FOREWORD

The decision to hold twin symposia was based on two factors, the experience of the Working Group on Growth Regulators in Fruit Production which used this format so successfully in 1972, and the rapidly increasing cost of travel which would have severely restricted truly international participation in a single meeting. This choice proved to be fortunate as the main emphases of the two Symposia were complementary with little duplication of material. The North American meeting had greater emphasis on new approaches to the problem of juvenility whereas the European meeting had greater emphasis on placing current knowledge of juvenility in perspective.

Only the briefest summary of these Symposia is possible here. Nevertheless some of the salient points discussed should be mentioned. One of these is the continuing lack of agreement on definition of terms used in discussing juvenility. As an example, the juvenile period is often defined as that time from seed germination until the plant attains the ability to flower. This definition precludes plants from flowering in the juvenile phase. Yet some maintain that flowering in the juvenile phase is possible, citing the induction of flowering in very young conifer seedlings with gibberellins among other examples. Since it is most unlikely that these young conifers have attained the mature phase, the common definition obviously does not hold. Which of these two viewpoints represents the general situation regarding the relationship between flowering and juvenility cannot be ascertained now. Whenever discussing juvenility, it is imperative that one defines terms so that others will know from which viewpoint the problem is being approached.

Gibberellins seem to play a central role in control of phase change although the effects are different in different species. Gibberellins can induce flowering in some conifers but they inhibit flowering in a number of angiosperms. Which of the many gibberellins are present, their concentration, their interactions with other endogenous regulators and their sites of action are important considerations. Differences between gibberellins exported by the roots and those produced in the shoots need to be determined. The requirement of some plants to attain a certain minimum height (size) before flowering as well as the results of many girdling experiments might now be explained in terms of gibberellin production and transport within the plants. Certainly this area merits further investigation.

It was generally agreed that the apical meristem is the site of phase change. Again this fits with the data mentioned above regarding plant size and the effects of gibberellins exported from the roots. The evidence is not clear as to what extent the apical meristem acts as a developmental organizer and the extent to which it responds to stimuli from other parts of the plant. Evidence for both modes of action can be cited.

Finally there is the problem of generalizing from results obtained with one species to others. This may be possible where the phylogenetic relationship is close but it leads to great difficulties in some cases, for example the differing effects of gibberellins on young conifers as compared to angiosperms. The applicability of results obtained with Hedera to tree species must also be considered here. While Hedera has been widely used and may be an excellent model for certain aspects of juvenility, the data obtained by using it may not describe the situation in trees with any great reliability.

Many people have contributed much time and energy to the organization of these Symposia and to the preparation of this volume of Acta Horticulturae. I wish to thank the members of the organizing committees, particularly Drs. Romberger and Kender of the North American Committee and Drs. Visser and Karnatz of the European Committee, for their efforts in making these Symposia successful. Dr. H. Jonkers worked diligently to survey the interest of the European workers and assisted in many other ways in the early organizational work of the Working Group. Dr. Miklos Faust deserves special thanks for his unstinting support of the North American Symposium and for the publication of this volume. My assistant, Mrs. Olivia Broome, performed many tasks related to organizing the North American Symposium and to editing this volume for which I am very grateful.

R. H. Zimmerman