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Citrus plants exhibit positive floral response under water stress conditions, however, the mechanistic understanding of floral induction remains largely unexplored in water deficit. In this study, DNA methylomic and transcriptomic analyses were integrated to explore the flowering bud formation as well as branches building after light drought stress. While comparing with the conventional watering group (CK), the light drought group treated with five months (LD) showed a significant increase in the flowering branches, whereas an apparent decrease in vegetative branches. Global DNA methylation analysis showed that the LD Group acquired DNA methylation in more than 70,090 genomic regions and lost DNA methylation in about 18,421 genomic regions compared with normal watering group, this indicates that water deficiency leads to a global increase in the expression of DNA methylation in citrus. In the same time, we verified that the increase of DNA methylation level in LD group was correlated with the decrease of DNA demethylase related gene expression. Interestingly, in transcription analysis, it was found that the promoting flower genes of the LD group did not increase but decreased similarly with repressing genes, which is contrary to the intended result. Thus, we thought the lower expression of suppressors FLC and BFT were the key influencing factor to stimulate the flowering branches formation after LD treatment. Moreover, there was a strong negative correlation between the genes expression level and methylation level of the flowering induction/flower development genes. In general, we thought high global DNA methylation level induced by water deficit regulate the flowering branches building by reducing FLC and BFT genes expression., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To understand the effects of different fertilizer applications on soil quality and arbuscular mycorrhizal colonization, we examined the changes in soil physical and chemical properties, mycorrhizal colonization and propagules, and their relationships in citrus under inorganic fertilization (IF), organic fertilization (OF), combined organic and inorganic fertilization (CF), and no fertilization (CK) treatments. Results showed that all fertilization treatments improved the content of rhizospheric soil organic carbon (SOC), nutrient contents, and electrical conductivity (EC). Both CF and OF significantly increased soil pH, soil aggregate stability, activities of urease, catalase, and sucrase, and the colonization and reproduction of arbuscular mycorrhizae fungi (AMF) in citrus rhizosphere. However, IF treatment significantly decreased soil pH and the colonization and reproduction of AMF in citrus rhizosphere. The number of mycorrhizal colonization and propagation was positively correlated with soil aggregate stability, SOC content, total nitrogen content, total phosphorus content, urease activity, and soil pH. Combined with the principal component analysis, we concluded that application of inorganic fertilizer alone could cause soil acidification and inhibit AMF colonization in citrus orchards. In contrast, organic fertilizer combined with inorganic fertilizer in citrus could improve the soil quality and AMF colonization. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]