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Key message Geography and tree diameter were the major factors in determining oleoresin yield in Sindora glabra. The tangential diameter and area of secretory canals are reliable indicators of the oleoresin amount. Abstract Sindora glabra is a native tropical plant of China that exudes copious amounts of oleoresin from wounds on the stem. Elucidation of the mechanism affecting the oleoresin yield is of particular importance for development and selection of superior germplasm. In this study, the variation in oleoresin yield and anatomical traits of the stem of S. glabra were investigated in four main natural populations in Hainan. The results showed that annual oleoresin yield varied considerably among populations. The oleoresin yield exhibited an increasing trend with the increment of tree stem diameter until the diameter was over 80 cm. Moreover, oleoresin production in the wet season was higher than in dry season. The analysis of oleoresin chemical composition indicated that the four populations belong to the same chemotype despite the amount of four types of sesquiterpenes varying greatly among areas. Anatomical analysis revealed that quantitative traits of secretory canals including tangential diameter, area, density and frequency were significantly different among populations and tree diameters. Furthermore, the tangential diameter and area of secretory canals were positively correlated with oleoresin yield. These results suggested that the tangential diameter and area of secretory canals are reliable indicators of the oleoresin amount and can be used for selection purposes in S. glabra.

Mytilaria laosensis is a fast-growing tropical broadleaf tree that is extensively used for wood production and has significant ecological benefits. To investigate the genetic diversity and population structure of M. laosensis, eight major natural popu­lations in China were analyzed by using simple sequence repeat (SSR) markers. A total of 88 microsatellite-containing fragments were obtained by the method of magnetic bead enrichment, among which 26 pairs of SSR primers were scree­ned out and used to generate a total of 190 alleles among 152 individuals. The average of observed number of alleles, Shannon’s information index and polymorphism information content per locus were 18.3, 1.1577 and 0.7759, respectively, implying a high level of genetic diversity in M. laosensis popu­lations. The variation within populations accounted for 81.74 % of total variation based on analysis of molecular variance. Clus­ter analysis divided the eight populations into four groups, among which five populations from the southern parts of Guangxi province were classified as one major group. Mantel test showed that there was highly significant correlation bet­ween Euclidean genetic distance and geographic distance, suggesting that geographic isolation contribute to the high genetic diversity of M. laosensis. Together, these could provide support for the feasibility of exploration and utilization of M. laosensis in subtropical areas of East Asia including Jiangxi, Hunan and Fujian province of China.

Sindora glabra, one of the second-class national protective plants in China, has important ecological and commercial values. To understand the genetic diversity and structure of S. glabra, eleven natural populations from four areas of Hainan Island in China were analyzed by inter-simple sequence repeat (ISSR) markers. Thirteen primers were screened out and used to amplify 157 DNA samples. A total of 122 bands were obtained, among which 114 (93.4%) bands were polymorphic. Genetic parameters including average number of effective alleles (1.547), Nei's gene diversity (0.321), Shannon's information index (0.482), and gene differentiation coefficient (0.1944) revealed a high level of genetic diversity maintained in S. glabra populations. The variation within populations accounted for 85.6% of total variation based on analysis of molecular variance. Genetic distance analysis showed that 11 populations could be divided into three groups and populations from the same areas were classified as one group, suggesting that high genetic diversity of S. glabra was attributable to geographic isolation. Together, introduction of germplasm from distant natural distribution areas would be a sound strategy for germplasm resource conservation of S. glabra and selection of elite individuals from populations of far relationship for hybridization is of great importance for breeding improvement programs in future.

A male-specific SCAR DNA marker was developed using a RAPD DNA marker specific for male plants of Salacca zalacca var. zalacca (salak palm). The marker is 1579 bp long and has a GC content of 38.5 %. Its sequence contains 1 or 2 open reading frames, indicating the marker is probably a coding region. No highly similar sequences were found in a search of the GenBank database. Sexes were identified using the SCAR DNA marker for three kinds of seedlings grouped by the number of seeds per fruit (1, 2 or 3). The sex ratio of female to male did not differ significantly from 1:1 for the three kinds of seedlings, implying that the number of seeds per fruit is not a reliable index to identify the sex of a seedling.

Terpenes serve important physiological and ecological functions in plants. Sindora glabra trees accumulate copious amounts of sesquiterpene-rich oleoresin in the stem. A transcriptome approach was used to determine the unique terpene biosynthesis pathway and to explore the different regulatory mechanisms responsible for the variation of terpene content among individuals. Analysis of de novo-assembled contigs revealed a complete set of genes for terpene biosynthesis. A total of 23,261 differentially expressed unigenes (DEGs) were discovered between high and low oil-yielding plants. DEG enrichment analysis suggested that the terpene biosynthesis process and the plant hormone signal transduction pathway may exert a major role in determining terpene variation in S. glabra. The expression patterns of candidate genes were further verified by quantitative RT-PCR experiments. Key genes involved in the terpene biosynthesis pathway were predominantly expressed in phloem and root tissues. Phylogenetic analysis and subcellular localization implied that S. glabra terpene synthases may evolve from a common ancestor. Furthermore, two sesquiterpene synthase genes, SgSTPS1 and SgSTPS2, were functionally characterized. SgSTPS1 mainly generated β-caryophyllene from farnesyl pyrophosphate. SgSTPS2 is a versatile enzyme that catalyzes the formation of 12 sequiterpenes from farnesyl pyrophosphate and synthesis of three monoterpenes using geranyl pyrophosphate. Together, these results provide large reservoir for elucidating the molecular mechanism of terpene biosynthesis and for exploring the ecological function of sesquiterpenes in S. glabra.

A greenhouse pot experiment was conducted to study the effects of nitrogen fertilization on Calophy- llum inophyllum seedlings grown with 0, 50, 100, 150, 200, 300, 400 and 600 mg N per seedling according to exponential functions. Seedling height, root collar diameter, leaf area and total biomass increased with increasing fertilization from 0 to 200 mg N per seedling and decreased with further increase in fertilization from 300 to 600 mg N per seedling. The net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate of C. inophyllum seedlings showed a unimodal parabolic trend, with peak values of 7.29 mmol·m−2·s−1, 0.071 mol·m−2·s−1, 220 mmol·mol−1 and 1.34 mmol·m−2·s−1, respectively, when the rate of fertilization was 200 mg N per seedling. Photosynthetic gas exchange parameters were significantly different among nitrogen treatments. Based on the critical values of leaf N and P concentration and N/P ratio, the optimum amount of nitrogen of C. inophyllum seedlings was 200–400 mg per seedling for leaf N and P concentration, and 100–400 mg per seedling for N/P ratio. It was concluded that 200–400 mg N per seedling was the most suitable nitrogen range for C. inophyllum seedlings.