Showing papers in "Biological Reviews in 1981"

Phycology and heavy-metal pollution

Abstract: Summary 1. All heavy metals, including those that are essential micronutrients (e.g. copper, zinc, etc.), are toxic to algae at high concentrations. 2. One characteristic feature of heavy-metal toxicity is the poisoning and inactivation of enzyme systems. Many of the physiological and biochemical processes, viz., photosynthesis, respiration, protein synthesis and chlorophyll synthesis, etc., are severely affected at high metal concentrations. 3. Some algae inhabit waters chronically polluted with heavy-metal-laden wastes from mining and smelting operations; Nodularia sp., Oscillatoria sp., Cladophora sp., Hormidium sp., Fucus sp. and Laminaria sp., etc., occur in metal-rich waters. These algal forms are probably more capable of combating the toxic levels of heavy metals and this attribute is a result of physiological and/or genetic adaptations. The sensitivity or tolerance to heavy metals varies amongst different algae. The phenomena of multiple tolerance and co-tolerance may be exhibited by some algae. 4. Heavy-metal pollution causes reduction in species diversity leading to the dominance of a few tolerant algal forms. The primary productivity also decreases after metal supplementation. 5. The uptake and accumulation of heavy metals can be active (energy-dependent), passive (energy-independent), or both. 6. Heavy metals can be safely stored as intranuclear complexes by some algae. Notwithstanding this, some changes in the cell wall can enable the algae to tolerate heavy metals by checking the entry of the metals (exclusion mechanism). 7. The metal content of algae growing in a waterbody may yield valuable information for simulating heavy metal pollution: several species of Cladophora and Fucus have been extensively used for this purpose. 8. Several factors affect and determine toxicity of heavy metals to algae. At low pH, the availability of heavy metals to algae is greatly increased, as a consequence of which pronounced toxicity is evident. Hard waters decrease metal toxicity. Some ions, e.g., calcium, magnesium and phosphorus, can alleviate toxicity of metals. 9. The presence of other metals can influence toxicity of a heavy metal through simple additive effect or by synergistic and antagonistic interactions. Similarly, other pollutants can influence heavy-metal toxicity. 10. The toxicity of heavy metals depends upon their chemical speciation. Various ionic forms of a metal characterized by different valency states, may be differentially toxic to a test alga. 11. Amino acids, organic matter, humic acids, fulvic acid, EDTA, NTA, etc. can complex with heavy metals and render them unavailable. This may eventually lead to less toxicity. 12. Heavy-metal toxicity largely depends upon algal population density: the denser the population the more numerous the cellular sites available, leading to decreased toxicity.

The molecular and cellular basis of preimplantation mouse development

Abstract: Summary The formation of a mouse blastocyst involves the use of genetic information derived from the expression both of maternal genes in the preovulatory oocyte and of embryonic genes from the mid-2-cell stage onwards. Evidence is reviewed suggesting that maternal RNA transcripts made up to 20 days prior to ovulation are available for use after fertilization, that no significant KNA synthesis occurs between germinal vesicle breakdown and the mid-2-cell stage and that during this interval of transcriptional inactivity production of protein is regulated at a post-transcriptional level. Embryonic genes first become active at the mid-2-cell stage and at, or shortly after, this time most maternal mRNA is inactivated. Thereafter, the available evidence suggests that although embryonic mRNA has considerable stability, much of it is translated immediately after it is transcribed. This tight linkage of mRNA transcription and utilization appears to be crucial for certain key events in development. The cellular interactions operating during early development are described. It is suggested that asymmetry of cell contacts in the 8-cell embryo imparts a mosaic structure to the embryo, and that this mosaicism is conserved and elaborated with division to the 16- and 32-cell stages. The mechanisms are discussed by which mosaicism is induced and through which the two cell lineages of the blastocyst are founded. Temporal events in preimplantation development show a periodicity of about 27 h. It is proposed that within each period differentiative events are cumulative but reversible, and that the boundaries between each developmental period constitute points of commitment at which reversal of development becomes no longer possible.

The origins of adaptive interspecific territorialism

Abstract: Summary 1. In order to understand fully the evolution of a behavioural trait one must not only consider whether it is adaptive in its present environment but also whether it originated as an adaptation to existing selective forces or as a fortuitous consequence of selection for a different role in other environments (i.e., as a pre-adaptation) or of selection for different traits (e.g., as a pleiotropic effect). In this paper interspecific territorialism is examined in species of humming-birds, sun-birds, tropical reef fishes, stingless bees, stomatopods, crayfish, and limpets as a means of determining its adaptiveness and its origins. 2. Humming-birds form complex assemblages with species sorted out among the available resources. Dominant species establish feeding territories where flowers provide sufficient nectar. A few large, dominant species, usually uncommon, are marauders on others' territories. Subordinate species establish territories where flowers are more dispersed or produce less nectar, or they fly a circuit from nectar source to nectar source when flowers are even more dispersed, a foraging pattern called ‘traplining’, or they steal nectar from the territorial species by being inconspicuous while foraging. Two species, Amazilia saucerottei and Selasphorus sasin, subordinate in one-to-one encounters, are able to take over rich resources by establishing several small territories within a territory of a dominant and forcing it to forage elsewhere. 3. Among humming-birds, territorial individuals attacked not only subordinate competitors but marauding humming-birds and some insects, which stayed in the territory and foraged at will, and seemingly inappropriate targets, such as non-competitors. This suggests that the stimulus for aggression is ‘any flying organism near the food resources’, regardless of its appearance. The behaviour rather than the identity of the intruder is the stimulus. 4. Sun-birds resemble humming-birds to the extent that dominants establish territories on rich nectar sources and subordinates establish territories on less rich nectar sources or steal from the territories of dominants. The diversity of foraging patterns is not so great as in humming-birds, perhaps because so few species of sun-birds have been studied. However, the advantage of territorialism has been measured in the sun-bird Nectarinia reichenowi. Individuals with territories lose much less nectar to competitors than do those without territories. 5. Field work on three species of tropical reef fishes involved a single aggressive species whose individuals attacked a wide range of species intruding on their territories. The stimulus for aggression in Pomacentrus jenkinsi seemed to be an “object moving through [its] territory”. As suggested for humming-birds, the stimulus is the behaviour rather than the identity of the intruder. 6. The relationships found in stingless bees, stomatopods, crayfish, and limpets are simpler. The dominant and subordinate species divide the resources in their habitat, the dominants' aggression preventing the subordinates from using resources that were otherwise available to them. 7. A general pattern emerges. Mutual interspecific territorialism occurs between species that (i) have different geographic ranges, (ii) occupy different habitats, or (iii) use different resources within the same habitat. Examples of two species holding separate territories on the same resources within the same habitat are rare and occur when the dominant species is rare relative to the available resources. These observations are contrary to the usual view that interspecific territorialism is an adaptation that permits co-existence of potential competitors within the same habitat. 8. Interspecific territorialism is sometimes adaptive and sometimes maladaptive, depending upon the species and the situation. 9. The general pattern of occurrence of the behaviour and the general nature of the stimulus for aggression, i.e., the behaviour rather than the identity of the intruder, suggest that interspecific territoriality is a fortuitous consequence of selection for intraspecific territorialism, the latter being not only an adaptation to the presence of conspecific competitors but a pre-adaptation to the presence of competitors of other species, should they occur.

Cuticular lipids of terrestrial plants and arthropods: a comparison of their structure, composition, and waterproofing function

Abstract: Summary 1. Lipids deposited on the surface or embedded within the cuticle of terrestrial plants and arthropods are primarily responsible for the observed low rates of water loss through the cuticle. 2. These lipids are a mixture of long-chain compounds which include hydrocarbons (saturated, unsaturated, branched), wax esters, free fatty acids, alcohols, ketones, aldehydes, and cyclic compounds. 3. The cuticle of both plants and arthropods is a continuous, non-cellular multilayered membrane which overlies the epidermal cells. 4. In arthropods, horizontal division of the cuticle into layers is clearly visible. In plants, the layers comprising the cuticle are not sharply demarcated. 5. The substance responsible for the structural integrity of the plant cuticle (cutin) is composed of cross-esterified fatty acids; structural integrity in arthropod cuticle is provided by a chitin-protein complex. 6. Cuticular lipids are probably formed near the surface in both plants and arthropods; however, specific sites of synthesis are known for only a few species and little is known about their transport from these sites to the surface. The elaborate pore canal and wax canal system of arthropod cuticle is absent from plants. 7. The physical structure and arrangement of the lipid deposits on the cuticular surface that are so important in controlling water movement depend on both quantity and chemical composition, and are, in turn, specific to each species in relation to its environment. 8. Different lipid components are not equally efficient in reducing transpiration. Maximum waterproofing effectiveness is provided by long-chain, saturated, non-polar molecules containing few methyl branches. 9. Plants and arthropods can, within genetic constraints, alter the composition of their cuticular waxes to improve impermeability when conditions require increased water conservation. 10. None of the models proposed to explain the change in arthropod cuticular permeability which occurs at a species-specific temperature (‘transition temperature’) in many species is supported by the experimental data now available.

The organization and evolution of microtubular organelles in ciliated protozoa

Abstract: Summary (1) Ciliated protozoa are viewed as unicellular organisms structured in a hierarchy of organizational levels that include the macromolecular, suborganellar, unit organellar, organellar complex, and organellar system. (2) The ciliate cortex is divided into two major functional regions, the somatic region and the oral region. The fundamental component of the cortex is an organellar complex, the kinetid, whose organizing centre is the kinetosome with which are associated three fibrillar associates diagnostic of ciliates. These three fibrillar associates are the periodically striated kinetodesmal fibril and two microtubular ribbons, the transverse and postciliary ribbons. (3) Somatic and oral kinetids are found to be of three major types: monokinetids are composed of one kinetosome and its fibrillar associates; dikinetids are composed of two kinetosomes and their fibrillar associates; polykinetids are composed of more than two kinetosomes and their fibrillar associptes. (4) The mechanisms underlying kinetid function and development remain largely unexplored. Research into the molecular biology and ultrastructure, especially of mutant forms, should provide basic insights in the near future. (5) The conservation of kinetid structure across major phyla of organisms suggests that this subcellular structure should be useful in phylogenetic analysis despite the concepts of ‘chemical identity’ and ‘organic design’. (6) The evolutionary rate of change of oral features is greater than that of somatic features, probably due to developmental and ecological factors. Nevertheless both cortical regions are constrained by the phenomenon of structural conservatism; that is, the conservation of structure through time is inversely related to the level of biological organization. (7) Eight major groupings of ciliate species are recognized, based on ultrastruc-tural features of the cortex. Several examples of differences between these eight groups and the groups presently recognized are discussed.

The hydrogen metabolism of cyanobacteria (blue‐green algae)

Abstract: ( 2 ) Hydrogenases . . . . . . . . . . . (3) Species distributions . . . . . . . . . 111 . Hydrogen formation . . . . . . . . . . ( I ) Pathways . . . . . . . . . . . . ( 2 ) Factors influencing nitrogenase-mediated hydrogen formation . (a) Species and strains . . . . . . . . . (b) Growth conditions . . . . . . . . . ( c ) Incubation conditions . . . . . . . . (3) Hydrogenase-mediated hydrogen formation . . . . . (I) Nitrogenase . . . . . . . . . . .

Insect hormones in development

Abstract: Summary Advances in studies of prothoracicotropic hormone (ecdysiotropin), ecdysteroids and juvenile hormones in the past decade are considered: 1. Until recently there has been little progress with prothoracicotropic hormone. The development of a sensitive bioassay for the hormone promises to produce rapid advances. 2. Current methods of hormone analysis are described, with detection limits. The application of these methods in studies of hormones at different stages and in different tissues of insects have revealed a far greater complexity in hormone titres than was predicted from classical studies. 3. Very few studies employ chemical characterization of hormones and some assays do not distinguish biologically inactive metabolites of the hormones from the active hormones. Many studies have thus failed to reveal the numerous rapid fluctuations in hormone titre necessary for insect development. 4. While ecdysteroids act, via a receptor, on specific chromosome sites, the cellular mode of action of juvenile hormone in larval development is still unknown. Recent evidence suggests that juvenile hormone acts prior to the time at which its effects are realized by ecdysteroids. 5. Insect hormones produce dramatic changes in gene activity and their co-ordinate control of specific protein synthesis has been the basis for a number of ‘model systems’ of gene control in higher eukaryotes.

Genetic control of very early mammalian development.

Abstract: Summary The overall picture of mammalian preimplantation development is one of complex but integrated molecular activity. As development progresses from the 1 cell stage, blastomeres acquire properties that differ from those of cells of previous stages and eventually differ from one another even at the same stage. The evidence concerning the role of the embryonic genome during this process can be summarized under two headings: (1) Expression of embryonic genome (a) Quantitative and qualitative changes in RNA and protein synthesis begin to occur soon after fertilization. (b) Changes in structural proteins (enzyme activities, transport systems and intercellular junctions) occur throughout early development. (c) Paternal gene products can be detected as early as the 2-cell stage. (2) The need for expression of the embryonic genome (a) Inhibitors of RNA and protein synthesis result in abnormal and lethal development. (b) Several mutations (T/t, Ay, Os, c25H, Om, Ts) are lethal during early development. (c) Chromosomal imbalance (monsomy, haploidy, nullisomy) has adverse effects on early development. Even though studies in vitro on the inhibition of translation indicate that stable maternal messenger RNAs are present in the developing mouse embryo, the evidence outlined in this article strongly supports the belief that embryonic gene expression occurs very early in development (at least by the 2 cell stage) and that this expression is required for normal development.

The mechanism of lymphocyte-mediated cytotoxicity.

Abstract: Summary 1. Three classes of cytotoxic lymphocyte are discussed: thymus-derived T cells, antibody-dependent K cells and NK (natural killer) cells. Each of these cytotoxic lymphocytes has receptors allowing the formation of adhesions (contact) with a target cell (the cell to be killed). The type of receptor and the corresponding ligand on the target cell is different in each class. Cytotoxic T cells (and probably NK cells) react with a target cell antigen, in a manner rather like antibody-antigen reactions (although not involving classical serum antibody). K cells have a receptor for the Fc part of immunoglobulin (IgG) and hence can make contact with antibody-coated target cells. 2. It seems likely that all three classes of cytotoxic lymphocyte have a similar basic mechanism of killing, which is different from the membrane leakage occurring in complement-mediated lysis. Much more information is available on cytotoxic T cells than on the other types of cell. 3. Cytotoxic T cell killing can be divided into two phases. A reversible phase in which the T cell is in contact with the target cell, but causes no apparent damage. This phase can vary from a few minutes up to several hours, when a single T cell interacts with a single target cell. If the T cell detaches or is inactivated the target cell survives. The second phase is irreversible, once the lethal event has occurred, and the target cell will progress to eventual lysis in the absence of the Tc cells. 4. The lethal event initiates a period of zeiosis (membrane blebbing) in the target cell, which is accompanied by increased effiux of 86rubidium. Cell lysis occurs at a variable time after the initiation of zeiosis, when the soluble contents of the cytoplasm burst out of the target cell. The fact that both these phases are of variable length leads to the accumulation of cytoplasmic markers (such as 51chromium) in the medium in an approximately linear fashion. 5. The nature of the lethal event is unknown, but it is suggested that it involves changes inside the target cell rather than at the target cell membrane. Remarkable long projections from the T cell (and also seen from K cells and NK cells), apparently arising as a result of the receptor-ligand interaction, may be involved in the delivery of the lethal event.

Effects of urban air pollution on plant growth

Abstract: It is not surprising that effects of air pollutants on plants have been investigated in more detail in rural than in urban situations, because the economic implications for agriculture and forestry are of more widespread concern than the fate of plants of amenity value. Apart from areas near well defined sources of pollution, such as factories located in the countryside, amounts of pollution are usually less in rural than in urban areas. The types of pollutants experienced are not, however, totally different in nature. A recent survey by Martin & Barber (1980) has, for example, shown that at a rural site in England remote from known sources of pollution, nitrogen oxides (nitric oxide and nitrogen dioxide, collectively known as NO,) occur alongside SO, and 0,, and that there are approximately equal amounts of NO and NO,. The latter are usually considered to be urban pollutants, related to the density of road traffic, but Martin & Barber found that their concentration in the countryside exceeded that of SO, by 50 per cent. Experiments designed to assess the effects of long-term exposures to individual pollutants at concentrations characteristic of rural areas have produced conflicting results, as revealed by a recent review of effects of SO, on grasses (Bell, 1981). On

Spatial order in microbial ecosystems

Abstract: Summary (1) It is suggested that microbiologists are concentrating too much at present on homogeneous laboratory models such as the chemostat to simulate natural microbial ecosystems, which are not usually homogeneous but structured in time and space. (2) Natural microbial ecosystems that are spatially heterogeneous are reviewed and discussed. (3) Microbial interactions are briefly discussed in so far as they are relevant to spatially organized ecosystems. (4) Concepts such as ‘niche’ and ‘habitat’ are defined and discussed in terms of their applicability to microorganisms growing at specific points in solute gradient systems. The definitions currently accepted lack precision because they do not take into account the importance of spatial and temporal coordinates. Use of the words ‘compartment’ and ‘domain’ is advocated. The former corresponds to a cell, the latter to a region around it where the compartment has some influence, acting either as a source or as a sink for particular solutes. The terms ‘niche’ and ‘habitat’ are redefined for the activity domains of a compartment and the habitat domains of the ecosystem. The importance of the vectorial flow of solutes is stressed. (5) Current methodology in microbial ecology is briefly reviewed. (6) Methods specifically designed for investigating heterogeneous ecosystems are described, These include: (i) The gradostat, a system of interlinked culture vessels allowing the establishment of steady-state bidirectional solute gradients. (ii) Percolating columns. (iii) Capillary methods. (iv.) The thin film fermenter. (v) The use of two-dimensional diffusion plates or stopped time-dependent gradient plates. (vi) Gel-stabilized diffusion models established vertically in glass containers such as beakers. (vii) The use of one-dimensional gradient-plate techniques to determine the habitat characteristics of different microbial species. (viii) Investigations into naturally structured systems such as the bacterial colony.

Cell behaviour and molecular mechanisms of cell-cell adhesion.

Abstract: Summary 1. At the behavioural level, cell adhesion is generally non-specific. The search for molecular mechanisms of adhesion should be conducted on this basis. 2. Cells in general, be they from slime moulds or vertebrate epithelia, possess multiple molecular adhesive mechanisms. In epithelial cells this is shown by the number of their different ultrastructurally recognizable intercellular junctions. Elucidation of the structure and composition of such intercellular junctions will make a valuable contribution to the understanding of cell adhesion. 3. The measurement of cell adhesion is fraught with difficulties. Commonly used assays by aggregation cannot give a true representation of the normal adhesive interactions of cells in tissues, and the results they yield must be interpreted with caution. This is because it takes dissociated tissue cells up to 24 h to develop their full adhesiveness after making initial contact. 4. Cell-cell adhesion probably depends largely upon the interaction of complementary molecules on adjacent cell surfaces. Glycoproteins seem the most likely candidates but, as yet, there is no compelling evidence in any individual case and mechanisms of cell adhesion still remain obscure.

Production and role of interferon in physiological conditions

Abstract: Summary 1. There is convincing evidence that a number of agents such as bacteria, viruses, endotoxins, foreign proteins, smokes and chemicals come into contact under physiological conditions with the lymphoid tissue associated with the gut and/or bronchial systems. Endogenous lectins and proteases may also act as mitogens on the central and peripheral immune system. 2. It is suggested that these agents act as inducers of interferon (and some also as immunogens), so that local production of interferon is turned on successively from cell to cell depending upon their responsiveness and upon the periodic inflow of inducers. 3. On the basis of a number of different features, it is proposed to distinguish between an ‘acute’ and a ‘physiological’ interferon response. In the latter, the interferon-producing cell influences the neighbouring cells by short-range humoral transmission (paracrine control) and possibly by cellular interaction, while the route of the general circulation is preferentially used in the former response. 4. It is suggested that the physiological interferon response, although previously overlooked, has great biological importance because production of interferon at strategic sites can maintain active defence systems essential for survival. 5. It is to be expected that the physiological interferon response, although amenable to experimental verification, may be difficult to detect. On the basis that interferon is normally absent from serum, it is suggested that most of the released immune-type interferon is either bound by cells surrounding the site of its synthesis or catabolized locally. 6. It is postulated that the progressive decline of the physiological interferon response with increasing age may represent one of the factors favouring the insurgence of autoimmune diseases and tumours in the process of ageing. It is also suggested that the involution of the thymus may in part be due to intrathymic production of interferon induced by proteases released from macrophages.

Electron‐probe x‐ray microanalysis: techniques and recent applications in biology

Abstract: Summary (1) X-ray microanalysis is a powerful technique, allowing the quantitative measurement of many elements of physiological interest, at physiological concentrations and with a spatial resolution typically of a few micrometres in bulk specimens, and a few hundred nanometres in thin sections. (2) The basic requirements are a focussed, high-energy electron beam, X-ray spectrometers and a means of visualizing the specimen. These facilities are found in a number of different types of commercial microanalysers, which may be based on either the transmission electron microscope, or the scanning electron microscope. Scanning microanalysers offer a lower image resolution, but are considerably more versatile than instruments based on the transmission electron microscope. (3) Preparing the specimen in a form that will withstand electron bombardment under high vacuum, and yet in which the elements to be analysed are retained in their original locations, is clearly the most critical step. For analysing diffusible elements, especially water-soluble electrolytes, the only reliable method is to freeze as rapidly as possible, and analyse the tissue without any chemical treatment. (4) The best results are obtained if frozen sections are cut and analysed on a cold-stage with their water content retained as ice. Procedures have been worked out for doing this, and for quantitative interpretation of the results, so that original tissue concentrations of the common electrolytes, phosphorus, sulphur and other elements of interest can now be measured with confidence. The original water-content of different cell and tissue compartments can also be estimated from the mass-loss on drying. (5) A simpler alternative is to freeze-dry the frozen sections before analysis. The distribution of diffusible elements is probably not too much disturbed, spatial resolution is improved, and the visual image becomes much clearer, but quantitation is made more difficult and less reliable. Nevertheless this technique is frequently used. (6) For precipitated materials and for fluid samples, much simpler methods of preparation can be used. (7) The technique is the subject of a large and rapidly expanding literature, and is providing new information on the sub-cellular distribution of electrolytes and other elements in many different tissues from animals and plants. (8) Some of the earliest applications to the study of diffusible ions were in the analysis of micropuncture samples from kidney tubules, where it has been possible to analyse many very small samples, and for several elements at once. Some preliminary information has also been obtained on the intracellular ion concentrations in kidneys subjected to different physiological conditions. (9) A particularly successful field has been the study of transporting epithelia, including vertebrate and insect digestive and excretory tissues, where the distribution of ions along the intercellular spaces has been shown not to agree with that predicted from the ‘standing gradient’ theory of osmotic coupling. The regulatory mantle epithelium in a mollusc has also been investigated, and some new information obtained on intracellular ion distributions in the frog skin. (10) Studies on nervous tissue are still at a preliminary stage, because of the structural complexity of the tissue. In muscle, however, it has been possible to demonstrate the re-uptake of calcium by intracellular structures, following its release into the cytoplasm during contraction. (11) Information has been obtained on the distribution of ions in the nuclei and chromosomes of cells at different stages in development and division. Nuclear sodium and potassium levels are generally similar to those in the cytoplasm: the very high sodium concentrations found in nuclei isolated anhydrously are shown to be artefactual. (12) A variety of plant cells has been investigated, problems of particular interest being the regulation of salt uptake by roots and leaves, and the role of potassium ions in causing opening of the stomata by osmotic swelling of the guard-cells. (13) Some applications in pathology are briefly mentioned, including studies on the fate of accidentally or deliberately introduced minerals, and on differences in the ion content of normal and diseased muscle cells. (14) Numerous observations on the subcellular distribution of calcium are collected together. In general, measurable calcium uptake by mitochondria appears to occur only in damaged cells; in muscle the cell-membrane and the sarcoplasmic reticulum seem to be the main organelles responsible for re-uptake of calcium following a contraction. Direct involvement of calcium with a contractile system has been shown in the vorticellid protozoan Zoothamnium, and a connection between calcium and exocytosis has been demonstrated in digestive and other epithelia.

Photocontrol of stomatal movements

Abstract: Summary 1. Opening in light is a feature common to the majority of functional stomata, but the current argument is against the traditional view that light is the principal environmental promoter of opening, because stomata can open in the dark in response to CO2 removal and/or temperature increase. In this review, evidence is provided that light is more efficient and effective than other physical factors in both producing and maintaining wide opening. However, light acts on stomata both directly and indirectly, in conjunction with changes in, for example, CO2 balance, water regime and temperature of the leaf tissue. 2. Three general categories of light effects on stomata are recognized: (a) photosynthetic effects driven by metabolic processes, induced or enhanced by light, (b) hydrophotic effects mediating through light-induced changes in epidermal turgor, and (c) photothermal effects arising from light-dependent changes in leaf temperature. 3. Photosynthetic effects involve both CO2 depletion, and starch mobilization, malate synthesis, H+ extrusion, and accumulation of K+ and C1- in guard cells; these processes are triggered by light of different qualities: (a) Both blue and red light are involved in photosynthetic CO2 fixation, utilizing energy from photosynthetic light reaction(s), which provides C precursors for synthesis of stornatal starch. (b) Blue light, but not red, enhances starch mobilization, PEP carboxylase activity and respiration. Accordingly, blue light is postulated to enhance hydrolysis of stornatal starch providing C3 precursors for malate synthesis via PEP-fixation of endogenous CO2; the active extrusion of H+, derived from malate, is coupled with K+ influx to guard cells. Malate and C1- are competitive anions, for K+, and one begins to play a progressively more important role as the other becomes limiting; in intact leaves, however, malate plays a more decisive role. These processes are driven by the energy from blue-light-enhanced respiration. (c) Both photosynthetic fixation and PEP carboxylation act as CO2 sensors, but the exact role of CO2 in the stornatal mechanism has yet to be determined. 4. Hydrophotic and photothermal effects facilitate guard cell expansion by releasing epidermal pressure through enhanced evaporative water loss, and are, therefore, indirect effects of light; photothermal effects may also contribute to metabolic processes outlined in paragraph 3. 5. Stomatal closure in the dark accompanies starch synthesis, malate reduction, efflux of K+ and C1- from guard cells, and accumulation of CO2 in substomatal cavities. Malate may be converted to starch via C2 compounds. Guard cells release K+ and C1- into apoplastic space, from which they are removed by neighbouring cells. The entry of K+ into neighbouring cells is supposed to be coupled with H+ extrusion. These processes are dependent on respiratory energy. 6. The differential abaxial and adaxial stomatal light responses are related to inherent metabolic differences between the two epidermes, but the biochemical basis is not known.

Regulation of haemopoiesis in altered gravity.

Abstract: Summary (1) The aetiology of one of the most striking physiological changes occurring during space-flight, the loss of red blood cells, remains unknown, and its precise time-pattern in flight has not yet been studied. (2) It is suggested that the changes during space-flight responsible for loss of red blood cells in man are (a) loss of plasma volume resulting from disappearance of hydrostatic pressure in the circulation during weightlessness and (b) reduced energy expended in maintenance of form, posture and locomotion resulting from elimination of the usual gravitational load on the muscles. Quadrupeds, like rats, would be expected to suffer minimal blood shifts in weightlessness and therefore have an unchanged plasma volume. However, since in weightlessness the activity-related energy expenditure by the muscles is reduced, the accompanying reduced oxygen demand by the tissues would cause a reduction in erythropoietin levels and so in the production of red blood cells, and a progressive lowering of the total red blood cell mass toward a new steady-state level. (3) Loss of plasma volume alone does not explain the observed loss of red blood cells in astronauts because, in the three manned Skylab missions, as the duration of the missions increased, loss of red blood cell mass decreased, whereas loss of plasma volume increased. This discrepancy is, however, well accounted for by the above hypothesis by taking into consideration the increased level of exercise of the astronauts as the duration of the mission increased. (4) Though water submersion of human subjects does mimic the effects of weightlessness, such effects were overriden in sea mammals because of adaptation to other factors associated with a life in the sea. (5) From the presented analysis of haemopoietic changes observed in spaceflight, an experiment can be designed for a future flight to uncover the causes and mechanisms of these changes and provide a basis for developing protective measures. Thus, the space environment can be used as an investigative tool to enhance the knowledge of the function of the haemopoietic system, which is a major homeostatic system of man and other vertebrates.