The experiment was conducted in the LED plant factory experimental room of China Agricultural University (116.3 E, 40.0 N). Sixty strawberry plants (Fragaria × ananassa Duch. cv. “Akihime”) with three fully expanded leaves and 9.4 ± 0.6 mm crown diameter were selected as mother plants for runner plant propagation. Strawberry mother plants were planted in 1.5 L pots (L100 mm × W100 mm × H150 mm) with the mixed substrate (vermiculite: perlite: peat = 1:1:1, V/V/V). Twelve pots were placed evenly in two rows on the long side of the cultivation bed (L1200 mm × W900 mm × H70 mm), keeping the spacing between plants and rows at 15 cm. The rest of the cultivation bed was covered with a cultivation cover for horizontal extension of the runners and runner plants produced by the mother plant. During the experiment, the produced runner plants with three leaves were harvested promptly based on a single runner-plant excision method (He et al., 2018; Figure 1B ). The nutrient solution was prepared according to Yamazaki strawberry formulation (N 77, P 15.5, K 117, Ca 40, Mg 12, S 16, Fe 2, Mn 0.2, B 0.2, Zn 0.02, Cu 0.01, Mo 0.005 mg L⁻¹), and EC and pH were maintained in the range of 0.6–0.8 mS cm⁻¹ and 6.0–6.5, respectively, with sub-irrigation once a day for 30 min. Environmental conditions for the growth of strawberry mother plants were set as follows: air temperature was controlled at 25 ± 1°C in the photoperiod and 20 ± 1°C in the dark period; relative humidity was 75% ± 10%; CO₂ concentration was controlled at 800 ± 50 µmol mol⁻¹ in the photoperiod and without control in the dark period.

The photoperiod of LED lighting treatments was set to 16 h d⁻¹ with the photosynthetic photon flux density (PPFD) of 200 μmol m⁻² s⁻¹ and 8 h d⁻¹ in the dark period. The LED plant growth lamps (the information was shown in Supplementary Table S1 ) were installed 15 cm from the top of the mother plants’ canopy in different LED lighting treatments. The full-spectrum LED lighting treatments were labelled as W₁₀₀ (white LEDs provided the PPFD of 200 μmol m⁻² s⁻¹), W₈₄R₁₆ (White LEDs provided the PPFD of 168 μmol m⁻² s⁻¹, red LEDs provided the PPFD of 32 μmol m⁻² s⁻¹), and W₅₅R₄₅ (White LEDs provided the PPFD of 110 μmol m⁻² s⁻¹, red LEDs provided the PPFD of 90 μmol m⁻² s⁻¹), respectively. On the basis of the red and blue LEDs lighting treatment (RB₁₀₀: the PPFD provided by red LEDs was 156 μmol m⁻² s⁻¹, the PPFD provided by blue LEDs was 44 μmol m⁻² s⁻¹), the blue LEDs were partly replaced with the green LEDs to obtain the treatment labelled RB₈₀G₂₀ (the PPFD provided by red LEDs was 156 μmol m⁻² s⁻¹, the PPFD provided by blue LEDs was 35 μmol m⁻² s⁻¹, and green LEDs provided the PPFD of 9 μmol m⁻² s⁻¹). The spectrum compositions between 300 and 800 nm were measured at 15 cm below the lamps using a fiber spectrometer (AvaField-2, Avates, Apeldoorn, the Netherlands). The PPFD was measured by the quantum meter (LI-250A, LI-COR Inc., Lincoln, NE, USA). According to the spectrum composition, the photon flux densities of ultraviolet light (UV, 300–399 nm), blue light (B, 400–499 nm), green light (G, 500–599 nm), red light (R, 600–700 nm), and far-red light (Fr, 701–800 nm) were integrally calculated, and the red light to blue light ratio (R: B ratio) and the red light to far-red light ratio (R: Fr ratio) were calculated by the photon flux of red light waveband to blue light waveband, the photon flux of red light waveband to far-red light waveband, respectively. The spectrum compositions and PPFD of different LED lighting treatments in this study were shown in Figure 2 . The entire experimental period was 115 days.

A total of 5 LED lighting treatments was used in this experiment, with 12 strawberry mother plants planted in each treatment. The experiment was replicated twice. One-way analysis of the variance (ANOVA) of the data was based on Duncan’s multiple comparison method for the comparison of means (P < 0.05). The significance analysis for the growth component analysis was conducted using an independent samples t-test. Statistical analysis and graphing of the data were done using SPSS 23.0 software (IBM SPSS Statistics for Windows, version 23.0, Armonk, NY, USA), Microsoft Excel 2016, and Origin Pro software, respectively.

White LEDs contributed to the morphogenesis and biomass accumulation of strawberry mother plants ( Table 1 ). The mother plants W₁₀₀ exhibited a total of 11.3 new leaves per plant and the crown diameter was 14.3 mm, both of which were substantially higher compared with those under white and red LEDs and/or narrow-spectrum LEDs. The TDM per mother plant in W₁₀₀ increased by 20%, 22% to 13.66 g compared with W₅₅R₄₅ and RB₁₀₀, respectively. The net photosynthetic rates of mother plants in W₁₀₀ were consistently higher than those of the other treatments ( Figure 4A ). The average net photosynthetic rates of strawberry mother plants throughout the experiment under narrow-spectrum combination LEDs were only 82% of that under white LEDs, which were at 6.0 μmol m⁻² s⁻¹ ( Figure 5B ). Moreover, the Fv/Fm values of mother plants under different LED light qualities remained consistently above 0.80 ( Figure 4B ). In full-spectrum LED lighting treatments, the total leaf area of strawberry mother plants was significantly reduced when replacing white light with red light. The total leaf area of mother plants in W₅₅R₄₅ was only 76% of that in W₁₀₀ ( Figure 5A ). In contrast, there were no significant differences in the total leaf area and net photosynthetic rate of mother plants in RB₁₀₀ and RB₈₀G₂₀ ( Figure 5 ).

The length of the primary runner was increased by red and blue LEDs, which were approximately 13% longer compared with white LEDs ( Figure 6 ). The partial replacement of blue light by green light also significantly reduced the length of the primary runner. The harvesting time of primary runner plants was the shortest in W₁₀₀ and RB₈₀G₂₀, significantly shorter than in W₈₄R₁₆ and W₅₅R₄₅. Red light replacing white light was not favourable for early runner plant harvesting. Figure 6 showed that a shorter runner length can reduce the time required for harvesting runner plants.

The total number of runner plants produced by a strawberry mother plant was determined by the number of runners produced by it and the number of runner plants produced by a single runner. In this experiment, the earliest runner sprout from strawberry mother plants in different LED lighting treatments produced four runner plants. Under full-spectrum LEDs, excessive increase in red light reduced the number of runners. The number of runners in W₁₀₀ was significantly higher than that in W₅₅R₄₅, reaching 6.6 per mother plant ( Figure 7 ). Narrow-spectrum LEDs did not have a significant effect on the number of runners. Full-spectrum LEDs did not affect the production of runner plants, but the number was higher than under red and blue LEDs. The total number of runner plants in W₁₀₀ was 1.3 times that in RB₁₀₀, reaching 15.0 per mother plant. Additionally, partial replacement of blue light by green light significantly increased the number of runner plants produced by mother plants under narrow-spectrum LEDs.

Compared with W₈₄R₁₆ and W₅₅R₄₅, W₁₀₀ increased the dry mass of total runner plants by 30% and 23%, respectively ( Figures 8A, B ). Under full-spectrum LEDs, W₁₀₀ significantly enhanced the TDM of strawberry plants. Still, it did not significantly impact the fraction of dry mass partitioning to runner plants. As the proportion of white light replaced by red light increased, W₁₀₀ significantly boosted the increase in the total leaf area of strawberry plants from 21% to 33%. This was the main reason for the increase in the TDM of strawberry plants in W₁₀₀. Compared with narrow-spectrum LEDs, W₁₀₀ significantly increased the dry mass of total runner plants by 28% to 83% ( Figures 8C, D ). Taking Figure 8D as an example, the increase in the TDM of strawberry plants (+44%) and the fraction of dry mass partitioning to runner plants (+37%) together contributed to the 83% increase in the dry mass of total runner plants. Additionally, compared with RB₁₀₀, RB₈₀G₂₀ did not result in significant differences in the net photosynthetic rate and total leaf area of mother plants. However, RB₈₀G₂₀ significantly increased the leaf area of total runner plants by 55%, ultimately leading to a 15% increase in the TDM of strawberry plants ( Figure 8E ).

White LEDs improved the quality of runner plants. The crown diameter, leaf area, and dry mass of a single primary runner plant under white LEDs (W₁₀₀) represented an increase of 7%, 16%, and 14% compared with W₅₅R₄₅ ( Table 2 ). The partial replacement of blue light by green light significantly increased the leaf area and dry mass of primary runner plants under narrow-spectrum LEDs. In contrast, the leaf area and dry mass of primary runner plants under red and blue LEDs were significantly lower than those under full-spectrum LEDs, measuring only about 75% of those under white LEDs.

The vertical axis and horizontal axis of Figure 9 represented PY and EY, respectively. The closer the value of the LED lighting treatment was to the upper right of the chart, the more suitable the light source was for plant growth in PFALs. As shown in Figure 9A , the difference in EY of runner plants under full-spectrum LEDs was small but significantly higher than that of narrow-spectrum LEDs. Additionally, more runner plants were produced in W₁₀₀ and RB₈₀G₂₀, resulting in a better PY compared with RB₁₀₀. The changing trend of the dry mass of total runner plants per mother plant under each LED lighting treatment was consistent with the number of runner plants. The dry mass of total runner plants per mother plant under white LEDs was close to the upper right, while it under red and blue LEDs was close to the lower left ( Figure 9B ). In full-spectrum LED lighting treatments, the partial replacement of white light by red light reduced PY and EY in the number and dry mass of the runner plant.

The accumulation of biomass in plants is primarily determined by the net photosynthetic rate of the leaves and the total leaf area influencing the plant’s light interception capacity (Higashide, 2022). Our study demonstrated that under full-spectrum LEDs, an increase in the proportion of red light at the expense of white light significantly reduced both the leaf net photosynthetic rate and leaf area in strawberry mother plants and runner plants ( Figures 4 , 5 ), ultimately leading to decreased biomass. In the context of supplemental lighting for strawberry greenhouse production, the addition of red light significantly reduced the maximum net photosynthetic rate of leaves by inhibiting processes such as the mesophyll conductance and the maximum rate of carboxylation of Rubisco (Lauria et al., 2021). On the other hand, the daily photosynthetic rate of strawberry leaves increased significantly under white light (23% red light) compared with full-spectrum light with a high proportion of red light (43% and 59% red light, respectively) (Wu et al., 2012). This increase in photosynthetic rate under white light can be attributed to the ability of lower red light proportions to maintain higher stomatal conductance, thereby enhancing CO₂ uptake. Similarly, the net photosynthetic rate change of strawberry mother plant leaves was significantly higher under white LED compared with white and red LEDs (W₈₄R₁₆ and W₅₅R₄₅) in this experiment ( Figure 4 ). It is notable to note that while supplementing red light under white light has been reported to reduce the leaf area of plants like green onions (Gao et al., 2021) and strawberries (Wu et al., 2012), the addition of white and red LEDs increased the leaf area of wheat seedlings (Li et al., 2022) and lettuce (Lin et al., 2013). Therefore, this experiment concluded that replacing white light with red light did not enhance the photosynthetic rate or expansion of leaf area in plants, ultimately leading to a reduction in biomass accumulation.

The average leaf net photosynthetic rate of mother plants in W₅₅R₄₅ was similar to RB₁₀₀, but the leaf area was significantly lower in W₅₅R₄₅ ( Figure 5 ). It is worth noting that the TDM of mother plants in W₅₅R₄₅ was not significantly different from RB₁₀₀ ( Table 1 ). We hypothesize that the green light in the 500–599 nm range of the full-spectrum light enhances photosynthesis in plant leaves and/or the middle and lower parts of the canopy, leading to increased plant dry mass accumulation. Nishio (2000) has shown that photosynthesis is enzyme limited rather than light limited in the upper one-third of the leaf, while photosynthesis at the abaxial surface of the leaf is light limited. Difference in transmittance between red light and green light in the leaves can result in a 15% difference in biomass accumulation (Ptushenko et al., 2015). Hence, wavelengths in the 500–599 nm range may compensate for or even exceed the losses caused by the reduction of red and blue light. The absence of green light likely contributes to the lower dry mass under red and blue LEDs compared with full-spectrum LEDs. In narrow-spectrum LEDs, the partial replacement of blue light by green light also significantly increased the TDM of runner plants ( Table 2 ). Green light also induces shade avoidance responses, such as stem elongation and leaf expansion (Meng et al., 2019). In this experiment, replacing part of the blue light with green light significantly increased the leaf area of runner plants, contributing to increased dry mass, though it did not affect the mother plants ( Figure 8E ). Thus, green light plays a crucial role in promoting canopy dry mass accumulation and enhancing light interception through shade avoidance responses.

The emergence of runners and the production of runner plants are key indicators of vigorous vegetative growth in strawberries. During the process of vegetative propagation, runners served as the main reproductive organs and their formation involved two stages: axillary bud germination and axillary bud growth (Liang et al., 2022). The direction and growth of axillary buds were regulated by signals such as light and hormones (Hytönen et al., 2009). Our study discovered that the partial replacement of white light by red light had a negative impact on the germination of axillary buds into runners. For example, the number of runners in W₁₀₀ was 1.3 times higher than in W₅₅R₄₅ ( Figure 7 ). In contrast, partially replacing white light with blue light had no significant effect on runner numbers (Lee et al., 2023). This suggested that red light had a greater influence on runner formation than blue light, with higher proportions of red light inhibiting this process. Red light and far-red light often worked together in hormone-mediated pathways that regulate runner formation. The low R: FR (rich in far-red light) upregulated gibberellic acids (GAs) anabolism and downregulate catabolic genes in plants, leading to an increase in GAs levels (Yamaguchi et al., 1998; Seo et al., 2006). Elevated GA levels induced cell division in axillary buds, promoting runner formation (Hytönen et al., 2009). In this experiment, the low R: FR (W₁₀₀) significantly increased the number of runners compared with high R: FR (W₅₅R₄₅) and red and blue LEDs (without far-red light) ( Figure 7 ). White LEDs, which contain more far-red light, may have a greater advantage in promoting runner formation compared with narrow-spectrum LEDs.

White LEDs significantly promoted strawberry runner growth by increasing dry mass accumulation in crowns. Experimental data showed that the dry mass of crowns treated with white LEDs was significantly higher than in other treatment groups ( Table 1 ). Crowns acted as major carbohydrate storage sites, with fructose, glucose, and sucrose playing crucial roles in plant development (Macías-Rodríguez et al., 2002). Higher biomass in crowns could more effectively mobilize reserves, promoting leaf and axillary bud sprouting while temporarily increasing sugar content (Rivero et al., 2022). This rise in sugar content helped break axillary bud dormancy and was positively correlated with runner numbers (Li et al., 2020). However, the number of runner plants was influenced by both the number of runners and the number of runner plants produced per runner. Compared with W₈₄R₁₆ and W₅₅R₄₅, W₁₀₀ reduced the number of runner plants produced per runner, although there was no significant difference in total runner plant numbers among the three treatments ( Figures 8A, B ). Fewer runner plants per runner facilitated the concentrated supply of dry mass, thereby improving the quality of primary runner plants in W₁₀₀ ( Table 2 ), which was one reason why it outperformed white and red LEDs.

Our study revealed that partially replacing white light with red light in full-spectrum LEDs significantly reduced the TDM of strawberry runner plants ( Figures 8A, B ). This reduction was primarily attributed to the high proportion of red light, which significantly decreased the total leaf area of both mother plants and runner plants, ultimately leading to diminished overall photosynthetic productivity. Compared with blue and red LEDs, the increase in TDM observed in runner plants under white LEDs could only be partially linked to the increase in the overall dry mass of the strawberry plants. The increased the fraction of dry mass partitioning to runner plants contributed to more than 40% of the total runner plant dry mass ( Figure 8D ). Enhancing the fraction of dry mass partitioning to the sinks (runner plants), it was crucial to enhance the sink strength and numbers (Marcelis, 1996; Heuvelink, 1997). The relative sink strength of a sink organ could be determined by its dry mass. Strawberry runner plants acted as sinks and relied on the mother plant for their water, minerals, and photosynthate supply before they took roots (Caraco and Kelly, 1991). As the leaves of runner plants mature and initiate photosynthesis, the proportion of photosynthates transported from the mother plant decreases, complicating the assessment of the sink strength of runner plants. Unlike reproductive organs such as fruits, dry mass was not an ideal indicator of sink strength for photosynthetically active vegetative organs.

The number of runner plants significantly influenced the fraction of dry mass partitioning to runner plants. While the number of runner plants per runner affected the total number of runner plants produced by the mother plant, this effect was not statistically significant. Therefore, under full-spectrum LEDs, a substantial increase in the number of runners led to an overall increase in the number of runner plants, which in turn increased the fraction of dry mass partitioning to runner plants. Wu et al. (2011) found that partially replacing blue light with green light promoted runner growth. Similarly, our experiment showed that, compared with RB₁₀₀, the RB₈₀G₂₀ treatment significantly increased the number of runners by 19%. This increase in runner number was a primary factor in the significant rise in the fraction of dry mass partitioning to runner plants under this treatment, contributing to the overall increase in total runner plant dry mass. Neither the RB₈₀G₂₀ nor the RB₁₀₀ significantly affected dry mass accumulation in mother plants ( Table 1 ). It remained unclear whether the RB₈₀G₂₀ treatment increased the transport of photosynthates from the mother plant to the runner plants, resulting in no significant change in the mother plant’s dry mass. However, it was evident that the 55% increase in leaf area observed under RB₈₀G₂₀, compared with RB₁₀₀, positively impacted dry mass accumulation in runner plants.

PY and EY are valuable for assessing the effectiveness and energy efficiency of LED lamps in PFALs for crop cultivation (Fang, 2019). This study demonstrated that white LEDs outperformed white and red LEDs, with their performance improving as the proportion of red light decreased ( Figure 9 ). This improvement was primarily due to the positive effect of white LEDs with a low red light proportion on the growth of strawberry runners and runner plant. Although the addition of red LEDs increased the photon efficacy of white and red LEDs, the cost was significantly higher compared with white LEDs. Therefore, optimizing the light spectrum must balance energy efficiency with the potential to maximize crop production. Our findings suggested that for strawberry runner plant propagation, low-cost, high-efficiency white LEDs satisfied these essential requirements. Full-spectrum LEDs generally outperformed narrow-spectrum LEDs, providing advantages in terms of growth promotion and energy savings. These results were consistent with the increasing popularity of white LEDs as light sources for horticultural production.