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Ten 16-year-old trees were used as test materials to investigate the effect of foliar calcium fertilizer on the sugar content of ‘Feizixiao’ litchi (Litchi chinensis Sonn.) pulp. The experiment began 35 days after anthesis (DAA) in 2020 and 2021, and the treatment was a foliar spray application of 0.3% CaCl2 aqueous solution, whereas the control was a foliar spray application of water. The sugar content, sucrose-metabolizing enzymes, and ATP-dependent phosphofructokinase (PFK) activities in pulp were measured in 2020 and 2021. Transcriptome sequencing (RNA-seq) was performed on RNA samples from treatment and control fruit pulps at 35, 63, and 69 DAA (full mature stage) in 2020, and 10 genes were chosen for confirmation by real-time polymerase chain reaction (PCR) in 2020 and 2021. At full maturity, the soluble sugar content in the calcium-treated group was extremely significantly or significantly higher than that in the control group. After 63 DAA, the net sucrose-metabolizing enzyme activity in the calcium-treated group was significantly higher than that in the control group. Furthermore, at full maturity, the calcium-treated group had significantly higher sucrose synthase cleavage activity and significantly lower PFK activity than the control group. Fifty-four highly expressed genes in the glycolytic pathway (EMP) were screened from transcriptome data, including hexokinase, PFK, and pyruvate kinase genes; 87% of these genes were downregulated in the treatment group compared with the control group at 69 DAA in 2020. The linear regression between RNA-seq and real-time PCR results was significant in 2020 (r = 0.9292) and 2021 (r = 0.8889). When the fruit is fully ripe, calcium treatment increases net sucrose-metabolizing enzyme activity by increasing sucrose synthase cleavage activity, promoting the accumulation of reducing sugars, and it downregulates phosphofructokinase gene expression in EMP, promoting sugar accumulation.

During the period of ‘Feizixiao’ litchi fruit pericarp’s full coloring, there is a phenomenon of “sugar withdrawal” in the pulp, and the mixed foliar nutrients of calcium and magnesium (Ca+Mg) can effectively overcome this phenomenon. One of the reasons for this may be that it is related to the influence of the mixed nutrients of Ca+Mg on the bypass respiratory pathways of the pulp. The major fruit quality indicators, the rates of cytochrome and cyanide-resistant respiratory pathways (CP and AP) in the pulp and the activities of their key enzymes, were observed continuously in 2021 and 2022, and the deferentially expressed genes (DEGs) related to the two bypass respiratory pathways in the pulp were screened by RNA-seq analysis, with a qPCR of the random genes performed to verify the results. Ca+Mg treatment kept the content of the total soluble sugar in the pulp stable and higher than that the control in the ripening stage; Ca+Mg treatment increased the activities of electron-transferring enzymes in the electron transport chain, such as NADH dehydrogenase (ND), succinate dehydrogenase (SDH), cytochrome bc1 complex, and cytochrome c (Cyt c) through up-regulating their gene expression. In terms of the rate-limiting enzymes in the pulp, Ca+Mg treatment increased the activity of cytochrome oxidase (COX) in the CP pathway by up-regulating the expression of COX genes, then increased the CP respiratory rate and inhibited the CP respiratory rate decrease; meanwhile, it also inhibited the activity of AOX (alternate oxidase) in the pulp in the AP pathway by down-regulating the expression of AOX genes, then inhibited the increase in the AP respiration rate. The qPCR validation of randomly selected DEGs showed a significant unitary linear correlation between their expression levels and the results of the RNA-seq analysis. Therefore, one of the physiological mechanisms on the mixed foliar nutrients of Ca and Mg overcoming the phenomenon of “sugar withdrawal” in the ‘Feizixiao’ litchi pulp could be to promote CP and to inhibit AP, and then to delay the ripening and senescence of the pulp.

To investigate the causes of the “sugar receding” in ‘Feizixiao’ litchi (Litchi chinensis Sonn.) pulp, the main sugar contents and sucrose metabolism enzyme activities were measured in pulp obtained in 2020 and 2021. Pulp RNA obtained in 2020 was extracted at 35, 63, and 69 days after anthesis (DAA) for transcriptome sequencing analysis. The differential expression of genes was verified by real-time PCR for both years. The results showed that after 63 DAA, the contents of soluble sugars and sucrose decreased, and the contents of fructose and glucose increased in both years. The dynamic changes in sucrose metabolism enzyme activities were similar in both years. After 63 DAA, except for acid invertase (AI) in 2021, the activities of other enzymes decreased significantly, and the net activity of sucrose metabolism enzymes showed a strong sucrose cleavage activity. There were 18061, 19575, and 985 differentially expressed genes in 35 d vs. 63 d, 35 d vs. 69 d, and 63 d vs. 69 d, respectively. Ninety-one sugar metabolism genes were screened out, including sucrose synthase (SS), sucrose phosphate synthase (SPS), AI, neutral invertase (NI), hexokinase (HK), glucose 6-phosphate dehydrogenase (G6PD), 6-phosphogluconate dehydrogenase (6PGD), phosphofructokinase (PFK), and pyruvate kinase (PK) genes. In 63 d vs. 69 d, seventy-five percent of sucrose metabolism genes were downregulated, seventy-seven percent of genes in glycolysis (EMP) were upregulated and the PFK genes were significantly upregulated. There was a significant linear correlation between the expression of 15 genes detected by real-time PCR and the transcriptome sequencing results (r2020 = 0.9139, r2021 = 0.8912). These results suggest that the upregulated expression of PFK genes at maturity may enhance PFK activity and promote the degradation of soluble sugar in pulp through the EMP pathway, resulting in decreased soluble sugar and sucrose contents and “sugar receding” in pulp. Moreover, the downregulated expression of sucrose metabolism genes in pulp decreased the activities of these enzymes, but the net activity of these enzymes resulted in cleaved sucrose and replenished levels of reducing sugars, resulting in a stable reducing sugar content.

To investigate the characteristics of photosynthetic physiological changes in leaves of Mangifera indica L. cv. Guifei under enhanced ultraviolet (UV)-B radiation, natural light-exposed trees were used as controls and 96 kJ·m−2·d−1 enhanced UV-B radiation was artificially simulated in the field. The changes in fruit maturity and quality, the leaf net photosynthetic rate (Pn), photosynthetic pigment contents, photochemical reactions, the activities of photosynthetic enzymes and related gene expression levels were determined. Compared with the control, the percentage of mature fruits under the treatment significantly increased, and fruit quality improved. The net photosynthetic rate (Pn), photosynthetic pigment content, Hill reaction activity, and photochemical quenching coefficient (qP) of the treated leaves showed significantly higher values than those of the control leaves. The activities of Rubisco and Rubisco activating enzyme (RCA) and the expression levels of the Rubisco large subunit and Rubisco small subunit were significantly increased. Treatment with 96 kJ·m−2·d−1 enhanced ultraviolet-B radiation improved Rubisco activity by increasing the expression of the Rubisco large and small subunit genes, thereby enhancing the CO2-fixing capacity and dark reaction capacity of leaves. Thus, the net photosynthetic rate of leaves increased, which promoted the early maturity of ‘Guifei’ mango by the rapid accumulation of photosynthetic products.

Objective: In the future, the stress of enhanced UV-B radiation on the Earth will first affect the photosynthesis of plants, including mangoes. Therefore, it is necessary to study the effects of enhanced UV-B radiation on the photosynthesis of mangoes. Methods: ‘Tainong No 1’ mango trees in the field were selected as the experimental material and divided into 2 groups: one group was shined under 96 kJ·m−2·d−1 UV-B lamps for artificially simulated treatment of enhanced UV-B radiation, and the other group was shined under sunshine directly as the control (CK). The main photosynthetic physiological indicators were measured with conventional methods, and the expression levels of the genes encoding large and small subunits of the Rubisco enzyme were measured with fluorescent qPCR. The changes in stomatal morphology and chloroplast structure were observed with scanning electron microscopy and transmission electron microscopy. Results: The content of malondialdehyde (MDA) and the relative conductivity in the leaves of the treatment tended to be significantly higher than those of the CK. The net photosynthetic rate (Pn) of the treatment tended to decrease and be lower than that of CK. The dynamics of intercellular CO2 concentration (Ci) of the treatment and CK changed differently from each other but generally tended to decrease, and that of the treatment tended to be significantly higher than that of CK. The stomatal conductance (Gs) of the treatment and CK both generally decreased, and that of the treatment was always significantly lower than that of CK. The contents of chlorophyll a, chlorophyll b and total chlorophyll and the ratio of chlorophyll a/b of the treatment were lower than those of CK, while the carotenoid content showed the opposite trend. The stomata and the surface of leaves of the treatment were sunken and damaged, respectively. The palisade tissue, spongy tissue and upper epidermis thickened more, and the total thickness significantly increased. Meanwhile, the ratio of palisade tissue to spongy tissue decreased. During treatment, the chloroplasts were swollen and shortened, the number of chloroplasts was reduced, the starch grains were degraded, and the grana lamella were distorted, loosely arranged and blurred. The expression of the genes encoding the Rubisco large subunit (rbcL) in the treatment was significantly inhibited, while that encoding the Rubisco small subunit (rbcS) decreased first and increased later. In conclusion, 96 kJ·m−2·d−1 enhanced UV-B radiation treatment caused damage to the leaf cell membrane system. This led to stomatal limitation of photosynthesis by destroying the stomatal structure and nonstomatal limitation of photosynthesis by damaging the submicrostructure of the chloroplasts and downregulating the expression of rbcL. The leaves may resist the photosynthetic damage caused by enhanced UV-B radiation by upregulating rbcS expression as much as possible.

The sugar contents of ‘Feizixiao’ litchi (Litchi chinensis Sonn.) decrease at full maturity; calcium (Ca) and magnesium (Mg) foliar fertilizer can resolve this “sugar receding”. To investigate the physiological mechanism of Ca and Mg foliar fertilizer used to resolve the “sugar receding” phenomenon in ‘Feizixiao’ litchi pulp, 16-year-old litchi trees were treated with 0.3% CaCl2 + 0.3% MgCl2 foliar spraying or water as a control. We determined the pulp sugar content over a two-year period in 2020 and 2021. Pulp total RNA was extracted for transcriptome sequencing in 2020, and the expression pattern of 10 differentially expressed genes (DEGs) was verified by real-time PCR in 2020 and 2021. The results showed that the fertilizer treatment significantly increased pulp fructose and total soluble sugar contents at maturity in both years. According to Gene Ontology (GO) functional enrichment analysis, there were 155 DEGs divided into 35 GO categories, among which 49 DEGs were divided into 49 pathways according to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. We isolated sugar-metabolism-related enzyme genes, including sucrose synthase (SS), acid invertase (AI), neutral invertase (NI), sucrose phosphate synthase (SPS), and hexokinase (HK). All sucrose-metabolism-related enzyme (NI, AI, SS, SPS) genes were downregulated, and six of the seven HK genes were downregulated. The expression patterns of the 10 DEGs were verified by real-time PCR, which showed significant linear relationships (r2020 = 0.9127, r2021 = 0.8705). In conclusion, the fertilizer treatment inhibited the synthesis of sucrose and phosphorylation of hexose by downregulating the expression of the SS, SPS, and HK genes, thus increasing the fructose and total soluble sugar contents in ‘Feizixiao’ litchi.