Showing papers by "Ajay Kumar Mishra published in 2010"

Pressure-Induced Structural Transformations in Bis(glycinium)oxalate

Abstract: We report in situ high-pressure Raman spectroscopic as well as X-ray diffraction measurements on bis(glycinium)oxalate, an organic complex of glycine, up to 35 GPa. Several spectral features indicate that at ∼1.7 GPa it transforms to a new structure (phase II) which is characterized by the loss of the center of symmetry and the existence of two nonidentical glycine molecules. Across the transition, all the N−H···O bonds are broken and new weaker N−H···O bonds are formed. Our high-pressure X-ray diffraction studies support the possibility of a non-centrosymmetric space group P21 for phase II. Across 5 GPa, another reorganization of N−H···O hydrogen bonds takes place along with a structural transformation to phase III. The C−C stretching mode of oxalate shows pressure-induced softening with large reduction from the initial value of 856 to 820 cm−1 up to 18 GPa, and further softening is hindered at higher pressures.

Isozyme and PCR-based genotyping of epidemic Phytophthora colocasiae associated with taro leaf blight.

Abstract: The Oomycetous fungus Phytophthora colocasiae causing leaf blight of taro is widely distributed in India. Wide geographic range or sexual recombination provides genetic differentiation within this species. To determine how genetic variation is partitioned in P. colocasiae, 14 isolates were isolated from different regions of India, where the incidence of leaf blight is great. Molecular and biochemical techniques were employed for assessing and exploiting the genetic variability among isolates of P. colocasiae. Seven polymorphic enzyme systems revealed 23 isozyme patterns, each uniquely characterised by the presence or absence of electromorphs. Further, 10 oligodeoxynucleotide primers were selected for random amplified polymorphic DNA (RAPD) assays, which resulted in 123 polymorphic bands for 10 isolates of P. colocasiae. The data were entered into a binary matrix and a similarity matrix was constructed using a DICE similarity (SD) index. A UPGMA cluster based on SD values was generated using a NTSYS comput...

A species-specific polymerase chain reaction assay for rapid and sensitive detection of Sclerotium rolfsii

Abstract: The failure to adequately identify plant pathogens from culture-based morphological techniques has led to the development of culture-independent molecular approaches. The timely and accurate detection of pathogens is critical in the study of epidemiology and management of plant diseases. A polymerase chain reaction (PCR)-based method was developed for the identification and detection of Sclerotium rolfsii in Amorphophallus paeoniifolius plants. A PCR primer pair specific for S. rolfsii was designed based on the sequence of the internal transcribed spacer region. The designed primer pair SCR-F/SCR-R amplified a 540-bp product from S. rolfsii DNA and did not amplify DNA from A. paeoniifolius or several other fungi pathogenic to A. paeoniifolius. In conventional PCR, the limit of detection of pure fungal gDNA was 6 pg ml−1, which was reduced 2-fold within a plant DNA background. S. rolfsii DNA was detected in inoculated A. paeoniifolius and 12 h after inoculation in symptomless tuber samples. The protocol was assessed for the detection of S. rolfsii in infected soils.

Structural evolution of double perovskite Sr2MgWO6 under high pressure

Abstract: Angle dispersive X-ray diffraction and Raman scattering studies have been carried out on Sr 2 MgWO 6 up to ∼28 and ∼40 GPa, respectively. This compound is found to be structurally stable up to the highest pressure in this study. Bulk modulus of ambient phase is determined to be 128 ± 3.9 GPa which is in close agreement with the theoretical value of bulk modulus, 132 GPa, obtained using first principles calculations. Our first principles calculations also show that the initial tetragonal phase is more stable compared to the expected monoclinic structure up to at least 100 GPa.

Rapid and sensitive detection of Phytophthora colocasiae associated with leaf blight of taro by species-specific polymerase chain reaction assay

Abstract: The failure to adequately identify plant pathogens from culture-based morphological techniques has led to the development of culture-independent molecular approaches. The timely and accurate detection of pathogens is a critical aid in the study of epidemiology and biology of plant diseases. In the case of regulated organisms, the availability of sensitive and reliable assay is essential when trying to achieve early detection of pathogens. We developed and tested the PCR assay for detection of Phytophthora colocasiae, an oomycetes pathogen of leaf blight of taro and of rotting of taro tubers. The method described here is specific for P. colocasiae when tested across fungal, bacterial, and other Phytophthora species. In conventional (single-round) PCR, the limit of detection was 20 pg DNA for both primer sets, whereas in nested PCR the detection limit for both was 0.2 pg. In sampling studies, P. colocasiae-specific primers were used to detect leaf blight in infected leaves and tubers of taro cultivar. The causal pathogen P. colocasiae was detected by PCR from artificially infected tubers after 16 h of post inoculation, before any visible symptoms were present. The method was also tested to detect fungal DNA in infected leaves and infested soils. The PCR assay and direct tissue extraction methods provide tools which may be used to detect P. colocasiae pathogens in taro planting material and thus limit the transmission and spread of new, aggressive strains of P. colocasiae in taro-growing regions.

Zircon-monoclinic-scheelite transformation in nanocrystalline chromates

Abstract: Our in-situ high-pressure x-ray diffraction measurements on zircon structured nanocrystalline chromates show that the structural phase transformation from zircon to scheelite phase proceeds via an intermediate monoclinic phase, i.e., $\text{zircon}\ensuremath{\rightarrow}\text{monoclinic}\ensuremath{\rightarrow}\text{scheelite}$. Though there have been speculations about zircon-scheelite phase transformation proceeding through an unstable transient monoclinic phase in ${\text{ZrSiO}}_{4}$, we have experimentally demonstrated the existence of a similar intermediate stable state in a zircon structured compound. For our nanocrystalline samples, the transformation pressure is found to be higher than the bulk.