The species in the genus Populus, characterized by fast growth, good wood properties, strong resistance, and wide adaptability, are widely distributed over the northern hemisphere (Porth and El-Kassaby, 2015; Georgii et al., 2019). They are used not only in urban landscaping, ecological remediation, and rehabilitation of degraded lands, but also for wood and fiber production (Bannoud and Bellini, 2021). As an important source of industrial raw timber, an improvement on wood properties of Populus is a major focus of breeders. Wood and fiber properties, such as basic density (BD), lignin content (LC), and fiber length (FL), directly affect pulp value and paper quality.

Triploid breeding is an effective approach for the genetic improvement of tree species, which can achieve multitarget trait improvement on growth, wood properties, and stress resistance (Kang, 2020). In Acacia, allotriploid clones derived from crossing tetraploid Acacia mangium with diploid A. auriculiformis possessed a higher heartwood proportion and bark thickness than commercial diploid clones (Bon et al., 2020). Serapiglia et al. (2014, 2015) found that triploid genotypes of shrub willow could produce more biomass and possess lower LC than diploid genotypes. In Populus, some triploid varieties have been widely used for plantations, all of which exhibited favorable growth and pulpwood characteristics (van Buijtenen et al., 1958; Weisgerber et al., 1980; Zhu et al., 1995; Kang, 2016).

Unilateral sexual polyploidization based on the hybridization of uniparental 2n gametes is the main way for triploid breeding of Populus (Kang, 2016). Sexual polyploidization, integrating the contributions of hybridity and genome dosage, can cause extensive phenotypic variation (Fort et al., 2016). In Populus, therefore, understanding trait variation associated with triploidy is of considerable interest to breeders. Zhu (2006) indicated that not all triploid individuals of Populus are elite, so triploid breeding should follow the strategy of strong selection in a large candidate population. In hybrids of (P. alba × P. glandulosa) × P. tomentosa, although the 2-year seedling height and ground diameter of 12 triploids were 33 and 38% larger than those of the diploids on average, respectively, several triploid genotypes exhibited lower growth performance than the diploids (Li and Kang, 2007). Wu et al. (2013) found that the elite triploid genotypes of P. tomentosa hybrids had favorable wood density and fiber traits. However, the variation in wood and fiber traits of unselected allotriploids within the same cross-combination was considerable, and the contributions of ploidy and genotype on the variation of wood properties were not clarified.

As dioecious trees, members of the genus Populus provide opportunities to study the relationship between trait variation and sexual dimorphism in perennial woody plants. In a recent review, Melnikova et al. (2017) reviewed the sex-specific response to the stress of Populus, which demonstrated that the males of the Populus species were better adapted to the stress conditions and showed less damage, better growth, and higher photosynthetic capacity and antioxidant activity than that of the females. In P. purdomii, it was found that the males had a quicker energy-return strategy in high-altitude areas (Lei et al., 2016). For wood properties, in a collected natural diploid population of P. tomentosa, the females had a significantly larger FL and fiber width (FW) than the males on average (Du et al., 2014). However, in allotriploids, the effect of gender on wood and fiber traits remains poorly understood.

In our previous work, a full-sib family including triploid hybrids derived from crossing between induced 2n eggs of Populus pseudo-simonii × P. nigra ‘Zheyin3#’ and normal pollen of P. × beijingensis and diploid hybrids of the two parents were obtained (Wang et al., 2010). After cutting propagation, the diploid and triploid hybrids were planted in the trial field of Weixian County, Hebei Province, China, in 2010. In this study, the wood and fiber traits, including BD, LC, FL, FW, and fiber length/width (FL/W), were measured in the diploid and triploid hybrids, and contributions of ploidy, gender, and genotype effects on the variation of the wood and fiber traits were disaggregated. This enhanced our understanding of trait variation resulting from sexual polyploidization and provided additional data for the selection of elite genotypes.

Genus Populus is an important source of industrial raw timber in the northern hemisphere. In China, the lumber of Populus is widely used to produce pulpwood. The basic wood density, LC, and FL affect the pulp yield and paper quality (Kenney et al., 1990). Therefore, the improvement of pulpwood characteristics has attracted much attention. Triploid breeding has proved to be an effective way to improve the growth, wood properties, and abiotic stress of Populus (Kang, 2016). In white poplar, however, it was found that the BD of triploids was lower than that of the diploids (Kang, 2016). The low BD could decrease the yield of the pulp. In the triploid breeding program of the Populus, the selection of elite genotypes with high BD, low LC, and large FL are our key breeding objectives.

Polyploidization plays an important role in plant speciation and evolution and contributes to plant breeding (Van de Peer et al., 2009; De Storme and Mason, 2014; Sattler et al., 2016). According to the genomic composition, polyploid can be classified into two basic types: auto- and allopolyploid. In allopolyploids, phenotypic variation is commonly affected by both genomic heterozygosity and polyploidy. In Arabidopsis thaliana, the plant size of allotriploids was related to their parental genome dosage, but the tetraploid plants showed a low growth rate, suggesting that both the genotype and ploidy level contributed to variation in the plant size (Fort et al., 2016). In the present study, we took advantage of a full-sib progeny, including diploid and triploid genotypes, to provide new insights into the genetic effects of ploidy, gender, and genotype on the wood and fiber traits of Populus. All the wood and fiber traits studied here showed high repeatabilities (0.55–0.76). Genotype and ploidy were the main contributors to trait variation, suggesting that integration of heterosis and ploidy vigor is important for polyploid breeding. However, because the number of induced triploid hybrids is limited, and from a single parental combination, it is not known how generalizable these findings are.

An increased genome dosage is thought to cause many phenotypes associated with polyploidy, such as increased cell size, improved biomass yield, and enhanced production of secondary metabolites (Dhawan and Lavania, 1996; Kondorosi et al., 2000; Serapiglia et al., 2015; Iannicelli et al., 2020). Shang et al. (2020) found that ploidy notably affected the stomatal traits of Populus, but did not affect the serration number on the leaf margin and petiole length of the leaf. Triploid elite plants in white poplar hybrids showed better wood properties and fiber traits compared with diploid control (Kang, 2016). In Eucalyptus grandis × E. urophylla, triploid and tetraploid plants presented wider trunks, taller trees with longer stems, wider crowns, and higher BD compared with diploid plants (Longui et al., 2021). In our study, the diploid and triploid groups differed significantly in FL. The significant difference of FL/W between diploids and triploids could be attributed to a consequence of collinearity between FL and FL/W, because there was no significant difference in FW between the ploidy levels. Further analyses of variance components showed that ploidy contributed to the variation of FL and FL/W at 77 and 50%, respectively, indicating the huge potential of triploid breeding for the improvement of fiber traits in Populus.

The utilization of heterosis is an important topic of plant breeding. Compared with the selfing crops, hybridization breeding of the tree species usually results in extensive allelic segregation and assortment in the progeny due to the high heterozygosity of the parents. Generally, wood property and fiber traits vary with genotype in tree species (Zobel and van Buijtenen, 1989). Pliura et al. (2007) found significant genotypic variation in the wood density and growth traits of poplar hybrids. A remarkable difference of FL from wood disks at 1.5-m height was detected among genotypes in Populus (DeBell et al., 2002). In triploid white poplar, the genotype also resulted in significant differences in BD, FL, FW, and FL/W traits (p < 0.001, Wu et al., 2013). In our study, it was found that the wood and fiber traits varied significantly among the genotypes. The contribution of the genotype effect to the measured wood and fiber traits ranged from 7 to 62% (Figure 5). For the BD, LC, and FW traits, the genotype effect was the dominant source of their variation. Therefore, no matter in cross-breeding or triploid breeding of Populus, genotype screening in progeny based on the multiple parental combinations is important for the improvement of wood property and fiber traits.

In addition, gender dimorphism is an important evolutionary transition in many plant families (Miller and Venable, 2000). For dioecious plants, gender dimorphism may lead to phenotypic divergence. In Silene latifolia, the male plants had significantly wider calyx than the female plants (Yu et al., 2011). Maldonado-López et al. (2014) found that the female plants of Spondias purpurea showed a higher plant size and nutritional quality than the male plants. In a collected natural population of P. tomentosa, both FL and FW of the female group were significantly larger than those of the male group, but there were no significant differences between the female and male groups in wood chemical compositions (Du et al., 2014). In our study, however, no statistical difference was detected between the female and male groups in any measured BD, LC, and fiber traits within the diploid or triploid groups. The analyses of variance components showed that gender explained 23% of the variation, but rarely contributed to the variation of the LC, FL, FW, and FL/W traits. The different effects of gender dimorphism between our study and the earlier ones might be attributed to the differences in species and population for investigation. Recently, male varieties of Populus were recommended for plantation in China, because fluff catkins of female plants may cause severe environmental pollution in rural and urban areas, even potential fire risk (Janković et al., 2021). The minor effects of gender on wood properties suggest that the selection of male genotypes might not reduce the pulp-industrial value of Populus in our experimental family.

An understanding of the relationships between these wood properties is of value in the development of a breeding program for the pulp and paper industry. In our study, both phenotypic and BLUP correlations between the traits were analyzed. The results showed that the phenotypic correlations had similar changes with the BLUP correlations, though the significance levels in several trait pairs varied, which reflected on the difference between the phenotypic value and BLUP-adjusted genotypic value. BLUP, presented by Henderson (1975), is a traditional method for predicting genetic parameters and has been commonly used in plant breeding (Resende, 2016). The BLUP method has good predictive accuracy for genetic parameters when the sample sizes are variable (Piepho et al., 2008). Compared with the phenotypic correlation, BLUP reflects the genetic correlation between traits, which can provide a better understanding of how traits are interrelated. In triploids, BD was significantly positively correlated with FL in BLUPs (r = 0.65, p = 0.030), suggesting that BD could be improved together with FL during triploid breeding. In the triploids of P. tomentosa, a significantly positive genotypic correlation between BD and FL was also found with the correlation coefficient ranging from 0.41 to 0.98 at different sites (Wu et al., 2013). There was a moderately negative BLUP correlation between LC and FL in the triploid group (r = −0.46, p = 0.159), in our study, suggesting that the selection of FL trait might lead to the decrease of LC in triploid breeding.

In this study, the CV_G for each measured trait was low, ranging from 4 to 11%, which indicates that the genetic variation is quite small for these wood traits. It means that changes from direct selection and correlated responses will be quite small for these traits in the family of our study. The CV_G values in triploids were no larger than that of the diploids in this family. In the triploids of P. tomentosa, the CV_G values of the wood and fiber traits were also very small, ranging from 1.4 to 6.4 at different sites (Wu et al., 2013). Therefore, to improve the wood and fiber traits, it is necessary to produce many hybrids and triploids for selection. To produce triploids of Populus, various methods for sexual polyploidization, such as 2n pollen induction, 2n female gamete induction, and crossing diploids with tetraploids, have been developed (Einspahr, 1984; Kang et al., 2000; Wang et al., 2010, 2012a, b; Li et al., 2019). The approach of 2n female gamete induction could produce more than 60% triploids (Wang et al., 2010, 2012a, b; Lu et al., 2013), which should be used to increase the number of triploids in this family. Alternatively, other species could be introduced as parents to improve the wood and fiber traits. In our study, the parents are both Aigeiros–Tacamahaca intersection hybrids (the female parent ‘Zheyin3#’ is derived from P. pseudo-simonii × P. nigra and the male parent BJY was generated by crossing P. nigra var italica with P. cathayana), resulting in limited variations of the traits in progeny. In future, more species, such as P. simonii, P. nigra, and P. deltoides, could be used for parental selection with large combining ability, and triploid induction should be conducted using the parents.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

JW conceived and designed the research. X-YH, JS, and Y-HZ collected wood disks and conducted laboratory experiments. JS and D-LL analyzed the data. L-JS managed the field trial. X-YH, D-LL, and JW wrote and revised the manuscript. All authors read and approved the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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