Background: The growth and yield of pepper (Capsicum annuum L.) is often affected by the critical salt stress. Salicylic acid (SA) can improve plants’ stress tolerance by promoting growth and regulating ion absorption and transportation. Methods and results: To uncover the alleviated mechanism of salt stress by SA in pepper, we conducted morphological, physiological, cytological, and transcriptomic analyses under a single SA treatment and NaCl with and without SA pre-treatment for 9 days. Seedlings under NaCl treatment showed yellow shrunken leaves, this tatus were alleviated by NS treatment (NaCl with SA pre-treatment). Compared with plants under NaCl treatment, those in the NS treatment showed reduced lipid peroxidation, and significantly increased contents of chlorophyll and osmotic regulators (proline, soluble sugars). Treatment with SA balanced the Na⁺/K⁺ ratio. We conducted transcriptome sequencing and identified differentially expressed genes (DEGs) contributing to alleviation of salt stress by SA in pepper. Besides photosynthesis related genes, GO and KEGG analyses revealed that the DEGs were enriched in ‘sequence-specific DNA binding’, ‘transcription regulator activity’ and ‘DNA binding transcription factor activity’ by GO terms. And our results showed that TFs, such as MYB, bZIP, BBX, AP2/ERF, NAC, etc., probably make a great contribution in the alleviation of salt stress by SA. Conclusions: These results reveal that SA can improve plants’ stress tolerance by balancing ion absorption, gene expression and transcriptional regulation, which provide new ideas and resources for subsequent research on the mechanism of salt tolerance in pepper.

Insufficient light in autumn–winter may prolong the production periods and reduce the quality of plug seedlings grown in greenhouses. Additionally, there is no optimal protocol for supplementary light strategies when providing the same amount of light for plug seedling production. This study was conducted to determine the influences of combinations of supplementary light intensity and light duration with the same daily light integral (DLI) on the morphological and physiological properties of cucumber seedlings (Cucumis sativus L. cv. Tianjiao No. 5) grown in a greenhouse. A supplementary light with the same DLI of 6.0 mol m⁻² d⁻¹ was applied with the light duration set to 6, 8, 10, or 12 h d⁻¹ provided by light-emitting diodes (LEDs), and cucumber seedlings grown with sunlight only were set as the control. The results indicated that increasing DLI using supplementary light promoted the growth and development of cucumber seedlings over those grown without supplementary light; however, opposite trends were observed in the superoxide dismutase (SOD) and catalase (CAT) activities. Under equal DLI, increasing the supplementary light duration from 6 to 10 h d⁻¹ increased the root surface area (66.8%), shoot dry weight (24.0%), seedling quality index (237.0%), root activity (60.0%), and stem firmness (27.2%) of the cucumber seedlings. The specific leaf area of the cucumber seedlings decreased quadratically with an increase in supplementary light duration, and an opposite trend was exhibited for the stem diameter of the cucumber seedlings. In summary, increased DLI or longer light duration combined with lower light intensity with equal DLI provided by supplementary light in insufficient sunlight seasons improved the quality of the cucumber seedlings through the modification of the root architecture and stem firmness, increasing the mechanical strength of the cucumber seedlings for transplanting.

Adding far-red (FR, 700–800 nm) light to photosynthetic active radiation (400–700 nm) proved to be a possible approach to increasing plant biomass accumulation for lettuce production in indoor vertical farms with artificial lighting as a sole-source lighting. However, how FR light addition influences plant growth, development, and metabolic processes and the optimal value of FR photon flux density for greenhouse-grown lettuce under sunlight are still unclear. This work aims to quantify the value of supplementary FR light with different intensities on lettuce morphological and physiological characteristics in a greenhouse. Lettuce ‘Dasusheng’ (Lactuca sativa L.) was grown in a greenhouse under seven light treatments, including white plus red LEDs with FR photon flux density at 0, 10, 30, 50, 70, and 90 µmol m⁻² s⁻¹ (WR, WR + FR10, WR + FR30, WR + FR50, WR + FR70, and WR + FR90, respectively), and lettuce grown with sunlight only was marked as natural light (NL). FR light addition improved the electron transport flux per cross section and performance index (PI_abs, PI_total) and decreased the changes in relative variable fluorescence of lettuce leaves compared to plants under NL. Specifically, the PI_abs of lettuce leaves were 41%, 41%, 38%, 33%, 26%, and 25% lower under control than in plants under treatments WR + FR90, WR + FR70, WR + FR50, WR + FR30, WR + FR10, and WR, respectively. Leaf number, leaf area, and biomass accumulation of lettuce followed a quadratic function with increasing FR light intensity and were the highest under treatment WR + FR50. The shoot fresh weight and dry weight of lettuce were increased by 111% and 275%, respectively, under treatment WR + FR50 compared to NL. The contents of vitamin C, reducing sugar, total soluble sugar, and starch in lettuce showed a similar trend with biomass accumulation. In conclusion, with commonly used photosynthetic photon flux density (PPFD, 400–700 nm) around 200 μmol m⁻² s⁻¹, supplementary FR light intensity of 30~50 μmol m⁻² s⁻¹ was suggested to enhance the photochemistry efficiency, biomass accumulation, and carbohydrates’ contents in greenhouse-grown lettuce.

Light is a crucial environmental signal and a form of photosynthetic energy for plant growth, development, and nutrient formation. To explore the effects of light quality on the growth and nutritional qualities of greenhouse-grown lettuce (Lactuca sativa L.), lettuce was cultivated under supplementary white (W) light-emitting diodes (LEDs); white plus ultraviolet A LEDs (W+UV); white plus far-red LEDs (W+FR); and the combination of white, far-red, and UV-A LEDs (W+FR+UV) for 25 days, with lettuce grown under natural sunlight used as the control. The results indicate that the leaf length and leaf width values for lettuce grown under the W+FR+UV treatment were significantly higher than those of lettuce grown under other supplementary light treatments. The highest values of shoot fresh weight, shoot dry weight, root fresh weight, and root dry weight were recorded under the W+FR treatment (4.0, 6.0, 8.0, and 12.4 times higher than those under the control treatment, respectively). Lettuce grown under the W+FR treatment exhibited the highest total chlorophyll content (39.1%, 24.6%, and 16.2% higher than that under the W, W+UV, and W+FR+UV treatments, respectively). The carotenoid content of lettuce grown under the W+FR treatment was the highest among all treatments. However, the root activity of greenhouse-grown lettuce was the highest under the W+FR+UV treatment. Soluble sugar content, cellulose content, and starch content in the lettuce responded differently to the light treatments and were highest under the W+UV treatment. In summary, supplementary light promoted growth and nutrient accumulation in lettuce. Specifically, white plus far-red light promoted lettuce growth, and white plus UV increased some specific compounds in greenhouse-grown lettuce. Our findings provide valuable references for the application of light-supplementation strategies to greenhouse lettuce production.

Insufficient solar light in winter inside the greenhouse may lead to a lower quality of vegetable seedlings, and supplemental light is an effective technique to solve this problem. This study evaluated the impacts of supplementary white (W)-light-emitting diodes (LEDs), ultraviolet A LEDs (UV-A), white and blue LEDs (WB), the combinations of white and UV-A LEDs (W-UVA), and white, blue, and UV-A LEDs (WB-UVA) on the leaf morphology, photosynthetic traits, biomass accumulation, root architecture, and hormone content of cucumber (Cucumis sativus L. cv. Tianjiao No. 5) seedlings grown in the greenhouse. The results indicated that supplementary LED lighting led to a decreased plant height, shorter hypocotyl length, bigger leaf area, and thicker leaf compared with those grown with solar light only, regardless of light quality. The shoot fresh weight, root fresh weight, and seedling quality index of cucumber seedlings grown under the combinations of white, blue, and UVA radiations increased by 30.8%, 3.2-fold, and 1.8-fold, respectively, compared with those grown with natural light only. However, no significant differences were exhibited in the biomass accumulation of greenhouse-grown cucumber seedlings between the control and the UVA treatment. The cellulose content and stem firmness of greenhouse-grown cucumber seedlings grown under the combinations of white, blue, and UVA radiations increased by 49.9% and 13.1%, respectively, compared with those grown under white light only. Additionally, the cytokinin content of cucumber seedlings was promoted by over 36.7% by applying supplementary light. In summary, the combinations of white, blue, and UVA radiations led to compact morphological characteristics, superior mechanical properties, and preferable growth performance, which could be applied as an available lighting strategy to obtain the desired morphological and quality properties of vegetable seedlings.

Temperature and light are the key factors that affect the quality of pumpkin rootstock seedlings’ growth process. Responses to temperature and light are an important basis for optimizing the greenhouse environment. In order to determine the quantitative effects of temperature and light on the growth and development of pumpkin (Cucurbita moschata cv. RTWM6018) rootstock seedlings, relationships between temperature, light, and pumpkin rootstock seedlings growth were established using regression analysis. The results indicated that the daily average temperature had a significant negative correlation with the development time of pumpkin rootstock seedlings, and the shoot dry weight of pumpkin rootstock seedlings increased within a certain range of the daily light integral (DLI). We established a prediction model of pumpkin rootstock seedling quality indicators (hypocotyl length, stem diameter, shoot dry weight, root dry weight, root shoot ratio, and seedling quality index) based on thermal effectiveness and photosynthetic photon flux density (TEP). The coefficient of determinations (R²) of the hypocotyl length and seedling quality index prediction models of pumpkin rootstock seedlings, based on accumulated TEP, were 0.707 and 0.834, respectively. The hypocotyl length and seedling quality index prediction models of pumpkin rootstock seedlings, based on accumulated TEP, were y₁ = 0.001 x² − 0.180 x + 13.057 and y₂ = 0.008 x^0.722, respectively, which could be used for predicting the growth of pumpkin rootstock seedlings grown under different temperature and light conditions.