The Sprays of Nanoparticles of Zinc Oxide and Titanium Dioxide Enhance Fruit Coloration in Litchi

Feizixiao litchi ( Litchi chinensis Sonn.), the most widely cultivated commercial variety in China, holds significant economic and social value. However, uneven coloration on the fruit surface markedly diminishes its market appeal. This study investigated the effect of foliar application of ZnO (100 mg/L) and TiO 2 (50 mg/L) nanoparticles (NPs) on the color and quality of Feizixiao during fruit expansion, with water sprayed fruits serving as the control. At harvest, fruits treated with ZnO NPs (63.4 ± 9.5%) and TiO 2 NPs (70.4 ± 8.8%) exhibited significantly enhanced skin coloration rates compared to the control (53.3 ± 9.2%). RNA sequencing of pericarp tissues revealed that the genes associated with anthocyanin biosynthetic enzymes (DFR and UFGT) and transcription factors ( MYB, bHLH, WD40 , and NAC ) were upregulated by both ZnO and TiO 2 NPs sprays , leading to enhanced anthocyanidin synthesis and concomitant fruit pigmentation. Meanwhile, ZnO NPs treatment upregulated the expression of genes encoding Mg and Zn transporters, leading to elevated pericarp Mg and Zn. However, TiO 2 NPs spray reduced S assimilation in pericarp by downregulating the expression SULTR3;4 gene, which was negatively associated with enhanced photosynthesis as well as photorespiration. The alteration of pericarp Mg, Zn, and S was conducive to anthocyanidin synthesis and accumulation. Therefore, ZnO NPs treatment promoted fruit pigmentation by increasing pericarp Mg, Zn, Cy3R, and phenolic copigments accumulation (EC, rutin), while TiO 2 NPs spray enhanced coloration by reducing pericarp S and increasing Cy3R and copigments (PC B2, Qu3G, ferulic acid, and Ka3G). Overall, this study offers a viable strategy for improving fruit coloration in litchi.