increased, the overgrowth of the hypocotyl length intensified and the quality of the seedling decreased, but the rootstocks were not affected by the temperature. Since PFAL is isolated from the external environment, it is hardly affected by external temperature in environmental creation, but since it consumes cooling and heating energy through heat exchange, it is greatly affected by external temperature according to the season in terms of energy cost. As a result of this experiment, it was confirmed that sensitive temperature control is required when cultivating cucumber scions, but the effect of temperature is not significant when cultivating rootstocks. Therefore, it is considered practical to set the temperature at 28 °C in summer and 25 °C in winter when producing cucumber rootstocks using PFAL in farms and commercial nurseries. Summarizing the results, when producing high-quality cucumber grafted seedlings in PFAL, setting the daytime temperature at 25 to 28 °C and the illumination at 260 μmol·m⁻²·s⁻¹ according to the season will help stabilize farms and commercial nurseries by using energy efficiently.