Search

SUMMARY The juvenile leaves of many tropical and a few temperate plants are translucent and apparently colourless or are white, red or brown in colour rather than green. It is already known that the distinctive colours of the young leaves can result from a delayed increase in chlorophyll content and/or from the masking effects and changing levels of other pigments. The present study deals with the patterns of chloroplast development during leaf growth in Coffea arabica L., Durio zibethinus Murray, Mangifera indica L., Photinia x Fraseri Dress, Red Robin and Theobroma cacao L. Even in very young leaves the mesophyll cells contain chloroplasts, but these are small and few in number. Although conspicuous greening usually takes place only after the leaves are fully expanded, chloroplast development proceeds without interruption throughout leaf growth. The late increase in chlorophyll content is associated with a noticeable increase in the long axes of the chloroplasts and/or their numbers within each cell section. The way in which the overall increase in chloroplast volume per cell is achieved varies between species.

The traditional theory of serial endosymbiosis envisages the origin of all chloroplasts from prokaryotic algal symbionts. Of the two membranes that immediately surround plastids, the inner is considered homologous with the plasma membrane of the symbiont and the outer homologous with the vacuolar membrane provided by the host. This theory has been modified to suggest that those chloroplasts that are surrounded by more than two membranes were derived, following a second act of symbiosis, from eukaryotic rather than prokaryotic symbionts; these may have been whole eukaryotic algae or chloroplasts isolated from them. This suggestion is partly based on homologies between the various membranes that surround chloroplasts of different taxonomic groups and those that surround photosynthetic symbionts known today. If chloroplasts did indeed evolve from photosynthetic symbionts, then algal phylogeny must take account not only of the individual characteristics of the host cells and their symbionts but also of modifications of these characteristics arising from interactions between host and symbiont.

SUMMARY The sequence of ultrastructural changes which take place during chloroplast development in leaves of plants of Phaseolus vulgaris grown in the light is compared with that of plants grown in the dark. Further comparisons are made with the developmental sequence found in the hypocotyls of Phaseolus and in the leaves of Zea mays. It is concluded that there is a single basic pathway of chloroplast development. Variations in this pathway are related to blocks in the sequence resulting in the accumulation of storage materials such as phytoferritin and prolamellar bodies, characteristic of the species or tissue concerned. An attempt has been made to assess the ubiquity and duration of an association between plastids and the E.R. in lower plant species compared with the angiosperms. A plastid-E.R. association is common and persistent in lower plant species whereas in angiosperms it is apparently transitory and confined to immature or specialized cells.

Summary In the water fern, Azollapinnata, all the cells of the root are ultimately derived from a single apical cell. The programme of sequential cell divisions which take place as the root develops can be identified with precision. A median longitudinal thin section of this small root provides an ultrastructural transect in which successive stages of cell development are displayed. From a series of sections of roots of different ages it was established that the state of plastid development and the size of the chloroplast population within a cell depend on (1) the age of the root (2) the type of cell and (3) the distance of that cell from the single apical cell.

Summary Making use of serial thin sections of conventionally fixed tissue we have constructed a model of the thylakoid system in a pre-granal, dividing chloroplast. The three-dimensional configuration of this plastid is compared with the thylakoid configurations seen in 1μm thick sections of the zinc iodide-osmium impregnated tissue which were examined with a high voltage electron microscope. A comparison has also been made between the differential staining characteristics of the various membranous invaginations, vesicles and tubules which are frequently associated with the plastid envelope and which may play a part in development of the thylakoid system.

SUMMARY It is suggested that developmental changes in plastid structure follow a cyclic pathway which may involve the dedifferentiation of mature chloroplasts to proplastids and the subsequent redifferentiation of proplastids to mature chloroplasts. The proposal of a cyclic plastid relationship revives the earlier concept of Schimper. This has generally been discarded in favour of a unidirectional concept which discounts dedifferentiation as a normal process. Justification of the cyclic concept depends largely on the verification that plastid dedifferentiation and re differentiation do indeed take place. Evidence for this comes not only from the literature, but also from personal observations.

Chromoplasts are heterogeneous organelles and their carotenoids can be associated with several different structural elements including globules, tubules and membranes. In the flower petals of some species, carotenoids are absent or present only in trace amounts. Nevertheless, the plastids in such flowers can closely resemble or even be indistinguishable in structure from chromoplasts.

Summary In general angiosperms do not synthesize chlorophyll in the dark. It has, however, been pointed out that the large or woody fruits of a few species may be opaque, but that, nevertheless, their embryos become green. In some species of citrus, seeds occasionally germinate within the intact, ripe fruit. The cotyledons of such seedlings expand and become dark green. An electron microscope investigation showed that the cotyledons contained chloroplasts with few stroma lamellae but with true grana. The quality of light reaching the seeds and embryos in ripe lemons was found to be poor in blue light but rich in the red wavelengths necessary for the conversion of protochlorophyll to chlorophyll. The quantity of light penetrating to the embryos was low but of an order of magnitude known to be adequate for chlorophyll synthesis and chloroplast development in a number of species.

Summary When seeds of Phaseolus vulgaris are allowed to develop normally in fruits on the mature plant, the plastids in the primary leaves of the embryo within the seed do not develop into mature chloroplasts. However, when the developing seeds are removed from the parent plants, the cotyledons detached, and the embryonic axes grown on agar, primary leaf plastids differentiate into mature chloroplasts within 7 days. In embryos with attached cotyledons, and similarly grown for a week on agar, plastids fail to differentiate. In primary leaves of dormant seeds allowed to germinate on agar, lamellar development begins earlier in seedlings from which the cotyledons have been removed than in those with attached cotyledons, although there is no significant difference between the two groups of seedlings in the time required for the chloroplasts to become fully mature. Plastid differentiation in primary leaves of developing seeds of Phaseolus therefore appears to be inhibited initially by the presence of the cotyledons. This inhibitory effect apparently declines during seed ripening and dormancy.

SUMMARY The sizes and numbers of plastids were determined in the cells of the upper epidermal and upper palisade layers during development of the primary leaf of Phaseolus vulgaris. Plastids of both epidermal and palisade cells divide at all stages of plastid development, but division ceases soon after the plastids become mature. The process of plastid division involves both extension growth and fission. During growth of the primary leaf within a seed developing on the parent plant, plastids in both the epidermal and the palisade cells show little overall growth in size. During germination epidermal cell plastids remain small but plastids in palisade cells increase considerably in size as the leaf expands. Populations of potentially dividing plastids can be identified by the characteristic distribution of their size classes which, when set out graphically, give curves of distinctive shape. Cell division takes place during development of the primary leaf both prior to and following dormancy. Plastid division also occurs during these two periods. However, within each cell the plastids do not divide in synchrony. Nor are the periods of cell and plastid division synchronized, for the plastids continue to divide during the phase of maximum leaf expansion, when cell division has ceased but while cell extension continues. In the primary leaf of Phaseolus the overall number of cycles of plastid division exceeds that of cell division by between two and three cycles.

The petals of Caltha palustris are unusual in that they contain chromoplasts belonging to two different ultrastructural classes. Other forms of plastid are also present and one of these provides the petal with a reflective starch layer. These different types of plastid and several distinctive features of petal morphology all point to a type of petal which is particularly well-adapted to encourage pollination of the flower by insects. chromoplast. It is common for plastids in adjacent cell layers of the same organ to differ from each other in structure and for these differences first to become apparent during very early stages of organ development. The developing chloroplasts in adjacent layers within the primary leaf of Phaseolus vulgaris L. and those within the green root of Azolla pinnata R. Br. both show such cell layer-related differences (Whatley, 1979; Whatley & Gunning, 1981). However, these layer-related variations in plastid ultrastructure are mainly minor and quantitative, as illustrated by small differences in plastid size and in the amounts of starch and thylakoid material present. More fundamental qualitative differences in plastid ultrastructure are typical of only a few specialized types of cell like the bundle sheath cells of some C4 plants and the sieve elements of seed plants (Whatley, 1983a). The petals of Caltha are unusual in that just such major qualitative differences in ultrastructure are shown by the plastids (including the two classes of chromoplast) in each of the three uppermost cell layers. In addition, a fourth type of plastid - a true chloroplast - is present in cells adjacent to the veins.

SUMMARY Differentially thickened wall bars are formed in cells of the anther endothecium as well as in the xylem. In Phaseolus vulgaris both types of wall bar appear to be lignified. As thickening takes place, Golgi bodies become numerous, endoplasmic reticulum proliferates and both types of organelle occupy characteristic sites within the cell. Thickening begins at an early stage of xylem development but in the endothecium it is long delayed. Subsequent loss of cytoplasm is associated with increasing vacuolation of xylem elements, but may be associated with plasmolysis in the endothecium.

Summary During the period of cellular differentiation, plastids in all cells throughout the primary leaf of Phaseolus vulgaris undergo the same sequence of structural changes. However plastids in different types of cell can, at an early stage, be distinguished from each other quantitatively, i.e. in size, in the amount of starch accumulated, and, later, in the extent of their thylakoid systems. These quantitative differences depend in part on the position of the cell within the lamina and in part on the type of cell involved, and they persist throughout development. Further modifications in plastid structure affect only a few specialized types of cell. These later modifications in plastid structure take place only as the cells become fully differentiated. Plastid division occurs in two separate phases, during seed development on the parent plant and during germination. It is probable that differing rates of plastid division and cell division in epidermal, palisade and mesophyll cells during early stages of germination are responsible for the characteristically different numbers of plastids present in each type of cell in the mature leaf.

SUMMARY The primitive giant amoeba, Pelomyxa palustris, contains a population of endosymbiotic bacteria. The distinctive ultrastructure of one of the species of bacteria is described in detail. A particularly close relationship exists between these bacteria and the nuclei of Pelomyxa. The significance of some aspects of this relationship is discussed with reference to the endosymbiotic theory of the origin of mitochondria from bacterial precursors.

SUMMARY The fine structure is described of Prochloron, a unicellular prokaryotic alga which contains both chlorophyll a and chlorophyll b. The cell wall resembles that of a blue-green alga. The thylakoids and cytoplasm together occupy a wide peripheral band. However, the thylakoids are present not as single lamellae, as in blue-green algae, but in pairs or, sometimes, in thicker stacks. Both thylakoids and cytoplasm are absent from a large central zone which is generally electron-transparent, but may contain electron-dense granules and fibrils. The function of the central zone is not known.

Summary 1. Plastid inheritance in most green algae and land plants is uniparental. In oogamous species, plastids are usually derived from the maternal parent; even when inheritance is biparental, maternal plastids usually predominate. Only a few species of conifer are known to have essentially paternal plastid inheritance. In spite of the overall strong maternal bias, there exists a spectrum of species in which plastid inheritance ranges from purely maternal to predominantly paternal. 2. Factors that influence the pattern of plastid inheritance operate both before (often long before) and after fertilization. For example, several different mechanisms for exclusion of plastids from particular cells, none of which is completely effective on its own, may operate sequentially during both gametogenesis and embryo-genesis. There appears to exist a general trend such that the more highly evolved the organism, the more numerous the mechanisms employed and the earlier they first come into operation. The pattern of plastid inheritance shown by a species represents the efficiency or lack of efficiency of these combined mechanisms. 3. In the newly-formed zygote of many unicellular algae, the plastids from both gametes are present and there is direct competition between them. Often the plastid from one mating type (usually the ‘invading’ male gamete, where this can be identified) quickly degenerates. Species such as Chlamydomonas are unusual in that the plastids from the two gametes fuse. In spite of this, inheritance of plastid DNA is normally uniparental. How this is accomplished remains unclear. In oogamous algae, the paternal plastids which enter the egg cell are frequently fewer in number and smaller in size than those contributed by the female gamete. The reduced contribution of paternal plastids can result from asymmetrical cell division or from differential timing of cell and plastid division during spermatogenesis. 4. In species ranging from unicellular algae to angiosperms, plastids may be partially or completely debarred from particular cells at critical stages during the reproductive cycle. An important factor in this form of plastid elimination is their postioning with respect to the nucleus prior to a cell division. When plastids closely encircle the nucleus, they are usually incorporated equally into the two daughter cells; when the plastids are concentrated at some distance from the nucleus, they are frequently excluded from one daughter cell. 5. Elimination of plastids from a gamete prior to plasmogamy prevents direct competition between the two types of plastid in the zygote or embryo. Perhaps the most effective method of excluding paternal plastids from the egg cell has been achieved by some lower land plants; the plastids migrate to the posterior part of the spermatozoid, and are discarded from there in a discrete vesicle before the egg is reached. 6. Plastid inheritance in conifers appears to be unique. In those species in which the derivation of plastids in the pro-embryo can be determined, it has been found that they come only from the male gamete. Maternal plastids are positively excluded from the pro-embryo and later degenerate. 7. In most angiosperm species plastid inheritance is maternal; in only a few species is it regularly biparental. The first step towards exclusion of paternal plastids often takes place in the uninucleate pollen grain where the plastids may be concentrated at the pole of the cell farthest from the site of the future generative cell. Any plastids that succeed in entering the generative cell may degenerate before the gametes are released from the pollen tube. Even if paternal plastids reach the egg, they are at a disadvantage because they are (a) entering an environment that is essentially alien, and (b) normally present in much smaller numbers than maternal plastids. Later, when the zygote divides, the few paternal plastids may fail to become incorporated in the small terminal cell which gives rise to the embryo proper. 8. There appears to be no consistent evolutionary progression in the use of more efficient mechanisms to influence plastid inheritance; most of the mechanisms associated with exclusion of paternal plastids in angiosperms, for example, can also be found in one or other species of green alga. The primary factors that influence plastid inheritance appear to be (I) direct competition in the zygote between plastids of the two parental types – the principal mechanism operating in isogamous algae, but also operating in some angiosperms; and (2) the divergent evolution of the two types of gamete - on the one hand a small male gamete with a minimum of cytoplasm which is capable of moving (spermatozoid) or being moved (pollen) efficiently, and, on the other hand, a large egg cell with numerous organelles, which is well able to act as ‘host’ for the future zygote. Many of the additional mechanisms that influence the pattern of plastid inheritance seem to be the more or less ‘accidental’ result of other evolutionary events.

SUMMARY The sequence of development of chloroplasts in coiled and newly uncoiled croziers of Pilularia globulifera L. is described. Changes in chloroplast shape and lamellar orientation could be correlated with particular states of crozier coiling or unfolding.

Summary Plants of Phaseolus vulgaris were grown for a period of 4 weeks in nutrient solutions deficient in S, Mg, K, N, P and Fe, and compared with controls growing in complete nutrient solutions. At weekly intervals, tissue from all leaf groups fully expanded at the time of sampling was prepared for examination under the electron microscope. It was possible to examine chloroplast ultrastructure, firstly, during leaf development under each of the nutrient deficient conditions and, secondly, to compare the chloroplasts of similar leaf groups produced under different deficiency conditions. Although distinctive chloroplast types could be associated with conditions of Mg, K, N and Fe deficiency, chloroplasts from S and P deficient plants were variable in appearance. The chloroplast patterns produced were found to be consistent in palisade parenchyma tissue but in spongy mesophyll tissue cells frequently appeared plasmolysed and chloroplasts varied considerably in appearance. Particularly in the spongy mesophyll distinctive crystalline configurations were found within the chloroplasts.

Summary Parenchyma cells in the leaf sheath of Equisetum telmateia form linearly arranged series of maturing and senescing cells. These series of cells contain chloroplasts showing a progressive sequence of developmental stages easily recognizable because of the spatial relationships involved. The cell sequence may be classified into four successive zones, characterized by the typical structure of the chloroplasts present in the cells of each zone. Zone I. Meristematic cells contain immature chloroplasts with little internal lamellar structure. Zone II. Adjacent to the above are cells containing mature chloroplasts with a well-developed internal lamellar system. Zone III. Next in sequence are cells in which the chloroplasts retain a mature internal lamellar system but are distinguished from the previous group by the presence of irregular osmiophilic deposits around the outer double membranes. Zone IV. Finally there occur cells with usually rounded chloroplasts which show the osmiophilic deposits described above, but in which the outer double membrane is often difficult to distinguish. Internally the lamellar system is highly sinuous, and contains many swollen locules. In spite of a general similarity in appearance between chloroplasts of Equisetum and those of angiosperms, Equisetum plastids are characterized by a distinctive feature more commonly associated with algae than with higher plants. Young chloroplasts are almost completely surrounded by rough endoplasmic reticulum. This very close relationship between reticulum and chloroplast is retained as the chloroplast matures, though the degree of envelopment becomes less with increasing age.

Summary The ultrastructure of mature guard cells of Phaseolus vulgaris L. grown under continuous light regime showed remarkable stability, remaining constant in appearance under a wide range of conditions of nutrient deficiency. The ultrastructure was quite unlike that of leaf parenchyma cells of normal plants, but had certain characteristics similar to those found in palisade parenchyma cells suffering from magnesium and potassium deficiency.