The Development of Chloroplast Ultrastructure and Hill Reaction Activity During Leaf Ontogeny in Different Maize (Zea Mays L.) Genotypes

Changes in Hill reaction activity (HRA) and ultrastructure of mesophyll cell (MC) chloroplasts were studied during the ontogeny of third leaf of maize plants using polarographic oxygen evolution measurement, transmission electron microscopy, and stereology. The chloroplast ultrastructure was compared in young (actively growing), mature, and senescing leaves of two different inbreds and their reciprocal F1 hybrids. Statistically significant differences in both HRA and MC chloroplast ultrastructure were observed between different stages of leaf ontogeny. Growth of plastoglobuli was the most striking characteristic of chloroplast maturation and senescence. The chloroplasts in mature and senescing leaves had a more developed system of thylakoids compared to the young leaves. Higher HRA was usually connected with higher thylakoid volume density of MC chloroplasts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Senescence induced structural reorganization of thylakoid membranes in Cucumis sativus cotyledons; LHC II involvement in reorganization of thylakoid membranes

We report the formation and appearance of loosely stacked extended grana like structures along with plastoglobuli in the chloroplasts isolated from 27-day old senescing cucumber cotyledons. The origin and the nature of these extended grana structures have not been elucidated earlier. We isolated Photosystem I complexes from 6-day-old control and 27-day-old senescing cotyledons. The chlorophyll a/b ratio of the isolated Photosystem I complex obtained from 6-day cotyledons was 5–5.5 as against a ratio of 2.9 was found in Photosystem I complexes obtained from 27-day-old senescing cotyledons. We also found that the presence of LHC II in the Photosystem I complexes isolated from 27-day cotyledonary chloroplasts. The presence of LHC II in Photosystem I complexes in senescing and not in control samples, clearly suggest the detachment and diffusion of LHC II complexes from stacked grana region to Photosystem I enriched stroma lamellar region thereby, forming loose disorganized extended grana structures seen in the transmission electron microscope. Furthermore, we show that under in vitro condition the senescing cotyledon chloroplasts exhibited lower extent of light induced phosphorylation of LHC II than the control samples suggesting a possible irreversible phosphorylation and diffusion of LHC II in vivo during the progress of senescence in Cucumis cotyledons. From these findings, we suggest that the senescence induced phosphorylation of LHC II and its migration towards Photosystem I may be a programmed one some how causing the destruction of the thylakoid membrane. The released membrane components may be stored in the plastoglobuli prior to their mobilization to the younger plant parts. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Heterogeneity of Structural and Functional Photosynthetic Characteristics of Mesophyll Chloroplasts in Various Parts of Mature or Senescing Leaf Blade of Two Maize (Zea Mays L.) Genotypes

Differences in ultrastructural parameters of mesophyll cell (MC) chloroplasts, contents of photosynthetic pigments, and photochemical activities of isolated MC chloroplasts were studied in the basal, middle, and apical part of mature or senescing leaf blade of two maize genotypes. A distinct heterogeneity of leaf blade was observed both for structural and functional characteristics of chloroplasts. In both mature and senescing leaves the shape of MC chloroplasts changed from flat one in basal part of leaf to nearly spherical one in leaf apex. The volume density of granal thylakoids decreased from leaf base to apex in both types of leaves examined, while the amount of intergranal thylakoids increased in mature leaves but decreased in senescing leaves. The most striking heterogeneity was found for the quantity of plastoglobuli, which strongly increased with the increasing distance from leaf base. The differences in chloroplast ultrastructure were accompanied by differences in other photosynthetic characteristics. The Hill reaction activity and activity of photosystem 1 of isolated MC chloroplasts decreased from leaf base to apex in mature leaves. Apical part of senescing leaf blade was characterised by low contents of chlorophyll (Chl) a and Chl b, whereas in mature leaves, the content of Chls as well as the content of total carotenoids (Car) slightly increased from basal to apical leaf part. This was reflected also in the ratio Chl (a+b)/total Car; the ratio of Chl a/b did not significantly differ between individual parts of leaf blade. Both genotypes examined differed in the character of developmental gradient observed along whole length of leaf blade.     

Changes in Tomato Leaves Induced by NaCl Stress: Leaf Organization and Cell Ultrastructure

The alterations of organization of leaf tissues and cell ultrastructure as a consequence of salt stress (75 and 150 mM NaCl) were studied in two tomato (Lycopersicum esculentum Mill.) cultivars showing different salinity tolerance. The salinity brought changes in cell shape, volume of intercellular spaces and chloroplast number, shape and size. These characteristics were specific in each cultivar. The ultrastructural changes were also different in the two tomato cultivars studied and the most important ones were in the number and size of starch granules in chloroplasts, the number of electron-dense corpuscules in the cytoplasm, the structure of mitochondria, and number of plastoglobuli. This revised version was published online in July 2006 with corrections to the Cover Date.     

Decline in photosynthesis as related to alterations in chloroplast ultrastructure of a cotton leaf during ontogeny

Field studies were conducted to investigate ontogenic changes in leaf photosynthesis and chloroplast ultrastructure of a single cotton (Gossypium hirsutum L.) leaf subtending the fruit. A 20-d old leaf was the most physiologically active with net photosynthetic rate (PN) of 16.5 mol m-2 s-1 and nitrogen (N) concentration of 168 mmol m-2. These values declined with leaf age and a close relationship existed between them. Concurrent with declines in PN, ultrastructural alterations occurred in the chloroplast: the 20-d old leaf had increased grana number and thylakoids per granum and a few plastoglobuli. Afterwards, the grana number and thylakoids per granum declined with leaf age indicating disintegrated grana and stroma lamellae. Concomitant with disintegrated membrane system was the presence of numerous large plastoglobuli. The PN was closely related to grana number and thylakoids per granum suggesting that the decline in PN with leaf age was associated with ultrastructural changes in the chloroplast.     

The function of carotenoids during chloroplast development. V. Correlation between carotenoid content, ultrastructure and chlorophyll b to chlorophyll a ratio

The ultrastructural effect of carotenoid deficiency in wheat ( Triticum aestivum L. ) was studied after adding the herbicide SAN-9789 to the growth medium. The presence of SAN-9789 (28 mg I^-1) resulted in an almost complete absence of carotenoids. For plants grown in darkness the lack of carotenoids was accompanied by a reduction in partitions between primary thylakoids as well as a change in appearance of the plastoglobuli from small and black (osmiophilic) to large and greyish white (less osmiophilic). When plants were grown in weak red light (16 mW m^-2), the presence of SAN-9789 also resulted in an almost complete absence of grana, a decrease in the ratio of chlorophyll b /chlorophyll a from 0.25 to 0.1, and an almost complete absence of prolamellar bodies. The greatest differences in carotenoid content, in amount of grana, in chlorophyll b /chlorophyll a ratio, and in number of prolamellar bodies, all occurred between 0.28 and 2.8 mg I^-1 of SAN-9789. The change in appearance of plastoglobuli occurred at a lower SAN-9789 concentration, between 0.028 and 0.28 mg I^-1. The ultrastructural responses to the SAN-9789 treatment could be related to the effect on the carotenoid synthesis in different ways. The disappearance of thylakoid partitions was connected to a structural role of carotenoids, while the disappearance of the prolamellar bodies was dependent on the photoprotective role of carotenoids. The change in shape and size of plastoglobuli was not correlated to the presence of carotenoids. However, a connection to the accumulation of the carotenoid precursors is still possible.     

Effect of Glycinebetaine on Chloroplast Ultrastructure, Chlorophyll and Protein Content, and RuBPCO Activities in Tomato Grown under Drought or Salinity

The effect of foliar-applied glycinebetaine (GB) on chloroplast ultrastructure, the amount of chlorophyll and proteins and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity in stressed tomato leaves were analysed. Initial, and total activity and activation state state of RuBPCO were also measured. RuBPCO activities, chloroplast area, and the number of plastoglobuli remained unaffected, while the relative area of starch granules increased in GB-treated, salt-stressed tomato leaflets. Under drought-stress, the relative area of plastoglobuli increased upon GB application. The primary effect of GB was, however, the increased protein and chlorophyll content.     

Observations in Hemichloris antarctica Tschermak-Woess & Friedmann (Chlorophyceae) and the occurrence of a second Hemichloris species, Hemichloris polyspora n. sp

The chlorococcalean genus Hemichloris is characterized by the possession of two chloroplasts per vegetative cell. The occurrence of a second species of the genus is reported (H. polyspora sp. nov.). Just as H. antarctica it grows cryptoendolithically in sandstone in Southern Victoria Land, Antarctica. In H. antarctica propagation by two autospores prevails over four, whilst in the new species H. polyspora in general four or eight (rarely 16 or 32) autospores are produced and Borodinella-stages do occur typically. Sexuality and zoosporulation do not exist in both species. Internal structures of chloroplasts can be observed by light microscopy more regularly in H. polyspora than in H. antarctica and under various conditions. Investigations of both Hemichloris species by transmission electron microscopy show them to go back to more or less extended assemblages of plastoglobuli. In both species the plastoglobuli are arranged around tubular inflations of thylakoids and apparently attached to the thylakoids. Keeping the cultures for three (even up to seven) months without light makes them survive and causes coming forth of the chloroplast structure throughout.     

Cadmium Induced Ultrastructural Changes in Shoot Apical Meristem of Elodea Canadensis Rich.

The ultrastructure of cells of ground meristem in the peripheral zone of shoot apical meristem in Elodea canadensis Rich. was studied after treatment with cadmium in concentrations 0.5, 1, 2, 3, 5, and 6 g(Cd²⁺) m⁻³. After 5 d treatment, changes in the structural organisation of the plastid apparatus were found, namely in proplastid, amyloplast, and amoeboid stages of plastid development. This revised version was published online in June 2006 with corrections to the Cover Date.