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.

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.

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.