Jacaranda mimosifolia is regarded as a prized ornamental tree in the urban landscape with attractive, abundant and long-lasting violet-colored flowers and graceful tree form. It has been widely cultivated in recent years in many Chinese cities. However, the lack of scientific and practical guidance to cultivate the exotic species has brought about planting failures in some areas, incurring substantial economic losses and landscape decline. A comprehensive understanding of the current spatial pattern and climatic conditions of J. mimosifolia in China can inform species choice, planting and management. We collected data on the geographical coordinates of 257 planting cities and acquired additional information from the literature and field surveys. The limiting factors for cultivation were investigated using principal component analysis (PCA) of 19 bioclimatic parameters of the sampled sites. The potentially suitable habitats were predicted by BIOCLIM modeling using eight selected ecological-important climatic parameters. We found that the present cultivated areas were focused in the low-altitude parts of the subtropical zone, mainly covering the provinces of Yunnan, Sichuan, Guangxi, Guangdong and Fujian. The PCA results indicated that temperature was the clinching determinant of the current cultivation patterns, especially annual mean temperature, the minimum temperature of the coldest month and the mean temperature of the warmest quarter. Moisture was a necessary but not critical secondary factor. The predicting model for potential habitats was graded as “excellent” by objective validation measures. The findings can provide science-based evidence to plan the expansion of the biogeographical range of cultivation into hitherto unplanted cities and rationalize urban tree introduction and management practices.

Eucalyptus camaldulensis Dehn. is one of the most morphologically and genetically variable Eucalyptus species. Growth, Leptocybe invasa Fisher & La Salle susceptibility, pilodyn penetration and other traits up to age 36 months were assessed in a seed source/family trial in China comprising 112 seedlots representing five natural stand and six exotic seed sources. Genetic diversity and population structure of this trial population were also analyzed using 48 simple sequence repeat (SSR) markers. The key objective was to examine whether the genomic data could provide value over information obtained from just quantitative trait data. Significant genetic variation was found among seed sources and among families within seed sources for most quantitative traits. The ratio of variance among seed sources to variance among families within seed sources, based on variances estimated from quantitative trait data, varied from 0.1% (height at 9 months) up to 75.2% (bark thickness). Equivalent ratios estimated from the AMOVA on SSR loci data were similar for height (ages 24 and 36 months) and also pilodyn penetration at 36 months, but not for 9-month height or 36-month bark thickness. From 48 SSR loci examined, the genetic differentiation coefficient (among seed sources) was 0.086, indicating low genetic differentiation among seed sources. While overall genetic diversity in the trial population examined was high, the levels within the different seed sources varied markedly. Prior to this study, genetic distances among families from the three exotic seed sources (from domesticated Indian populations) in the trial, along with their genetic distances from, and relatedness to, families from five natural stand seed sources (Australian) in the trial were unknown. The SSR loci data removed uncertainties and revealed that the exotic sources increased the breadth of genetic origins represented in the trial population—information that could not have been obtained from just the quantitative trait data.

With the depletion of peat resources, local agricultural and forestry wastes represent potential seedling substrate materials. We investigated the effects of mixed substrates supplemented with peat replacements, such as decomposed eucalyptus bark, coconut bran, and carbonized rice husk, on the growth of Handroanthus chrysanthus (Jacq.). We screened suitable organic substrates for the production of seedlings of H. chrysanthus to provide a theoretical basis for its scientific cultivation. The proportions of 15 types of substrates were determined by using simplex center-of-gravity mixing. The physical properties of different mixed substrates were studied, and the growth, biomass, root development, and photosynthesis of seedlings were analyzed. The quality of seedlings was comprehensively evaluated, and the optimal substrate ratio was obtained through regression analysis. Substrate compositions had significantly different effects on growth, biomass, root morphology, and photosynthesis. The results of these single-index analyses were corroborated by using principal component analysis. Adding decomposed eucalyptus bark was beneficial to the growth of seedlings, whereas coconut bran and perlite restricted growth. The results show that decomposed eucalyptus bark can replace peat as a container seedling substrate. The substrate formula can be used to provide technical support and a theoretical basis for the scientific cultivation of high-quality light matrix seedlings of H. chrysanthus.

Handroanthus and Tabebuia are known for their ornamental and medicinal value, which are attributed to metabolites. However, the mechanisms underlying the synthesis of these metabolites are poorly understood. In this study, the expression levels of secondary metabolites and the mechanism of flavonoid biosynthesis in the bark and leaves of Handroanthus chrysantha (Jaq.) were examined using transcriptomic and metabolomic techniques. Metabolic analysis identified several differentially accumulated metabolites (DAMs), most of which were flavonoids, isoprenoids, and sterols. Additionally, 30 flavonoids were identified in the bark and leaves of H. chrysantha. Transcriptomic analysis identified 69 genes involved in flavonoid biosynthesis, among which 49 were significantly different between the bark and leaves. qRT-PCR analysis of eight genes involved in flavonoid biosynthesis showed that the expression patterns of the genes were consistent with the transcriptome sequencing data. Integrative transcriptomic and metabolomic analysis showed that 20 differentially expressed genes (DEGs) associated with flavonoid biosynthesis were strongly correlated with seven DAMs, confirming the involvement of the DEGs in flavonoid biosynthesis. These findings considerably contribute to the understanding of the biosynthesis of secondary metabolites in H. chrysantha and serve as a reference for further pharmacological studies.

Handroanthus chrysanthus is a remarkable landscape tree species with significant research and development potential. However, it remains unclear whether the varieties introduced to China are one species or two species. Since the morphological characteristics of seeds are extremely valuable in plant systematics research, in this work, 126 germplasm samples of H. chrysanthus were genotyped by genotyping-by-sequencing (GBS) for the first time. Subsequently, the phylogenetic tree, kinship and population structure were analyzed. Additionally, a genome-wide association analysis study (GWAS) was performed to identify candidate genes for seed morphology traits in H. chrysanthus. After applying various filtering criteria, 124 574 high-quality single-nucleotide polymorphisms (SNPs) were obtained. Most germplasms showed no obvious genetic relationship. The phenotypic analysis indicated that the four seed morphology-related traits had high variability. The coefficients of variance ranged from 9.50 to 12.07%, and they followed a continuous normal distribution. A total of 29 significant SNPs were identified by GWAS based on mixed linear model (MLM (QK)), and the number of significant SNPs associated with NWL, NWW, WL and WW were 14, 16, 2 and 9, respectively, among which 9 were pleiotropic loci. Within a 50-kb range of the flanking sequence of these significant SNP loci, 68 candidate genes potentially related to seed morphological traits were identified. Then, 29 candidate genes were screened by integrating expression level and functional annotation. Combined with quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analysis, CDL12_15522, CDL12_15254, CDL12_11100, CDL12_11105 were ultimately selected as the important candidate gene. This study reveals the genetic basis of seed traits in H. chrysanthus, offering new perspectives on into its introduction history. The identification of markers closely related to these seed traits can be utilized for classification and breeding programs, thereby providing a theoretical basis for the genetic background and classification of H. chrysanthus.