In this study, we investigated the effects of the combined application of different doses of potassium and organic fertilizers on the nutrient uptake, yield, and quality of Newhall navel oranges (Citrus sinensis Osb. cv.). The citrus trees were treated with potassium fertilizer alone or with one of three combinations of organic and potassium fertilizers. Spring shoots, autumn shoots, and fruit were collected when they reached maturity, and the relevant indexes were determined by conventional methods. The soil carbon flux was measured 15, 35, 65, and 95 days after fertilizer application by a soil gas flux measuring instrument. The results showed that the dry weights of leaves and twigs of the spring shoots were 43.1% and 99.8% higher in the OM15+K05-treated (15 kg organic manure and 1.05 kg potassium sulfate per plant) than in the OM0+K25-treated (1.25 kg potassium sulfate per plant) plants, respectively. Combined potassium and organic fertilizer application helped the shoots and fruits to absorb more mineral nutrients. As a result, the fruit yield increased, fruit surface color improved, and skin thickness became thinner. The internal quality of the fruit was also improved by the combination of different potassium fertilizers and organic fertilizers. The fruit quality was optimal for the OM10+K15-treated plants. The recommended doses of organic and potassium sulfate fertilizers for Newhall navel orange orchards with purple paddy soil in southwestern China are 5–10 kg/plant and 1.15–1.20 kg/plant, respectively.

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.

The Satsuma mandarin, a prominent fresh citrus variety cultivated in Asia, is susceptible to fruit cracking, a physiological disorder that significantly impacts yield and economic efficiency. This phenomenon occurs during the fruit expansion phase. The present study sought to further elucidate the correlation between citrus fruit cracking and fruit peel development or mineral elements, as well as to propose efficacious management measures. The present experiment was conducted on Citrus unshiu Marc. cv. ‘Miyagawa Wase’ over two successive seasons—2022 and 2023. The dynamic changes in fruit morphology were recorded using calipers, and the peel strength was assessed via a Plus Texture Analyzer. Paraffin sectioning technology was used to observe the morphological structure of peel cells. At 10 days after full bloom (DAFB), the peel cells exhibited vigorous proliferation, and the fruit and peel thicknesses underwent rapid expansion. At 50–60 d after full bloom, the longitudinal and transverse diameters of the fruit exhibited a marked increase in the growth rate of the former over the latter. At 80 d after full bloom, both the peel thickness change and the fruit growth rate exhibited a marked deceleration, and the albedo layer cells began to show signs of perforation. The following two time points were preliminarily proposed as the key points for the control of citrus fruit cracking: key point one was 50–60 days after full bloom; and key point two was 80–90 days after full bloom. The nitrogen (N), phosphorus (P), and potassium (K) contents in the different orchards were measured via the semi-micro Kjeldahl nitrogen method, the molybdenum–antimony colorimetric method, and flame photometry, respectively. The determination of other mineral elements was conducted by means of inductively coupled plasma spectroscopy. Principal component analysis was employed to analyze the 21-parameter indices of mineral elements in soil and leaf samples from the three orchards with different levels of fruit cracking. The study found that high concentrations of leaf Fe, P, and soil Cu, as well as organic matter content, contributed negatively to the extent of fruit cracking. The impact of diverse control measures on the incidence of fruit cracking was subsequently observed, following the implementation of tree crown spray treatments. The application of 0.5% calcium superphosphate and 0.006% EDTA-Fe, in combination with 10 ppm GA₃ sprayed during two critical periods, significantly reduced fruit cracking and did not adversely affect the internal or external quality of the fruits. The study emphasises the necessity of customising management measures according to the developmental characteristics of citrus fruits, given the observed varietal and regional distinctions in susceptibility to cracking. These findings are pivotal for advancing research in the field of fruit cracking and promoting the healthy development of the industry.

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.