Introduction

Properties of Reaction Wood

Reaction wood is modified in its anatomical and chemical properties and is generally formed in response to a non-optimal orientation of the stem or branch caused by prevailing winds, snow, slope, loads, or asymmetric crown shape. When a herbaceous plant is brought out of its equilibrium position in space, inclined or horizontally positioned for example, a longitudinal growth promotion is initiated on its lower side, thus making the stem bend upward. In the case of woody plants, however, an increased longitudinal growth on the lower side is not sufficient for the upward bending. The righting is achieved by the formation of reaction wood, which develops while the tree “reacts” to its changed environment. The properties and induction of reaction wood have been investigated and reviewed by many wood anatomists and physiologists557273796756. The type of reaction wood formed in gymnosperms is referred to as compression wood. It develops on the lower side of an inclined shoot or stem, where the tissues seem to be under a “compressive” stress. The most characteristic feature of compression wood is the rounded shape of the tracheids, resulting in intercellular spaces. The cell walls of compression wood are thicker than those in normal wood, and contain more lignin but less cellulose. The primary wall is the same as in normal wood, but the secondary wall lacks the innermost S3 layer, containing only S1 and S2 layers. The tracheid length is reduced and the microfibril angle is much higher than in normal wood. Helical cavities are another very characteristic feature of compression wood, and are responsible for some of the physical properties of this tissue. As a result of these changes, compression wood cells can expand longitudinally during the cell maturation process to perform the function of “pushing” up the shoot or stem77786739.

Reaction wood in the arboreal, dicotyledonous angiosperms is called tension wood and is formed on the upper side of a leaning stem, where the tissues are held in “tension”. The most outstanding characteristic of tension wood fibers is the presence of a thick, inner cell-wall layer that consists of highly crystalline cellulose, in which the microfibril orientation is nearly longitudinal and parallel to the fiber axis. This so-called gelatinous layer (G-layer) is almost invariably unlignified and is loosely attached to the other cell-wall layers. As G-layer is blue-stained with a fast-green solution, tension wood fibers are easily recognized using this feature subjected to light microscopy. During maturation, tension wood fibers shrink strongly in the longitudinal direction, thereby creating a very strong “tensile” stress to “pull” the leaning stem upright. Besides, the formation of both compression and tension wood is usually paralleled by increased radial growth on the side, causing the stem to be eccentric50511066636. The characteristic anatomical and chemical features of tension wood have been the focus of extensive study because they are closely related to the quality of wood and its products54722250512351820216162.

There are also some special cases where the patterns of reaction wood formation do not correspond with the group of gymnosperms or angiosperms that the species belongs to. In the Cycadales, which produce very little xylem, no reaction wood has been detected. The Gnetales do not form compression wood. Instead, they can form tension wood tissues. The primitive angiosperm Buxus, however, was found to produce compression wood. These genetic characteristics are correlated with the origin and evolution of the species55678368.

Stimuli Inducing Reaction Wood Formation

More than one factor may contribute as an initial stimulus to the induction of reaction wood formation. Compressive and tensile stresses are the originally proposed candidate stimuli thus giving the names for compression and tension wood, respectively67. The loop experiments of shoots or branches probably contributed most to the confirmation of gravity vector as the stimulus for reaction wood formation. These kind of experiments were conducted by several investigators3233315264715840 and have been repeatedly cited and discussed by later researchers5573777840131467817. In the case of vertical loops of conifers, they found that compression wood was formed on the lower side in both the upper and the lower halves of the loops, but not in the vertical portions. Similarly, in the tested broadleaf trees, tension wood was formed on the upper side of both upper and lower portions of the loop, but not in the vertical portions

Internal control mechanism

Reaction wood formation can also be generated to maintain a proper status of the entire tree even in the absence of any gravitational, compressive, or tensile stimulus307980. A well known example is that which is formed in branches of the uppermost whorl when the apical shoot is removed. Reaction wood formation in this case causes a side shoot to bend upward and replace the lost leader. In an experiment of removing one of the branches from the highest whorl of pine trees, compression wood formation was observed along the out flank of the two adjacent branches, and the two branches bent toward each other to reduce the gap between them76. The process of reaction wood formation in these cases is controlled by an internal mechanism, but can also be regarded as a response to changed environments.

Hormonal Control of Reaction Wood Formation

Auxin

Plant hormones, auxin in particular, have been intensively studied in revealing the biology of gravitropic responses. The upward bending of tilted herbaceous plants is interpreted by the Cholodny-Went theory as a result of lateral redistribution of auxin, its accumulation on the lower side of tilted stems resulting in the acceleration of longitudinal growth on the same side1975.

Many investigations indirectly demonstrated a correlation of elevated auxin level to compression wood formation by local applications of exogenous auxins or auxin regulators. These include examples of artificially induced compression wood formation by the application of IAA or naphthaleneacetic acid (NAA) to vertical shoots or to the upper side of horizontal or inclined shoots535548737440. The old model for tension wood formation postulates that gravistimulation (e.g. tilting of stems) causes a difference in auxin level around the stem and an auxin deficiency on the upper side induces tension wood formation. Applied radiolabeled IAA was detected to move towards the lower side of bent or horizontally placed willow and beech stems opposite to the side of tension wood formation3835. Direct evidence that auxin concentration is lower on the upper side of inclined hardwood shoots where tension wood forms has been detected using bioassay by several investigators4836. A number of application experiments support this postulation55341617242544451260

Ethylene

Ethylene is a volatile hormone which contributes to many plant functions such as cell division, cell wall synthesis, elongation growth, swelling, curvatures, xylem formation, and response to gravity127. Ethylene evolution in tree stems or branches can be enhanced by various kinds of physical stresses such as shaking, bending, and tilting371559491165825741. These stresses often locally stimulate cambial activity and wood formation, including reaction wood. A possibility that ethylene plays a role in reaction wood formation has accordingly been considered and investigated. A role for ethylene produced by stem inclination in compression wood formation of conifers has been suggested by several investigators1594129. The evidence supporting this hypothesis includes findings that ethylene evolution was greater from the lower sides where compression wood formed than from the upper sides in branches or tilted stems of cypress, spruce, and metasequoia114128.

Gibberellin

Gibberellins (GAs) are another group of plant hormones that are essential to cambial growth in both woody angiosperms and gymnosperms. Several GAs have been identified in the cambial region, and both endogenous and applied GAs have been found to be possibly related to stem radial increment23443697042. Much evidence implies that GAs are involved in tension wood formation. It was reported that GA3 induced tension-wood-like fibers in vertical cherry shoots26. Drooping mutants of cherry trees restarted vertical shoot growth following GA3 application, and the formation of tension wood on the upper side was promoted in this occasion46478. Throughout this review it seems that information about the mechanism of reaction wood formation has been accumulating less rapidly in recent years than was the case several decades ago. This work highlights the properties and role of plant hormones in the regulation of reaction wood formation.