Flowering is a necessary condition and basis for yield in the life cycle of woody fruit trees. Although there has been considerable interest in the regulatory mechanisms underlying floral induction and flowering, the associated epigenetic modifications remain poorly characterized. We identified genome-wide DNA methylation changes and the transcriptional responses in axillary buds of ‘Qinguan’ (QA) and ‘Fuji’ (FA) varieties with contrasting flowering behaviors. The DNA methylation levels were 19.35, 62.96 and 17.68% in FA, and 19.64, 62.49 and 17.86% in QA in the CG, CHG and CHH contexts, respectively. The number of hypermethylated and hypomethylated differentially methylated regions (DMRs) in different regions contributed to significantly up- and downregulated gene expression. DNA methylation can positively or negatively regulate gene expression depending on the CG, CHG and CHH contexts and their locations in different regions. Additionally, the huge differences in transcription of MIKCc-type MADS-box genes, and multiple flowering genes in multiple flowering pathways (i.e., light, aging, GA and sugar) by changing DNA methylation, contributed to contrasting flowering behaviors in both QA and FA. Specifically, the floral meristem identity genes (i.e., FT, LEAFY, AP1 and SOC1) exhibited significantly higher expression in QA than FA, but the floral repressors (i.e., SVP, AGL15, and AGL18) showed the opposite trend. Significant differences in multiple hormone levels were due to differentially expressed genes (DEGs) and their DMRs in hormone synthesis pathways, leading to both contrasting axillary bud outgrowth and flowering behaviors. These findings reflect the diversity in the epigenetic regulation of gene expression and may be helpful for elucidating the epigenetic regulatory mechanism underlying the axillary bud flowering in apple.

The root system is pivotal for plant development, enabling both vegetative growth and tolerance to abiotic stresses like salinity. However, the molecular mechanisms governing root adaptive development in response to salt stress remain poorly understood in apple (Malus domestica Borkh.). In this study, we identified the salt stress-responsive WRKY transcription factor MdWRKY75. Overexpression of MdWRKY75 in transgenic apple negatively regulates adventitious root (AR) formation and salt stress tolerance, whereas reducing MdWRKY75 expression yields the opposite phenotype. Moreover, MdWRKY75 directly binds to the promoter of MdSAUR15 (SMALL AUXIN UP RNA15) and transcriptionally represses the expression of MdSAUR15, which, when overexpressed, promotes AR formation and enhances salt stress tolerance. We further demonstrated that MdWRKY75 interacts with MdWOX11, a WUSCHEL-related homeobox (WOX) transcription factor, both in vitro and in vivo. MdWOX11 expression is upregulated and enhances AR formation under salt stress. Additionally, MdWOX11 reduces the binding of MdWRKY75 to the MdSAUR15 promoter, and alleviates the MdWRKY75-mediated inhibitory effect on MdSAUR15 expression. Collectively, our study provides a MdWOX11-MdWRKY75-MdSAUR15 module regulating root adaptation in response to salt stress in apple.

Fruit development and ripening are complex processes involving various physiological and biochemical changes. Apple rootstocks significantly regulate fruit ripening, quality, and nutrient accumulation. To study the impact of SXND6 and M26 rootstocks on fruit development and maturation, we investigated fruit traits including single fruit weight, shape index, firmness, soluble solids content, titratable acidity, starch content, index of absorbance difference (IAD), endogenous hormone levels, as well as the transcriptome and metabolic profiles. At the transcript level, 9219 differentially expressed genes (DEGs) were identified in apple fruits grafted on SXND6 and M26 rootstocks at 135, 150, and 165 DAFB. Additionally, 116 differentially expressed primary metabolites were identified between the two rootstocks at 165 DAFB. An integrative analysis of the transcriptome and primary metabolic expression patterns revealed some key pathways and DEGs involved in early maturity and better quality mediated by various dwarf intermediate rootstocks. This primarily included starch and sucrose metabolism (SS, AMY, BAM, SORD, PFK, SPS, and SUS) and plant hormone signal transduction pathways, especially abscisic acid (NACED, PP2C, SnRK, and ABF) and ethylene (ACS, ACO, ETR, and ERF). Moreover, we employed weighted gene co-expression network analysis (WGCNA) to investigate the co-expression patterns of genes encoding transcription factors linked with sugar, organic acid, starch, and IAD. AGL9, bZIP6, and ERF5 were identified as key hub genes within the MEturquoise module, as well as NAC, MADS, MYB, and C2C2 genes, which positively regulated sugars metabolism and fruit ripening. In contrast, WRKY17 and NAC18 were found in MEbrown module, as well as MYB related genes and MADS family genes, which were negatively correlated with sugar and fruit maturity. These findings provide new insights into the regulatory mechanisms by which rootstocks enhance fruit quality and promote fruit ripening. This knowledge contributes to improved rootstock selection for apple cultivation and management, while also facilitating the development of early-ripening apple varieties with high economic value in the future.

With global climate change, extremely high temperatures are occurring frequently during summer, seriously affecting development of the apple industry. Heat-shock factors (HSFs) are important signaling proteins for plants to respond to heat stress. However, research on the effects of the apple HSF gene on the regulation of heat-stress tolerance is lacking. In this study, we analyzed the expression and function of MdHSFA2, an HSF, in apples. The MdHSFA2 expression level significantly increased under heat treatment, and Arabidopsis overexpressing MdHSFA2 showed significantly higher tolerance than that in the WT plants. In addition, we found that apple MdGolS4/6 had the highest expression levels among the eight MdGolSs under heat treatment. Using electrophoretic mobility shift (EMSA) and dual-luciferase assays, we found that MdHSFA2 could directly bind to the MdGols4 promoter region thereby promoting transcription. MdGolS4 overexpression in Arabidopsis enhances heat-stress tolerance. Our findings suggest that MdHSFA2 directly promotes MdGolS4 transcription, thereby enhancing heat-stress tolerance in apples. These results enhance our understanding of heat-stress tolerance mechanisms.

Flower bud formation is a critical process that directly determines yield and fruit quality in fruit crops. Floral induction is modulated by the balance between 2 flowering-related proteins, FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1); however, the mechanisms underlying the establishment and maintenance of this dynamic balance remain largely elusive. Here, we showed that in apple (Malus × domestica Borkh.), MdFT1 is predominantly expressed in spur buds and exhibits an increase in expression coinciding with flower induction; in contrast, MdTFL1 exhibited downregulation in apices during flower induction, suggesting that MdTFL1 has a role in floral repression. Interestingly, both the MdFT1 and MdTFL1 transcripts are directly regulated by transcription factor basic HELIX-LOOP-HELIX48 (MdbHLH48), and overexpression of MdbHLH48 in Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) results in accelerated flowering. Binding and activation analyses revealed that MdbHLH48 functions as a positive regulator of MdFT1 and a negative regulator of MdTFL1. Further studies established that both MdFT1 and MdTFL1 interact competitively with MdWRKY6 protein to facilitate and inhibit, respectively, MdWRKY6-mediated transcriptional activation of target gene APPLE FLORICAULA/LFY (AFL1, an apple LEAFY-like gene), ultimately regulating apple flower bud formation. These findings illustrate the fine-tuned regulation of flowering by the MdbHLH48-MdFT1/MdTFL1-MdWRKY6 module and provide insights into flower bud formation in apples.

Somatic variations can give rise to bud sports with advantageous traits, serving as the foundation for bud sport breeding in perennial plants. Here, we report a fully phased genome assembly of 'Fuji' apple, enabling comprehensive identification of somatic variants across 74 clonally propagated 'Fuji' varieties. Phylogenetic analysis indicates that spur-type and early-maturation traits in 'Fuji' sport varieties arise from multiple independent events. Several putative functional somatic variants have been identified, including a spur-type-specific deletion in the promoter of the TCP transcription factor gene MdTCP11. DNA methylation level of the deletion-associated miniature inverted-repeat transposable element is lower in spur-type varieties compared to standard-type varieties, while the expression of MdTCP11 is significantly higher. Overexpression of MdTCP11 in apple decreases plant height, highlighting its important role in the development of spur-type apple varieties. This study sheds light on the cloning history of 'Fuji' and provides valuable resources for apple breeding. Spur-type of growth is preferred for apple production. Here, the authors employ the trio-based phasing approach to assemble a fully phased Fuji genome and reveal the association of a 167-bp deletion in the promoter region of the MdTCP11 gene with the spur-type varieties.

Harvest maturity significantly affects the quality of apple fruit in post-harvest storage process. Although the regulatory mechanisms underlying fruit ripening have been studied, the associated epigenetic modifications remain unclear. Thus, we compared the DNA methylation changes and the transcriptional responses of mature fruit (MF) and immature fruit (NF). There were significant correlations between DNA methylation and gene expression. Moreover, the sugar contents (sucrose, glucose, and fructose) were higher in MF than in NF, whereas the opposite pattern was detected for the starch content. The expression-level differences were due to DNA methylations and ultimately resulted in diverse fruit textures and ripeness. Furthermore, the higher ethylene, auxin, and abscisic acid levels in MF than in NF, which influenced the fruit texture and ripening, were associated with multiple differentially expressed genes in hormone synthesis, signaling, and response pathways (ACS, ACO, ZEP, NCED, and ABA2) that were regulated by DNA methylations. Multiple transcription factor genes involved in regulating fruit ripening and quality via changes in DNA methylation were identified, including MIKCC-type MADS-box genes and fruit ripening-related genes (NAP, SPL, WRKY, and NAC genes). These findings reflect the diversity in the epigenetic regulation of gene expression and may be relevant for elucidating the epigenetic regulatory mechanism underlying the ripening and quality of apple fruit with differing harvest maturity.

“Genetics and Breeding of Fruit Trees”, published in Horticulturae, is a collection of 11 manuscripts focusing on commercially important fruit crops, such as apple, peach, pear, kiwi, persimmon, blueberry, and red bayberry [...]

The firmness of the flesh fruit is a very important feature in the eating process. Peach fruit is very hard during development, but its firmness slightly decreases in the later stages of development. While there has been extensive research on changes in cell wall polysaccharides during fruit ripening, little is known about the changes that occur during growth and development. In this study, we investigated the modifications in cell wall components throughout the development and ripening of peach fruit, as well as its impact on firmness. Our findings revealed a significant positive correlation between fruit firmness and cellulose content at development stage. However, the correlation was lost during the softening process, suggesting that cellulose might be responsible for the fruit firmness during development. Members of the chitinase-like protein (CTL) group are of interest because of their possible role in plant cell wall biosynthesis. Here, two CTL homologous genes, PpCTL1 and PpCTL2, were identified in peach. Spatial and temporal expression patterns of PpCTLs revealed that PpCTL1 exhibited high expression abundance in the fruit and followed a similar trend to cellulose during fruit growth. Furthermore, silencing PpCTL1 expression resulted in reduced cellulose content at 5 DAI (days after injection), this change that would have a negative effect on fruit firmness. Our results indicate that PpCTL1 plays an important role in cellulose biosynthesis and the maintenance of peach firmness during development.

Abiotic stress of plants has serious consequences on the development of the apple industry. Nuclear pore complexes (NPCs) control nucleoplasmic transport and play an important role in the regulation of plant abiotic stress response. However, the effects of NPCs on apple salt and osmotic stress responses have not been reported yet. In this study, we analyzed the expression and function of NUCLEOPORIN 62 ( MdNup62) , a component of apple NPC. MdNup62 expression was significantly increased by salt and mannitol (simulated osmotic stress) treatment. The MdNup62 -overexpressing (OE) Arabidopsis and tomato lines exhibited significantly reduced salt stress tolerance, and MdNup62 -OE Arabidopsis lines exhibited reduced osmotic stress tolerance. We further studied the function of HEAT SHOCK FACTOR A1d (MdHSFA1d), the interacting protein of MdNup62, in salt and osmotic stress tolerance. In contrast to MdNup62 , MdHSFA1d -OE Arabidopsis lines showed significantly enhanced tolerance to salt and osmotic stress. Our findings suggest a possible interaction of MdNup62 with MdHSFA1d in the mediation of nuclear and cytoplasmic transport and the regulation of apple salt and osmotic stress tolerance. These results contribute to the understanding of the salt and osmotic stress response mechanism in apple.

Flower bud formation in the apple tree life cycle is associated with multiple biological processes. To explore the physiological and molecular mechanisms underlying the protein and metabolite changes in buds with different flowering capabilities, axillary buds with no flowering (Ab), long-shoot buds with a low flowering rate (Lb), and spur buds with a higher flowering rate than the Lb (Sb) were analyzed using a Tandem Mass Tag™ proteomic technique in combination with nLC–MS/MS analyses. We identified 471 (88 up- and 383 down-regulated), 459 (176 up- and 283 down-regulated), and 548 (387 up- and 161 down-regulated) differentially expressed proteins in Sb vs. Lb, Sb vs. Ab, and Lb vs. Ab, respectively, that were involved in carbohydrate, amino acid and lipid transport, and metabolism. Additionally, 110 (91 increased and 19 decreased), 89 (71 increased and 18 decreased), and 99 (37 increased and 62 decreased) metabolites having significantly different levels were identified in Sb vs. Lb, Sb vs. Ab, and Lb vs. Ab, respectively. The identified metabolites were related to amino acids and their isoforms, sugars and polyols, and organic acids, and occurred at significantly greater levels in the Sbs than the other buds. Thus, flower bud formation is a complex process that involves various biochemical materials and signals, such as carbohydrates, amino acids and their isoforms, and organic acids.

Peach (Prunus persica) is a typical climacteric fruit, having an obvious peak of ethylene release during ripening. Transcriptional regulators play key roles in fruit ripening. NAP (NAC-like, activated by AP3/P1) protein is a subfamily of the NAC (NAM, ATAF1/2 and CUC2) transcription factor (TF) family. Here, we discovered the function of peach NAP subfamily NAC TF, PpNAP6, homologous to the tomato ripening-related TF NOR. In the pulp of ‘Zaofengwang’ peach, PpNAP6 is highly expressed, and its transcript level decreased gradually throughout the whole development period, however, it is up-regulated at onset of peach ripening. Transient transformation experiments showed that PpNAP6 over-expression in peach fruit increased ethylene release and accelerated fruit ripening, while PpNAP6 down-expression inhibited ethylene biosynthesis. Dual-luciferase reporter assay (DLR) and electrophoretic mobility shift assay (EMSA) proved that PpNAP6 can directly bind to the promoters of key ethylene biosynthesis genes PpACS1 and PpACO1, and activate their transcription. Moreover, PpNAP6 was heterologously overexpressed in tomato nor mutant, and the transgenic lines showed the overexpression of PpNAP6 could partially recover its ripening-inhibited phenotype. Interestingly, PpNAP6 activates ethylene biosynthesis during peach ripening, whereas its transcription was inhibited treated with ethephon and enhanced with 1-MCP treatment. In summary, our work revealed that PpNAP6 regulates peach fruit ripening through ethylene biosynthesis, and may be involved in the fine regulation of ethylene via a feedback mechanism.

Fruit ripening is a highly coordinated process at the transcriptional level, requiring a hierarchical regulatory network formed by transcription factor and ripening-related genes. MADS-box transcription factors are core regulators that play a key function in the ripening process of fruit, targeting a large amount of downstream ripening-related genes. However, the upstream regulators of MADS-box transcription factors are poorly understood. In this study, we identified an ethylene response factor PpERF61 by yeast one-hybrid screening with the promoter of PpSEP1 (MADS-RIN homologs). The expression pattern of PpERF61 was closely associated with ethylene biosynthesis and fruit softening across six peach cultivars. The transient overexpression and VIGS assay showed that overexpression or silencing of PpERF61 in peach promoted or inhibited fruit softening, while ethylene production was increased or decreased, respectively, demonstrating that PpERF61 has effect on ethylene generation and fruit softening during peach ripening. As a nuclear-localized transcription factor, PpERF61 activated the ripening-related genes transcription, including PpACO1, PpACS1, PpPL1, and PpPL15, by binding directly to their promoters. Interestingly, PpERF61 acts as upstream of PpSEP1 and binds directly to the PpSEP1 promoter, thereby forming PpERF61-PpSEP1 module to promote the PpPG2 and PpPG3 transcription. Based on these findings, we conclude that PpERF61 positively regulates ethylene biosynthesis and fruit softening by directly activating ripening-related genes or activating the PpERF61-PpSEP1 signaling cascade. Collectively, these results enrich knowledge of the complex cascade network for peach fruit ripening.

Adventitious root (AR) formation plays an important role in vegetatively propagated plants. Cytokinin (CK) inhibits AR formation, but the molecular mechanisms driving this process remain unknown. In this study, we confirmed that CK content is related to AR formation and further revealed that a high auxin/CK ratio was beneficial to AR formation in apple (Malus domestica). A correlation between expression of CK-responsive TEOSINTE BRANCHED1, CYCLOIDEA, and PCF17 (MdTCP17) and AR formation in response to CK was identified, and overexpression of MdTCP17 in transgenic apple inhibited AR formation. Yeast two-hybrid, bimolecular fluorescence complementation, and co-immunoprecipitation assays revealed an interaction between MdTCP17 and WUSCHEL-RELATED HOMEOBOX11 (MdWOX11), and a significant correlation between the expression of MdWOX11 and AR ability. Overexpression of MdWOX11 promoted AR primordium formation in apple, while interference of MdWOX11 inhibited AR primordium production. Moreover, a positive correlation was found between MdWOX11 and LATERAL ORGAN BOUNDARIES DOMAIN29 (MdLBD29) expression, and yeast one-hybrid, dual luciferase reporter, and ChIP-qPCR assays verified the binding of MdWOX11 to the MdLBD29 promoter with a WOX-box element in the binding sequence. Furthermore, MdTCP17 reduced the binding of MdWOX11 and MdLBD29 promoters, and coexpression of MdTCP17 and MdWOX11 reduced MdLBD29 expression. Together, these results explain the function and molecular mechanism of MdTCP17-mediated CK inhibition of AR primordium formation, which could be used to improve apple rootstocks genetically.

Adventitious root (AR) formation is a bottleneck for the mass propagation of apple rootstocks, and water stress severely restricts it. Different hormones and sugar signaling pathways in apple clones determine AR formation under water stress, but these are not entirely understood. To identify them, GL-3 stem cuttings were cultured on polyethylene glycol (PEG) treatment. The AR formation was dramatically decreased compared with the PEG-free control (CK) cuttings by increasing the endogenous contents of abscisic acid (ABA), zeatin riboside (ZR), and methyl jasmonate (JA-me) and reducing the indole-3-acetic acid (IAA) and gibberellic acid 3 (GA3) contents. We performed a transcriptomic analysis to identify the responses behind the phenotype. A total of 3204 differentially expressed genes (DEGs) were identified between CK and PEG, with 1702 upregulated and 1502 downregulated genes. Investigation revealed that approximately 312 DEGs were strongly enriched in hormone signaling, sugar metabolism, root development, and cell cycle-related pathways. Thus, they were selected for their possible involvement in adventitious rooting. However, the higher accumulation of ABA, ZR, and JA-me contents and the upregulation of their related genes, as well as the downregulation of sugar metabolism-related genes, lead to the inhibition of ARs. These results indicate that AR formation is a complicated biological process chiefly influenced by multiple hormonal signaling pathways and sugar metabolism. This is the first study to demonstrate how PEG inhibits AR formation in apple plants.

Lipid phosphate phosphatases (LPPs) are a key enzyme in the production and degradation of phosphatidic acid (PA), which plays an important role in plant growth, development, stress resistance and plant hormone response. Thus far, little is known about the LPP family genes in kiwifruit (Actinidia spp.). According to this study, 7 members in the AcLPP family were identified from the whole genome of kiwifruit, the subcellular localization predictions were mainly on the plasma membrane. Chromosomal localization analysis showed that the AcLPP genes were unevenly distributed on 5 chromosomes, it was determined to have undergone strong purifying selection pressure. There were 5 duplicate gene pairs and all underwent segmental duplication events. The LPP genes of kiwifruit were conserved when compared with other plants, especially in terms of evolutionary relationships, conserved motifs, protein sequences, and gene structures. Cis-regulatory elements mainly included hormone response elements and abiotic response elements. Functional annotation of GO revealed that AcLPP genes were closely related to phosphatase/hydrolase activity, phosphorus metabolism and dephosphorylation. AcLPP genes family were predicted to be targets of miRNA. Transcript level analysis revealed that the AcLPP family played diverse functions in different tissues and during growth, development, and postharvest storage stages. qPCR analysis showed that the members of AcLPP gene family might be regulated by ETH, ABA, GA3, and IAA hormone signals. The family members were regulated by the stress of salt stress, osmotic stress, cold stress, and heat stress. These results would provide a basis and reference for studying the agricultural characteristics of kiwifruit and improving its stress resistance.

Pectin is the major component in the primary cell wall and middle lamella, maintaining the physical stability and mechanical strength of the cell wall. Pectate lyase (PL), a cell wall modification enzyme, has a major influence on the structure of pectin. However, little information and no comprehensive analysis is available on the PL gene family in peach (Prunus persica L. Batsch). In this study, 20 PpePL genes were identified in peach. We characterized their physicochemical characteristics, sequence alignments, chromosomal locations, and gene structures. The PpePL family members were classified into five groups based on their phylogenetic relationships. Among those, PpePL1, 9, 10, 15, and 18 had the higher expression abundance in ripe fruit, and PpePL1, 15, and 18 were upregulated during storage. Detailed RT-qPCR analysis revealed that PpePL1 and PpePL15 were responsive to ETH treatment (1 g L-1 ethephon) with an abundant transcript accumulation, which suggested these genes were involved in peach ripening and softening. In addition, virus-induced gene silencing (VIGS) technology was used to identify the roles of PpePL1 and PpePL15. Compared to controls, the RNAi fruit maintained greater firmness in the early storage stage, increased acid-soluble pectin (ASP), and reduced water-soluble pectin (WSP). Moreover, transmission electron microscopy (TEM) showed that cell wall degradation was reduced in the fruit of RNAi-1 and RNAi-15, which indicated that softening of the RNAi fruit has been delayed. Our results indicated that PpePL1 and PpePL15 play an important role in peach softening by depolymerizing pectin and degrading cell wall.

Flowering-related problems in "Fuji" apple have severely restricted the development of China's apple industry. Nuclear pore complexes (NPCs) control nucleoplasmic transport and play an important role in the regulation of plant growth and development. However, the effects of NPCs on apple flowering have not been reported. Here, we analysed the expression and function of MdNup54, a component of apple NPC. MdNup54 expression was the highest in flower buds and maintained during 30-70 days after flowering. MdNup54-overexpressing (OE) Arabidopsis lines displayed significantly earlier flowering than that of the wild type. We further confirmed that MdNup54 interacts with MdHSP70, MdMYB11, and MdKNAT4/6. Consistent with these observations, flowering time of MdHSP70-OE Arabidopsis lines was also significantly earlier. Therefore, our findings suggest a possible interaction of MdNup54 with MdHSP70 to mediate its nuclear and cytoplasmic transport and to regulate apple flowering. The results enhance the understanding of the flowering mechanism in apple and propose a novel strategy to study nucleoporins.

In the apple tree, insufficient flower bud production is an intractable challenge, and very little information is available in this field due to the fact that research done in this sector is very rare owing to its extended life cycles and low rate of genetic transformation. Here we display novel changes and events in spur buds of Malus × domestica trees after they were exposed to salicylic acid (SA) treatment during the flower induction period. We found a significant increase in morphological indexes, followed by a wider and well-defined shoot apical meristem in SA-treated spur buds. Additionally, we observed increased oxidative stress markers and enzymatic antioxidants in control-treated buds during the flower induction period, while non-enzymatic antioxidants were recorded higher in SA-treated buds. Maximum flowering was observed in SA-treated trees in the next year. Furthermore, ultra-high-performance liquid chromatography (u-HPLC) analysis displays that SA treatment enhances SA and indole acetic acid (IAA), while having an antagonistic effect on gibberellin (GA). At different time points, transcriptome analysis was conducted to analyze the transcriptional response of CK and SA treated buds. Pathway enrichment was detected in differentially expressed genes (DEGs). Agamous (AGL) and SQUAMOSA-promoter binding protein-like (SPL) family related flowering genes display a positive signal for the increased flowering in SA-treated trees, which confirms our findings. As far as we know, there is no report available on the response of spur buds to SA treatment during the flower induction period. This data provides a new theoretical reference for the management of apple tree flowering and also provides an essential basis for future analysis of the regulation and control of flowering in M. domestica.

The subsequent plant growth is determined by tomato seedling quality which is affected by light intensity. The present study proposes a method for obtaining the regulatory target value by using the U-chord length curvature method based on the dry weight model-fitted curve characteristics to obtain the optimum light intensity at the seedling stage. Tomato seedlings were selected as the experimental material, and six photosynthetic photon flux densities (PPFDs) were set under completely artificial side lighting. Depending on the scattering trend, the tomato shoot dry weights (SDWs) were fitted by the polynomial, logarithmic, and half logistic function models after optimization of parameters by using the differential evolution algorithm (DE-HLFM). According to the curve variation characteristics, the discrete curvature values corresponding to the models calculated using the U-chord length curvature method were 335, 108, and 241.3 mu mol.m(-2.)s(-1), respectively. Physiological tomato growth indicators exhibited that L4 had the smallest root-shoot ratio and the highest level of seedling index and SDW significance. Moreover, L4 had the highest SDW per PPFD improvement efficiency. Thus, tomato seedlings under L4 exhibited the best quality and highest efficiency among those under the six PPFDs. DE-HLFM optimization was closest to the optimal PPFD L4 (243.17 +/- 4.37 mu mol.m(-2).s(-1)), indicating that DE-HLFM is the most suitable model with a coefficient of determination (R-2) of 0.99897 and root-mean-square error of 0.00554. Thus, the PPFD corresponding to the maximum curvature point of the appropriate model exhibited physiological and production regulation significance. In conclusion, based on the curve characteristics, the curve feature point obtained using the U-chord curvature method can be used as the control target value for the precise control of the artificial controllable light environment.