Despite major advances in forest biotechnology, clonal regeneration by somatic embryogenesis or organogenesis is still difficult for many woody species and is often limited to the use of juvenile explants. Adventitious regeneration of plants from gymnosperms older than zygotic embryos, and frequently even from highly immature zygotic embryos, is often difficult or has not yet been achieved. A number of experimental approaches that could eventually lead to overcoming recalcitrance are suggested in this review. When cloning trees of various ages, it is important to determine first which part of the individual contains the most responsive cells and at what time of the year these cells are in the most responsive state. This allows selection of the most useful explants. In hardwood trees and a few gymnosperms, responsive tissues are found in root or stump sprouts and in tissues near the site of meiosis at about the time that meiosis takes place. Another potentially active area is the shoot apex with most or all of its leaf or needle primordia removed. Apomixis is a natural form of clonal regeneration but occurs naturally in only one gymnosperm species. As the genetic mechanism of apomixis has been in part elucidated, the induction of apomixis by experimental means may soon be possible. The cytoplasm plays a major role in the expression or repression of nuclear genes that control embryogenesis. Expression of nuclear genes can be manipulated by nuclear transfer into de-nucleated cells (e.g., the cytoplasm of egg cells). Cytoplasmic control also plays a role in regeneration by androgenesis, asymmetric cell division and cell isolation. A short overview is presented of the genetic mechanisms involved in embryo initiation, maturation and germination and how manipulation of these mechanisms by genetic transformation could help in overcoming recalcitrance. It is expected that rapid development in the fields of research areas discussed in this review will over time eliminate the problem of recalcitrance in many instances where it is currently prevalent.

Nectar and pollination drops contain sugars as the main components, but the total concentration and the relative proportions are different. They also contain amino acids, of which proline is frequently the most abundant. Proteomic studies have revealed the presence of common functional classes of proteins such as invertases and defence-related proteins in nectar (floral and extrafloral) and pollination drops. Invertases allow for dynamic rearrangement of sugar composition following secretion. Defence-related proteins provide protection from invasion by fungi and bacteria. Currently, only few species have been studied in any depth. The chemical composition of the pollination drop must be investigated in a larger number of species if eventual phylogenetic relationships are to be revealed. Much more information can be provided from further proteomic studies of both nectar and pollination drop that will contribute to the study of plant reproduction and evolution.

This review focuses on clonal propagation of mature trees by tissue culture. Most trees have marked phase changes that result in a decline in their potential for somatic embryogenesis or micropropagation. By altering conditions of the source material ex vitro, or by changing in vitro conditions encountered by the explant, rejuvenation and increased propagation can sometimes be accomplished. Various methods of enhancing micropropagation are reviewed, with particular emphasis on manipulations that involve application of osmotic, temperature or hormonal stress.

-Conifers have a diversity of pollination mechanisms that assist in the capture of pollen during pollination. Pollination mechanisms can be divided into a number of general types depending on whether they have an ovular secretion that interacts with the pollen. These types include mechanisms that never have a secretion, or those that have a delayed secretion, or the most common type in which a pollination drop is formed. This review outlines the evolutionary context of ovular secretions, describes the origins of these secretions within the ovule, their function in the two types of pollination mechanisms, and details the biochemical composition of these liquids. Not only do ovular secretions provide a germination medium for pollen, but they may also play a significant part in reducing pollen pollution by foreign species. pollination mechanism / conifer / ovular secretion / pollination drop Résumé -Les sécrétions ovulaires : leurs rôles dans les mécanismes de pollinisation des conifères. Les conifères possèdent divers mécanismes de pollinisation qui aident à la capture des grains de pollen lors de la pollinisation. Ces mécanismes peuvent être classés en quelques types généraux selon qu'une sécrétion ovulaire interagissant avec le pollen existe ou non. Ces différents types comprennent des mécanismes sans sécrétion, avec sécrétion retardée ou, et c'est le type le plus répandu, avec formation d'une goutte de pollinisation. Cet article décrit le contexte de ces secretions en terme d'évolution, leurs origines ovulaires, leurs fonctions dans les deux types de mécanis-mes de pollinisation, et leur composition biochimique. Les sécrétions ovulaires non seulement fournissent un milieu favorable à la germination du pollen, mais peuvent aussi diminuer de façon importante la pollution pollinique due à des pollens étrangers. mécanisme de pollinisation / conifère / sécrétion / goutte de pollinisation

Chemical profiles of ovular secretions of ambophilous gymnosperms show a clear signal of pollinator-driven selection, including higher levels of carbohydrates than anemophilous taxa, lower levels of amino acids, and the presence of specific amino acids, such as β-alanine, that are known to influence insect feeding behaviour and physiology.