A Scheme for Speed Breeding of Tomato Through Modification of the Light Environment

Abstract

This study aimed to determine optimal light recipes for speed breeding of tomato (Solanum lycopersicum L.) in a plant factory. Two tomato cultivars, Zuanhongmeili and Xiaokeai, were investigated. In Experiment 1, conducted under a 12 h photoperiod, both cultivars showed accelerated development with increasing light intensity. The optimal light intensity range of 300–400 μmol·m⁻²·s⁻¹ promoted development and seed maturation. Under these conditions, Zuanhongmeili and Xiaokeai achieved flower bud emergence in the shortest times, at 24.91 ± 0.13 and 24.91 ± 0.12 days after sowing (DAS), respectively. Furthermore, for the two cultivars, anthesis initiation occurred at 39.08 ± 0.62 and 35.78 ± 0.19 DAS, fruit setting at 41.31 ± 0.61 and 38.54 ± 0.24 DAS, and the breaker stage at 83.05 ± 1.05 and 69.78 ± 0.29 DAS, respectively, under these conditions. Critically, germinable seeds were harvested from each cultivar as early as 63 and 60 DAS, projecting a theoretical annual generational turnover of up to six cycles. Based on these results, a baseline irradiance of 350 μmol·m⁻²·s⁻¹ was selected for Experiment 2, which independently assessed the impact of photoperiod. Zuanhongmeili and Xiaokeai both showed accelerated development with increases in photoperiod. The optimal photoperiod of 20 h promoted development and seed maturation. Under a 20 h photoperiod, Zuanhongmeili and Xiaokeai achieved flower bud emergence in the shortest times, at 25.12 ± 0.09 and 23.76 ± 0.13 DAS, respectively. Furthermore, anthesis initiation occurred at 41.21 ± 0.66 and 37.27 ± 0.34 DAS, fruit setting at 44.51 ± 0.15 and 40.25 ± 0.08 DAS, and the breaker stage at 91.19 ± 0.59 and 77.47 ± 0.36 DAS, respectively, under these conditions. The shortest times to harvest of germinable seeds from the two cultivars in this experiment were 76 and 72 DAS. Overall, this study demonstrates that tailored light environments, particularly the light intensity regime identified in Experiment 1, can dramatically accelerate tomato growth and development, enabling production of six generations per year in a controlled environment.

1. Introduction

Tomato (Solanum lycopersicum) is one of the most important cultivated plants in the Solanaceae family. As the leading vegetable crop globally, it accounts for approximately 16% of total vegetable production [1], and is cultivated in over 161 countries, with an annual output exceeding 180 million tons (FAO). Beyond its agricultural and economic significance, it is a vital dietary source of health-promoting compounds such as lycopene, vitamins, and minerals. The escalating demand from a growing global population, coupled with the increasing pressures of climate change, underscores an urgent need to develop new tomato varieties with enhanced yields, improved resilience, and diversified quality traits.

However, long generation times are required for developing new crop varieties [2]. For instance, 7–9 years are required for the generation of homozygous lines in crops after hybridization [3]. In regions with one annual cropping season, 7–8 years are needed for the development of a phenotypically stable new variety, due to the time constraints of traditional breeding cycles. To overcome these limitations, innovative strategies such as gene editing and environmental control have been developed to accelerate generation times [4,5,6]. However, the use of gene editing technology in breeding has drawbacks such as potential off-target effects, ecological risks, ethical concerns related to genetic discrimination, regulatory challenges due to unclear rules, and issues with consumer acceptance [7]. In recent years, a system termed speed breeding (SB) has garnered significant attention [8]. SB enables plant breeders to drastically shorten crop life cycles and achieve more generations per year through optimization of photoperiod, light quality, light intensity, temperature, humidity, and CO₂ concentration within controlled environments. Compared to gene editing, this approach of leveraging controlled environments is generally considered safer and more straightforward to implement. Among the various environmental factors, light regulation is particularly promising due to its precision and cost-effectiveness.

A plant factory, as the most advanced form of modern protected agriculture, allows for precise control of all environmental factors required for plant growth, and is thus the most suitable environment for rapid breeding. Light is a crucial environmental factor affecting plant development in a plant factory, with changes in photoperiod serving as signals for plants to regulate seasonal growth and flowering [9]. Arabidopsis thaliana grows faster in a long-day photoperiod [10]. Extended photoperiods were applied to achieve significantly shortened generation times (40–60% of traditional duration) for wheat, barley, and other crops in controlled-environment growth chambers, resulting in up to 5–6 generations per year for wheat and 5.4 generations annually for barley [8]. In hot peppers, the time to anthesis emergence was significantly shortened, by 2 days, using an extended photoperiod [11]. In addition to photoperiod, light intensity can influence plant growth. In red firespike, higher light intensity accelerated flowering and increased inflorescences [12]. High light intensity accelerated hot pepper growth and development, reducing durations of breeding cycles to enable up to four generations yearly [11]. Shade nets delay flowering in tomato plants by reducing light intensity [13].

Conventional breeding programs are severely constrained by the long generation times of tomato crops. Even in favorable climates, field-grown tomatoes are typically limited to 3–4 generations per year, and the development of speed breeding protocols for tomatoes lags behind development of similar protocols for major cereal crops [14,15]. This slow pace constitutes a fundamental bottleneck, delaying the timely release of improved cultivars. To overcome this bottleneck, accelerating the breeding cycle through controlled environments is critical. This technique is known as speed breeding. This study investigates the effects of the light environment—specifically, LED-based light intensity and photoperiod—on accelerating the breeding cycles of two representative tomato cultivars (large-fruit and cherry tomato) in plant factories. By evaluating the impact of these two factors on key developmental stages and seed germination, this study aims to optimize growth conditions to shorten the life cycle and increase annual generational turnover.

2. Materials and Methods

2.1. Plant Material and Growth Conditions

Two red tomato cultivars with contrasting phenotypes were used: the large-fruited, mid-early Zuanhongmeili (averaging 165 g) and the small-fruited, early Xiaokeai (averaging 20 g). Their differing sink strengths (fruit load) and potential growth rates might lead to divergent responses under modified light-intensity and photoperiod regimes aimed at acceleration. The interaction between genetic background (fruit type) and environmental manipulation might ensure that speed breeding recommendations could be effective for a broader range of tomato varieties.

The tomato seeds were sown into damp sponge blocks and then placed in a dark environment. Plants were initially grown under a 250 µmol·m⁻²·s⁻¹ photosynthetic photon flux density (PPFD) white LED light (peaking at 440 nm and 660 nm; Chenghui Equipment Co., Ltd., Guangzhou, China), with a 12 h photoperiod (6:00–18:00) until complete cotyledon development; then, they were exposed to variable light regimes for growth. All experiments were conducted in a plant factory (South China Agricultural University) under preset conditions of 21 ± 2 °C temperature and 60 ± 10% relative humidity, utilizing LED lighting systems and Hoagland’s solution (EC: 1.5 mS cm⁻¹, pH: 6.5). A specific light recipe developed based on our preliminary experiments, which effectively promotes vegetative growth and development in tomato seedlings, was applied. This recipe maintains a photosynthetic photon flux density (PPFD) ratio of 3:2:1 for white light (peaking at 440 and 660 nm), red light (660 nm), and blue light (460 nm), respectively. Based on these findings, two experiments were implemented to investigate the effects of light intensity and photoperiod on tomato growth and development. In Experiment I, four PPFDs (250, 300, 350, and 400 µmol·m⁻²·s⁻¹) were used with a 12 h photoperiod. In Experiment II, four photoperiods (14, 16, 18, and 20 h) were used with a PPFD of 350 µmol·m⁻²·s⁻¹.

2.2. Growth and Morphology Assessment

Tomato plants were regularly monitored to record the timing of the following growth stages: flower bud emergence (average time to flower bud appearing); anthesis initiation (average time to flower full bloom); the fruit setting stage (average time to start of fruit development); and the fruit breaker stage (average time to initial red coloration in fruit). The experiment was conducted using 8 biological replicates (individual plants). Each plant had 2 fruit trusses, each retaining 4 fruits. Eight fruits were sampled from each plant, resulting in a total of sixty-four fruits for observation.

2.3. Sampling and Germination Ability Test

To evaluate seed germination capacity across different maturity stages, fruits were harvested at 18–42 days after anthesis. Tomato fruit pulp was manipulated on gauze to remove residual pectin via gentle compression, followed by seed extraction and oven-drying at 30 °C for 2 days. To break physical dormancy, the dried seeds were subjected to a hydro-thermal treatment; this involved soaking in water maintained at 50–55 °C, with constant agitation, for 15 min. Germination assays were conducted by placing seeds on moist filter paper in a constant-temperature incubator (28 °C). Germination rates were determined as the proportion of seeds exhibiting radicle emergence after 7 days of incubation.

2.4. Statistical Analysis

One-way ANOVA analysis was conducted to compare treatment effects, with post hoc comparisons performed using Duncan’s multiple range test at a significance level of α = 0.05. Data visualization was achieved through graphical representation generated by Origin 2018 (Origin Lab, Northampton, MA, USA).

3. Results

3.1. Effects of Light Intensity on the Growth and Development of Tomato

The time taken to reach key growth stages (flower bud emergence, anthesis initiation, fruit setting stage, and fruit breaker) in tomato plants varied with photosynthetic photon flux density (PPFD) (Figure 1).

The shortest times to reach the growth stages occurred at elevated light intensities (PPFD). Under 300–400 μmol·m⁻²·s⁻¹ PPFD, Zuanhongmeili exhibited accelerated development across multiple growth stages. Compared with 250 μmol·m⁻²·s⁻¹ PPFD, the time from sowing to flower bud emergence was significantly reduced, by 3.21–3.75%, under 300–400 μmol·m⁻²·s⁻¹ PPFD, with the shortest duration of 24.91 ± 0.13 DAS at 300 μmol·m⁻²·s⁻¹. Similarly, the time from sowing to anthesis initiation was significantly shortened, by 3.49% and 2.92%, respectively, under 300 and 400 μmol·m⁻²·s⁻¹, with the shortest duration of 39.08 ± 0.62 DAS observed at 400 μmol·m⁻²·s⁻¹. In addition, the time from sowing to the fruit setting stage showed significant reductions, of 4.02% and 5.08%, respectively, at 300 and 400 μmol·m⁻²·s⁻¹, with the shortest duration of 41.31 ± 0.61 DAS achieved at 400 μmol·m⁻²·s⁻¹. However, no significant differences were detected in the times taken from sowing to the fruit breaker stage. Seed germination capacity varied with fruit maturity and light intensity (Figure 1c). For Zuanhongmeili, late-stage maturation triggered dramatic germination improvement of 0–3.96% at 24 days after anthesis (DAA), compared with 44.73–83.33% at 42 DAA. Increased light intensity (300–400 μmol·m⁻²·s⁻¹) improved seed germination capacity and shortened the time to harvest of germinable seeds, with the highest germination percentage (83%) occurring at a light intensity of 300 μmol·m⁻²·s⁻¹ at 42 DAA.

In the case of Xiaokeai, increased light intensity (300–400 μmol·m⁻²·s⁻¹) also accelerated plant development (Figure 2). Compared with 250 μmol·m⁻²·s⁻¹ PPFD, the time from sowing to flower bud emergence was significantly reduced, by 3.23–3.77%, with the shortest duration of 24.91 ± 0.11 DAS observed at 300 μmol·m⁻²·s⁻¹. The time from sowing to anthesis initiation was also significantly shortened, by 5.87–6.73%, with the shortest duration of 35.78 ± 0.19 DAS observed at 400 μmol·m⁻²·s⁻¹. Times taken from sowing to the fruit setting stage showed significant reductions of 5.12–6.57%, with the shortest duration of 38.54 ± 0.24 DAS achieved at 400 μmol·m⁻²·s⁻¹. Times taken from sowing to the breaker stage showed significant reductions of 4.51–4.72%, with the shortest duration for this stage (69.78 ± 0.29 DAS) achieved at 300 μmol·m⁻²·s⁻¹. Late-stage maturation also triggered a dramatic improvement in germination: 0–3.13% at 24 DAA, compared with 93.65–97.95% at 33 DAA (Figure 2). Moreover, increased light intensity (350–400 μmol·m⁻²·s⁻¹) shortened the time to harvest of germinable seeds.

3.2. Effects of Photoperiod on the Growth and Development of Tomato

Photoperiod influenced the growth and development of Zuanhongmeili (Figure 3). Compared with a 14 h photoperiod, flower bud emergence in Zuanhongmeili was significantly advanced by a 20 h photoperiod, which resulted in a minimum duration of 25.1 ± 0.1 DAS and a 5.16% reduction. No significant differences were observed in anthesis initiation, the fruit setting stage, or the fruit breaker stage, which were 40.78–41.24, 44.08–44.95, and 90.42–91.28, respectively (Figure 3). Seed germination capability varied with maturity stage and photoperiod (Figure 3c). Late-stage maturation triggered germination improvement: 4.8–17.7% at 36 DAA, compared with 4.8–21.74% at 42 DAA (Figure 1c). Compared with a 14 h photoperiod, a 20 h photoperiod improved seed germination capability, with the highest germination rate (21.74%) occurring at a photoperiod of 20 h at 42 DAA.

Photoperiod also significantly influenced the growth and development of Xiaokeai (Figure 4) Compared with a 14 h treatment, the time from sowing to flower bud emergence was significantly shortened, by 1.87% and 7.30%, under 18 h and 20 h treatments, respectively, with the shortest duration of 23.76 ± 0.13 DAS observed under a 20 h treatment. The time from sowing to the fruit setting stage was significantly shortened, by 5.95%, under 20 h treatment, achieving a minimum duration of 40.25 ± 0.08 DAS. The time from sowing to the fruit breaker stage was significantly reduced, by 3.53–4.60%, by 16–20 h treatments, with the shortest duration of 77.5 ± 0.36 DAS observed under 20 h treatment. Moreover, in Xiaokeai, late-stage maturation triggered dramatic germination improvement: 65–85% at 33 DAA, compared with 95–100% at 42 DAA. Compared with a 14h photoperiod, a prolonged photoperiod (16–20 h) shortened the time required to harvest seeds with higher germination capacity.

The study demonstrates that light environment optimization is a powerful tool for accelerating tomato growth and breeding development. While absolute timeframes varied between experimental runs, a result which could be attributed to variations in nutrient solution batches or to subtle differences in plant starting material, the results collectively confirmed that breeding cycles of tomato can be substantially shortened in plant factories. The shortest growth cycle from seed sowing to new germinable seeds (60 days) was achieved in Experiment 1, suggesting a potential for up to six generations per year. Further refinement integrating both light intensity and photoperiod under stable conditions is warranted.

4. Discussion

Among environmental factors, light is a primary factor governing plant growth and development. It is well established that light intensity acts as the primary limiting factor for photosynthesis when it is below the light saturation point. Generally, higher light intensity supplies greater photosynthetic energy, which is essential for the synthesis of florigenic compounds that promote flowering [16]. Moreover, under higher light intensity, enhanced photosynthesis increases levels of photosynthates, which are transported to seeds and fruits to boost biomass, growth, and yield [13,17]. In Eustoma grandiflorum, increasing the light intensity from 100 to 400 µmol·m⁻²·s⁻¹ resulted in a higher leaf amount accompanied by a reduction of 7 to 10 days in the time to first flowering [18]. In strawberry, a significant promotion of flower development was observed under LED light treatment, whereas a clear delay occurred under shaded conditions [19]. Exposure to higher light intensities (300 and 420 µmol·m⁻²·s⁻¹) resulted in a reduction in time to anthesis and an increase in leaf number, respectively, in the hot pepper varieties Xiangyan55 and Xiangla712, compared to growth at 240 µmol·m⁻²·s⁻¹ [11]. Compared to low light intensity, moderate light intensity is more effective at supplying the essential photosynthates required for development in the large-fruited tomato [20]. Compared to low light intensities (50, 150, 200 μmol m⁻² s⁻¹), higher light intensities (300, 450, 550 μmol m⁻² s⁻¹) improved photosynthate distribution in cherry tomatoes, which in turn enhanced leaf photosynthesis and plant growth [21]. Consistent with previous findings, under light intensities of 300–400 µmol·m⁻²·s⁻¹ PPFD, the durations from sowing to flower bud emergence, anthesis initiation, and the fruit setting stage were all significantly shortened in the large-fruited tomato and cherry tomato (Zuanhongmeili and Xiaokeai) cultivars studied in the present work (Figure 1 and Figure 2). In one previous study, the breeding cycle of sorghum was substantially shortened through direct germination of immature seeds [22]. In another work, the germination rate of hot pepper seeds rose substantially from 28 ± 3.12% at 77 DAS to 82 ± 3.13% when harvested at 82 DAS (43 days after anthesis) [11]. In the present work, the shortest time (63 DAS) for seeds of Zuanhongmeili to achieve germination capability was observed under 350 µmol·m⁻²·s⁻¹ PPFD (Figure 1), while the shortest time for Xiaokeai seeds (60 days) occurred under 350 µmol·m⁻²·s⁻¹ (Figure 2).

Photoperiod is a critical environmental regulator of plant development, influencing processes such as vegetative growth, metabolic activity, flowering, and fruiting. Anthesis in a plant is determined by its genotype, and a plant can be classified as a long-day, short-day, or day-neutral plant according to its photoperiodic response. A 42-day reduction per cycle was achieved in oat (a long-day plant) using a 22 h photoperiod that advanced phenological stages and flowering by 11 days across eight genotypes, and through speed breeding with early harvesting at 21 days, which shortened the average lifecycle from 114 to 72 days, increasing the number of potential generations per year from 3.2 to 4.9 [23]. However, in the case of the short-day plant Bougainvillea glabra Sao Paulo, reducing the photoperiod from 10.5 to 8 h markedly advanced floral bud differentiation by 119 days, thereby effectively promoting the development of complete flowers and inflorescence formation [24]. Although tomato is a day-neutral plant, the flowering of which is not reliant on photoperiod, photoperiod does nevertheless influence its vegetative growth and biomass accumulation. This, in turn, accelerates the development and reproductive cycle [12]. Prolonged light exposure (12 h, 16 h, and 20 h) significantly enhanced large-fruited tomato growth, with the highest levels of photosynthetic activity and chlorophyll content observed under a 20 h photoperiod [25,26]. There is limited research on the effects of photoperiod on the growth and development of cherry tomatoes. In this study, an extended photoperiod (20 h) significantly advanced flower bud emergence in both large-fruited tomato and cherry tomato (Zuanhongmeili and Xiaokeai), by 1 and 2 days, respectively, and accelerated fruit setting in Xiaokeai by 2 days (Figure 3 and Figure 4). Similarly, in day-neutral hot pepper plants, a longer photoperiod (20 h) significantly accelerated flowering relative to a 12 h regime, reducing the time to flowering by an average of 2 days [11]. A prolonged photoperiod can shorten the time required to obtain germinable seeds by promoting fruit maturation. In this study, increasing the photoperiod from 14 to 20 h enabled Zuanhongmeili and Xiaokeai to produce germinable seeds 3 days earlier. Given the trade-off between a shortened developmental period and energy expenditure, a photosynthetic photon flux density (PPFD) of 350 µmol·m⁻²·s⁻¹ under a 20 h photoperiod is proposed as an optimal regimen for speed breeding of tomato.

It is important to note that this study focused on optimizing seed development for next-generation usable seeds. The impact of the described speed-breeding light regimens on quality parameters of mature fruit—such as content levels of lycopene, soluble solids, and vitamins—remains to be evaluated. Future work should involve fruits grown to full maturity under these accelerated conditions to enable comprehensive assessments of yield and nutritional quality, both of which are essential for determining the commercial viability of lines developed using this method.

5. Conclusions

This study independently assessed the effects of light intensity (Exp. 1) and photoperiod (Exp. 2) on speed breeding of tomato. Most significantly, under the conditions of Experiment 1 (12 h photoperiod, 300–400 μmol·m⁻²·s⁻¹ PPFD), a complete seed-to-seed cycle was achieved within 60 days in Xiaokeai, and 63 days in Zuanhongmeili. These results project to a theoretical maximum of six generations per year—a substantial increase over conventional methods (Figure 5). Experiment 2 confirmed that a longer photoperiod at 350 μmol·m⁻²·s⁻¹ promotes flowering. The longer seed maturation period observed in Exp. 2 highlights that the full breeding cycle is sensitive to environmental factors beyond light, and that achieving maximum generational turnover requires integrated and stable control of all growth parameters. Future work will focus both on identifying the combined optimal light recipe and on mitigating the impact of environmental fluctuations, to achieve consistently shortened cycles.