No abstract

Open-pollinated seeds were collected from loblolly pine (Pinustaeda L.) selections made in natural stands in eastern North Carolina, then planted in a short-term, closely spaced test and in a conventional genetic test. Wood density measurements from seedlings, from older trees, and from parents aged 40–75 years old were used to estimate the genetic covariance structure between juvenile and mature wood. These data were also used to determine if the genetic relationship between juvenile and mature wood varied with the estimation method used or with the fertilizer and irrigation treatments in the short-term test. Age–age relationships were moderately to highly positive and these results were corroborative using several methods: parent–offspring regression and coefficient of genetic prediction, half-sib analyses, and graphical use of type I selection mistakes. Strong age–age relationships (rg = 0.76 to 0.90) were expressed between juvenile wood in the short-term test and older-tree wood density in the genetic test. Moderate to high heritabilities (h2 = 0.55–0.76) were estimated for juvenile wood in short-term tests. The latter estimates tended to be higher than heritability estimates based on parent–offspring regression (h2 = 0.23–0.25). There was negligible family × treatment interaction due to rank change between short-term testing treatments. Height and specific gravity expressed a slight positive correlation at all ages.

A long-term series of experiments to map QTL influencing wood property traits in loblolly pine has been completed. These experiments were designed to identify and subsequently verify QTL in multiple genetic backgrounds, environments, and growing seasons. Verification of QTL is necessary to substantiate a biological basis for observed marker-trait associations, to provide precise estimates of the magnitude of QTL effects, and to predict QTL expression at a given age or in a particular environment. Verification was based on the repeated detection of QTL among populations, as well as among multiple growing seasons for each population. Temporal stability of QTL was moderate, with approximately half being detected in multiple seasons. Fewer QTL were common to different populations, but the results are nonetheless encouraging for restricted applications of marker-assisted selection. QTL from larger populations accounted for less phenotypic variation than QTL detected in smaller populations, emphasizing the need for experiments employing much larger families. Additionally, 18 candidate genes related to lignin biosynthesis and cell wall structure were mapped genetically. Several candidate genes colocated with wood property QTL; however, these relationships must be verified in future experiments.

In recent years, several studies have examined the effect of microfibril angle (MFA) on wood quality. However, little research has been conducted upon the genetic mechanisms controlling MFA. In this study, we examined the heritability of MFA in loblolly pine, Pinus taeda L.,and its genetic relationships with height, diameter, volume, and specific gravity. Increment cores were collected at breast height from 20 to 25 progeny from each of 12 to 17 crosses (among 11 parents) in two modified partial-diallels in different locations in southern Arkansas. Specific gravitywas measured on segments containing rings 1 through 5 and on segments containing rings 6 through 20. MFA was measured on the earlywood and latewood sections of rings 4, 5, 19, and 20. Rings 4 and 5 were chosen as representative of core wood and rings 19 and 20 as representative of outer wood. Analyses of variance revealed statistically significant genetic and environmental influences on MFA. Significant general combining ability (GCA), specific combining ability (SCA), and SCA × block effects indicated that there are both additive and nonadditive genetic influences on MFA. Individual-tree, narrow-sense heritability estimates were variable, ranging from 0.17 for earlywood (ring) 4 MFA to 0.51 for earlywood (ring) 20 MFA. Genetic correlations between MFA, specific gravity, and the growth traits were nonsignificant due to large estimated standard errors. South.J. Appl. For. 28(4):196–204.

We report the identification of quantitative trait loci (QTL) influencing wood specific gravity (WSG) in an outbred pedigree of loblolly pine (Pinus taeda L.). QTL mapping in an outcrossing species is complicated by the presence of multiple alleles (> 2) at QTL and marker loci. Multiple alleles at QTL allow the examination of interaction among alleles at QTL (deviation from additive gene action). Restriction fragment length polymorphism (RFLP) marker genotypes and wood specific gravity phenotypes were determined for 177 progeny. Two RFLP linkage maps were constructed, representing maternal and paternal parent gamete segregations as inferred from diploid progeny RFLP genotypes. RFLP loci segregating for multiple alleles were vital for aligning the two maps. Each RFLP locus was assayed for cosegregation with WSG QTL using analysis of variance (ANOVA). Five regions of the genome contained one or more RFLP loci showing differences in mean WSG at or below the P = 0.05 level for progeny as grouped by RFLP genotype. One region contained a marker locus (S6a) whose QTL-associated effects were highly significant (P > 0.0002). Marker S6a segregated for multiple alleles, a prerequisite for determining the number of alleles segregating at the linked QTL and analyzing the interactions among QTL alleles. The QTL associated with marker S6a appeared to be segregating for multiple alleles which interacted with each other and with environments. No evidence for digenic epistasis was found among the five QTL.

Wood properties vary within stems of trees according to genera, species, and individuals. Such variation is largely systematic and is very pronounced in many pines, notably Pinus radiata and P. taeda. While gradual transitions predominate, there are practical needs for categorizing wood, in cutting stems to log lengths, followed by sorting and segregation for processing. Even radial segregation of wood is often possible. In such pines, there are strong, widely recognized progressions, in various wood properties, from pith to bark. Based largely on variation in density, the first few rings from the pith are widely termed juvenile wood, which intergrades outward into wood termed mature wood. Alternative terminology used for this radial progression has been from corewood into outerwood. Critical examination shows the juvenile versus mature wood terminology to be inadequate, on two counts: (1) various other wood properties, some of which can be important, show substantial axial variation at equal ring number from the pith; (2) this categorization based solely on radial variation does not fit the well-established botanical concept of maturation. However, this axial variation closely parallels the vertical variation in shoot morphology and the onset of reproduction that reflect maturation. This argues for a two-dimensional characterization of wood properties: juvenility versus maturity for the progression up the stem, and corewood versus outerwood for the radial progression from the pith to bark. FOR. SCI. 50(4):399–415.