This study investigated the impact of various light qualities on the growth, photosynthesis, antioxidant capacity, anthocyanin accumulation and associated gene expression in purple lettuce. The results showed that stem diameter, leaf number and dry mass of purple leaves lettuce increased significantly under white light plus red and blue light (WRB) plus 10 µmol·m⁻²·s⁻¹ UV-A (SUV₁), plus 20 µmol·m⁻²·s⁻¹ UV-A (SUV₂) and plus 30 µmol·m⁻²·s⁻¹ UV-A (SUV₃) treatments compared to white light plus red and blue light (WRB). Leaf expansion decreased with increasing UV-A doses, while fresh leaf mass was higher under SUV₁ and SUV₂ treatments. Photosynthesis parameters were improved under WRB, SUV₁ and SUV₂ treatments, with an increase in net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) and a decrease in intercellular carbon dioxide concentration (Ci) under SUV₃ treatment. Superoxide radical generation rate, hydrogen peroxide and malondialdehyde (MDA) content and relative conductivity increased significantly under SUV₃ treatment. Anthocyanin content increased significantly with increasing doses of UV-A treatment, while related structural gene expression levels were upregulated more significantly by SUV₂ and SUV₃ treatments than WRB treatment. In summary, moderate UV-A supplementation can enhance the antioxidant system and promote anthocyanin accumulation in purple lettuce. Specifically, WRB plus 20 µmol·m⁻²·s⁻¹ UV-A (SUV₂) is recommended as an optimal light recipe for cultivating purple lettuce in protected horticulture.

Purple lettuce (Lactuca sativa L. cv. Zhongshu Purple Lettuce) was chosen as the trial material, and LED intelligent light control consoles were used as the light sources. The purpose was to increase the yield and quality of purple lettuce while lowering its nitrate level. By adding various ratios of NO₃⁻-N and NH₄⁺-N to the nutrient solution and 20 µmol m⁻² s⁻¹ UV-A based on white, red, and blue light (130, 120, 30 µmol m⁻² s⁻¹), the effects of different NO₃⁻-N/NH₄⁺-N ratios (NO₃⁻-N, NO₃⁻-N/NH₄⁺-N = 3/1, NH₄⁺-N) and UV-A interaction on yield, quality, photosynthetic characteristics, anthocyanins, and nitrogen assimilation of purple lettuce were studied. In order to produce purple lettuce hydroponically under controlled environmental conditions, a theoretical foundation and technological specifications were developed, taking into account an appropriate UV-A dose and NO₃⁻-N/NH₄⁺-N ratio. Results demonstrate that adding a 20 µmol m⁻² s⁻¹ UV-A, and a NO₃⁻-N/NH₄⁺-N treatment of 3/1, significantly reduced the nitrate level while increasing the growth, photosynthetic rate, chlorophyll, carotenoid, and anthocyanin content of purple lettuce. The purple leaf lettuce leaves have an enhanced capacity to absorb nitrogen. Furthermore, plants have an acceleration of nitrogen metabolism, which raises the concentration of free amino acids and soluble proteins and promotes biomass synthesis. Thus, based on the NO₃⁻-N/NH₄⁺-N (3/1) treatment, adding 20 µmol m⁻² s⁻¹ UV-A will be helpful in boosting purple lettuce production and decreasing its nitrate content.

Drought stress severely limits crop growth and yield. With the atmospheric CO₂ constantly increasing, plants will be affected by multiple effects of drought and increased CO₂ in the future. Abscisic acid (ABA) plays vital roles in plant stress tolerance, especially drought stress. However, little is known about the effects of elevated CO₂ concentration (e[CO₂]) and exogenous ABA in cucumber (Cucumis sativus L.) response to drought stress. In the present study, we investigated the effects of e[CO₂] and exogenous ABA on the drought tolerance of cucumber seedlings under the simulated drought stress induced by 5% polyethylene glycol 6000. The experiment was a split-plot design, in which the main factor was CO₂ concentrations; atmospheric and elevated CO₂ concentrations (~400 and 800 ± 40 μmol mol⁻¹, respectively). The subplot factor was the combinations of exogenous ABA and its synthesis inhibitor sodium tungstate (Na₂WO₄); deionized water (control), 20 μM ABA, 2 mM Na₂WO₄, and 2 mM Na₂WO₄ + 20 μM ABA, which were applied to plant leaves. The results showed that compared with exogenous ABA application only, e[CO₂] combined with exogenous ABA significantly increased the biomass, chlorophyll content, and net photosynthetic rate (P_n) of cucumber seedlings under drought stress. Meanwhile, e[CO₂] and exogenous ABA were more efficient in reducing the contents of reactive oxygen species and malondialdehyde, promoting the accumulation of proline, soluble sugar, soluble protein, free amino acid, ascorbic acid, and glutathione. The ratios of ascorbic acid/dehydroascorbic acid (ASA/DHA), glutathione/oxidized glutathione (GSH/GSSG), as well as the activities of antioxidant enzymes were increased. In conclusion, e[CO₂] and exogenous ABA synergistically alleviated oxidative damage of drought stress on cucumber seedlings by increasing antioxidant enzyme activities and accelerating the ASA–GSH cycle in cucumber seedlings, which in turn improved the drought tolerance of cucumber seedlings, and provided theoretical and practical support for further studies on the alleviation of drought stress in protected horticulture.