SUMMARY

Carbon dioxide is a significant component of volcanic fiimaroles, and its concentration can vary within large limits in different environments. A high level of hazard can be associated to natural gas emissions, and a concentration of about 30% CO2 in the atmosphere is considered as a life threatening threshold. The oldest historical record of fatalities produced by natural degassing dates back to the Pompeii eruption of Vesuvius (AD 79); more recently, 142 people were killed in 1979 on the Dieng plateau (Indonesia), and gas emissions from volcanic lakes of Cameroon, mainly consisting of CO2, caused 37 victims at Monoun in 1984 and over 1700 fatalities at Nyos in 1986. Significant outputs of carbonic gases can be expected at any active or dormant volcanic system, so that potential hazards from toxic gases are associated to any area of recent or present volcanism. With reference to Italy, two sites appear to deserve our attention when considering hazards connected with gaseous emissions: Phlegrean Fields, NW of Naples, and the island of Vulcano, where civil settlements are located well inside the potentially hazardous areas.

INTRODUCTION

Because of the low solubility of carbon in silicic melts, carbon oxides are preferentially released from magma bodies as the confining pressures decrease: accordingly, any volcanic activity produced by the ascent of magmas from depth is characterized by significant outputs of carbon dioxide, whose stability is much higher with respect to carbon monoxide in all natural conditions.

SUMMARY

Plant species native to CO2 springs have evolved along gradients of naturally elevated atmospheric CO2 concentrations that differ as much as predicted for anthropogenic increases in CO2 over the next 100-200 years. This study characterized differences in gro wth and dry matter partitioning under controlled conditions in two species, Agrostis canina L. spp. montelucci (Selvi, 1994) and Plantago major L., adapted to contrasting atmospheric CO2 concentrations. When plants were grown at twice the present CO2 level there was a pronounced increase in biomass production, which was largely due to an initial stimulation in relative growth rate. In Agrostis canina, CO2 enrichment caused an increase in the relative partitioning of dry matter to the root, but effects were less pronounced in Plantago major. Specific leaf area was reduced by elevated CO2 in both species. Plants adapted to growth at high CO2 produced greater biomass and exhibited higher initial RGRs than plants adapted to lower CO2 concentrations, and in the case of Agrostis canina these differences were maintained at ambient CO2. Differences in original seed weight between populations were small (<10% for Agrostis and < 20% for Plantago). A positive correlation was, however, found between seed weight and the long-term atmospheric CO2 concentration at the site of collection and larger seeds were associated with higher initial plant RGRs under CO2 enriched conditions.

There has been substantial discussion by politicians, scientists and the general public about the likely effects of the changing composition of the atmosphere of the Earth. The CO2 concentration of the Earth's atmosphere continues to increase in response to land-use change and the burning of fossil fuels. There is no question that this increase is directly linked to the ever increasing global human population and our demand for energy. This is one of the most easily quantified aspects of global environmental change.

It is well accepted that the enhanced CO2 concentrations associated with global change will directly influence the process of photosynthesis in most species of plants. Early observations suggested that increased rates of photosynthesis would lead to increased growth rates of plants. There was a proliferation of experimental studies investigating the effects of CO2 on plants. As more data were collected these became increasingly difficult to interpret. The majority of studies were conducted under artificial conditions in growth rooms, glasshouses or open-top chambers. The great deficiency of most studies was that vegetation was exposed for relatively short periods of time. There seemed to be no way forward that would permit the study of the longterm effects of exposure of vegetation to enhanced CO2 concentrations.

In the early 1990s a few groups of scientists faced with this problem suggested using sites with naturally enhanced atmospheric levels of CO2 as natural experiments. There followed a number of scientific programmes in several locations around the world where vegetation was exposed to enhanced CO2.

SUMMARY

In 1984 and 1986, massive eruptions of carbon dioxide from two lakes in Cameroon killed at least 1800 Cameroonian villagers. Countless head of cattle were also asphyxiated. Since then, measurements have shown that the amount of CO2 still dissolved in these lakes is very high (20,000 tons and 500,000 tons at Monoun and Nyos, respectively). The danger of a future gas burst (the so-called “limnic eruption”) could be eradicated by drawing off dissolved CO2. A gas-lift experiment at Lake Monoun in April 1992 allowed CO2 to be released at a flow-rate of 15 to 150 l.s-1 (STP), depending on the diameter of the pipe used. The large high grade CO2 resource from these two west African lakes provides an exceptional opportunity to conduct large scale and long-term experiments on the effect of increased atmospheric CO2 concentrations on biotic systems in the tropical region. A description of the planned experiments is presented.

INTRODUCTION

In several volcanic regions of the world, large amounts of gas containing mainly CO2 are sporadically released. In some instances, the gas released may asphyxiate humans. In the last decade, this phenomenon has been reported in Indonesia (Le Guern, Tazieff & Faivre-Pierret, 1982), and later in Cameroon at lake Monoun in 1984 (Sigurdsson et al., 1987) and at Lake Nyos in 1986. These gas eruptions were all of nearly pure carbon dioxide of deep (mainly magmatic) origin, expanding from a near-surface reservoir.

SUMMARY

Carbon dioxide is the main geosphere product affecting the biosphere. Its migration in the ground has several geological constraints whose understanding is essential for the study of its occurrence and behaviour in shallow environments, such as soil and groundwater. In industrialized, urban and rural areas high CO2 concentrations in the ground may be associated with volatile organic compounds whose effects on the biosphere may be remarkable. In these cases any environmental monitoring should consider all the contaminants, both endogenetic and anthropogenetic, occurring in the ground. This paper is an overview of the main geological factors controlling the migration of endogenetic gas to the surface and, in particular, the CO2 occurrence in soil and groundwater. Some guidelines are given for identifying and monitoring leakages of CO2 and other volatile contaminants based upon soil-gas, exhalation and groundwater surveys.

INTRODUCTION

The occurrence of high concentrations of volatile compounds at the Earth's surface, viz. in soil and groundwater, is the object of a wide series of researches and practical applications within the framework of environmental studies, exploration geology and earthquake prediction. In the first case the researches are particularly focused to those gases, both endogenetic and anthropogenetic, which may have toxic relevance or excite modifications in the biosphere (e.g., CO2, VOCs, CH4, CFCs, SO2, H2S, COS, CS2, HC1, HF, Rn). In the second and third case importance is given to those natural volatiles which may be used as “tracers” or “pathfinders” of subsurface energy sources (geothermal reservoirs, hydrocarbon or ore deposits), and which are sensitive to seismic stresses, respectively (e.g., He, Rn, CO2, H2, CH4, Ar, S-compounds).

SUMMARY

The steady increase in the atmospheric concentration of carbon dioxide has important implications for the future growth and productivity of natural and managed ecosystems and is of particular interest to determine the carbon sinks useful for maintaining carboxylation efficiency in plant response to elevated CO2 by eliminating the excess reduced carbon. The aim of this study was to evaluate the impact of exposure to naturally elevated CO2 concentrations on the leaf carbon economy of a Mediterranean oak, Quercus pubescens. Measurements of net photosynthesis, leaf conductance to water vapour, transpiration, isoprene emission, and chlorophyll a fluorescence parameters were made on one-year-old seedlings (transplanted ten months prior to the experiment) and indigenous trees growing within the vicinity of a CO2 spring and at an adjacent control site (4 km from the spring) on a clear day in August, 1993. Data on tree leaves were provided for comparison only. After measuring the leaves in situ, they were detached and allowed to dry for one hour, after which they were resampled for gas exchange and chlorophyll a fluorescence measurements. In addition, leaves from both seedlings and trees were sampled to enable specific leaf weight and tannin concentrations to be determined. Seedlings and the tree growing near the spring exhibited equal or slightly higher rates of photosynthesis, while leaf conductance was significantly lower, in comparison with plants growing at the control site. Instantaneous leaf water use efficiency was higher in plants growing near the spring than in control plants.

Completion of embryogenesis in angiosperms results in the formation of a full-term embryo, which is an assemblage of organs whose cells and tissues are specialized for functions essential for the initiation and continuation of the sporophytic life of the plant. As seen in Chapter 13, our current understanding of embryogenesis owes much to extensive descriptive accounts gained from light and electron microscopic studies and experimental work relating to the culture of embryos. Based on these accounts, it is convenient to consider embryogenesis in terms of a series of developmental processes aimed at creating a recognizable morphological structure. Most important among these processes are the establishment of the precise spatial organization of cells derived from the first few rounds of division of the zygote, the differentiation of cells of common origin to create diversity in the resulting embryo, and biochemical preparations for embryo maturation, desiccation, and dormancy. As part of the mature seed, the embryo unleashes another developmental program to initiate germination; however, since germination-related events are not considered in this book, the topic will not be a part of the discussion here. The developmental episodes during embryogenesis have little in common, yet there is a common background because they are at the root of many multidimensional processes involving different levels of interactions.

The primary endosperm nucleus, born out of fusion of the second sperm cell with the polar fusion nucleus during double fertilization, is the starting point for the development of the endosperm. Repeated divisions of this nucleus within the confines of the central cell generate a cluster of free nuclei or a cellular tissue known as the endosperm. Although the endosperm is reckoned as a triploid tissue in the vast majority of angiosperms, the ploidy level of the fusion nucleus obviously determines the final nuclear constitution of this tissue. Theoretically, the ploidy level of the endosperm might be expected to vary from diploid in the Oenothera type of embryo sac, to pentaploid in the Penaea, Fritillaria, and Plumbago types, and as high as 9N in the Peperomia type. Although both diploid and pentaploid endosperms have been described, it is, however, doubtful whether a 9N endosperm is formed as a permanent tissue in any seed types investigated.

The endosperm has been studied from a number of viewpoints using light and electron microscopy, tissue culture, and biochemical and molecular techniques. For many years, the patterns of division of the primary endosperm nucleus, the morphological nature of the tissue formed, and the interaction of the tissue with the developing embryo were of prime concern to investigators whose studies have been an important factor in revealing that the developing embryo is nourished by the food materials of the endosperm.

One of the most promising approaches currently available to improve the quality, diversity, and yield of our crop plants is genetic engineering. In a broad sense, the genetic engineering of plants connotes a manipulation of plant cells and organs at the molecular level leading to the introduction, integration, and expression of specific and useful segments of foreign genetic material in a host plant. The application of genetic manipulative techniques has resulted in the production of plants with altered metabolic pathways and useful agronomic traits such as insect, viral, or herbicide resistance; it has also generated custom-made male-sterile plants useful in hybrid seed production. Other genetic engineering approaches, such as improving the post-harvest qualities of fruits and vegetables and creating crops that synthesize useful vaccines, Pharmaceuticals, or chemicals, are already at an advanced stage on the drawing board or are well into the developmental phase.

Historically, engineering of the first transgenic plants relied on the ability of the natural vector Agrobacterium tumefaciens to introduce recombinant DNA molecules into plant cells; in later years, this has become the most widely used procedure for the genetic transformation of various dicotyledonous plants. Cocultivation of isolated protoplasts with bacteria was the method of choice in some of the early attempts at Agrobacterium-mediated transformation.

A series of disconnected early observations on anther and pollen development in angiosperms led to the discovery that male function can be selectively abolished by gene action at any time from the stage of initiation of the anther primordium to the stage of pollen maturation. Occasionally, male function is inhibited nongenetically by chemical or environmental manipulations of the flower. Irrespective of the means of interfering with sex expression, phenotypically this results in the generation of male steriles or plants that fail to produce anthers with viable pollen grains, combined with side effects on other aspects of anther development. Since sterility interferes almost solely with anther and pollen developmental episodes, its study may assist in the analysis of gene expression during male sporogenesis and gametogenesis. For, if we cannot discern the reasons for the abnormal development of pollen grains, we will be sorely limited in our efforts to understand the molecular basis for their normal development. From a practical point of view, male sterility is akin to self-emasculation of the flower, so an understanding of the mechanism involved in pollen degeneration could have important ramifications in breeding programs and in the production of hybrid seeds without the need for labor-intensive procedures.

The stigma and style are the two differentiated organs of the flower that carry out the unique function of gamete screening and selection during pollination. These floral organs undergo subtle morphological and physiological changes from the time they are carved out on the receptacle, and by the end of their growth phase they develop a set of unusual features for the specified functions. It was pointed out at some length in Chapter 9 that the current models for pollen–stigma–style interactions during self-incompatibility predict the existence of a precise control system involving expression of the S-locus both in the pollen grains and in the stigmatic papillae or the style. Supporting this prediction, S-gene products from the pistil were identified as specific glycoproteins, although thus far no pollen component has been found. Based on this model, the widely accepted view is that self-incompatibility is due to the composite reaction of two somewhat similar, if not identical, molecules. The past two decades have witnessed major advances in the isolation and characterization of S-locus genes of the stigma–style complex and of their protein products which mediate in self-incompatibility reactions in some model systems. Included in the list of model systems are plants that display either gametophytic and sporophytic types of self-incompatibility. This chapter will deal with the fundamental components of S-locus genes and their protein products.

In the majority of angiosperms the pollen grain matures at the two-celled stage, enclosing a vegetative cell and a generative cell. In some plants, pollen maturation occurs at the three-celled stage, when the generative cell divides to produce two sperm cells. Irrespective of the number of cells they enclose, mature pollen grains are released by the dehiscence of the anther and are passively carried to the receptive surface of the stigma of another flower in the act of pollination. This is the beginning of a cascade of events that ensure double fertilization in the embryo sac. This chapter will consider how the pollen grain makes its way through the stigma and style toward the ovary and ovule. Although our knowledge of the intimate details of individual events in the odyssey of the pollen grain is far from complete, there is a considerable body of descriptive information relating to these events.

The environment of the stigma and style where the events subsequent to pollination take place is so overwhelmingly complex that it almost defies analysis. Fortunately, recent advances in biochemical and cell biological methods have gone in tandem with exploitation by the electron microscope, with the result that a detailed account of the structure of most of the participating cells in the stigma and style has become available.

Recent advances in molecular biology and genetic engineering have brought new and powerful methodologies to bear upon investigations into the reproductive biology of flowering plants or Angiospermae (angiosperms). These studies, which have been undertaken using a few model systems, have provided novel insights into the role of genomic information during a dynamic phase in the life of higher plants. Naturally, the modern approaches owe their origin to foundations laid in the past; taken together, the old and the new studies have led to the conclusion that the steps in the reproductive biology of plants are integrated systems of changes that occur at levels ranging from morphological to molecular. The purpose of this introductory chapter is to highlight the impact of novel conceptual approaches as well as emerging technologies in unraveling the complex mechanisms that underlie the various phases of sexual and asexual reproduction in angiosperms. Since most of our food and many other natural commodities are generated directly or indirectly from angiosperms, these studies collectively hold great promise of opening new frontiers to improve our commercial exploitation of flowering plants. Thus, the story to be told in the following chapters is as intellectually exciting as it is economically important, with many ramifications of a practical nature.

The anther is a morphologically simple organ of the flower concerned with some unique functions, such as microsporogenesis and the production of pollen grains. The size, shape, and orientation of the anther on the stamen and the diversity of the cells and tissues of the anther make it a morphogenetic system of great interest. A typical anther is a two-lobed organ with two locules or microsporangia in each lobe and, so, the anther is functionally a group of four microsporangia. A single vascular strand is embedded in the connective, which is the tissue found in the central region of the anther between the two lobes. In most plants, anthers are readily accessible to observations and manipulations; for these reasons, and in recognition of their role in the sexual reproduction of plants, anther developmental biology is one of the most extensively studied topics in plant embryology. Over the years, the goal of these studies, aptly termed “classical,” has been to gain insight into the mechanisms by which the anther reacts to developmental information communicated to it. As a result, there has been an accumulation of considerable data on the histology underlying the differentiation of specialized cells and tissues of the anther and the physiology of anther growth, as well on the isolation and characterization of genes preferentially expressed during anther development.

The work reviewed in the previous chapter has established that meiotic division of the microsporocyte results in the formation of four haploid microspore nuclei. They remain encased in the original callose wall of the microsporocyte to form a tetrad – the four-celled stage at the end of meiosis. There are two basic patterns of wall formation followed by these nuclei before they attain the status of cells. In most monocotyledons, immediately after each meiotic division of the microsporocyte, cell plate formation occurs in concert with a centrifugally expanding phragmoplast to produce the tetrad (successive cytokinesis). Alternatively, the norm in dicots is a type of division in which the four nuclei are walled off at the end of meiosis II (simultaneous cytokinesis). In either case, the first wall delimiting the microspore nuclei from each other is constituted of callose and not of cellulose. Later, after its release from the tetrad, each microspore forms its own wall comprising the exine and intine. According to an informal morphological concept, the microspore represents the beginning of the male gametophytic generation, with the term “pollen grain” being reserved for the older microspore, particularly after its release from the tetrad. In some accounts, the first haploid mitosis is considered to terminate the life of the microspore and usher in the reign of the pollen grain. However, the terms “microspore” and “pollen grain” continue to be used interchangeably and synonymously in standard embryology literature to refer to the first cell of the male gametophytic generation of angiosperms.