Tengqi Wang

Our previous study revealed a significant effect of red and blue light on steady-state and dynamic photosynthesis in cucumber seedlings. To further investigate the mechanism by which red and blue light affect photosynthesis, cucumber (Cucumis sativus L. cv. Jinyou 365) seedlings were exposed to white light (W), monochromatic red light (R), monochromatic blue light (B), and mixed red–blue light with red:blue photon flux ratios of 9:1 (9R1B), 7:3 (7R3B), 5:5 (5R5B), 3:7 (3R7B), and 1:9 (1R9B). We measured photosynthetic pigment contents, induction curves of photosynthetic gas exchange and chlorophyll fluorescence parameters during low-to-high-light transition, as well as the activities and gene expression of key Calvin cycle enzymes. The results showed that B yielded higher photosynthetic pigment contents than R, although both remained lower than W. Mixed red–blue light with a blue-light ratio above 50% produced significantly higher pigment contents than W. Under low light, blue light exhibited negligible effects on photosynthesis. In contrast, under high light, 7R3B, 5R5B, 3R7B and 1R9B elevated net photosynthetic rate (A) and stomatal conductance (Gs). Among these, 7R3B, 5R5B and 3R7B also accelerated photoinduction, as reflected by a shorter time to reach 90% steady-state net photosynthetic rate (T_90%A). Chlorophyll fluorescence parameters changed rapidly and attained a steady state within 5 min during light induction. During photoinduction, biochemical limitation was severe in the early stage but declined rapidly, while stomatal limitation appeared in mid-induction and persisted into the late stage; photochemical limitation remained minor, and the severity of these limitations was modified by light quality. Red and blue light exerted negligible effects on chlorophyll fluorescence parameters under low light. However, blue light enhanced the relative electron transport rate of PSII under high light by elevating the fraction of open PSII centers. Furthermore, blue light significantly increased the activities and gene expression of key Calvin cycle enzymes. Correlation analysis revealed that higher chlorophyll contents were positively associated with A and Gs under high light but showed no significant correlation with photoinduction. Higher A and Gs under both low- and high-light intensities were associated with faster photoinduction.

Adventitious roots (ARs) are crucial for grafted seedlings, playing vital roles in nutrient absorption, stress resistance, and grafting efficacy. However, how scions regulate endogenous hormones to influence AR formation remains poorly understood. In this study, we constructed watermelon seedlings (WP) using “HXX” as the scion and “Tie Zhen No. 3” as the rootstock. Removing scion cotyledons (WP-1) significantly promoted AR development, while removing true leaves (WP-2) had minimal effect. Simultaneous removal of both (WP-3) increased endogenous levels of IAA (Indole-3-Acetic Acid), abscisic acid (ABA), and jasmonic acid (JA). Transcriptome sequencing revealed that differentially expressed genes (DEGs) are enriched in various hormone signaling pathways. On the fourth day, when the number of differential genes was the highest, the DEGs significantly expressed in all three treatment groups were enriched in the activation signaling pathways and responses of JA, auxin, ethylene, and cytokinins. Transcription factors such as bHLH, ERF, MYB, and NAC were significantly expressed during the development of ARs, playing a key regulatory role. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified 82 DEGs across five hormone signal transduction pathways. Weighted gene co-expression network analysis (WGCNA) identified modules positively correlated with AR hormones, highlighting hub genes such as ethylene transcription factors (CRF4, ABR1, ERF054, ERF098), auxin response factors (SAUR21, SAUR32), and other regulators (CSA, HSP, bHLH93, ZAT5, ZAT13, NAC, MYB, C3H). These findings provide molecular insights into how scions regulate AR development through hormones, offering a foundation for improving watermelon grafting practices.

The effects of red and blue light on growth and steady-state photosynthesis have been widely studied, but there are few studies focusing on dynamic photosynthesis and the effects of LED pre-treatment on cucumber seedlings’ growth, so in this study, cucumber (Cucumis sativus L. cv. Jinyou 365) was chosen as the test material. White light (W), monochromatic red light (R), monochromatic blue light (B), and mixed red and blue lights with different red-to-blue ratios (9:1, 7:3, 5:5, 3:7, and 1:9) were set to explore the effects of red and blue light on cucumber seedlings’ growth, steady-state photosynthesis, dynamic photosynthesis, and subsequent growth under fluctuating light. The results showed that compared with R and B, mixed red and blue light was more suitable for cucumber seedlings’ growth, and the increased blue light ratios would decrease the biomass of cucumber seedlings under mixed red and blue light; cucumber seedlings under 90% red and 10% blue mixed light (9R1B) grew better than other treatments. For steady-state photosynthesis, blue light decreased the actual net photosynthetic rate but increased the maximum photosynthetic capacity by promoting stomatal development and opening; 9R1B exhibited higher actual net photosynthetic rate, but the maximum photosynthetic capacity was low. For dynamic photosynthesis, the induction rate of photosynthetic rate and stomatal conductance were also accelerated by blue light. For subsequent growth under fluctuating light, higher maximum photosynthetic capacity and photoinduction rate could not promote the growth of cucumber seedlings under subsequent fluctuating light, while seedlings pre-treated with 9R1B and B grew better under subsequent fluctuating light due to the high plant height and leaf area. Overall, cucumber seedlings treated with 9R1B exhibited the highest biomass and it grew better under subsequent fluctuating light due to the higher actual net photosynthetic rate, plant height, and leaf area.

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.