Role of ethylene and auxin in regenerative differentiation and orientationof tracheids in Pinus pinea seedlings

A low auxin concentration (0.1% naphthalene acetic acid) induced tracheids with longitudinal polarity parallel to the hypocotyl axis in young Pinus pi nea seedlings. Application of 0.1% ethrel laterally and 0.1% naphthalene ac etic acid apically disturbed axial tracheid polarity and promoted the diffe rentiation of tracheids with a lateral orientation. Ethrel by itself, with no auxin background, did not affect tracheid differentiation. Apical applic ation of 1% gibberellic acid with the low auxin, reversed the polarity diso rder induced by ethrel. Disturbance of axial tracheid polarity was observed under a high auxin concentration (0.5% naphthalene acetic acid) which was similar to the combined effect of ethrel and auxin. The high auxin concentr ation increased tracheid number significantly. This effect was curtailed fo llowing treatment with inhibitors of ethylene formation (Co2+; 1-aminoethox y-vinylglycine) and action (Ag2+). The role of ethylene in controlling the differentiation of radial tracheids, which characterize the vascular rays o f pines, is discussed.