Fundamental helical geometry consolidates the plant photosynthetic membrane

Plant photosynthetic (thylakoid) membranes are organized into complex networks that are differentiated into 2 distinct morphological and functional domains called grana and stroma lamellae. How the 2 domains join to form a continuous lamellar system has been the subject of numerous studies since the mid-1950s. Using different electron tomography techniques, we found that the grana and stroma lamellae are connected by an array of pitch-balanced right- and left-handed helical membrane surfaces of different radii and pitch. Consistent with theoretical predictions, this arrangement is shown to minimize the surface and bending energies of the membranes. Related configurations were proposed to be present in the rough endoplasmic reticulum and in dense nuclear matter phases theorized to exist in neutron star crusts, where the right- and left-handed helical elements differ only in their handedness. Pitch-balanced helical elements of alternating handedness may thus constitute a fundamental geometry for the efficient packing of connected layers or sheets.

A new small-angle X-ray scattering model for polymer spherulites with a limited lateral size of the lamellar crystals

As is well known, polymers commonly form lamellar crystals, and these assemble further into lamellar stacks and spherulites during quiescent crystallization. Fifty years ago, Vonk and Kortleve constructed the classical small-angle X-ray scattering theory (SAXS) for a lamellar system, in which it was assumed that the lamellar stack had an infinite lateral size [Vonk & Kortleve (1967), Kolloid Z. Z. Polym. 220, 19–24]. Under this assumption, only crystal planes satisfying the Bragg condition can form strong scattering, and the scattering from the lamellar stack arises from the difference between the scattering intensities in the amorphous and crystalline layers, induced by the incident X-ray beam. This assumption is now deemed unreasonable. In a real polymer spherulite, the lamellar crystal commonly has dimensions of only a few hundred nanometres. At such a limited lateral size, lamellar stacks in a broad orientation have similar scattering, so interference between these lamellar stacks must be considered. Scattering from lamellar stacks parallel to the incident X-ray beam also needs to be considered when total reflection occurs. In this study, various scattering contributions from lamellar stacks in a spherulite are determined. It is found that, for a limited lateral size, the scattering induced by the incident X-ray beam is not the main origin of SAXS. It forms double peaks, which are not observed in real scattering because of destructive interference between the lamellar stacks. The scattering induced by the evanescent wave is the main origin. It can form a similar interference pattern to that observed in a real SAXS measurement: a Guinier region in the small-q range, a signal region in the intermediate-q range and a Porod region in the high-q range. It is estimated that, to avoid destructive interference, the lateral size needs to be greater than 11 µm, which cannot be satisfied in a real lamellar system. Therefore, SAXS in a real polymer system arises largely from the scattering induced by the evanescent wave. Evidence for the existence of the evanescent wave was identified in the scattering of isotactic polypropylene. This study corrects a long-term misunderstanding of SAXS in a polymer lamellar system.

Assimilatory pigments, photosynthetic activity and ultrastructure of chloroplasts of the variegated-leaf chimera of Acer platanoides L.

The assimilatory pigment composition, photosynthetic activity and ultrastructure of chloroplasts were studied in the chlorophyll - deficient chimera of <i>Acer platanoides</i> L. Part of the crown of this chimera is a virescent mutant with variegated leaves. It was found that there exists no qualitative difference in the pigment composition between normal and variegated leaves. The accumulation of chlorophyll in the mutated part is more delayed that the accumulation of carotenoids. The photosynthetic rate on a chlorophyll basis is much higher in variegated than in green leaves. This difference gradually falls off with development. In the early spring, chloroplasts from the yellow spots of leaf blade have no lamellar system but only many vesicles are dispersed in the stroma. Occasionally also a single granum consisting of a few thylakoids occurs in the stroma. At the end of summer chloroplasts from yellow spots of variegated leaves possess a poorly developed lamellar system.

Effects of some inhibitors of protein synthesis on the chloroplast fine structure, CO2 fixation and the Hill reaction activity

A comparative study concerning the effects of chloramphenicol (100 μg ml^-1, actidione (10 μg ml^-1), 5-bromouracil (190 μg ml^-1), actinomycin D (30 μg ml^-1) and DL-ethionine (800 μg ml^-1) on the chloroplast fine structure, ¹⁴CO₂ incorporation and the Hill reaction activity was the subject of the experiments presented in this paper. The experiments were conducted on bean seedlings under the conditions when chlorophyll accumulation was inhibited only partially. The results obtained indicate that chloromphenicol is responsible for the reduction of the number of grana per section of plastid and for the formation of numerous vesicles in the stroma. In the presence of actidione, actinomycin D or DL-ethionine the lamellae are poorly differentiated into .stroma and granum regions and there occur disturbances in the typical orientation of lamellae within chloroplasts. Only in the presence of 5-bromouracil the development of chloroplast structure resemble that in control plants. A comparison of the results obtained with those published earlier (Więckowski et al., 1974; Ficek and Więckowski, 1974) shows that such processes as assimilatory pigment accumulation, the rate of CO₂ fixation, the Hill reaction activity, and the development of lamellar system are suppressed in a different extent by the inhibitors used.

chlorophyll autofluorescence in globular and heart-shaped embryos of some dicotyledons

The regions in early embryos of several species display chlorophyll autofluorescence in a certain order. First, autofluorescence in <em>Pisum sativum</em> appears in the basal part of globular embryos; in <em>Lathyrus vernus</em> in the basal part of early heart embryos; in <em>Cardamine pratensis</em> at the sides of the hypocotyl or in <em>Phaseolus vulgaris</em> in the hypocotyl of elongating heart-shaped embryos. Chlorophyll autofluorescence in an embryo proper of <em>Pisum</em> coincides with the development of a lamellar system in the plastids. The suspensorial plastids remain undifferentiated with one or two DNA positive nucleoids. <em>Cardamine</em>, <em>Lathyrus</em>, <em>Phaseolus</em> and <em>Pisum</em> suspensors give no chlorophyll autofluorescence.

Chloroplast ultrastructure in leaves of Cucumis sativus chlorophyll mutant

The developing and young leaves of <em>Cucumis sativus</em> chlorophyll mutants are yellow, when mature they become green and do not differ in their colour from those of control plants. The mesophyll of yellow leaves contains a diversiform plastid population with a varying degree of defectiveness, which is mainly manifested in the reduction or disorganization of the typical thylakoid system. DNA areas, ribosome-like particles and aggregates of electron-dense material are preserved in the stroma of mutated plastids. Starch grains are deficient. Apart from mutated plastids, chloroplasts with a normal structure, as in control plants, were also observed.The leaf greening process is accompanied by a reconstruction and rearrangement of the inner chloroplast lamellar system and an ability to accumulate starch. However, in the mutant chloroplasts as compared with control-plant ones, an irregular arrangement of grana and reduced number of inter-grana thylakoids can be seen. An osmiophilic substance stored in the stroma of mutated plastids and the vesicles formed from an internal plastid membrane take part in restoration of the membrane system.

Genetic and ultrastructural studies on an orange coloured chlorophyll mutant of winter rye (Secale cereale L.)

The feature of the orange colour of young seedlings, which appeared in inbred generation S₂ of a short straw form of winter rye cultivated in Jeleniec is described. Genetic analysis revealed that this feature is determined by one recessive gene, marked with the symbol cl₂ (chlorophyll lethal), because the plants with a double dose of this gene (recessive homozygotes) die four weeks after germination. In contrast to typical chloroplasts in normal plants (green), mesophyll cells of mutants (of orange colour) contain plastids devoid of grana thylakoids. The lamellar system of these plastids is composed only of more or less parallel single thylakoids. Short thylakoids often demonstrate a "staircase" arrangement forming in this way long thylakoids. Moreover, such plastids are characterized by the presence of 1-4 clusters of osmophilic globules. Starch grains were lacking in plastids of mutated plants.

Chlorophyll content and culture of flax embryos in vitro

During <em>in vitro</em> cultivation of green flax embryos excised 14 days after fertilization on White's medium the chloraphylls a and b are gradually destroyed and the lamellar system of plastids is disintegrated. It was possible, to prolong somewhat the presence of chlorophyll and to enhance embryo growth by culture in the dark and by adding kinetin. Light has a stimulating effect on the germination of embryos. Streptomycin inhibits chlorophyll synthesis not earlier than at germination. Young, green embryos are unable to sumvive and develop on media without sucrose. It is supposed that photosynthesis does not occur in flax embryos <em>in vitro</em>, and that green embryos are not more autotrophic <em>in vitro</em> than those of leucoembryophytes.