Background: Lemon peels are crucial quality traits and important sources of nutraceuticals. However, the regulation of peel development remains largely misunderstood. Here, we delved into the dynamic molecular mechanisms that control smooth lemon peel formation through transcriptomics and metabolomics analyses of developing peels at five different stages, including C1 (30 DAF, day after flowering), C2 (60 DAF), C3 (90 DAF), C4 (120 DAF), and C5 (150 DAF). Results: Our analyses revealed that peel development is stage-specifically regulated, with the transition from stage C2 to stage C3 being the most critical period. Notably, major metabolic adjustments and transcriptional switches occurred between C2 and C3. Particularly, phytohormones and their related genes, flavonoids, terpenoids, phenolic acids, vitamin C, and citric acid were down-regulated at stage C3. Differentially expressed genes between the transition from C2 to C3 were mainly enriched in hydrolase activity, nucleic acid binding transcription, transcription factor activity, sequence-specific DNA binding and pathways such as plant hormone signal transduction, starch and sucrose metabolism, MAPK signaling pathway-plant, plant-pathogen interaction, and circadian rhythm-plant biosynthesis. Plant hormone signaling was crucial for peel development process. Specifically, we found that crosstalks between jasmonic acid, abscisic acid, salicylic acid, gibberellic acid, and ethylene dynamically and coordinately regulate peel development process. ABC transporter family genes and green-degrading promoting genes, including protein SGR1 ( LOC18041639 ), ervatamin-C ( LOC18051181 ), and many ethylene-responsive transcription factors were significantly induced from C2. Furthermore, we screened out stage-specific most regulated genes. Conclusions: Our findings offer molecular understanding of peel formation process and fundamental resources for the functional dissection of its regulatory networks and for genomics-assisted control of peel shape and fruit quality in lemon.

Lemon is an important economic crop in the world and can bloom several times each year. The Dehong prefecture of Yunnan province is one of the main lemon-producing areas in China, and is a suitable area for planting high-quality lemons in China. However, the hot and rainy summers in Yunnan are not conducive to flower bud differentiation, which leads to low lemon yield. Therefore, normal flower bud differentiation is important to guarantee lemon production and quality. In this study, we selected some lemon trees for a pot experiment, and we sprayed the lemon leaves with gibberellin and paclobutrazol to regulate lemon flower formation. We set four separate concentration gradients for each regulator: 50, 100, 150, and 200 mg/L. The PBZ concentration gradients were 200, 400, 600, and 1200 mg/L. After the experiment, we determined and analyzed the morphological index and fruit quality of the lemon trees. The results showed that under the same cultivation and management conditions, spraying paclobutrazol substantially inhibited the growth of lemon shoots, increased the fruit setting rate, and improved the fruit yield. However, gibberellin considerably reduced the number of lemon flowering branches and promoted the vegetative growth of the lemons. When the concentration of paclobutrazol was 600 mg/L, the amount of lemon fruits reached the maximum, which remarkably increased the titratable acid and soluble solids contents of the fruit and ultimately increased the fruit quality compared with the control. Altogether, selecting the appropriate concentration of regulators to control the flowering and fruit setting of fruit trees is important and has value in guiding actual production.

Fatty acids tend to undergo dynamic changes during the growth and development of fruits. In this study, we analyzed the variations in fruit morphology and kernel fatty acid fractions and contents at seven post-flowering stages in the fruit of ‘A4’ and ‘OC’, two main macadamia cultivars in Yunnan, China. The single fruit weight and longitudinal and transverse diameters showed a ‘fast–slow–stable’ growth trend, and the fruit shape index gradually decreased with fruit development. A total of 13 saturated fatty acids, 18 monounsaturated fatty acids, and 10 polyunsaturated fatty acids were detected in macadamia kernels at seven developmental stages. The total fatty acid content in ‘OC’ and ‘A4’ tended to first increase and then decrease. The fatty acid content accounted for 8.81% and 6.33% of the total fatty acids at 50 days after flowering (DAF), and peaked at 95 DAF and 125 DAF (the fatty acid content accounted for 25.61% and 20.69% of the total fatty acids), indicating that these two periods are critical for fatty acid accumulation in the two cultivars. In addition, oleic acid, palmitoleic acid, cis-Vaccenic acid, and hexadecenoic acid were determined as the main fatty acids. This study reveals the dynamic changes in fatty acid composition and content in ‘OC’ and ‘A4’ during fruit development, providing a scientific basis for determining the appropriate harvesting time for macadamia nuts.

As the second largest family of transcription factors (TFs) in plants, basic helix–loop–helices (bHLHs) play key roles in regulating plant growth and development and responding to environmental stress. As the fastest growing Citrus variety in China in recent years, Orah mandarin has vital economic and nutritional value. Although a comprehensive genome-wide analysis of the bHLH TF family has been performed in many plants, a systematic study of the genes of this family has not been carried out in Orah mandarin. In this study, 114 bHLH TFs were identified in Orah mandarin via genome-wide analysis and were classified into 27 subfamilies according to the evolutionary tree. The gene expression profile revealed that five genes were significantly upregulated at 12 h and 24 h after low-temperature stress treatment. In addition, soluble sugars, soluble proteins, and proline contents increased with increasing low-temperature stress, which promoted the expression of the CrbHLH46 gene, thus mediating the interconversion pathway of pentose and glucose to improve the cold tolerance of Orah mandarin. The results help explore the characteristics and functions of CrbHLH genes and provide a basis for further research on the Orah mandarin resistance to low-temperature stress.

Background: The Orah mandarin is an economically important variety of Citrus reticulata for citrus growers in Yunnan Province, China. Generally, the fruit peel is smooth, an attractive feature for consumer preferences. Recently, rough peels have been observed in several orchards, making the fruit aesthetically less desirable. Little is known about the mechanism of rough skin development. Methods: In this study, we used global metabolomics and a comparative transcriptomic approach to characterize the differences between smooth (CK) and rough (CP) Orah mandarin peels. Results: Our results indicate that CP fruits have a significantly larger diameter, peel weight and thickness, total soluble solids, and titratable acid content compared to CK. Metabolomic analysis detected 810 metabolites, of which 192 were differentially accumulated in CP and CK. CP is characterized by higher levels of flavonoids, amino acids and derivatives, terpenoids, and alkaloids. We also report nine compounds detected exclusively in CP, including dambonitol, 3-methyl-L-histidine, deacetylnomilinic acid, obacunoic acid, and 6-O-acetylarbutin. The transcriptome results showed that the expression of genes enriched in flavonoids, lipid, and amino acid metabolism and related pathways were consistent with the metabolome profiles. We also discuss the possible involvement of phytohormones in peel roughening. Conclusions: Overall, we present, for the first time, a detailed comparative metabolome and transcriptome profile in smooth and rough Orah mandarin peels. Our data and discussion highlight the potential mechanisms and provide a theoretical basis for the improvement of rough peel Orah mandarins.

This study investigated the effects of Ethephon (CEPA) and Gibberellic acid (GA₃) treatments on the post-harvest flavor quality of ‘Yunning No. 1’ green lemon. A comprehensive analysis was conducted on the changes in primary metabolites (sugars, organic acids, amino acids, alcohols) in the pulp and peel, as well as those in major volatile compounds in the peel during fruit storage. The results showed that CEPA treatment initially increased volatile compounds like monoterpenes and sesquiterpenes in the fruit peel during early storage, but later decreased these compounds along with total sugar and amino acid content in the pulp. Conversely, GA₃ treatment markedly delayed the decline in sugars and organic acids in the fruit peel, preserved the amino acid content in the pulp and the alcohol content in the peel, and delayed the decrease in volatile compound content in the peel. In conclusion, GA₃ treatment effectively delayed the decline in primary metabolites and volatile compounds to maintain the storage quality of green lemon; therefore, GA₃ represents a suitable strategy for the preservation of green lemons. CEPA temporarily improved aroma but accelerated quality deterioration, making it better suited for short-term degreening. This study offers a theoretical foundation for optimizing post-harvest degreening and preservation techniques of green lemons.