Tuyển tập các báo cáo nghiên cứu về lâm nghiệp được đăng trên tạp chí lâm nghiệp Original article đề tài: Phloem S. Delrot loading and unloading. | 786s Ann. Sà. For. 1989 46 suppl. 786s-796s Forest Tree Physiology E. Dreyer et al. eds. Elsevier INRA Phloem loading and unloading s. Delrot and . Bonnemain Laboratoire de Physiologie et Biochimie Végétales CNRS URA81 25 rue du Faubourg Saint-Cyprien 86000 Poitiers France Introduction Phloem transport of assimilates provides the materials needed for the build up of the herbaceous plant or the tree. Understanding this mechanism is therefore important to control the edification of the plant. Considerable work has been devoted to transport in the past for recent reviews see Giaquinta 1983 Delrot and Bonnemain 1985 Delrot 1987 1989 Van Bel 1987 but much further work is needed especially on woody species because the information available on basic processes such as loading into and unloading from the sieve tubes mainly concerns herbaceous species. Therefore this short overview will often refer to herbaceous species but the general principles which will be given may be used to understand assimilate transport in trees. Actually the scant information available shows wide variety in the anatomical physiological and biochemical situations involved in assimilate transport. General background Nature of translocated substances Long distance transport of assimilates occurs in specialized cells sieve tubes characterized by their osmotic pressure. The high osmotic pressure of the phloem sap is due to the presence of many solutes sugars amino acids ions Ziegler 1975 . Concerning sugars in many species sucrose is the predominant mobile sugar This is the case for most herbaceous plants and for tree species belonging to gymnosperms Picea abies Pinus strobus or angiosperms monocotyledons palm-tree dicotyledons willow . In other plants in addition to sucrose the phloem sap contains oligosaccharides belonging to the raffinose family and characterized by the attachment of one or more galactose residues to the sucrose molecule. Some members of Bignonia-ceae Tiliaceae and Ulmaceae belong
