Granulation of juice sacs is a physiological disorder, which affects pomelo fruit quality. Here, the transcriptome and ubiquitinome of the granulated juice sacs were analyzed in Guanxi pomelo. We found that lignin accumulation in the granulated juice sacs was regulated at transcription and protein modification levels. In transcriptome data, we found that the genes in lignin biosynthesis pathway and antioxidant enzyme system of the granulated juice sacs were significantly upregulated. However, in ubiquitinome data, we found that ubiquitinated antioxidant enzymes increased in abundance but the enzyme activities decreased after the modification, which gave rise to reactive oxygen species (ROS) contents in granulated juice sacs. This finding suggests that ubiquitination level of the antioxidant enzymes is negatively correlated with the enzyme activities. Increased H₂O₂ is considered to be a signaling molecule to activate the key gene expressions in lignin biosynthesis pathway, which leads to the lignification in granulated juice sacs of pomelo. This regulatory mechanism in juice sac granulation of pomelo was further confirmed through the verification experiment using tissue culture by adding H₂O₂ or dimethylthiourea (DMTU). Our findings suggest that scavenging H₂O₂ and other ROS are important for reducing lignin accumulation, alleviating juice sac granulation and improving pomelo fruit quality.

Fruit quality and yield of citrus orchards are co-regulated by complex interactions among soil properties, microbial communities, and plant physiological processes. However, systematic studies that integrate the soil–microbe–plant–fruit continuum remain limited. This study selected four representative ponkan orchards based on yield and fruit quality performance, and systematically determined and correlated key indicators in the soil–plant–fruit continuum. The results showed that the orchards with higher comprehensive performance exhibited more suitable soil pH, higher contents of soil organic matter and available nutrients, as well as higher activities of soil enzymes including urease and acid phosphatase. Compared with the orchards with lower comprehensive performance, soil bacterial and fungal Chao1, Shannon, and Simpson indices were higher in the orchards with higher comprehensive performance. Among the dominant phyla, the relative abundance of Proteobacteria was significantly higher, while that of Actinobacteria was significantly lower. Leaf photosynthetic indexes (chlorophyll content, net photosynthetic rate, Rubisco activity) of the higher-performing orchards were also significantly higher. Correlation analysis showed that soil microbial diversity and Proteobacteria were significantly positively correlated with soil nutrients, enzyme activities, leaf photosynthesis, fruit quality and yield, while Actinobacteria showed the opposite trend. These results provide a theoretical basis for soil management and high-quality cultivation of ponkan orchards.

The ponkan (Citrus reticulata Blanco cv. Ponkan), an important citrus crop, is increasingly threatened by soil acidification. This study evaluated the efficacy of various soil amendments, including lime alone (L), lime with gypsum and organic fertilizer (LGOF), lime plus K₂CO₃ (LK), and lime with chicken manure ash (LCMA), in mitigating soil acidification and improving ponkan seedling growth. Surface-applied lime raised topsoil pH and acid buffering capacity while reducing exchangeable Al. However, combined amendments (LGOF, LK, LCMA) more effectively alleviated acidity throughout the soil profile. They significantly increased pH and buffering capacity, decreased exchangeable H and Al in the 20–40 cm layer, and elevated exchangeable base cations (K⁺, Ca²⁺, Mg²⁺). These changes reduced Al content in roots, stems, and leaves, promoted deeper root growth, and increased biomass and nutrient uptake (N, P, K). Physiologically, combined amendments enhanced photosynthetic performance (chlorophyll, Pn, ΦPSII) and increased activity of key metabolic enzymes (Rubisco, SS, SPS, NR, GS), promoting sucrose, starch, and protein accumulation. LK rapidly raised subsoil pH and potassium levels, ideal for K-deficient orchards. LGOF and LCMA improved overall fertility by supplying Ca and Mg, with LGOF additionally enhancing soil structure in poorly structured acidic soils.

Citrus fruits produced in China are often affected by granulation. Granulation is an altered physiological state of citrus fruits occurring usually before harvest but whose underlying mechanisms remain elusive. In this study, cDNA-AFLP technology enabled the identification of 116 granulation-associated genes in pummelo (C. grandis) juice sacs. Differentially expressed transcript-derived fragments (TDFs) were shown to be mainly involved in biological regulation and signal transduction, carbohydrate and energy metabolism, nucleic acid, protein metabolism, stress responses, and cell metabolism. Therefore, granulation in pummelo juice sacs seems to involve the following alterations: (1) changes in hormone levels; (2) activation of metabolic pathways related to ATP and sugar synthesis to produce more energy; (3) nucleic acid accumulation and increased protein degradation; (4) activation of stress-responsive metabolic pathways; (5) accelerated juice sac senescence. Our findings provide an overview of differential responses occurring at the transcriptional level in granulated juice sacs, thus revealing new insights into the adaptive mechanisms underlying this altered physiological state in 'Guanximiyou' pummelo (C. grandis) juice sacs.

To clarify the effects of shade-net covering on sunburn control and fruit quality of ponkan, white, gray, black, and blue nets with differing light transmittance were applied to assess the canopy microenvironment, leaf photosynthesis, peel antioxidant system, and fruit quality. All shading treatments significantly reduced fruit surface temperature and light intensity compared with the unshaded control. Light transmittance was significantly higher under white (26.49%) and blue (25.69%) nets than under gray (12.97%) and black (12.58%) nets. Consequently, sunburn incidence decreased significantly from 31.41% in the control to 5.47–11.13% under shading. Regarding leaf photosynthesis, white and blue nets maintained a significantly higher net photosynthetic rate (P_n) and PSII photochemical efficiency than gray and black nets, whereas gray and black nets showed no significant difference in P_n from the control. All shading treatments significantly lowered reactive oxygen species (ROS) and malondialdehyde (MDA) levels, alleviating peel oxidative injury. By contrast, the sunburn peel of the unshaded control accumulated high levels of ROS and MDA and activated SOD, POD, CAT, and the AsA-GSH cycle, thereby elevating total antioxidant capacity (T-AOC). At fruit maturity, shading generally improved juice yield and nutritional quality. White and gray nets enhanced soluble solids content (SSC), white and blue nets improved the sugar–acid ratio, and gray, black, and blue nets increased vitamin C content, while the white net showed the best performance in soluble sugar accumulation. These results indicate that white and blue shade nets are more effective in optimizing the canopy microenvironment of ponkan. Shading followed by post-shading light recovery alleviates fruit sunburn and enhances fruit quality.

Citrus is a highly important global fruit crop that faces significant challenges from citrus Huanglongbing (HLB). The nucleotide binding site leucine-rich repeat (NBS-LRR) gene family plays a central role in plant immunity. However, comprehensive studies on the function of NBS-LRR genes in HLB resistance or tolerance remain limited. In this study, we identified 278 NBS-LRR genes from Citrus clementina. These genes exhibit 7 domain architectures. Physicochemical analyses revealed significant subfamily divergence in length (91-2101 aa), molecular weight (10.57-237.18 kDa), and isoelectric point (pI, 4.61–9.35). Over 82% of proteins localize to the nucleus/cytoplasm, indicating roles in signal transduction and transcriptional regulation. NBS-LRR genes cluster non-randomly among chromosomes, forming tandem repeats on chromosomes 3, 5, and 7. Phylogenetic analysis grouped the NBS-LRR family into 5 clades. Conserved domain analysis identified 10 motifs; Motif 1 and 2 are highly conserved, potentially involved in pathogen recognition. Promoter analysis detected 36 cis-acting elements linked to diverse processes (e.g., hormone/stress response). Synteny analysis revealed 9 genes are orthologous genes of Arabidopsis thaliana. Crucially, transcriptome data showed that significantly higher NBS-LRR expression in Huanglongbing-infected plants, pinpointing candidate gene Ciclev10018143m.1. This study provides a foundation for citrus disease resistance breeding and candidate genes for functional validation.