Russell Washusen

Gene expression in Eucalyptus branch wood with marked variation in cellulose microfibril orientation and lacking G‐layers

In response to gravitational stresses, angiosperm trees form tension wood in the upper sides of branches and leaning stems in which cellulose content is higher, microfibrils are typically aligned closely with the fibre axis and the fibres often have a thick inner gelatinous cell wall layer (G-layer). Gene expression was studied in Eucalyptus nitens branches oriented at 45 degrees using microarrays containing 4900 xylem cDNAs, and wood fibre characteristics revealed by X-ray diffraction, chemical and histochemical methods. Xylem fibres in tension wood (upper branch) had a low microfibril angle, contained few fibres with G-layers and had higher cellulose and decreased Klason lignin compared with lower branch wood. Expression of two closely related fasciclin-like arabinogalactan proteins and a beta-tubulin was inversely correlated with microfibril angle in upper and lower xylem from branches. Structural and chemical modifications throughout the secondary cell walls of fibres sufficient to resist tension forces in branches can occur in the absence of G-layer enriched fibres and some important genes involved in responses to gravitational stress in eucalypt xylem are identified.

THE ASSOCIATION BETWEEN CELLULOSE CRYSTALLITE WIDTH AND TENSION WOOD OCCURRENCE IN EUCALYPTUS GLOBULUS

The association between cellulose crystallite width and the occurrence of tension wood was assessed for Eucalyptus globulus Labill., a commercially important plantation hardwood species. Crystallite width (uncorrected for instrumental broadening) was determined from X-ray diffraction patterns collected on SilviScan-2, an instrument developed for the rapid assessment of wood microstructure. Comparisons of crystallite widths were made using 66 samples of tension wood and normal wood selected randomly from one tree known to have abundant tension wood. Tension wood was found to have significantly wider crystallites than normal wood. The mean crystallite widths were 3.6 nm for tension wood and 3.2 nm for normal wood. The normal wood crystallite widths were consistent with those reported in previous studies, allowing for an experimental broadening equivalent to about 0.5 nm in this work. This study demonstrates that SilviScan-2 is useful for the detection of tension wood in solid wood samples such as increment cores.

Relationship between transverse shrinkage and tension wood from three provenances of Eucalyptus globulus Labill

Eucalyptus globulus was studied using 12 mm increment cores taken from straight, vertical and dominant trees of three provenances from a plantation near Tarpina, South Australia. Tension wood fibre percentages were determined from microscopic examination of stained transverse sections and transverse shrinkage was measured from cores dried to 17% equilibrium moisture content. Shrinkage before and after steam reconditioning, as well as associated collapse, were related to the proportions of tension wood. Contrary to normal expectations, high levels of tension wood produced greater shrinkage in the sapwood than in the heartwood in material from all provenances. Collapse was significantly greater in material from the Jeeralang and Western Tasmania provenances than in the King Island provenance. While the mean proportion of tension wood was highest (4.3%) in material from Jeeralang and least from King Island (2.4%), no significant difference among the three provenances was evident for proportions of tension wood fibres when considering only the last years growth. Variation in the width of the gelatinous layer (G-layer) was recorded for the tension wood fibres. Fibres with thin walled G-layers were usually scattered among normal fibres and collapsed regions containing such material generally recovered after reconditioning. On the other hand, regions containing the more typical tension wood fibres, with thick walled G-layers, exhibited high levels of shrinkage and collapse, and poor recovery after reconditioning.

Relationships between Density, Shrinkage, Extractives Content and Microfibril Angle in Tension Wood from Three Provenances of 10-Year-Old Eucalyptus globulus Labill

Summary Density and microfibril angle (MFA) of tension wood and normal wood were assessed in the sapwood and heartwood, from three provenanaces of 10-year-old Eucalyptus globulus Labill. Density was measured using a modified saturation method that also enabled the calculation of the extractives lost during saturation. Microdensity and MFA were determined by SilviScan 2, a rapid X-ray densitometry and X-ray diffraction system developed at CSIRO. Significant differences were found in density and extractives between provenances and also density between the sapwood and adjacent heartwood from each provenance. This result may explain some of the drying differences between provenances found in an earlier study (Washusen and Ilic 2000). Sapwood samples with high percentages of tension wood fibres had high density and a significant positive correlation was found between microdensity and tension wood fibre percentage. MFA was found to be very low in normal wood in the sapwood, where most tension wood was found, so tension wood could not be identified by MFA. The positive association between tension wood and wood density suggests that caution should be taken when selecting trees for high wood density in tree improvement programs.

Longitudinal growth strain as a log and wood quality predictor for plantation-grown Eucalyptus nitens sawlogs

Peripheral longitudinal growth strain (LGS) was measured on a total of 81, 22-year-old pruned Eucalyptus nitens trees from five thinning treatments in a plantation thinning trial in Tasmania. Growth strain data were gathered at breast height on each tree using the CIRAD-Forêt method. The effects of thinning treatment and tree diameter on LGS and its relationships to solid-wood traits determined in a processing trial were examined. LGS was significantly higher in the direction of the prevailing wind and was not significantly affected by either thinning treatment or tree diameter. An index of log end splitting was positively related to tree diameter, sawlog position (upper/lower), and LGS, which explained up to an additional 20% of the variance after tree diameter and log position were accounted for in a linear regression model. High LGS and log end splitting were significant indicators of increased board end splitting.

Prediction of wood tangential shrinkage from cellulose crystallite width and density in one 11-year-old tree of Eucalyptus globulus Labill

Summary The relationship between variation in wood shrinkage and cellulose crystallite width determined by X-ray diffraction was assessed using SilviScan-2 (a system developed at CSIRO for rapid assessment of wood microstructure). Cellulose crystallite width, density and microfibril angle were determined for 600μm wide zones on small wood samples with known tangential shrinkage. The tangential shrinkage measurements included shrinkage to 12% moisture content after reconditioning (MC AR); and a shrinkage differential calculated from tangential shrinkage to 12% MC AR recorded for each zone; and the minimum shrinkage recorded in adjacent wood of similar cambial age. Spearman correlations and forward stepwise regressions showed that the cellulose crystallite width was a good predictor of the shrinkage measures and that density was a minor predictor. Together, cellulose crystallite width and density could explain 75% of the variation in tangential shrinkage for randomly selected locations throughout the tree, and 87% of the variation in tangential shrinkage in samples selected from the lower 5% of tree height. The results suggest that SilviScan-2 can be used to predict tangential shrinkage during drying in increment cores, and may therefore be useful in developing nondestructive sampling strategies in tree improvement programs for E. globulus.

Recovery of dried appearance grade timber from Eucalyptus globulus Labill. grown in plantations in medium rainfall areas of the southern Murray-Darling Basin.

Summary The potential of 40 mm-thick, back-sawn boards of 15-year- old Eucalyptus globulus Labill, subsp. globulus to produce dried appearance products was assessed. Two samples of boards from plantation grown logs described in Washusen et al. (2000) representing (i) the cross-section through each log (Sample 1), and (ii) outer heartwood boards (Sample 2) were dried to 12% moisture content. Sample 1 was dried using sheltered air-drying followed by kiln-drying. Sample 2 was kiln-dried from green without preliminary air-drying. Dried recovery results were poor in both samples and extensive degrade substantially reduced the recovery of undried boards graded select and better that were reported by Washusen et al. (2000). The dried recovery of select grade and better represented reductions of 69% and 60% for Sample 1 and 2 respectively, and based on Sample 1 the final recovery was estimated to be less than 2% of the original log volume. Tension wood was identified as one of the causes of drying degrade in boards from the outer heartwood. The results suggest that the species has little prospect for the production of high-quality solid wood products from unpruned logs from plantations in the southern Murray-Darling Basin unless there can be significant improvement in drying performance.

Tension wood occurrence in Eucalyptus globulus Labill. I. The spatial distribution of tension wood in one 11-year-old tree

Summary In preliminary work for a project to develop methods for rapid detection of tension wood in standing trees, the spatial distribution of tension wood was assessed in one 11-y-old tree of Eucalyptus globulus Labill, from East Gippsland, Victoria, Australia. Tangential shrinkage was assessed at about 500 locations within the tree by drying small wood samples to 12% moisture content after reconditioning, and high tangential shrinkage was used to indicate tension wood. A shrinkage differential was calculated as the difference between the tangential shrinkage at the site of maximum shrinkage and the minimum normal shrinkage in tissue of similar cambial age at the same height in the stem. In earlier studies regression analysis indicated that cellulose crystallite width (which is a good indicator of the presence and severity of tension wood) and microdensity (measured on SilviScan-2; a system developed at CSIRO for rapid assessment of wood microstructure) were significant predictors of the shrinkage differential. In addition, at a shrinkage differential of 3.0% or greater, tension wood was easily identified histochemically in thin transverse microscope sections. Hence the shrinkage differential could be used to map tension wood occurrence. Tension wood was located predominantly below 30% of tree height and increased in extent and severity toward the base. The spatial distribution suggests that sample cores from low in the stem could be used to detect tension wood in standing trees, and the results warrant expansion of the work to more trees in order to further develop detection methods.

The Relationship between Longitudinal Growth Strain, Tree Form and Tension Wood at the Stem Periphery of Ten- to Eleven-Year-Old Eucalyptus globulus Labill.

Summary The potential for growth strain measurements for detection of tension wood was assessed in trees from two plantations of 10- to 11-year-old Eucalyptus globulus. Tension wood had commonly developed at or near the stem periphery of straight, vertical and dominant trees. At a localized level growth strain was found to be a good indicator of tension wood. However, in some cases moderate to low growth strain was also detected in some trees where tension wood had been overgrown with small amounts of normal wood. On a whole tree basis the relationship was not as clear. In this case growth strain values determined from multiple measurements appear to be influenced primarily by tissue close to the site of measurements and not by wood at relatively remote locations. In some cases low growth strain values were found in trees with significant tension wood and in others high growth strain values where little tension wood had developed.

A Study of Eucalyptus Grandis and Eucalyptus Globulus Branch wood Microstructure

Experimental measurements of cellulose crystallite width and microfibril angle (MFA) by X-ray diffractometry on SilviScan-2 and by conventional microtechniques revealed that the branch wood of the two species exhibited very similar trends in cellulose crystallite width and MFA. Cellulose crystallite width was greater on the upper side of the branches. Tension wood, as defined by the occurrence of gelatinous fibres, was found where cellulose crystallite width was greater than 3.0 nm and 3.1 nm in Eucalyptus grandis and E. globulus respectively. In the tension wood zones, MFA was lower than in the rest of the samples and so could be used to differentiate tension wood. On the lower side of the branches MFA determined from X-ray diffractometry unexpectedly exceeded 40° and fibres were often buckled in both the tangential and radial directions in both species. This local variation in the direction of the fibre axes contributed only slightly to the magnitude of the MFA determined by SilviScan-2. Even given this misalignment, the additional evidence gained from pit angles and cracks in fibre walls suggested that the MFA was indeed around 40° in the lower radius of the branches. This MFA is considerably larger than would be expected for eucalypt stem wood and it is suggested that opposite wood in eucalypt branches may provide a complimentary structural role to that of the tension wood. Experimental measurements of crystallite width produced by SilviScan-2 may be used to accurately locate tension wood zones in both species.