Crop breeding for high yields and quality is an important measure to ensure food security. In conventional breeding, a long generation time is required. Speed breeding could accelerate the flowering and fruiting of crops by providing suitable environmental conditions in order to reduce the generation times. This study aimed to determine a speed breeding scheme for hot peppers. Two hot pepper varieties, ‘Xiangyan 55’ and ‘Xiangla 712’, were investigated for their growth and development under different light intensities, photoperiods, and red-to-far-red ratios. Hot pepper plants bloomed at 39.88 ± 0.74 days after sowing under photosynthetic photon flux density (PPFD) 420 µmol·m⁻²·s⁻¹ and a 12-h photoperiod and had seed with acceptable germination rates at 82 days after sowing. Blooming was 2–3 days earlier when the photoperiod was extended to 20 h, but the fruit and seed development were not significantly improved. Supplementation of far-red light (R:FR = 2.1) significantly accelerated the red ripening of pepper fruit and improved seed germination rates. The modification of the light environment accelerated hot pepper growth and development, reduced breeding cycles, and could produce up to four generations per year.

The effects of different ratios of red:white light on the biomass, photosynthetic capacity, phytochemical accumulation, and antioxidant capacity of pak choi were investigated. There were three treatments: red:white = 1:2 (LR), red:white = 1:1 (MR), red:white = 2:1 (HR) with white light as control (CK) at photosynthetic photon flux density (PPFD) of 250 μmol·m⁻²·s⁻¹. In LR treatment, chlorophyll fluorescence and nitrate content were reduced, while the photosynthetic rate (Y(ll)), the contents of soluble protein, soluble sugar, vitamin C, flavonoids, polyphenols, glucosinolates, FRAP, and 2,2-diphenyl-1-bitter acyl radical scavenging increased. The chlorophyll content was enhanced in MR treatment, and the photosynthetic rate and glucosinolate content increased in HR treatment. The most suitable light for the cultivation of pak choi in the plant factory was red:white = 1:2.

To explore the effect of different nutrient-solution depths on the growth and phytochemicals accumulation of kale, three different nutrient-solution depth treatments (De-1: 1 cm, De-2: 2 cm, and De-3: 3 cm) were applied in the plant factory with artificial lighting. The maximum levels of plant height, stem diameter, total leaf area, total root length, and root surface area as well as fresh and dry weight of the shoot and root were all noted in De-2 at 24 days after treatment. Low nutrient-solution depth treatments (De-1 and De-2) were beneficial for total chlorophyll accumulation and nutrient ions uptake (N, P, K, Ca, S, and Zn). However, there were no obvious differences in DPPH and FRAP as well as contents of total polyphenols and flavonoids. In high nutrient-solution depth treatment (De-3), the contents of carotenoid, soluble sugars, and vitamin C were higher than those in De-1 and De-2. The contents of total glucosinolates (GLs), aliphatic GLs, and indole GLs significantly increased in kale under De-3. Higher contents of reactive oxygen species (ROS), hydrogen peroxide (H₂O₂), oxalate oxidase (OXO), and proline were observed in kale roots under De-2 and De-3. Overall, 2 cm of nutrient-solution depth could be used to promote kale growth, and 3 cm may represent a potential approach for improving kale quality in a plant factory.

The effects of supplemental UV-A (385 nm) period and UV-A intensity for 5 days before harvest (DBH) on growth, antioxidants, antioxidant capacity, and glucosinolates contents in Chinese kale (Brassica oleracea var. alboglabra Bailey) were studied in plant factory. In the experiment of the UV-A period, three treatments were designed with 10 W·m⁻² UV-A supplement, T1(5 DBH), T2 (10 DBH), and no supplemental UV-A as control. In the experiment of UV-A intensity, four treatments were designed with 5 DBH, control (0 W·m⁻²), 5 w (5 W·m⁻²), 10 w (10 W·m⁻²), and 15 w (15 W·m⁻²). The growth light is as follows: 250 μmol·m⁻²·s⁻¹; red light: white light = 2:3; photoperiod: 12/12. The growth and quality of Chinese kale were improved by supplemental UV-A LED. The plant height, stem diameter, and biomass of Chinese kale were the highest in the 5 W·m⁻² treatment for 5 DBH. The contents of chlorophyll a, chlorophyll b, and total chlorophyll were only highly increased by 5 W·m⁻² UV-A for 5 DBH, while there was no significant difference in the content of carotenoid among all treatments. The contents of soluble sugar and free amino acid were higher only under 10 DBH treatments than in control. The contents of total phenolic and total antioxidant capacity were the highest in 5 W·m⁻² treatment for 5 DBH. There was a significant positive correlation between total phenolic content and DPPH and FRAP value. After 5 DBH treatments, the percentages and contents of total aliphatic glucosinolates, sinigrin (SIN), gluconapin (GNA), and glucobrassicanapin (GBN) were highly increased, while the percentages and contents of glucobrassicin (GBS), 4-methoxyglucobrassicin (4-MGBS), and Progoitrin (PRO) were significantly decreased, especially under 10 W·m⁻² treatment. Our results show that UV-A LED supplements could improve the growth and quality of Chinese kale, and 5 W·m⁻² UV-A LED with 5 DBH might be feasible for Chinese kale growth, and 10 W·m⁻² UV-A LED with 5 DBH was better for aliphatic glucosinolates accumulation in Chinese kale.

This study investigated the effects of substituting B with FR and UVA (50 μmol·m⁻²·s⁻¹) at two growth stages on the growth and quality of loose-leaf lettuce (cv. Fangni). The basal light was red and blue LEDs at 250 μmol·m⁻²·s⁻¹. At stage I (the first 10 days of 20-day pre-harvest), there were three treatments: B substituted by FR (FR); B substituted by UVA (UVA); and no substituting (CKI). At stage II (next ten days after stage I), there were 9 treatments: FF (FR + FR), UF (UVA + FR), BF (B + FR), FU (FR + UVA), UU (UVA + UVA), BU (B + UVA), FB (FR + B), UB (UVA + B), and B + B (CKII). At stage I, compare with UV-A and CKI, the shoot fresh weight, leaf area, leaf width, leaf length, and vitamin C content highly increased under far-red light (FR), whereas specific leaf weight and the chlorophyll contents significantly decreased by FR. In CKI, nitrate contents and the antioxidant capacity (FRAP, DPPH) were significantly higher than in FR and UVA. At stage II, higher leaf width, leaf length, leaf area, and shoot fresh and dry weight were observed in FF, UF, and BF. The lowest biomass was shown in CKII. Higher chlorophyll contents were found in FU and FB. The soluble sugar contents significantly increased by all treatments. In addition to UB and BU, soluble protein contents increased by other treatments. There were higher vitamin C contents in UU, UB, and CKII. Large amounts of nitrates accumulated under CKII. The higher antioxidant capacity (DPPH, FRAP) was found in FB and CKII. The highest flavonoid content was found in UB, and higher polyphenols contents were found in UB and BU. In this study, substituting B with FR at 2 stages were the best way to increase lettuce biomass. The optimal measure to both increased lettuce nutrition quality and biomass was FB at stage II.

As an indispensable element in the morphology and phytochemical profile of plants, UV-A has proved to help promote the growth and quality of kale. In this study, UV-A supplementation in different photoperiods (light period supplemental UVA = LS, dark period supplemental UVA = DS, and light-dark period supplemental UVA = LDS) contributed to yielding greater biomass production (fresh weight, dry weight, and plant moisture content), thus improving morphology (plant height, stem diameter, etc.) and promoting higher phytochemicals content (flavonoids, vitamin c, etc.), especially glucosinolates. To fathom its mechanisms, this study, using RNA-seq, verified that UV-A supplementation treatments signally generated related DEGs of plant hormone signal pathway, circadian rhythm plant pathway, glucosinolate pathway, etc. Moreover, 2047 DEGs were obtained in WGCNA, illustrating the correlations between genes, treatments, and pathways. Additionally, DS remarkedly up-regulated related DEGs of the key pathways and ultimately contributed to promoting the stem diameter, plant height, etc., thus increasing the pigment, biomass, vitamin c, etc., enhancing the antioxidant capacity, and most importantly, boosting the accumulations of glucosinolates in kale. In short, this study displayed new insights into UV-A supplementation affected the pathways related to the morphology and phytochemical profile of kale in plant factories.