Waterlogging is a serious abiotic stressor that drastically hinders the growth and productivity of melon (Cucumis melo) around the world, due to the reduction in available oxygen levels in the waterlogged tissues. However, the mechanism underlying the responses to waterlogging stress in melon is largely unknown. In this study, physiological and transcriptome data of the waterlogging-sensitive accession ‘L39’ and the waterlogging-tolerant accession ‘L45’ were investigated under conditions of normal water supply and waterlogging stress. The results showed that ‘L45’ exhibited higher chlorophyll contents and lower REL (relative electrolyte leakage) and MDA (malondialdehyde) contents compared with ‘L39’ under waterlogging stress. Additionally, waterlogging stress only led to the stomatal closure and chloroplast damage of ‘L39’. In total, 1748 genes were differentially expressed in the leaves of waterlogging-stressed ‘L45’ compared with control, whereas 3178 genes were differentially expressed in ‘L39’. Further analysis indicated that genes related to chlorophyll synthesis and photosynthesis were more depressed in ‘L39’, while sugar cleavage, glycolysis, and fermentation genes were highly induced in ‘L39’ compared with ‘L45’. The expression of genes involved in ROS (reactive oxygen species) scavenging and hormone signaling significantly differed between ‘L39’ and ‘L45’ in their response to waterlogging stress. Moreover, a total of 311 differentially expressed transcription factors were waterlogging-responsive, among which members of the ERF (ethylene response factor), bHLH (basic helix-loop-helix), and WRKY families might play crucial roles in waterlogging tolerance in melon. This study unraveled the molecular responses to waterlogging stress in melon and could provide helpful candidate genes for further molecular breeding of waterlogging-tolerant melon varieties.

Low temperature (LT) and low illumination (LI) are common meteorological factors posing a great risk to plants. This study aimed to clarify and quantify the effects of LT, LI, and their combined stress (LTLI) on the photosynthetic physiological processes of strawberry plants during the flowering stage. The results indicated that LI stress increased Chla and b levels in strawberry plants while lowering the chlorophyll a/b ratio. In contrast, LT and LTLI stress reduced chlorophyll content. All stress conditions (LT, LI, and LTLI) decreased net photosynthetic rate, stomatal conductance, transpiration rate, the maximum photochemical efficiency of photosystem II, photosynthetic electron transport rate, and actual photochemical quantum efficiency. These stresses also raised intercellular carbon dioxide concentration, non-photochemical quenching coefficient, and levels of malondialdehyde, proline, hydrogen peroxide, and peroxide ion content. Moreover, LI stress treatment boosted the activity of superoxide dismutase, peroxidase, and catalase, while LT and LTLI stress initially raised the activity of these enzymes before it eventually declined. Importantly, the previously mentioned photosynthetic physiological parameters showed notable changes under the combined stress conditions. Ultimately, the TOPSIS model was used to quantitatively evaluate the impact levels of different stressors and treatment durations on the photosynthetic system of strawberry plants. In conclusion, the synergistic impact of LT and LI results in a reduction in photosynthetic rate and photosystem II activity, a disruption in the equilibrium of the antioxidant system, and an intensification of photoinhibition, ultimately leading to diminished photosynthetic efficiency in plants.

High temperatures significantly injure the flowering, pollination, fruit growth, and quality of plants. Photosynthesis, the fundamental process supporting plant life, is crucial. Nevertheless, the quantitative evaluation of the physiological activity of the photosynthetic system of Nanfeng tangerine (NT) plants under high-temperature conditions remains a challenge. This research utilized NT plants, a distinctive citrus variety in Jiangxi Province, as the experimental subject. The study investigated the effects of varying degrees of high-temperature stress and duration on 16 photosynthetic physiological parameters of NT plants. The study examined the impact of four varying high-temperature treatment levels (32/22 °C, 35/25 °C, 38/28 °C, and 41/31 °C) for durations of 2, 4, 6, and 8 days, respectively. Principal component analysis was utilized to identify the key indicators of photosynthetic physiological activity in NT plants, with F_v/F_m, P_max, LCP, H₂O₂, MDA, and POD being selected as key parameters. The high-temperature stress index model previously constructed was used to calculate the high-temperature stress index value of the NT plants exposed to varying degrees and durations of high temperature, in order to provide a comprehensive assessment of the photosynthetic system of NT plants under high-temperature stress. Subsequently, the high-temperature stress levels were categorized into five levels based on the calculated values: Level 0 for 0 < HSI ≤ 2, Level 1 for 2 < HSI ≤ 4, Level 2 for 4 < HSI ≤ 6, Level 3 for 6 < HSI ≤ 8, and Level 4 for HSI > 8. The research results provide valuable data for agricultural meteorological departments to carry out disaster risk zoning and risk assessment in the future.

The mandarin is an important fruit crop worldwide, and ‘Xinyu mandarin’—a local Citrus reticulata variety—is widely cultivated in Jiangxi Province, in China. Autopolyploidy has frequently been used for the improvement of crop varieties. In a previous study, we identified a tetraploid (4X) material of ‘Xinyu mandarin’ generated from its diploid (2X) mother seedling via a spontaneous mutation. However, the physiological and metabolic variations after polyploidy are not clear enough, which is not conducive to the early screening. In the present study, we analyzed the morphological, physiological, and metabolic differences between the tetraploid and diploid plants. The tetraploids had larger and thicker leaves, and the activities of key enzymes in photosynthesis, the total chlorophyll, phenolic, and proline contents, were all higher in tetraploids. In the non-targeted metabolomic profile, a total of 522 metabolites were identified, of which 61 were significantly different between diploids and tetraploids. The differential metabolites included similar proportions of primary and secondary metabolites, and most of these were up-regulated in tetraploids, especially stress-related metabolites such as phenolic acids, flavonoids, alkaloids, and so on, indicating that tetraploids may possess better stress tolerance ability and vigor. Therefore, the tetraploids of ‘Xinyu mandarin’ may serve as an excellent female parent for the improvement of citrus varieties.

Waterlogging-induced hypoxic stress severely impairs vegetative growth and crop yield of melon (Cucumis melo L.). The formation of adventitious roots represents a critical morphological adaptive strategy for melon seedlings to alleviate hypoxic damage and maintain viability under waterlogging conditions. Nevertheless, the synergistic molecular regulatory mechanisms governing waterlogging-triggered adventitious root development in melon remain largely uncharacterized at the proteomic and metabolomic layers. In this study, the waterlogging-tolerant melon line ‘L8’ with superior adventitious root production capacity was exposed to waterlogging treatment, and hypocotyl tissues were harvested at 0, 24, 48 and 72 h post-waterlogging for untargeted metabolomic and proteomic analyses. A total of 1337 differentially accumulated metabolites (DAMs) and 2898 differentially expressed proteins (DEPs) were identified across the pairwise comparisons. Functional enrichment analyses of DAMs and DEPs indicated that pathways related to linoleic acid metabolism, α-linolenic acid metabolism, phenylpropanoid biosynthesis, biosynthesis of secondary metabolites, and glutathione metabolism were centrally implicated in adventitious rooting induced by waterlogging. At the protein level, pivotal functional proteins associated with anaerobic respiration (pyruvate decarboxylase, alcohol dehydrogenase), ethylene biosynthesis (1-aminocyclopropane-1-carboxylate oxidase), cell wall remodeling and antioxidant defense were significantly up-regulated throughout adventitious root development. In addition, three transcription factors, namely the GRAS family protein MELO3C025904.1, MYB-related protein MELO3C007640.1, and ZF-HD protein MELO3C022921.1, exhibited differential expression across different time points compared to the control. Moreover, metabolomic profiling identified three prominent metabolites with regulatory functions, encompassing the terpenoid acorusnol, the piperidine alkaloid 1,4′-bipiperidine-1′-carboxylic acid, and the flavonoid 4′,7-dihydroxy-2′-methoxy-3′-prenylisoflavan. Omics correlation analysis revealed extensive concordance between metabolomic and proteomic profiles. Nine core DAMs, including N-methylserotonin, Val-Val and glyuranolide, were tightly correlated with hundreds of DEPs and key transcription factors, constructing a complex regulatory network governing waterlogging stress acclimation and adventitious root morphogenesis. This study systematically characterizes the coordinated proteomic and metabolomic reprogramming underlying waterlogging-induced adventitious root formation in melon. These findings deepen our understanding of the molecular mechanism of waterlogging tolerance and provide valuable candidate genes and metabolic targets for genetic improvement of waterlogging resistance in melon.