Heartwood formation

As ToF-SIMS also allows the mapping of the chemical species inside the sample, distributions of the different extractives in the cross-section of the wood are evaluated. They show clearly that hinokinin is predominantly localized in parenchyma cells other extractives are distributed randomly in both parenchyma and tracheid cells (Figure 15.8c). This could be very helpful in understanding the heartwood formation mechanism. 

Although the exact cause of heartwood formation is not known,... 

Pre-extraction of Wood. Since the major factor causing a reduction in the permeability of wood during heartwood formation is occlusion of the pit membranes with extraneous material, one would anticipate that pre-extraction of the wood with a suitable solvent would be a method of increasing the permeability of wood. This contention has been verified by a number of studies (15, 16, 17, 18, 19, 20, 3, 21). However, it appears doubtful that such treatments would be commercially feasible since the solvents are expensive and excessive time is required for the additional step in the treating process. 

Genus or species Ray parenchyma cells Vertical parenchyma cells Average pit size (pm) Amount in pulps (weight %) Reason for heartwood formation... 

Monolignol units usually maintain their trans-double bond in the side chain. During heartwood formation in gymnosperms, however, this double bond is reduced by phenylpropenal double bond reductases (PPDBR). The enzyme from Pinus taeda accepted dehydrodiconiferyl and coniferyl aldehydes but not the respective alcohols and NADPH as substrates (Kasahara et at, 2006). The structure of a very similar alkenal double bond reductase from Arabidopsis was studied (Youn et at, 2006b). 

The cells are physiologically alive in the sapwood. However, near the sapwood-heartwood boundary the depletion of oxygen and the formation of embolisms accompany the hydrolysis of starch to sugars that in turn breakdown, oxidize and polymerize to yield polyphenolics. Extractives are formed and deposited. This is the onset of heartwood formation. 

Until recently, for many plantation pines and spruces any interest in breeding has been to delay the onset of heartwood formation and to rely on preservative treatment to provide protection or in the short term to take advantage of the reduced amount of heartwood in fast grown, short rotation operations. Yet even today, heartwood of older southern pine is used for flooring in the US south. 

Delius, V. Mila, L Scalbert, A. Menard, C. Michon, V. Herve du Penhoat, C. L. M., Douglas-fir polyphenols and heartwood formation,... 

Gang, D. R., Fujita, M., Davin, L. D., and Lewis, N. G. (1998) The abnormal lignins mapping heartwood formation through the lignan biosynthetic pathway. In Lewis,... 

A recent review by Scheffer and Cowling (60) summarizes extensive evidence that the natural durability of many wood species is because of the toxicity of certain phenolic substances that are deposited in the process of heartwood formation. These substances act as poisons to the cellulolytic microorganism rather than by direct action on the enzymatic process of deterioration. 

This subtle mechanism is vital for trees, but causes some difficulties in wood drying. Indeed, pit aspiration occurs as soon as water is removed from the wood, sometimes even during the heartwood formation. In particular, it is impossible to avoid pit aspiration when drying softwoods under normal conditions. 

Hydroxylated derivatives of stilbene (stilbenoids) are secondary products of heartwood formation in trees that can act as phytoalexins (antibiotics produced by plants). 

Hauch S. and Magel E. 1998. Extractable activities and protein content of sucrose-phosphate synthase, sucrose synthase, and neutral invertase in trunk tissue of Robiniapseudoacacia L. are related to cambial wood production and heartwood formation. Planta 207 266-274. 

Higuchi T, Fukazawa K, Nakashima S 1964 Mechanism of heartwood formation. I. Histochemistry of wood tissue. Mokuzai Gakkaishi 10 235-241... 

Pinosylvin and pinosylvin monomethyl ether are formed in the reaction zone in Pinus spp. as a response to fungus infections (57, 61). There is also an interesting correlation between the stilbene production in Pinus radiata and the presence of ethylene (plant hormone) in the wood (105, 106). The processes involved in heartwood formation and the formation of phenolic compounds in reaction zones have been extensively investigated and reviewed by Hillis (59). 

The aucuparins appear to be significant constituents of only a limited number of genera and families and are therefore useful taxonomic markers. So far they have only been found in species belonging to the Rosaceae family. They are part of the defense system and are formed in response to infections or as a consequence of heartwood formation. Further members of this class of plant phenolics will certainly be found in the future. 

Shain L, Hillis W E 1973 Ethylene production in xylem of Pinus radiata in relation to heartwood formation. Can J Bot 51 1331-1335... 

Nelson N D 1978 Xylem ethylene, phenol-oxidizing enzymes and nitrogen and heartwood formation in walnut and cherry. Can J Bot 56 626-634... 

Research interests include wood chemistry, especially of western red cedar bark, leaves and wood heartwood formation especially of true firs bark adhesives HPLC analyses mill identification of lumber. 

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