Protein hydrolysates (PHs) and arbuscular mycorrhizal fungi (AMF) are environmentally friendly biostimulants that effectively promote crop growth and alleviate the damage from abiotic stress. However, the physiological and molecular regulatory mechanisms are still unclear. This study aimed to explore the effects of PHs and AMF on growth, mineral nutrient absorption, and expression of Aquaporins and SOSs in Goutoucheng (Citrus aurantium) under salt stress. Results showed that PH application and AMF inoculation significantly promoted plant growth and enhanced mineral element absorption and sodium effluxion in citrus under salt stress. The biomass, root activity, leaves mineral nutrition contents in PHs, AMF, and combined (PHs and AMF) treatments were significantly higher than those of control. Leaves sodium content in three treatments was significantly lower than in the control. AMF and combined treatments showed dominant effects than PHs alone. Besides, PHs interacted with AMF on growth, nutrient absorption, and sodium effluxion. Importantly, AMF and PHs induced stress-responsive genes. PIP1, PIP3, SOS1, and SOS3 expression in PHs and AMF treatments was significantly higher than control. Thus, it was concluded that AMF and PHs enhanced the salt tolerance of citrus by promoting nutrient absorption and sodium effluxion via up-regulating the expression of PIPs and SOSs. The mixed application of PHs and AMF had a better effect.

Potassium is an essential mineral nutrient for citrus growth and stress response. In this study, the HAK/KUP/KT gene family was identified from the genome of trifoliate orange (Poncirus trifoliata). The physical and chemical properties, chromosomal location, gene structure, evolutionary relationship, conserved motifs, and tissue expression characteristics were analyzed. The expression characteristics under low potassium and salt stress were analyzed by fluorescence quantitative PCR. The function of PtKUP10 was investigated by heterologous expression in Arabidopsis thaliana. The results showed that at least 18 PtKUPs were distributed in seven chromosomes. Phylogenetic analysis showed that four PtKUPs clustered in clade I, which mediated the high-affinity potassium absorption. Gene expression analysis showed that four PtKUPs were highly expressed in root, seven PtKUPs were up-regulated by low potassium stress, and nine PtKUPs were up-regulated by salt stress. The cis-acting elements on the promoter of PtKUPs were predominantly involved in stress and hormone responses. Overexpression of PtKUP10 in Arabidopsis thaliana could enhance salt tolerance by accumulating more potassium in the shoot and reducing sodium content in the shoots and roots. These results indicated that PtKUPs play important roles in potassium absorption and salt stress response, and PtKUP10 might enhance salt tolerance by maintaining potassium and sodium homeostasis.

Citric acid serves as the principal organic acid in citrus fruits, with its concentration critically determining fruit flavor and market acceptability. Isocitrate dehydrogenase (IDH), a key enzyme in citric acid metabolism, mediates the conversion of citrate to α-ketoglutarate. This study cloned six candidate genes encoding IDH from grapefruit (Citrus paradisi). Bioinformatics analysis showed that all six genes contained the typical characteristic structure of IDH. Gene expression analysis found that CpNADP-IDH1 is highly expressed in mature and low-acid varieties. Overexpression of CpNADP-IDH1 significantly increased IDH enzyme activity and decreased citric acid content in transgenic grapefruit callus. These results showed that at least six genes encoding IDH exist in grapefruit, among which CpNADP-IDH1 catalyzes the decomposition of citric acid and regulates the organic acid content in fruits at maturity. CpNADP-IDH1 can be used as a candidate gene for molecular breeding of low-acid citrus varieties and as an essential target gene for developing citrus cultivation technology for reducing acid content.

Facility-forcing cultivation could effectively improve the quality of ‘Beni Madonna’ citrus (Citrus nankao × C. amakusa) and advance the ripening period. However, segment drying (KS) before harvest caused fruit quality deterioration and commodity value loss. In this research, we investigated the physiological and molecular characteristics involved in citrus segment drying under facility-forcing cultivation. The juice yield, sugar, acid, vitamin C, and lignin contents in KS fruits were significantly decreased, and the contents of pectin and cellulose were significantly increased. The relative contents of abscisic acid and abscisic acid glucosyl ester in KS fruits were significantly decreased. A total of 1215 differentially expressed genes (DEGs) were screened by transcriptome sequencing. DEGs were significantly enriched in water metabolism, sugar metabolism, transportation, cell wall and phenylpropanoid biosynthesis, and plant hormone signal transduction. The decrease in water absorption and sugar synthesis, the increase of pectin and cellulose synthesis, and the decrease in ABA accumulation may be the main reasons that cause citrus fruit segments to dry under facility-forcing cultivation.

(1) Background: Huyou (Citrus changshanensis) is a traditional Chinese medicinal and edible citrus, yet cultivar- and tissue-dependent variations in metabolite profiles and their differential responses to thermal processing remain poorly characterized. (2) Methods: Physicochemical properties, targeted metabolite profiles (sugars, organic acids, amino acids, flavonoids, phenolic acids, limonoids), and antioxidant capacities were compared across five tissues (flavedo, albedo, segment membranes, pulp, seeds) of two Huyou cultivars, ‘Changhu No. 1’ (HY-N1) and ‘Huyou 01-7’ (HY-17), under freeze-drying and hot-air drying (60 °C). (3) Results: HY-N1 pulp had higher soluble sugars, acidity, and amino acids but lower juice yield and greater limonoid bitterness; HY-17 exhibited a more balanced sugar-acid profile and reduced bitterness, offering metabolic clues for its prospective fresh-consumption potential. Tissue-specific distribution revealed flavonoids concentrated in peel and segment membranes, while phenolic acids and limonoids accumulated in flavedo and seeds. Across tissues, HY-N1 maintained higher levels of most flavonoids and phenolic acids than HY-17. Hot-air drying degraded heat-labile amino acids and flavanone glycosides, yet enriched polymethoxyflavones and phenolic acids presumably through matrix-associated effects. Antioxidant capacity aligned spatially with metabolite allocation and showed variable responses to hot-air drying. (4) Conclusions: These findings inform cultivar-specific utilization, tissue-selective valorization, and thermal processing strategies of Huyou.