Formation of Earlywood, Latewood, and Heartwood David Wang s Wood formation Class Regulation of Formation of Earlywood and Latewood l l l Growing ring an annual ring in temperate and warm-temperate zones. The light and dark colors are ascribed to the different cells formed in the earlier and the later parts of the growing season. Wood formed in the first half of growing season is composed of cells with large diameters in the radial direction and thin cell walls. In the latter half is composed of cells with small diameters and thick cell walls. Thujopsis dolabrata Larix leptolepis Transition from earlywood to latewood
The process of earlywood and latewood formation, that is the frequency of division of cambial cells, the enlargement of newly formed xylem cells, and cell wall thicken are controlled by plant hormones.
Regulation of Formation of EW and LW Formation and Distribution of Auxins in Conifers l IAA is largely synthesized by buds in the tree crown, developing needles, shoots. l In the dormant period in winter, IAA is synthesized by dormant buds and old leaves. l The occurrence of small amounts of endogenous IAA in the cambial zone suggests the formation of IAA by cambial cells. l IAA synthesized by the tree crown is basipetally transported mainly to cambial cells and their differentiating cells. Regulation of Formation of EW and LW Formation and Distribution of Auxins in Conifers l When radioactive IAA was fed to Pinus echinata a high radioactivity was found in cambial and differentiating cells. l The transport rate of IAA was estimated to be about 5-7 mm/h in Pinus densiflora. l The transport rate did not change with seasonal changes or with active and resting periods of cambial activity. l The occurrence of IAA in the cambial zone (cambium, differentiating phloem, and xylem) of conifers has been confirmed in many tree species.
Regulation of Formation of EW and LW Effect of IAA on Formation of Tree Stems l When disbudding or defoliation was effected before initiation of cambial activity in early spring cambial activity was inhibited, except when IAA was administered. l The amount of xylem formed increased in accordance with the amount of IAA added. l A continuous supply of IAA to cambial cells was required to maintain fusiform initials, and IAA was lacking these cells were differentiated into axial parenchyma cells and not to tracheids.
Regulation of Formation of EW and LW Effect of IAA on Formation of Tree Stems l When Pinus sylvestris and Pinus resinosa were subject to a long-day treatment, continuous shoot elongation and formation of earlywood cells occurred, whereas on a short-day treatment shoot elongation ceased and latewood cells are induced. l When IAA was supplied externally to the latewood-forming sapling in the short-day treatments, a band of earlywood cells was formed. l This results indicate the changes in cell diameter are regulated by IAA. Regulation of Formation of EW and LW Effect of IAA on Formation of Tree Stems l Concentration of endogenous IAA in the cambial zone of a tree stem exhibits a seasonal change. l The concentration of IAA increased from spring to early summer, and then decreased toward the autumn to the level found in the spring. l In winter, IAA was present at a low level. The stage of rapid decrease of IAA coincides well with the change from earlywood to latewood formation. l IAA is a major factor in the control of elongation of xylem-differentiating cells, the formation of earlywood and latewood, and the transition of earlywood to latewood.
Regulation of Formation of EW and LW Effect of IAA on Formation of Tree Stems l Even if IAA is supplied, cambial activity ceased at a certain time of the season. l For termination of cambial activity (dormancy), other factors may be involved. l A change in the sensitivity of cambial cells to IAA has been suggested to be one of the factors involved. l When IAA was supplied to tree stems cambial activity increased, but the effect changed seasonally, and after summer the effect was very small. Regulation of Formation of EW and LW Effect of IAA on Formation of Tree Stems l The extract natural of any cambial sensitivity to IAA is scarcely known, but it seems that the change is induced by a change of temperature and day length, and is related to structural and histochemical changes in the cambial cells. l It was found that a reconstructure of cell membrane occurred before dormancy. If IAA receptor or carrier proteins are present on the plasma membranes these proteins may tentatively be transported of decreased, thus inducing a low sensitivity for IAA.
Regulation of Formation of EW and LW Effect of IAA on Formation of Tree Stems l When IAA was supplied to Picea abies, both the thickening period and the cell was thickness increased. l IAA is involved in enlargement of cells and promotion of cell division. l It is suggested that in differentiation of cambial cells, a tracheiddifferentiation factor (TDF) is essential, and that earlywood formation requires high concentrations of IAA and TDF, while latewood formation is induced by low concentration of IAA and high concentration of TDF. Regulation of Formation of EW and LW Abscisic Acid (ABA) The formation of latewood of Larix occidentalis is induced by growth inhibiting substances. The amount of these substances increases at the stage of latewood formation. It has been suggested that ABA, which is a dormancy-inducing substance, may be involved in the formation of latewood. ABA has been found in the cambial zone of conifers. When exogenous ABA was administered to stems, it inhibited division and enlargement of the cambial cells.
Regulation of Formation of EW and LW Abscisic Acid (ABA) A high concentration ABA was found in dormant shoots of Pseudotsuga menziesii. These results suggest that ABA acts as a growth inhibiting substances and regulates cambial activity. However, recently investigations on the concentrations of ABA in the cambial zones of several tree species have shown that the concentration of ABA did not change seasonally and that it did not change when earlywood was transformed into latewood or the termination of cambial activity. Regulation of Formation of EW and LW Cytokinins Cytokinins are substances promoting cell division, and many physiological effects of cytokinins are known. When cytokinins are added externally to tree stems, cambial activity is stimulated, and the stimulation is promoted by IAA. Cytokinins are essential for tissue and cell cultures. Native cytokinins are all adenine derivatives, and classified into free base, riboside, ribotide, and glucoside types.
Regulation of Formation of EW and LW Cytokinins The biosynthetic pathway of cytokinins is not fully elucidated, but it has been suggested that they are synthesized by the root apical meristem, and transported to the stem through vascular bundles. Several kinds of cytokinins were found in the roots of Pseudotsuga menziesii, while in the cambial zones of Abies balsamea trans-zeatin, and trans-ribosylzeatin were identified. In Cryptomeria joponica and Larix leptolepis zeatin, transribosylzeatin, cis-ribosylzeatin, isopentenyladenine, and isopentenyladenosine are present. Regulation of Formation of EW and LW Cytokinins Analysis of endogenous cytokinins in trees have not been successful so far. The reason for this is that there are many cytokinin derivatives with different polarities, they are present in very low quantities, and extraction and purification from plant materials is difficult.
Regulation of Formation of EW and LW Gibberellins n Gibberellins are synthesized via ent-kaurene, a diterpene, derived from mevalonic acid. n Gibberellins contain an ent-gibberellan skeleton (C 20 gibberellins) or ent-nor gibberellan skelton (C 19 gibberellins) and are numbered in the order of their identification from plants. n More than 70 gibberellins have been isolated, and their glucosides have been identified. Regulation of Formation of EW and LW Gibberellins n Gibberellins promote elongation growth, floral bud formation and fructification, and are involved in dwarf expression. n When a small amount of gibberellins was added to a stem of Pinus radiata, trunk xylem formation and tracheid diameter increased. n The effect of gibberellins is expressed in coordination with IAA.
Regulation of Heartwood Formation David Wang s Wood Formation Class Histological Characteristic of Heartwood n Intermediate wood and transition wood n Crib (1923) found a narrow band of pale wood around the heartwood of Taxus sp. and the water content of this area was lower than that of sapwood and heartwood. n Crib suggested that the pale wood was represented an initiation stage of heartwood formation.
Histological Characteristic of Heartwood n It is known that when cambium is differentiated to sapwood, prosenchyma cells are dead, but ray and axial parenchyma cells survive in the sapwood. n Living parenchyma cells of sapwood of several European conifers and hardwoods contain oval nuclei at initial stage, but the shape of the nuclei changes to round in aged sapwood. The nuclei later lose chromatin and disappear. Histological Characteristic of Heartwood n The following list are decreased when the differentiation of cambium to sapwood. n The volume of nuclei n The amount of RNA n The numbers of mitochondria n Golgi apparatus n Endoplasmic reticulum n Plastids n Ribosome n In contrast, the volume of vacuoles which store heartwood phenolics increase in accordance with the gradual decrease in physiological activity of the ray parenchyma cells.
Histological Characteristic of Heartwood n Chattaway (1952) showed that the physiological activity of intermediate wood increases considerably when tyloses are formed. n Hugentobler (1965) found that the volume of nuclei and nucleoli in ray parenchyma cells of intermediate woods is largest in particular trees that form a very dark heartwood. He suggested that the increase in the volume of the nucleoli is related to an increase in RNA production. Heartwood and Extractives n Various wood extractives such as terpenes, flavonoids, lignans, stilbenes, troplones, among others, are found in heartwood. n The content of polyphenols is generally higher in the heartwood of old trees than in that of young trees, and decreases with increasing stem height and from the periphery toward the central portion of the stem.
Heartwood and Extractives n The content of extractives was different in different specimens of the same species and in different parts of stem. n The heartwood of slow-growing large trees was generally darker and contained larger amounts of extractives than did young fast-growing trees. n The heartwood of birches and roots of Picea abies contained more lignans than the stem heartwood.
Heartwood and Extractives n The content of arabinogalactan was only 0.3% in the sapwood but 24.5% in the heartwood. n The arabinogalactan occurs largely in the lumen of the heartwood tracheids. Variation in arabinogalactan content with distance from the center of a Larix occidentalis. Heartwood and Extractives n In Pinus sylvestris and P. radiata, triglycerides decreased rapidly and free fatty acids increased in the narrow intermediate wood. n Saturated fatty acids were more common in the heartwood than unsaturated acids, while in the sapwood the content of unsaturated fatty acids was higher than that of saturated acids.
Heartwood and Extractives n Various lignans and tropolones were present mostly at the inner 50-100 annual rings from the dark heartwood boundary of Thuja plicata, and there tropolones were not synthesized by very young trees. n Red heartwood of Cryptomeria japonica contains significant amounts of several norlignans such as sequirin C, agatharesinol, sugiresinol, hydroxysugiresinol, yateresinol and hinokiresinol, whereas black heartwood has fewer norlignans together with hot-water soluble darkcolored substances (higher than 10k Da) composed of phenolic components, arabinogalactan, and arabinoglucuronoxylan. Sequirin-C Agatharesinol
Formation of Color Constituents in Sugi Heartwood Phenol oxidation Hydroxysugiresinol Dehydro-hydroxy sugiresinol Heartwood color material HPLC Chromatogram of Extracts from Taiwania Pith Si-60 column UV-254 nm y = 7.657E-5x + 0.2414 y = 0.0031x - 0.053 R² = 0.9994 R² = 0.999 Taiwanin A Helioxanthin Hinokiol y = 0.0031x - 0.0537 R² = 0.999 Ferruginol y = 0.0011x - 0.005 R² = 0.9993
The Main Constituents of Taiwania Distributed in Different Parts of Wood 1.0 0.8 % 0.5 0.3 0 Pith Inner Heartwood Outer Heartwood Transition Zone Sapwood Heartwood and Extractives n Chromatography, IR, and NMR analyses suggested that the phenolic components of the dark-colored substances consisted of oxidatively polymerized norlignans. n Extractives of C. joponica attained a maximum concentration on the inner inside of three annual rings from the boundary of the heartwood.
Metabolism of Ray Parenchyma Cells n Starch and fat are present in parenchyma cells of the sapwood. These are used with sugars (sucrose) transported via vascular bundles from the leaves, for wood and phloem growth and synthesis of heartwood extractives. n Sugars transported from leaves to ray parenchyma cells of intermediate wood could be converted to acetyl-coa via glycolysis-tca cycle, and then used for the synthesis of A rings of flavonoids and stibene via the malonyl-coa pathway, terpenes via the mevalonic acid pathway. A Metabolism of Ray Parenchyma Cells n Sugars could be converted to pentosephosphates via the pentosephosphate cycle, leading to the B rings of flavonoids and stilbenes, and hydrolyzable tannins via shikimatecinnamate pathway. n By cooperation of the biosynthetic reactions in these pathways, heartwood extractives could be synthesized in parenchyma cells of the intermediate wood.
Gene Expression in Heartwood Formation n It has been shown that PAL, a key enzyme leading to phenylpropanoid metabolism is encoded by a small family of genes in bean, parsley, rice, tobacco, and poplar. n In confier, PAL is encoded by a single gene, which is exceptional. n The isoforms of PAL genes are expressed differently in different organs and in response to different environmental stimuli, which include wounding, infection, light, and other factors.
Gene Expression in Heartwood Formation n Chalcone synthesis (CHS), which regulates the first step of flavonoid synthesis, is also encoded by a gene family comprising several members. n CHS gene family is founds in such plants as soybean and chick pea, petunia, and a French bean. n The various isoforms are differently regulated by various factors and environmental stimuli. Gene Expression in Heartwood Formation n Enzymes involved in the general phenylpropanoid metabolism are mostly encoded by several genes or gene families. n The expression of genes in the syntheses of heartwood extractives (flavonoids and anthocyamins etc.) should therefore be regulated by various biotic and abiotic factors. n The characterization of genes with promoters in flavonoid synthesis in intermediate wood should lead to better information regarding the mechanism of heartwood formation.
Gene Expression in Heartwood Formation n Concept of physiological activation in intermediate wood proposed by Chattaway n Alternation of the metabolism of sapwood ray parenchyma cells by ethylene, such as induction and activation of PAL, 4-CL, CHS, CHI, the pentose phosphate cycle, malonyl-coa pathways, synthesis of lipids and terpenes. n Activation of hydrolases and oxidases by destruction of vacuoles parenchyma cells in the intermediate wood to give heartwood.