Introduction

Self-incompatibility (SI) is the single most important outbreeding device found in Angiosperms. It is a mechanism under genetic control which regulates the acceptance or rejection of pollen during fertilisation. In many species SI is controlled by a single, multi-allelic (S-) gene (Darlington & Mather, 1949; de Nettancourt, 1977). Self-fertilisation is prevented when pollen carrying an S-allele which is genetically identical to that carried by the stigma on which it lands is discriminated from pollen carrying an S-allele which is not carried by the stigma; the former is inhibited, whereas the latter grows normally to achieve fertilisation. As such, self-incompatibility systems play a fundamental and crucial role in the processes involved in Angiosperm sexual reproduction.

SI systems may be conveniently divided into two groups: one is under sporophytic control; the other, which appears to be much more wide-spread, is under gametophytic control. Over the last ten years there has been considerable accumulation of new information, revealing much about the molecular and cellular processes involved in the two major SI systems. Most of the work on SI has concentrated on Brassica (which is under sporophytic control) and Nicotiana, together with other Solanaceous species (which are under gametophytic control). We are primarily interested in the SI system of Papaver rhoeas, which is determined by a single, multi-allelic gametophytically controlled S-gene (Lawrence, Afzal & Kenrick, 1978). In recent years we have begun to dissect the molecular events involved in SI in Papaver rhoeas.

Introduction

Petals have an important function: the attraction of pollinators. Conspicuous petals are, therefore, only found in plants that are pollinated by animals. Attraction of pollinators is terminated by wilting of the petals, by abscission of turgid petals, or by petal colour changes. A change in colour, if it occurs, normally precedes wilting or abscission.

Schumacher (1953), quoted above, expressed what has become a widely accepted hypothesis: the functional life of the petals is genetically determined at an early phase of development, resulting in the expression of genes that are involved in cellular breakdown of the petals in the species that show wilting, in the changes in the abscission zone cells in the species that show abscission, and in the colour changes if these occur. The nature of the physiological processes leading to cellular degradation, and those leading to abscission is only partially known.

Introduction

The programmed senescence of flower petals is a highly controlled developmental event and plays an important role in the overall reproductive strategy of many plants (see Stead & van Doom, this volume). The flower is a terminally differentiated complex structure composed of many organs performing a variety of functions, the ultimate goal being successful sexual reproduction. These functions include the production of pollen, pollination, fusion of gametes, and the development and dispersal of viable seeds. In many species, the petals function in the attraction of insects for pollination. Consistent with the petals' short-lived role in reproduction, pollination often serves as a signal for the initiation of petal senescence (Stead, 1992). In the flowers of carnation, the phytohormone ethylene serves as a signal for the initiation of petal senescence following pollination (Nichols, 1977; Nichols et al., 1983). In the absence of pollination petal senescence in carnation still occurs and is mediated by the increased production of ethylene (Nichols, 1966; Wang & Woodson, 1989). This paper attempts to summarise recent data on the regulation of programmed organ death in carnation flower petals with a particular focus on the molecular events associated with the induction of senescence by ethylene. It is not intended as an exhaustive review of flower senescence, for which the reader is referred to papers by Borochov and Woodson (1989), Cook and van Staden (1988), and Reid and Wu (1992).

Introduction

Flower development has intrigued man since the dawn of time. Firstly flowers are very attractive for their ornamental value and fragrance. Secondly, the flower forms the place in which the plants' reproductive organs and cells are formed. Therefore, development of efficient reproductive organs and cells is of vital interest for the maintenance of life on earth. For these reasons, flowers have also a considerable commercial value both in the area of the production of cut flowers and pot plants and in the area of plant breeding.

A typical dicot flower consists of four concentric whorls of floral organs. The outermost whorl contains the sepals. The main function of these floral organs is the protection of the vulnerable young flowerbud. The second whorl consists of the petals which are often very showy and serve an important function in the attraction of pollinators. Whorl three consists of the stamens in which the male gametes are formed. Whorl four consists of the carpels which are often fused to form a pistil. In the ovary of the pistil the egg cells are present within the ovules. Upon fertilisation, the pollen tube grows towards the ovules and the egg cells, and in a double fertilisation event the egg cell and the central cell are fertilised by the two male reproductive cells.

Floral induction, floral development and plant reproduction have been studied already for a long time especially at morphological and physiological levels.

Introduction

The hormonal theory of plant growth substances states that growth substances are limiting and, therefore, regulatory factors in plant growth and development. However, renewed interest in the role of the sensitivity of plant tissues to growth substances in growth and development was sparked of by a series of articles by Trewavas and his co-workers during the early 1980s (Trewavas, 1981, 1982; Trewavas & Jones, 1981). These articles pointed to the inadequacies of the hormonal theory, and emphasised the importance of sensitivity to growth substances as the primary limiting factor in the control of growth and development by these growth substances. The presence of both the growth substance and its receptor molecule is required for a biological response to occur. Since a biological response is caused by a receptor/growth substance complex, the response is dependent on the concentration of the growth substance as advocated by the hormonal theory. It is obvious that, if receptor availibility changes during growth and development, measurements of changes in growth substance concentration on its own has limited value.

The development of sensitivity to growth substances such as auxins, cytokinins and ethylene appears to precede the developmental process which it induces (Trewavas, 1982; Whitehead & Vasiljevic, 1993). In many flowers, ethylene plays an important role in the initiation and regulation of the processes that accompany corolla senescence.

Introduction

Callase is a complex of β-1,3-glucanase activities which play a crucial role in microsporogenesis. During microsporogenesis, archaesporial cells in the anther give rise to microsporocytes and tapetal cells (Fig. 1). In almost all higher plants a thick wall of callose, β-1,3-glucan polymer, is deposited between the cell membrane and the primary cell wall of the microsporocyte. As meiosis progresses, callose is also deposited along the cellular plates formed during cytokinesis, until each individual microspore is completely encased in callose. The tapetum forms a single layer of cells surrounding the anther locule. The tapetum is an extremely metabolically active tissue and is thought to play a nutritive role in microspore development (Chapman, 1987). Another critical function of the tapetum is the synthesis of callase which is required for the dissolution of the callose walls of the tetrad (Frankel, Izhar & Nitsan, 1969). After the primary and callosic walls of the tetrad have been degraded, the individual microspores are freed into the locule and continue their development into mature pollen grains. The developmental importance of callase activity is illustrated by the occurrence of mutants in petunia and sorghum where callase activity is premature (Frankel et al., 1969; Warmke & Overman, 1972) or delayed (Izhar & Frankel, 1971). These plants are male sterile because the inappropriate expression of callase results in microspore abortion. More recently, work by Worrall et al. (1992) demonstrated that premature secretion of an engineered β-1,3-glucanase from the tapetum of tobacco resulted in partial or total male sterility.

Summary

The genetic regulation of the development of the female gametophyte and early embryo of flowering plants is an area of considerable scientific interest. Cloning genes that are determinant in megagametogenesis and embryogenesis will be a major challenge for the future.

Using an enzymatic maceration technique, intact and viable embryo sacs have been isolated from mature ovules of Petunia hybrida. Total RNA was prepared and used as a template for the synthesis of cDNA. Due to the small amounts of material obtained, the cDNA was amplified, using the polymerase chain reaction (PCR), prior to cloning into λZAPII.

Using degenerate oligonucleotides, targeting conserved motifs in protein kinases expressed both maternally and zygotically in other organisms, ovule and embryo sac cDNA from P. hybrida was subjected to PCR amplification. cDNA fragments of about 160 bp were amplified and cloned. Two classes of ‘mini-clones’ have been characterised and shown to encode, respectively peptide sequences similar to the subdomains VI, VII and VIII of the protein kinases shaggy/zeste-white 3 and MAP/ERK (mitogen activated protein kinase or extracellular signal-regulated kinase). Probes derived from the above ‘mini-clones’ were used to screen cDNA libraries and several clones have been isolated and are being characterised.

In other systems, MAP and shaggy/zeste-white 3 kinases have been associated respectively with mitotic stimulation and establishment of embryo segment polarity. By analogy it might be possible that the homologous plant kinases are required for developmental processes during megagametogenesis and embryogenesis.

Forest lands and their development trends

Forest lands, FLs, are divided into classes as follows.

1. Forest (closed forest)

2. Exploitable EF

3. Unexploitable UEF

4. Other wooded land OWL

In principle, forest land represents sites capable of growing closed stands. In Northern Europe, the minimum yield of fully closed stands during rotation is defined to be 1 m3 per hectare and per year. Other wooded land can only grow scattered and stunted trees and bushes. Wetlands, stony mineral sites, and sites above the latitudinal and elevational boundary of closed forests are examples of other wooded land proper. Poor stocking, especially in Mediterranean Europe, is used as an additional criterion in defining the boundary between forest land and other wooded land.

Exploitable forest is primarily managed to serve the wood-producing function. Multi-benefit management, however, is common in large areas of exploitable forest. In unexploitable forests wood production proper is not economically feasible or it is restricted or prohibited because of functions other than production.

The total area of forest lands in Europe, 195 million ha, comprises 77% of forest and 23% other wooded land (Table 2.1). Other wooded land is concentrated in Northern Europe because of climatic and latitudinal effects and also elevational tundra, and in Mediterranean Europe (Table 2.2).

A significant part of the total area of other wooded land in Mediterranean Europe, 36 million ha, is a huge land reserve, should the increasing of forest and its biomass come to be considered a desirable objective.

General background

European forests and forestry are at a stage in which both the current situation and development trends need to be analysed and the policy implications formulated. This can be illustrated by the following observations.

Wood resources have increased constantly since the 1950s. They are greater now than at any time during the last 200-300 years. This has been achieved mainly by the management regimes developed and implemented in the 19th and 20th centuries. Moreover, as a result of the considerable developments in energy, production and traffic technologies, the use of wood for fuel and in buildings and other construction as well as utility commodities has diminished. Because of the decrease of the agricultural population, the use of wood for household purposes has also been reduced. At the same time, the increased efficiency of agricultural production, including pasturing, has released land for alternative uses, including forestry.

Side by side with the relatively decreasing importance of the commodity function of forests, protective, environmental, social and cultural functions have gained importance. Rising standards of living and purchasing power have given rise to wood substitution. The forest has become more of an environmental and social asset than a commodity asset in many countries.

The vitality of forests has become a dominating topic in discussions concerning forests and forestry practices. In addition to wild fires, increasing emissions from industry, traffic and consumption residues and their depositions in forests and other parts of the biosphere have killed trees and stands on some sites. They are assumed to contribute to a large-scale loss of vitality in forests.

The accuracy and consistency of the statistics of forest resources, fellings and natural losses reported by countries for use in the UN-ECE/FAO Forest Resource Assessments are insufficient with regard to their importance as a basis for estimating forest policy implications and for developing policies and management regimes for multi-benefit forestry in a Europe that is becoming economically and politically more integrated.

A way to improve the situation would be a system of repeated sampling measurements and observations by countries covering all relevant forest resource variables, including those concerning the health and multi-benefit management of forests. Sampling units should be tied to time and space coordinates, which permit the calculation of results for ecological and economic regions regardless of national boundaries.

Wood harvesting and utilization statistics should be of such a quality that reliable forest balance estimates are possible. The weakest point in the current utilization statistics concerns the amount of wood used for fuel and household purposes. Moreover, the definitions employed should be consistent so that country results would be comparable.

Analyses concerning the genetic, density, age and health situation and development trends of forests should be made with reference to the serial successions and climatic climaxes of forest plant communities under the effects of changing environmental factors.

In order to obtain a reliable assessment of the economic profitability of forestry, which has been decreasing and is negative in large areas, income and cost analyses should be made and applied to the rationalization and modernization of working methods, equipment and machinery, and to develop a pricing and valuation system for all material and non-material forest benefits, as well as a system for financing sustainable multi-benefit forestry.

The Timber Section of the ECE/FAO Agricultural and Timber Division has prepared five decennial assessments of the European forest resources. The last one, dated 1990, includes the industrial countries of the Temperate zone. The assessments are based on the information provided by the countries and they form an indispensable information basis for developing forestry – in its all forms – and forest industries in Europe.

The European Forest Institute started its activities in the beginning of 1993. Already in the planning phase of EFI the discussions with the Timber Section of the ECE/FAO Agricultural and Timber Division led to the conclusion that an independent institute could initiate a study based on all ECE/FAO Forest Resource Assessments. The cooperation between EFI and the Timber Section in developing a computerized database using the published assessments made it possible to produce the basic material for this kind of study.

European Forest Institute entrusted the work to Professor Kullervo Kuusela, leader of the Finnish National Forest Inventory for almost 30 years. He has also prepared the Finnish reports for the ECE/FAO Forest Resource Assessments 1970 and 1980, and contributed to the forest resource forecasts as a part of the 1980 Assessment. His earlier international work is focused on the study European Forest Resources and the Trade of Industrial Wood in 1950-2000, which was published in Finnish in 1985.

This study is the first issue in EFI's research report series. It consists of four main parts: introduction, wood resources and harvests by country groups, country statistics and conclusions on the ecological and economic basis of forestry in Europe.

Introduction

The natural forest, its tree-species composition and dynamics are functions primarily of climate and secondarily of soil parent materials. Soil, the living upper layer of soil, is created by climate, soil parent materials, flora and fauna. If a fire, storm or insect calamity destroys a tree stand at the climatic climax stage, pioneer plants occupy the site and start a serial succession towards the climatic climax plant community.

Natural forests proper are very rare in present day Europe. The majority of them are in the boreal coniferous zone of European Russia and in small areas of Finland, Sweden and Norway. Only isolated remnants can be found in remote and mostly inaccessible mountain areas in Europe. All other European forests are man-made tree communities maintained by silvicultural and logging measures. Some of them have been degraded by poor logging practices, pasturing and wild fires, often on lands degraded by erosion.

Whatever the current tree-species composition, age and density structure or management regime, there are natural forces inside a forest ecosystem affecting stand dynamics and awaiting the chance to commence succession towards a true climax. Knowledge of the natural forest ecosystems is therefore the basis for analysing the current and future condition of forests and formulating policies and management regimes for developing multi-functional forests which satisfy the needs of man and society (Fig. 4.1, part 1).

The effects of man's activities on forests are dependent on the nature of economy for which the forests have been used. The principal types of economies are presented in Fig. 4.1, part 2. They form the basis for studying and understanding the history of forests and the changing values which forests represent.