Biology Of Adventitious Root Formation

Charles E. Hess Department of Environmental Horticulture University of California Davis, CA 95616 Research in the biology of adventitious root formation has a special place in science. It provides an excellent forum in which to pursue fundamental research on the regulation of plant growth and development. At the same time the results of the research have been quickly applied by commercial plant propagators, agronomists, foresters and horticulturists (see the chapter by Kovar and Kuchenbuch, by Ritchie, and by Davies and coworkers in this volume). In an era when there is great interest in speeding technology transfer, the experiences gained in research in adventitious root formation may provide useful examples for other areas of science. Interaction between the fundamental and the applied have been and continue to be facilitated by the establishment, in 1951, of the Plant Propagators' Society, which has evolved into the International Plant Propagators' Society, with active programs in six regions around the world. It is a unique organization which brings together researchers in universities, botanical gardens and arboreta, and commercial plant propagators. In this synergistic environment new knowledge is rapidly transferred and new ideas for fundamental research evolve from the presentations and discussions by experienced plant propagators. In the past 50 years, based on research related to the biology of adventitious root formation, advances in plant propagation have been made on two major fronts.

This book contains the majority of the presentations of the Second International Symposium on the Biology of Root Formation and Development that was hcld in Jerusa lem, Israel, June 23---28, 1996. Following the First Symposium on the Biology of Adventi tious Root Formation, held in Dallas. USA, 1993, we perceived the need to include all kinds of roots, not only the shoot-borne ones. The endogenous signals that control root formation. and the subsequent growth and development processes, are very much alike, re gardless of the sites and sources of origin of the roots. Therefore, we included in the Sec ond Symposium contributions on both shoot-borne (i.e., adventitious) roots and root-borne (i.e., lateral) roots. Plant roots have remained an exciting and an intriguing field of sciencc. During thc years that followed the first symposium, an exceptional proliferation of interest in root biology has developed, associated with the intensive research activity in this field and the contemporary developments in the understanding of root function and development. New methods have been applied, and old ideas and interprctations werc rccxamined. Alto gether, it became necessary to update our viewpoints and to expand them.

Environmental, Physiological and Chemical Controls of Adventitious Rooting in Cuttings provides a review of the environmental, physiological and chemical controls of adventitious rooting in cuttings obtained from plants. In plants, adventitious roots, which are highly useful for vegetative propagation (or clonal propagation) are produced mainly from leaves, hypocotyls, stems or shoots. Vegetative propagation may occur naturally by using propagules such as roots, underground and aerial stems, leaves, buds and bulbils. It may also be done artificially through regenerative organs (rhizomes, bulbs, and corms) and by utilizing specialized methods, like cutting, grafting and layering. This book covers the latest tactics surrounding these processes. As a plethora of factors affect the adventitious rooting of cuttings, adding to the complexity of the phenomenon. The main factors which control adventitious root formation are types of cuttings, presence of leaf area on cuttings, types of hormones and their concentration, duration of hormonal treatment (quick dip, long soak, dry dip, spray dip, or total immerse method), maturation (juvenile or mature), genotype, explant position, and more, all of which are discussed here. Provides a comprehensive and exclusive book on the environmental, physiological and chemical factors associated with adventitious root formation in cutting, with up-to-date literature and lucid illustrations Presents a multidimensional approach and a broad range of explanation on adventitious root formation associated with mature and juvenile cutting Discusses a number of molecular, histological and physiological markers associated with adventitious root formation in numerous plant species Elaborates on how external and internal factors control the cell/tissue initiation, differentiation and overall adventitious root formation in cutting

Ontogeny and anatomy of lateral roots; Endogenous and exogenous influences on the regulation of lateral root formation; Adventitious roots of whole plants: their forms, functions and evolution; Anatomical changes during adventitiousn root formation; Metabolic processes in adventitious rooting of cuttings; Endogenous control of adventitious rooting in non-woody cuttings; Environmental influences on adventitious rooting in cuttings of non-woody species.

The formation of roots is in some respects one of the least fundamentally understood of all plant functions. Propagation by cuttings is the aspect that will occur first to most gardeners and horticulturists, and it is certainly the most useful application. But any observant traveller in the tropics can notice that some trees have the habit of forming roots in the air. Climbers like Cissus bear long fine strings of roots hanging down. Pandanus trees tend to have stout aerial roots issuing from the bases of the long branches, while the tangle of roots around the trunk of many of the Ficus species is characteristic. In Ficus bengalensis, in particular, stout cylindrical roots firmly embedded in the ground from a height of 3 to 5 meters give support to the long horizontal branches, enabling them to spread still further. In the big old specimen at Adyar near Madras, the spread of these branches all around the tree, each with a strong root growing out every few meters, makes a shaded area under which meetings of almost 5000 people are sometimes held. The history of how the formation of roots on stem cuttings was found to be under hormonal control is worth repeating here.

This text presents a comprehensive treatment of the striking of roots from above ground parts of plants. The subject is addressed from the points of view of genetics, plant nutrition and water status, plant enzymology, plant hormones (auxins, gibberellins, cytokinins) and other metabolic, soil, and environment factors. Practical propagation problems are also addressed.