A review on structures of secondary wall in reaction wood fiber of hardwood species

Due to a non-vertical orientation of the stem or branch, which may begin as responses of prevailing winds, snow, slope, or asymmetric crown shape, a specialized wood tissue forms, is called “reaction wood”. In hardwood species, the reaction wood tends to form on the upper side of a leaning stem or branch, is known to as “tension wood”. It is usually related with eccentric growth and changes in structure and chemistry of wood. During the formation of tension wood many changes found in wood cells, especially in wood fibers. The changes of wood properties in tension portion result different shrinkage characteristics during drying, creates a serious problem at adjacent portion of normal wood. However, tension wood is problematic in the hardwood industries and this wood is economically less valued. However, the studies on tension wood are necessary for beneficial applications in the wood industries sectors as well as research sectors. The knowledge on tension wood anatomy is still a matter for tree growers and timber processors, especially who are often involved in the furniture making. Besides, the understanding of tension wood formation provides a unique opportunity to obtain information on the molecular and biochemical mechanisms in the expression patterns of genes/proteins. The studies on reaction wood anatomy are not enough that we concluded any model. This review describes the features of fibers in tension wood todate, including secondary wall layers structure. Until date, six types structure of secondary cell wall layers for tension wood fibers such as, S1 + S2 + S3 + G, S1 + S2 + G, S1 + G, S1 + Ǵ, S1 + S2, and S1 + S2 + S3 have been reported, which considered being involved in plant evolution.