Effects of Flooding, Tilting of Stems, and Ethrel Application on Growth, Stem Anatomy and Ethylene Production ofPinus densifloraSeedlings

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Abstract
Flooding of soil, tilting of seedlings, application of ethrel to stems, and combinations of these treatments, variously altered the rate of growth and stem anatomy of 2-year-old Pinus densiflora seedlings. Either flooding or tilting increased stem diameter growth and induced formation of abnormal xylem. Whereas flooding decreased the rate of dry weight increment of roots and needles and increased growth of bark tissues, tilting of stems did not. However, tilting decreased the rate of height growth, stimulated tracheid production, and induced formation of well-developed compression wood with rounded, thick-walled tracheids, with a high lignin content but without an S3 layer in the tracheid wall. Ethylene appeared to have an important regulatory role in stimulating growth of bark tissues as shown by thicker bark in flooded seedlings or those treated with ethrel. Ethylene appeared to have a less important role in regulating formation of compression wood. Flooding increased the ethylene contents of stems and induced formation of rounded, thick-walled tracheids. However, these tracheids lacked such features of well-developed compression wood tracheids as a thick S2 layer, high lignin content, and absence of an S3 layer. Furthermore, application of ethrel to vertical stems greatly increased their ethylene contents but did not induce formation of well-developed compression wood. Furthermore, ethrel application blocked development of certain characteristics of compression wood when applied to tilted seedlings. For example an S3 wall layer was absent in tracheids of tilted seedlings but present in tracheids of tilted, ethrel-treated seedlings. Also lignification of tracheids was increased on the under side of tilted stems, but reduced in tilted, ethrel-treated seedlings, further de-emphasizing a direct role of ethylene in the formation of compression wood. Ethrel treatment induced formation of longitudinal resin ducts in the xylem whereas flooding or tilting of stems did not.