Mean-field analysis of collapsing and exploding Bose-Einstein condensates

The dynamics of collapsing and exploding trapped Bose-Einstein condensat es caused by a sudden switch of interactions from repulsive to attractive a re studied by numerically integrating the Gross-Pitaevskii equation with atomic loss for an axially symmetric trap. We investigate the decay rate of condensates and the phenomena of bursts and jets of atoms, and compare our results with those of the experiments performed by E. A. Donley {\it et al.} [Nature {\bf 412}, 295 (2001)]. Our study suggests that the condensate decay and the burst production is due to local intermittent implosions in the condensate, and that atomic clouds of bursts and jets are coherent. We also predict nonlinear pattern formation caused by the density instability of attractive condensates.Comment: 7 pages, 8 figures, axi-symmetric results are adde

Mean-field description of collapsing and exploding Bose-Einstein condensates

We perform numerical simulation based on the time-dependent mean-field Gross-Pitaevskii equation to understand some aspects of a recent experiment by Donley et al. on the dynamics of collapsing and exploding Bose-Einstein condensates of $^{85}$Rb atoms. They manipulated the atomic interaction by an external magnetic field via a Feshbach resonance, thus changing the repulsive condensate into an attractive one and vice versa. In the actual experiment they changed suddenly the scattering length of atomic interaction from positive to a large negative value on a pre-formed condensate in an axially symmetric trap. Consequently, the condensate collapses and ejects atoms via explosion. We find that the present mean-field analysis can explain some aspects of the dynamics of the collapsing and exploding Bose-Einstein condensates.Comment: 9 Latex pages, 10 ps and eps files, version accepted in Physical Review A, minor changes mad

Structural evidence of programmed cell death induction by tungsten in root tip cells of pisum sativum

Previous studies have shown that excess tungsten (W), a rare heavy metal, is toxic to plant cells and may induce a kind of programmed cell death (PCD). In the present study we used transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) to investigate the subcellular malformations caused byW, supplied as 200 mg/L sodium tungstate (Na 2 WO 4 ) for 12 or 24 h, in root tip cells of Pisum sativum (pea), The objective was to provide additional evidence in support of the notion of PCD induction and the presumed involvement of reactive oxygen species (ROS). It is shown ultrastructurally that W inhibited seedling growth, deranged root tip morphology, induced the collapse and deformation of vacuoles, degraded Golgi bodies, increased the incidence of multivesicular and multilamellar bodies, and caused the detachment of the plasma membrane from the cell walls. Plastids and mitochondria were also affected. By TEM, the endoplasmic reticulum appeared in aggregations of straight, curved or concentric cisternae, frequently enclosing cytoplasmic organelles, while by CLSM it appeared in bright ring-like aggregations and was severely disrupted in mitotic cells. However, no evidence of ROS increase was obtained. Overall, these findings support the view of a W-induced vacuolar destructive PCD without ROS enhancement. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

Tubulin acetylation mediates bisphenol a effects on the microtubule arrays of allium cepa and triticum turgidum

The effects of bisphenol A (BPA), a prevalent endocrine disruptor, on both interphase and mitotic microtubule array organization was examined by immunofluorescence microscopy in meristematic root cells of Triticum turgidum (durum wheat) and Allium cepa (onion). In interphase cells of A. cepa, BPA treatment resulted in substitution of cortical microtubules by annular/spiral tubulin structures, while in T. turgidum BPA induced cortical microtubule fragmentation. Immunolocalization of acetylated α-tubulin revealed that cortical microtubules of T. turgidum were highly acetylated, unlike those of A. cepa. In addition, elevation of tubulin acetylation by trichostatin A in A. cepa resulted in microtubule disruption similar to that observed in T. turgidum. BPA also disrupted all mitotic microtubule arrays in both species. It is also worth noting that mitotic microtubule arrays were acetylated in both plants. As assessed by BPA removal, its effects are reversible. Furthermore, taxol-stabilized microtubules were resistant to BPA, while recovery from oryzalin treatment in BPA solution resulted in the formation of ring-like tubulin conformations. Overall, these findings indicate the following: (1) BPA affects plant mitosis/cytokinesis by disrupting microtubule organization. (2) Microtubule disassembly probably results from impairment of free tubulin subunit polymerization. (3) The differences in cortical microtubule responses to BPA among the species studied are correlated to the degree of tubulin acetylation. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

Colchicine-Induced paracrystals in root cells of wheat (Triticum Aestivum L.)

Tubulin conformations other than microtubules in the meristematic cells of wheat roots grown in the presence of 2 mM colchicine solution were investigated by immunofluorescence and electron microscopy. In the affected cells microtubules disappeared and were replaced by tubulin fluorescent strands that occurred in the cortical cytoplasm. With increasing time of exposure to colchicine the tubulin strands became better organized and occurred also in the subcortical cytoplasm and finally they were restricted to the area around the nucleus. In prophase and preprophase cells thick strands occupied the cortical cytoplasmic zone where in normal cells a preprophase microtubule band (PMB) was expected to be assembled. In the colchicine-treated cells electron microscopy revealed an accumulation of paracrystalline aggregates, which initially occurred along the cell wall and later deeper in the cytoplasm, in the perinuclear regions and the cytoplasmic invaginations of the nucleus. In transverse planes the paracrystalline strands appear to consist of hexagonal subunits in a ’honeycomb’ arrangement, while in longitudinal and oblique sections they exhibit variable images. Since their distribution coincides with that of the tubulin strands visualized by immunofluorescence, they are considered to be the same structure. Therefore, the paracrystals consist of, or at least contain, tubulin. They are most likely to be polymers of tubulin-colchicine complexes. © 1995 Annals of Botany Company

Anthocyanin accumulation in poinsettia leaves and its functional role in photo-oxidative stress

The role of anthocyanin accumulation in poinsettia leaves exposed to photo-oxidative stress was evaluated by comparing green (anthocyanin less) and reddish (anthocyanin well equipped) leaves, co-occurring in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch). For the assessment we compared the photoprotective and antioxidant mechanisms in the two leaf types in response to photo-oxidative stress, generated in chloroplasts by the application of methyl viologen (MV). MV accepts electrons from photosystem I (PSI) with subsequent transfer to molecular oxygen to produce superoxide anions (O2 [rad] –) that are converted by the chloroplast superoxide dismutase (SOD) to hydrogen peroxide (H2O2) that is reduced by ascorbate peroxidase (APX) to water and oxygen. At 90ömin after MV application, the decrease in the activity of the H2O2-scavenging enzyme APX resulted in increased H2O2 levels in both leaf types, but to a significantly lower level in the reddish leaves, possibly due to the significantly higher anthocyanin and phenolic content that are considered as H2O2-scavengers. Reddish poinsettia leaves having higher antioxidant activity possessed more effective photoprotective and reactive oxygen species (ROS)-scavenging mechanisms, including higher enzymatic (SOD and APX), and non-enzymatic (anthocyanin and phenolic) content over the green ones. Anthocyanin well equipped poinsettia leaves were in advantage in response to photo-oxidative stress over the anthocyanin less ones that appeared not to be equally protected. This superiority in the avoidance of the photo-oxidative stress by the reddish leaves was also associated to a higher effective quantum yield of PSII photochemistry (ΦPSII), a lower excitation pressure (1- qp) and a lower H2O2 generation, compared to their green counterparts. © 2020 Elsevier B.V