Citrus creasing is a physiological rind disorder. Satsuma mandarin (Citrus unshiu Marc.) is the most widely cultivated mandarin variety worldwide and exhibits a high susceptibility to creasing. To investigate the physiological mechanisms underlying creasing, satsuma mandarin trees were treated with different drought stress during fruit expansion, then the relationship between the soil water content and creasing incidence was analyzed, while also examining the rind morphology, oil gland distribution in the flavedo, antioxidant enzyme activities, hormone concentrations, cell wall components, mineral content of creasing fruit, and the impact of creasing on fruit quality. Results showed that severe water stress (35% SRWC) increased the creasing incidence rate by 28% compared to well-irrigated treatments (80% SRWC). The creasing fruit oil gland diameter reduced by 35.7% and the density increased by 149.7% compared to healthy fruits. Simultaneously, the content of H₂O₂ and proline elevated by 47.1% and 8.3% respectively, and the activities of SOD, POD, and CAT of the creasing rind were enhanced significantly. Additionally, the content of IAA, ZR, and MeJA decreased by 17.2%, 7.8%, and 50.2%, respectively. Cell wall components such as cellulose, hemicellulose, and protopectin content reduced by 44.6%, 31.7%, and 33.1%, while soluble pectin increased by 36.3%. Significant alterations were observed in several minerals (Al, Fe, Na, Ni, V, Ga, Zn, Ba, Sn, Hg, Sc, Y, and La). However, fruit quality remained unaffected by creasing. These results demonstrate that drought is a key factor inducing creasing. Increased oil gland density, the degradation of cell wall components, elevated oxidative stress, reductions in phytohormones, and altered mineral element content work together to contribute to rind cells’ structural instability and lead to creasing in the satsuma mandarin.

Light regulates plant growth and secondary metabolism. To improve the nutritional quality and economic benefits of satsuma mandarin (Citrus unshiu Marc.) under greenhouse conditions with reduced natural light, we investigated the effects of supplementary lighting with different light qualities and intensities on fruit coloration, carotenoid accumulation, and fruit quality. Supplementary lighting was applied from 156 days after flowering using red, blue, red–blue combinations, and white light. All treatments significantly increased the color contribution index and total carotenoid content in both the rind and flesh compared with the control, with the red–blue light RB14 and white light W1 treatments being the most effective. β-Cryptoxanthin, the major carotenoid in satsuma mandarin, increased by 120.3% and 97.6% under RB14 and W1, respectively. Supplementary lighting also significantly increased the soluble sugar, total soluble solid, ascorbic acid, amino acid, flavonoid, and phenolic acid contents at most sampling time points. The leaf chlorophyll content increased, and chlorophyll degradation was delayed. qRT-PCR showed that RB14 and W1 upregulated the key carotenoid biosynthesis genes CitPSY, CitPDS, CitZDS, CitCRTISO, CitLCYE, CitLCYB, CitBCH2, CitZEP, and CitVDE and downregulated CitCCD4. These results suggest that supplementary lighting may enhance carotenoid accumulation and improve fruit quality by increasing the chlorophyll content and modulating carotenogenic gene expression, thereby providing a theoretical basis for citrus quality improvement.

Cuticular wax plays crucial roles in plant growth and various environmental stresses. Previous studies on cuticular wax biosynthesis have been reported in many plants. However, the molecular mechanisms of wax biosynthesis at the posttranscriptional level are less studied. Following the observation that csi-miR1134 was upregulated in the pericarp of wax-deficient navel orange fruit, transgenic overexpression of csi-miR1134 significantly reduced leaf cuticular wax, whereas silencing csi-miR1134 promoted wax biosynthesis and enhanced drought tolerance in citrus. Furthermore, R2R3-MYB transcription factor CsMYB16 was identified as a target of csi-miR1134 and positively regulated cuticular wax biosynthesis and drought tolerance. CsMYB16 was further verified to directly bind to the promoter of the aldehyde decarbonylase-encoding gene ECERIFERUM1 (CsCER1) and activate its expression. Finally, we revealed that the csi-miR1134-CsMYB16-CsCER1 module alters drought tolerance in citrus via cuticular wax biosynthesis. This study reveals a novel posttranscriptional regulatory framework for cuticular wax biosynthesis, thereby providing a theoretical basis for breeding new citrus varieties with enhanced drought tolerance.

In this study, we identified 34 HD-ZIP genes at the genome-wide level in P. trifoliata (Poncirus trifoliata (L.) Raf.), and conducted comprehensive bioinformatic analyses. The expression profiles of PtrHB genes in response to six phytohormones (ABA, SA, JA, BR, ETH, GA) and two abiotic stresses (dehydration, cold) were further determined. Phylogenetic analysis assigned all PtrHB proteins to four distinct subfamilies (I–IV) with highly conserved domain compositions, in line with the established classification system in Arabidopsis thaliana. Segmental duplication was revealed as the primary driver of PtrHB family expansion, whereas three tandem duplication events were also detected. Promoter cis-element profiling revealed subfamily-specific distribution of hormone- and stress-responsive elements. Several candidate genes with prominent stimulus-specific responses were highlighted, including PtrHB32 (strongly induced by dehydration, cold and ABA), PtrHB15 (JA-responsive) and PtrHB16 (SA-responsive). Collectively, this study provides the first systematic genome-wide analysis of the HD-ZIP transcription factor family in P. trifoliata, and offers valuable insights into the evolutionary expansion and functional divergence of this family. Our results also lay a theoretical basis for functional validation of candidate stress-regulatory genes and for molecular breeding of stress-tolerant citrus rootstocks.