摘要:

Summary1. Mature chloroplasts are able to synthesize a wide variety of compounds of low molecular weight in addition to carbohydrates.2. Mature chloroplasts from higher plants can synthesize fatty acids from acetate, and galactolipids from UDP‐galactose; but, thus far, there is no direct evidence that chloroplasts can produce their principal fatty acid, α‐linolenate, independently of the rest of the cell.3. Chloroplasts possess the enzymic machinery necessary to generate most of the common amino acids from inorganic nitrogen plus appropriate a‐keto analogs of amino acids. However, the plastids do not appear able to synthesize many α‐keto carbon compounds from the initial products of photosynthetic carbon dioxide fixation.4. Whether chloroplasts can generate their own supply of acetate remains in doubt.5. There is little evidence for or against the existence of chloroplastic enzymes catalysing synthesis of purines and pyrimidines.6. Recent evidence confirms that immature plastids possess the complement of enzymes required for synthesis of protochlorophyllide from 8‐aminolaevulinic acid but leaves open the possibility that extrachloroplastic cofactors may be involved in protochlorophyllide biosynthesis.7. The weight of the available evidence suggests that, despite its great metabolic versatility and possible reproductive autonomy, the chloroplast of the higher plant is not metabolically autonomous or nutritionally independent of the remainder of the plant cell. Therefore, if there is any validity to the oft‐repeated speculation that chloroplasts have evolved from ancient free‐living procaryotes, it appears that the evolution of the chloroplast has led to a considerable loss of nutritional autonomy concomitant with the development or preservation of photosynth

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1971.tb01051.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1971.tb01052.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

Summary1. This review aims to survey the process of translocation of solutes in the phloem, including the experimental observations of the process, hypothetical mechanisms with their consequences, and the compatibility of these mechanisms with the experimental information.2. Some properties of the sieve elements are summarized. The characteristic constituent of the sieve elements is a fibrillar protein, P‐protein, of 60–120 A. filaments, whose function and distribution in intact sieve elements are still the subject of debate.3. Apart from the very high levels of sucrose (0.3–0.9 m) and of specific amino acids and amides (10–100 mm), the contents of the sieve elements are characterized by close regulation of the ionic content; thus K (20–85mM) and Mg (2.3–23 mM) are very high relative to Na (0.06–0.3 mM) and Ca (0.25–0.5 mM) respectively; the pH is also very high.4. Convective movement (mass flow) is demanded by the very high rates of mass transfer. The longitudinal sucrose flux is about 2.5 times 106pmoles cm.‐2sec.‐1in petioles, and several times higher in fruits or trees; this is about 105times any reasonable transmembrane flux, and demands very large loading areas for each file of sieve elements. It also renders unlikely any mechanism demanding an associated trans‐membrane flux of any solute which approaches within several orders of magnitude of the sucrose flow.5. The evidence from tracer measurements (of14C or of heat) favour a mass flow of some kind in the sieve tube, with only restricted exchange between the flowing stream and other sucrose pools in the phloem (or out of it). It is not consistent with ready equilibration with a large stationary reservoir of sucrose, or with reverse flows. There is close correspondence between the input and output kinetics of a length of the trans‐location path, or of build‐up curves at different distances; hence lateral exchange from the moving stream is relatively minor.6. Tracer measurements show that loading into the translocation stream is relatively slow, and is the main determining factor in the time course of appearance of tracer down the stem, or in the profile of radioactivity against distance in the stem. This applies not only to the initial steep front of radioactivity in the stem, but also to the error function profiles found at longer times in some plants; those do not arise as has been suggested, by exchange in a two‐way system of transcellular strands, but are a reflexion of the loading kinetics.7. The evidence for or against bidirectional movement is equivocal. In conditions in which there is a strong source/sink gradient imposed, the movement of both labelled carbon and heat is consistent with a one‐way system, and is difficult to reconcile with two‐way movement. However, in the absence of any strong gradient there is evidence for bidirectional movement. It is suggested that the pattern of flow, as well as the direction and rate of flow, may be controlled by the source/sink relations along the path.8. Electro‐osmosis as a mechanism for translocation seems to be ruled out by a number of theoretical difficulties. The most basic of these is the fact that an electro‐osmotic mechanism is inherently incapable of the transport of both anions and cations, whereas the phloem can do both. There are further quantitative difficulties. The ratio of sucrose to potassium in the sieve elements is about 10, and if potassium provides the current a longitudinal potassium flux of about 2.5 times 106pmoles cm.‐2sec.‐lwould therefore be required in petioles, and considerably more in fruits or trees. This raises very great difficulties of potassium circulation to provide a complete current loop, in the path of recirculation, the size of the transmembrane fluxes required, and the energetics of pumping enough potassium to maintain the driving force for electro‐osmosis.9. Possibilities of activated mass flow, by a mechanism similar to that involved in protoplasmic streaming are discussed. Experimental work on streaming inNitellaand in the slime mouldPhysarumis reviewed, including the evidence that in both these systems, fibrils, made up of 50–70 Å. filaments, are responsible for the production of the motive force, and that these fibrils are akin to actomyosin.10. Possible ways in which fibrillar P‐protein might be organized in the sieve elements to produce translocation are discussed. The force generated byNitella‐typefilaments at the density of P‐protein in phloem exudate would be more than adequate for the observed rates of flow. Alternatively the fibrillar arrangement in the slime mould is capable of producing volume flows as large as those in phloem. This hypothesis provides a function for P‐protein, and is also consistent with the curious ionic concentrations characteristic of sieve elements.11. It is suggested that the control by the source/sink relations of the pattern, rate and direction of flow in the phloem might be achieved by the orientation of force‐generating microfilaments by a Münch‐type flow. Such a flow is inevitable if sucrose is pumped in at one end of the path and removed at the other; it seems to be inadequate to explain the rates of mass transfer, but it might be responsible for inducing

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1971.tb01053.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY

摘要:

Summary1. After defining social wasps, an outline is given of their classification and possible evolution. The Vespidae probably arose in the Malayan region at about the beginning of the Tertiary period and spread from there all over the world. They are now highly developed in South America and it is suggested that they may have reached that country over the Behring Straits, probably in two waves, the first of an ancestral Polybiine, the second perhaps ofPolistesonly.2. A biief account is given of the architecture of wasps' nests which is often specific to the genus. Nevertheless, there are examples of very similar wasps making very different nests and there does not seem any case for putting an overriding value on nest‐architecture in classification. While we know sufficiently well what we have to explain, only a very small beginning has been made in describing and analysing the behaviour which produces the elaborate constructions we find.3. In the familiar Vespinae the concepts of queen and worker are well defined and the two castes are discontinuous. In genera such asPolistesthe position is much more fluid, particularly in the tropical species in which it is not necessary to have a queen specialized for hibernation. Some time after nest‐production, however, a queen becomes recognizable, though often more by her behaviour than by her structure. In the Polybiini, queens and workers are sometimes well differentiated and sometimes not; but in most genera and species there is more than one queen in a nest. Other types of females not so well defined and of uncertain significance also occur. In temperate climates new colonies are founded by one or a few queens; in the tropics most colonies are founded by swarms of queens and workers.4. The main differences between queens and workers seem to be determined in the larval stage, perhaps by some secretion administered by the adults, but clearly influenced by other factors as well. The full behavioural differences between the castes are often finally established by social interactions between the adults.5. Males tend to be produced towards the end of the life of short‐lived colonies or towards the end of the reproductive cycles of longer‐lived colonies. There is need for more information about in‐ or out‐breeding patterns, particularly in relation to specific differences in the degree of sexual dimorphism.6. Very few pheromones are recognized with certainty and only one, the queen‐substance ofVespa orientalis, has been identified chemically. However, some species produce a substance from the female sixth gastral sternite (Van der Vecht's organ) which at least in one genus is an ant‐repellent. Some genera of Polybiines have a somewhat similar gland on the fifth sternite which may be connected with caste‐differentiation. Some wasps (Vespula) produce a footstep pheromone concerned with the recognition of the nest entrance and there is less good evidence for the existence of significant substances in the saliva of workers and in their poison glands (alarm substance).7. Trophallaxis, or the supply of salivary secretion by the larvae to adults more or less in exchange for the food provided from outside by the foragers, seems to be essential to the colonies ofVespasince the adults have no proteases. Trophallaxis also occurs in some Polistinae but its significance there is not known.8. Social hierarchies of ‘peck orders’ are always established amongst the adults ofPolistesand play a part in determining which of several potential queens becomes the acting one. They are also important in establishing the relations of the queen to the workers and of the latter amongst themselves. A similar hierarchy occurs inBelono‐gaster. Some sort of hierarchy also exists amongst adultVespulaand is probably important in relation to trophallaxis, but the hierarchies must become less definite in large colonies with hundreds of individuals, especially if there is more than one queen.9. The principal hurdle in the evolution of social behaviour is the establishment of genes which determine that some females lay most or effectively all of the eggs while others nurse, build and forage. This evolution may only be possible amongst females which are so closely related that their genomes are almost identical. In this connexion we need much more direct field evidence on the dispersal of queens, on mating systems and the incidence of multiple insemination. It appears that, once true social life is established, considerable diversification in nest architecture and social organization may happen relatively rapidly, probably much influenced by various ecological pressures, especially predation.10. A number of species ofVespula, VespaandPolisteshave become social parasites, in the first and the last cases without a worker caste. This seems like a wrong turning taken in the course of the normal evolution of the queen‐worker relationship.11. A brief account is given of the attempts to provide a theoretical framework for the population dynamics of wasp‐colonies.12. An account is given of the relations of wasps, chiefly as foragers, with pl

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1971.tb01054.x

出版商:Blackwell Publishing Ltd    

年代:1971

数据来源: WILEY