We have seen in the previous chapter that planar growth of the protonema in blue light is due to a change in the orientation of the plane of division of the terminal cell from transverse to oblique or longitudinal. This response to blue light may be the microscopically visible manifestation of a sequence of complex biochemical processes in the cell subsequent to the photochemical reaction. These processes are initiated by the transcription of a particular set of polysomal mRNA which moves into the cytoplasm to direct the synthesis of new proteins. The general view is that the pattern of nucleic acid and protein synthesis in the protonema might provide the operational criterion for gene transcription during planar growth. We will use this as a frame of reference for discussion in this chapter on the reprogramming of the cellular machinery of the gametophyte during photomorphogenesis. The approach chosen here is to introduce the various facets of earlier investigations and to focus on those concepts which have received renewed experimental attention in recent years. This subject is also considered in the reviews of Miller (1968a), DeMaggio and Raghavan (1973), and Raghavan (1974c).

Protein synthesis

A change in the rate synthesis of enzymatic, regulatory and structural proteins is the basic attribute of an organism embarking on a specific mode of growth.

It was noted in the introductory chapter that, in ferns, the genesis of a spore marks the beginning of the developmental process that produces a gametophyte. From the standpoint of understanding the evolution of the gametophyte in multicellularity, sexuality and functional competence, an understanding of the formation of the spore and of its structure is important. Therefore, in this chapter we focus on sporogenesis in ferns and on the normal course of morphogenesis of the spore to attain maturity. For the most part we will be concerned with description of events, with little emphasis on the underlying mechanisms. As we shall see later, some features of sporogenesis are surprisingly similar across the entire group of ferns, although the final products generated vary somewhat in their morphology.

Sporogenesis

Existing evidence suggests that the types of spores produced by ferns are of great significance in the evolution of the group. Although evolutionary considerations are not emphasized in this book, we shall nonetheless begin this part with an account of the two major spore types found in ferns.

Homospory and heterospory

One of the most important concepts generated from studies on sporogenesis in ferns is the recognition of homosporous and heterosporous plants (Fig. 2.1). Homosporous types produce only one kind of sporangium and just one kind of spore, as is characteristic of the vast majority of extant ferns.

Homosporous ferns produce an immense quantity of spores. Fern spores are relatively resistant to extremes of environmental conditions and persist in a viable, yet metabolically inactive state for long periods of time. Although no one can explain all that is going on in the spore during this period of enforced rest, the hiatus may well be valuable to the spore to plot a survival strategy and form the gametophyte when conditions are most propitious. While the spore is the first cell of the gametophytic phase, development of the gametophyte itself begins with the germination of the spore. It is therefore appropriate that attention is paid to the various factors that keep spores alive for prolonged periods and potentiate their germination. Accordingly, the present chapter is concerned with the physiological aspects of viability, dormancy, and germination of spores; the morphogenetic, metabolic, and biochemical changes during germination are treated in the next two chapters. Understanding of the physiological mechanisms of dormancy and germination of fern spores has benefited immeasurably in recent years from developments in the field of seed germination. However, references will be made in this chapter to the inescapable parallels that exist between seed germination and spore germination only when it is considered necessary to explain certain concepts further.

As the dominant phase in the life cycle of ferns and other pteridophytes, the sporophyte – the leafy fern plant with its characteristic foliage and growth habit – has received and continues to receive much attention in morphological, phylogenetic and evolutionary considerations, as well as in books and monographs. As a departure from this trend, the present book attempts to focus on the other phase of the fern life cycle – the unobtrusive gametophyte. Although slow to begin with, there has been a growing realization of the potentialities of the fern gametophyte for experimental studies, providing impetus for significant new interdisciplinary research on its growth, differentiation and sexuality. Since no unified summary of these investigations comprising the entire gametophytic phase exists, it is the purpose of this book to present a brief, but comprehensive account of the developmental biology of fern gametophytes. This work is also an inevitable outcome of my own interest in the study of gametophytes for nearly a quarter of a century.

The overall philosophy of the book has been to present the story of the gametophyte in a familiar developmental sequence beginning with the single-celled progenitor, going to the multicellular state and ending with the initiation of the sporophytic phase. The content of each chapter is focused around the physiological, cytological and biochemical background of the topic under discussion.

There has been increasing interest in recent years in the reproductive biology of ferns and much attention has been paid to the gametophytes. As is well-known, one of the traits that fern gametophytes have evolved is the ability for sexual reproduction. In sexually reproducing plants, the evolutionary importance of a mechanism that ensures outbreeding to preserve genetic diversity needs no emphasis. However, most homosporous ferns produce bisexual gametophytes that promote inbreeding and attainment of homozygosity in the progeny. Adaptations that prevent these deleterious features in the gametophyte facilitate genetic variability in the population and successful colonization of the species. In this chapter we will review some of the strategies in the reproductive biology of fern gametophytes that have contributed to genetic diversity and evolutionary potential of the species. A word of caution is necessary before we proceed further. Most of what we know in this area concerns homosporous ferns, those represented in nature by their green, thalloid gametophytes. Heterosporous ferns, as well as homosporous ferns with subterranean gametophytes have barely been studied.

The blight of homozygosity

To begin with, some comments about the breeding system of ferns are in order. Insofar as functional antheridia and archegonia are born on different gametophytes, heterosporous ferns are considered to be obligatorily heterothallic (self-incompatible, thus requiring two compatible gametophytes for sexual reproduction).

The potential of appropriately induced spores to germinate depends upon the sequential unfolding of a predetermined program of metabolic processes and regulatory mechanisms that control cell differentiation. Since germination of fern spores is accomplished in a closed system – that is, without the intake of any external nutrients, the synthetic processes must occur at the expense of the stored reserves. Thus, the first order of business of a germinating spore is hydrolysis of its storage reserves to simple compounds. Equally pressing is the need to replenish the structural proteins of the cytoplasm and the complement of enzymes required for general cellular metabolism. At the same time, a sustained biogenesis of membrane systems and organelles also occurs in the spore. Not long after these events, cell division ensues with all the complex biochemical processes it entails, including deoxyribonucleic acid (DNA) and protein synthesis and assembly of the mitotic spindle. Unlike the angiosperm seed, where major catabolic and anabolic activities during germination are segregated to morphologically different tissues, there is no conceivable division of labor between cells in the fern spore. Here, both degradative and synthetic processes preparatory to germination occur in the same cell.

This chapter is devoted to an analysis of the metabolic and synthetic activities during germination of spores of a selected number of ferns.

Introduction

In recent years the term ‘stress’ has been used in an uncritical and non-specific way by administrators of agricultural and plant research, and by some plant physiologists, plant breeders, and molecular biologists. As a consequence, the view is often propagated that crop varieties may ‘soon’ be produced which will be much more stress resistant or stress tolerant than those presently grown. In this concluding chapter, I shall urge caution over this optimistic view before drawing attention to objectives which it may be possible to achieve.

Stress – a normal condition of plants

In common with all living organisms, plants have to acquire the materials from which they are made from their surroundings. In the case of plants this acquisition usually has to be made against a concentration gradient. For land plants, tissue water has to be retained as well as acquired against gradients of water potential which can vary from small, as in humid tropical forests, to very large, as in hot, dry deserts. The concentrations of many essential ions in plants are often a thousand-fold greater than those in the soil solution from which their roots extract these ions. In the sense that plants have to expend metabolic energy to acquire their resources and invest in structures to acquire and conserve them in the face of limited supply, stress is a normal condition of plants, though of variable severity. Commonly, plants are considered to be under stress when they experience a relatively severe shortage of an essential constituent or an excess of a potentially toxic or damaging substance.

Historical background

Biological control, in its widest sense, has been used by man almost since the beginnings of organized arable agriculture. In the third and fourth millennia BC, fallowing, limited forms of crop rotation and mixed or inter-cropping were used in the fertile crescent of the Middle East and later by the Chinese. More recently the Saxon and medieval field systems of Europe also used simple rotation and fallow periods to try to reduce disease and increase fertility. The term biological control was coined in connection with the control of insect pests by the introduction of predators (Howard, 1916, Smith, 1919, in Cook & Baker, 1983; Baker, 1987). The use of non-pathogenic micro-organisms to control plant disease occurred at almost the same time, but was not specifically called biological control. Mechanisms of action were also being investigated, and in 1928 Fleming discovered a particular antibiotic, penicillin, which was later isolated, purified and chemically characterized by Florey, Chain and co-workers.

In the 1920s there was a sudden increase in the number of publications reporting the control of disease by antagonistic fungi, actinomycetes or general soil populations. Thus in 1921 Hartley introduced antagonistic fungi, isolated from soil, to control damping off in pine seedlings (Fig. 2.1) in partially sterilized soil, though the effect was apparent in field soil. There are complications in the modern interpretation of this experiment because there were obviously toxicity problems due to sterilization, and the added saprophytic inoculum did give growth improvement in the absence of the pathogen.

Introduction

There are some diseases that clearly can be described as stem diseases such as wound infections, timber decays and stem cankers on forest and orchard trees, and also wilts like Dutch elm disease. Crown gall classically infects stem bases, but may also cause galls on roots. We will consider all of these in this chapter, but there are some stem base diseases that are excluded and will be dealt with in Chapter 5 on roots: these include the cereal stem base complex of eyespot, sharp eyespot and Fusarium which are all trash- or soil-borne diseases. Similarly the seedling diseases such as damping off, which may affect stems, are considered in Chapter 7, and fire blight, which eventually causes death of stems, is described under infections of flowers (Chapter 6).

Most of the infections that we will consider are therefore of woody stems or twigs. There are rather few diseases of herbaceous annual stems, possibly because, in comparison with leaves, they are hard to penetrate and rather low in nutrients. Woody stems are a very specialized habitat, and one in which biological control has been most effective. They are generally covered by waterproof bark, rich in tannins and phenols, which successfully excludes most organisms. There are fungi and some bacteria growing on bark but they use it mainly as a growing surface and derive very few nutrients from it. The wood itself has very low numbers of micro-organisms in it when young and healthy and it is therefore relatively easy to introduce inocula and have colonization since there is very little competition.

Structure and microbiology of leaf surfaces

There are now many texts dealing with the microbiology of the phylloplane (the leaf surface) especially those based on a series of conferences which started in 1971 (Preece & Dickinson, 1971; Dickinson & Preece, 1976; Blakeman, 1981; Fokkema & van den Heuvel, 1987). The subject is also covered in many general texts on microbial ecology (Campbell, 1985) and Windels and Lindow (1985) have recently produced a small specialist study on biocontrol on the phylloplane that also contains some introductory material. The reader is referred to these texts for a general treatment of the microbiology of the phylloplane, and only an outline will be given here.

The surfaces of leaves are usually hydrophobic due to the presence of cutin and wax, the quantities of which vary with the plant species or cultivar and with the environmental conditions. This impervious layer not only restricts the loss of water from the leaf but also reduces the amount of nutrients which are leached from the leaf (cf. the root, section 5.1). Nutrients may also arrive on the leaf from dust and most importantly from the deposit of pollens. Very few leaves have flat or smooth surfaces: there are often crystals of wax of various shapes and the epidermal cells have convex surfaces with channels between them (Fig. 3.1). There may also be microscopic hairs even on apparently glabrous leaves. These topographical details give many microhabitats with improved water availability or nutrients or with protection from excessive radiation, the ultra-violet component of which is particularly damaging.

Introduction

The areas of land affected by soil salinity are ill defined, because detailed maps are available for few areas only, and consequently global estimates vary widely (Flowers, Hajibagheri & Clipson, 1986). Figures are more reliable for agricultural land in current or recent useage. On this basis some 2 (out of 15) million km2 of land used for crop production is salt affected, and 30–50% of irrigated land (Flowers et al., 1986). This makes salinity a major limitation to food production; indeed it has been recognised as the largest single soil toxicity problem in tropical Asia (Greenland, 1984). Land suited to intensive agriculture is a finite resource, and it has been estimated that the potential for increase is only some 67%, not much more than projected population increases by the end of the century (see Toenniessen, 1984). Dependence upon marginal land, and the need to reclaim land lost already to salinity, appears inevitable. If it is an agricultural necessity to grow plants in saline environments, then there are three possibilities:

1. Change the environment. We are used to solving environmental problems by relatively simple interventions; adding fertilisers, pesticides and water. However, it is much more difficult to remove an excess than it is to supplement a deficiency. A technological solution to the salinity problem has many possibilities but all are costly. In the case of staple foods then either their market value (commercial) or the resources of their consumers (subsistence) precludes such a solution.

2. Improve the resistance of the crop. The overwhelming effort in plant breeding is given over towards what may loosely be termed the ‘improvement of existing crops’.