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Black pepper is highly sensitive to water deficit stress especially during summer, resulting in significant losses in yield; therefore, strategies aimed at enhancing water stress tolerance are essential. Melatonin improves stress tolerance in plants; however, its method of application and optimum concentration in black pepper under water deficit stress remains unclear. Therefore, we conducted a two pot culture experiment during March and April, 2022 (var. Panniyur-1) to investigate the effects of foliar-sprayed and root-irrigated melatonin (50, 100 and 150 µM L-1) on the recovery per cent and physiological mechanism under water stress. The treatment details were, WW - Well-watered; WS - Water stressed; FM50 - Waterstress+50µM Melatonin (Foliar spray); FRM50 - Water stress + 50µM Melatonin (Foliar spray + Soil drenching @ 50ml/plant); FM100 - Water stress + 100µM Melatonin (Foliar spray); FRM100 - Water stress + 100µM Melatonin (Foliar spray + Soil drenching @ 50ml/plant); FM150 - Water stress + 150µM Melatonin (Foliar spray); FRM150 - Water stress + 150µM Melatonin (Foliar spray + Soil drenching @ 50ml/plant). The melatonin-induced enhanced stress tolerance could be attributed to improved recovery %, leaf relative water content, photosynthetic pigments, activity of antioxidant enzymes (SOD, POD, and CAT), and ultimately significantly relieved the inhibitory effects of water stress on leaves. After rehydration, melatonin-treated plants recovered more quickly than untreated plants. In addition, melatonin counteracted the water stress induced accumulation in proline content. Overall, the results of this study demonstrated that melatonin at 100 µM L-1 (Foliar spray and root irrigation) significantly alleviated the adverse effects of water deficit stress compared untreated plants. In addition, application of exogenous melatonin combined with root and foliar application is superior than foliar spraying alone.

Small cardamom (Elettaria cardamomum Maton), an eco-friendly high value spice and foreign exchange earner, is a native of Western Ghats of South India. India enjoyed near monopoly in its production till the seventies, but with the emergence of Guatemala as the main competitor, it has been relegated to second position both in production and export. The low production of cardamom up to eighties was due to lack of sound location specific agronomic package of recommendations. Hence, after a detailed survey of entire small cardamom growing areas in India and interaction with cardamom growers and other stake holders, a comprehensive location specific research programme was finalised and carried out for over 25 years. As a consequence of detailed agronomic investigations on cardamom and cardamom-based cropping systems, various agro techniques such as selection of elite lines of cardamom, rapid clonal multiplication technique, nursery techniques, systems and season of planting, irrigation, shade and weed management, apiculture, harvest and post harvest techniques, multistoried cropping system involving cardamom, coffee, arecanut, coconut etc. evolved. These high production technologies were effectively transferred to the cardamom growers based on local specificity which resulted in higher yields (average of 750 kg dry capsules ha-1 and a potential yield of 1945 kg dry capsules in demonstrations in farmers fields in Kodagu, Karnataka) from an initial benchmark average yield of 120-140 kg dry capsules ha-1.

Statistical tools such as analysis of variance, correlation, path coefficient analysis, Scott-Knott test and principal component analysis were used in the present study to characterize black pepper verities/hybrids for spike and berry traits. ANOVA indicated that fifteen traits under study were statistically significant. Traits like fresh pericarp weight and dry pericarp weight showed high positive correlation (>0.95) with spike weight. Path coefficient analysis revealed that berry weight and seed size are contributing directly to spike weight. Scott-Knott test identified Panniyur-1 and Nedumchola as the most contrasting genotypes for most number of traits studied. Based on Principal Component Analysis (PCA), first three principal components had an eigen value above unity and explained 88 per cent of cumulative variation. Principal component PC-1 accounted for maximum variation of about 42.4 percent which discriminated the genotypes based on fresh berry weight, dry seed weight and fresh pericarp weight. These traits serve as the selection criteria for improvement of yield in black pepper.

This review, dedicated to the memory of B. K. Sadashiva, concerns electrically driven patterns and defects in bent core nematics (BCNs) made of hard-core, soft-core or bent-rod type of molecules. I...

Black pepper is basically a rainfed crop in India. Drought is the chief abiotic stress causing up to 50-80% crop loss in black pepper. Lack of precise screening methodology to develop moisture stress tolerant lines is a limiting factor in black pepper productivity. To develop a rapid screening methodology, a laboratory experiment was conducted with rooted black pepper cuttings under hydroponic culture. The experiment was designed in a completely randomized with four replications. Moisture stress was imposed with six different concentrations of Polyethylene glycol-6000 (5, 8, 10, 12, 15 and 20 per cent) along with control in black pepper cv. Panniyur-1, IISR-Thevam, IISR-Sreekara and IISR-Girimunda. The results showed that Panniyur-1, IISR-Thevam, IISR-Sreekara and IISR-Girimunda at the PEG-6000 concentrations of 8%, 10%, 10%, 10% respectively reduced per cent cuttings survival almost by 50% Lethal dose (LD50) after 10 days of PEG stress and also, a significant increase in proline was recorded up to 10% PEG-6000. Hence, 10% PEG-6000 appears to be an ideal concentration for screening black pepper genotypes for moisture stress tolerance.

An experiment was conducted at the Indian Institute of Spices Research, Regional Station, Madikeri, Karnataka, to study physiology, yield and quality responses of six small cardamoms [Elettaria car...

Colloids formed of solid/fluid particle dispersions in oriented nematic liquid crystals are known to be an ideal means of realizing fundamentally significant topological defect geometries. We find, experimentally, that twist-bend nematic (NTB) droplets formed in the N-NTB biphasic regime, either of pure compounds or mesogenic mixtures, completely mimic colloidal particles in their ability to generate a rich variety of defects. In the biphasic regime, the topological features of both liquid crystal colloids and chiral nematic droplets are revealed by (i) topological dipoles, quadrupoles and their patterned clusters formed in planar nematic liquid crystals orientationally perturbed by coexisting NTB drops, (ii) the transformation of hyperbolic hedgehogs into knotted Saturn rings encircling the NTB drops dispersed in a 90°-twisted nematic matrix and (iii) the Frank-Pryce defect texture evident in smaller (relative to sample thickness) NTB drops. In larger drops with fingerlike outgrowths, additional line defects appear; most of these are deemed to be pairs of disclinations to which are attached pairs of screw dislocations intervening in the growth process of the NTB droplets.

Binary mixtures of the mesogen [1″,7″-bis(4-cyanobiphenyl-4'-yl)heptane] and a long chain amphiphile (e.g., 2-octadecoxypropanol) are examined for the structure, stability, and electro-optical behavior of nematic drops dispersed in the isotropic phase, in planar cells. Subjected to tangential boundary conditions, the drops adopt, besides the escaped concentric and untwisted bipolar geometries, the less common bound vortex geometry with a pair of half-strength disclination lines. The concentric drop, as it grows, switches its axis from an in-layer to the layer-normal direction corresponding to the stablest of all geometries. Bipolar drops in equilibrium have their axes parallel to the easy axis of the cell. Obliquely oriented bipolar drops rotate to attain the equilibrium disposition by the shorter of the clockwise and anticlockwise routes, the extent of rotation decreasing exponentially with time. The bipolar structure is marginally less stable than the concentric, and transforms to the latter geometry occasionally. In bound vortex drops, the separation between the lines varies as the drop diameter, the bipolar and concentric geometries appearing as the limiting cases. The complex course of Fréedericksz transition in all the different types of drops terminates in the division of the original large drop into many smaller drops, each with a surface charge 2, in conformity with the Poincaré-Hopf theorem. In low frequency electric fields, concentric drops exhibit flexoelectro-optic rotation in evidence of their escaped character.

Colloidal systems comprising solid or fluid particles dispersed in nematic monodomains are known to be a convenient means to study topological defects. Recent experiments have shown that twist-bend nematic $({N}_{\mathrm{TB}})$ droplets in a nematic matrix act as colloidal particles that lead to the formation of elastic dipoles, quadrupoles, and their ordered clusters. In this study, we examine the effect of low-frequency $(f\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}\mathrm{mHz})$ electric fields on such defect configurations. We find that (i) the hyperbolic hedgehogs of elastic dipoles shift toward the negative electrode in static fields and perform oscillatory motion in AC fields, indicating the presence of nonvanishing flexoelectric polarization in the field-free state; (ii) the elastic dipoles, propelled by forces of backflow due to coupled flexoelectric and dielectric distortions, drift uniformly along their axes with the ${N}_{\mathrm{TB}}$ drops in lead; (iii) the translational velocity ${v}_{d}$ increases linearly with both $f$ and the diameter of ${N}_{\mathrm{TB}}$ drops; and (iv) with increasing applied voltage U, ${v}_{d}(U)$ exhibits a monotonic, slightly nonlinear variation at $f\ensuremath{\le}200\phantom{\rule{0.16em}{0ex}}\mathrm{mHz}$, tending toward linearity at higher frequencies.

Masking of symptoms in winter and their re-appearance in black pepper (Piper nigrum L.) infected with piper yellow mottle virus (PYMoV) in summer is common, especially on new flushes that appear after pre-monsoon showers. Plants of nineteen cultivars of black pepper infected with PYMoV but without any visible symptoms were grown in a polyhouse under natural conditions and in a greenhouse under controlled conditions from January 2019 to January 2020. The number of plants expressing symptoms in the polyhouse increased gradually from 1% during the 3rd standard meteorological week (SMW) (16 January) to 41% during the 21st SMW (22 May), when the afternoon temperature was 30–40 °C and relative humidity (RH) was 75–93%, but began declining thereafter until the 53rd SMW (1 January), when the afternoon temperature was 30–36 °C and RH was 65–86%. The proportion of plants expressing symptoms varied with the cultivar. However, in the greenhouse, in which temperature and RH were maintained at approximately 26 °C and 80%, respectively, not more than 2% of the plants expressed symptoms. The number of symptomatic plants was positively correlated to maximum temperature (T Max) and maximum relative humidity (RH Max) in the afternoon. Based on this observation, a model for predicting the percentage of symptomatic plants was developed using stepwise regression analysis. Plants at the two sites did not differ significantly in the concentration of virus (virus titre) but differed significantly in the content of total carbohydrates, lipid peroxidase, and phenols. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13337-021-00686-3.

We report experimental studies on the phase behavior of binary mixtures of 1″,7″-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) and 4,4-diheptyloxyazoxybenzene, which exhibit, apart from the nematic (N) and twist-bend nematic (N_{TB}) phases, the induced smectic-A (Sm-A) phase for weight fraction of CB7CB between 0.05 and 0.70. In planar nematic layers, the N_{TB} phase separates as droplets of tactoidlike planform; the chirality of droplets manifests in the optical dissimilarity between their opposite angular ends. Our main result is that, in the appropriate two phase region, Sm-A nuclei with positive dielectric anisotropy change over to disks immersed in the nematic above some electric field, their edges decorated by periodic bright spots, a result which was earlier reported in another binary system exhibiting the induced Sm-A phase [R. Pratibha and N. V. Madhusudana, Physica A 224, 9 (1996)10.1016/0378-4371(95)00311-8]. We develop a simple theory for the threshold of this distortion, which is a periodic undulation of the edge of the disk, demonstrating that it arises from saddle-splay elasticity of Sm-A, the low Sm-A-N interfacial tension unable to suppress the distortion. The observed increases in the number of bright spots with field, and with the radius of the disk at a given field, in both the experimental systems are also accounted for by the model. The distortion, which results in the most direct visualization of saddle splay in Sm-A, is also exhibited by disks nucleating on surfaces treated for homeotropic anchoring.

We report wormlike flexoelectric structures evolving deep in the Freedericksz state of a nematic layer of the liquid crystal cyanobiphenyl-(CH2)7-cyanobiphenyl. They form in the predominantly splay-bend thin boundary layers and are built up of solitary flexoelectric domains of the Bobylev–Pikin type. Their formation is possibly triggered by the gradient flexoelectric surface instability that remains optically discernible up to unusually high frequencies. The threshold voltage at which the worms form scales as square root of the frequency; in their extended state, worms often appear as labyrinthine structures on a section of loops that separate regions of opposite director deviation. Such asymmetric loops are also derived through pincement-like dissociation of ring-shaped walls. Formation of isolated domains of bulk electroconvection precedes the onset of surface instabilities. In essence, far above the Freedericksz threshold, the twisted nematic layer behaves as a combination of two orthogonally oriented ...

Nanoscale structures in fluid media normally require techniques such as freeze fracture electron microscopy and atomic force microscopy for their visualization. As demonstrated in the present study, the surface modification due to nanoscale clusters occurring intrinsically in nematics made of bent-shaped molecules with either rigid or flexible cores leads to microscale structures, which are visible in an optical microscope. The underlying physical mechanism proposed here involves a quasiperiodic change in anchoring conditions on untreated glass plates for the medium made of islands of clusters surrounded by unclustered molecules. The resulting pattern of stripes outlines the director-normal field around line defects in the well-known schlieren texture. The instability, which is seen over most of the nematic range, with increasing visibility under continued cooling of the sample, sets the nematics made of bent-shaped molecules apart from the classical nematics of rod-shaped molecules.

microRNAs (miRNAs) are small non-coding RNAs that execute regulation of gene expression at the post-transcriptional level. Information on turmeric miRNAs is very rare. In the present study, we have identified 27 conserved and 40 non-conserved miRNA families by high-throughput Illumina sequencing and downstream in silico analysis. We could also identify 97 novel candidate miRNAs specific to turmeric. Selected miRNAs were validated by stem-loop RT-PCR and northern blotting. The targets were predicted for identified miRNAs and were found to be involved in plant growth and development, stress response and metabolism. Expression pattern of selected conserved miRNAs in response to drought stress was analyzed by stem-loop qRT-PCR. This is the first report on miRNA content of turmeric transcriptome, which might serve as a foundation for more future work on miRNA based gene regulation in this medicinally important spice crop.

In this experimental and theoretical study, we examine the equilibrium shapes of quasi-two-dimensional twist-bend nematic (Ntb) drops formed within a planarly aligned nematic layer of the liquid crystal CB7CB. Initially, at the setting point of the Ntb phase, the drops assume a nonequilibrium cusped elliptical geometry with the major axis orthogonal to the director of the surrounding nematic fluid; this growth is governed principally by anisotropic heat diffusion. The drops attain equilibrium through thermally driven dynamical evolution close to their melting temperature. They are associated with a characteristic twin-striped morphology that transforms into the familiar focal conic texture as the temperature is lowered. At equilibrium, large millimetric drops are tactoidlike, elongated along the director of the surrounding nematic fluid. This geometry is explained using a mathematical model that features two dimensionless parameters, of which one is the structural cone angle of the Ntb phase and the other is the relative strength of mismatch elastic energy at the drop's interface. Both parameters are extracted from the observations by measuring the aspect ratio of the equilibrium shapes and the inner corner angle of the cusps.

Black pepper (Piper nigrum L.) and small cardamom (Elettaria cardamomum Maton) are shade loving spice crops grown mostly in southern states of India viz. Kerala, Karnataka and Tamil Nadu mainly in Western Ghats region. Both the crops perform well in higher altitudes and high rainfall regions compared to lower altitudes and low rainfall regions. Nowadays, climate extremities such as floods and droughts, high temperature etc. have become more frequent in the growing regions leading to crop losses, increased incidence of pests and diseases and low yields. In this review, climate change impact on these crops and a few mitigation strategies are discussed. Black pepper (Piper nigrum L.) and small cardamom (Elettaria cardamomum Maton) are shade loving spice crops grown mostly in southern states of India viz. Kerala, Karnataka and Tamil Nadu mainly in Western Ghats region. Both the crops perform well in higher altitudes and high rainfall regions compared to lower altitudes and low rainfall regions. Nowadays, climate extremities such as floods and droughts, high temperature etc. have become more frequent in the growing regions leading to crop losses, increased incidence of pests and diseases and low yields. In this review, climate change impact on these crops and a few mitigation strategies are discussed.

We report on the instabilities in the Freedericksz state of the twist-bend nematic (NTB) liquid crystal 1′′,7′′-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB). The quasi homeotropic NTB state, into which a planar (untwisted or 90°-twisted) nematic CB7CB layer transits under a strong electric field, is found to be unstable despite the material being dielectrically positive. Close to the NTB melting point, destabilization occurs through the formation of metastable toric focal conic domains (TFCDs) that, in time, transform into parabolic focal conic domains (PFCDs) with the confocal parabolae in vertical planes through the layer normal. This transformation occurs by a novel process of continued dissociation of TFCDs. We outline how the extended Volterra process could help in a general appreciation of focal conic defects in the NTB phase. At relatively lower temperatures, stripes develop competingly with TFCDs. Identifiable as oily streaks, they are both localized and polarity sensitive; they form close to the substrates; and in low frequency square wave fields, they get suppressed at the cathode and augmented at the anode at each polarity switch. The study also dwells on the N–NTB–N sandwich region, found between the N and NTB states under a small temperature gradient.

Drought or water deficit stress is one of the main environmental stresses affecting plants, resulting in reduced productivity and crop loss. Black pepper, a major spice cultivated across the globe, is drought sensitive and water stress often results in plant death. The present study compared the difference in physiological parameters: relative water content (RWC) and cell membrane leakage, and also analyzed the differential expression of 11 drought responsive genes in drought tolerant and drought sensitive black pepper genotypes. Tolerant black pepper genotype exhibited significantly higher RWC and lower cell membrane leakage 10 days after stress induction than the sensitive genotype. The relative expressions of the 11 selected drought responsive genes were normalized against ubiquitin and RNA-binding protein which was identified as the most stable reference genes in black pepper under the present experimental condition using the RefFinder software. Dehydrin showed the highest transcript accumulation in both the black pepper genotypes under drought stress condition and the relative expression of the gene was higher in the tolerant genotype compared to the susceptible. Similar pattern of higher relative expression was also observed in the stress responsive gene, osmotin. The membrane protein aquaporin and the transcription factor bZIP were relatively down-regulated in the tolerant genotype. The differential expression of these important drought responsive genes in tolerant genotype of black pepper indicates its further usefulness in developing varieties with improved water stress tolerance.

Alternate bearing habit, a common phenomenon prevalent in some of the fruit trees is observed in black pepper variety Panniyur-1 also. In this study, an attempt was made to investigate the role of carbohydrates, mineral nutrients and plant hormones (IAA and zeatin riboside) in alternate bearing in Panniyur-1 variety. The experiment was conducted for three consecutive years at Murugarajendra estate, Madikeri, Karnataka, India (12.42。N, 75.73。E). Carbohydrate, mineral nutrient and plant hormone (IAA and zeatin riboside) statuses of the leaves as well as fruiting branches (stem) were compared between good fruiting year (on year) and low fruiting year (off year). Results revealed that concentration of mineral nutrients in leaves was more during on year compared to off year while the reverse was true on the stem. Both stem and leaf metabolite levels at harvest were higher in off year compared to on year suggesting that remobilization of nutrients to the developing berries must have been affected during off year, thus leading to accumulation of more nutrients at harvest in off year compared to on year. Leaf had higher concentration of most of mineral nutrients compared to stem. Spraying of 1% solution of complex fertilizer 19:19:19 (percent N:P:K) three times at an interval of 3 weeks from flower primordial initiation to flowering period enhanced the yield to the extent of around 30% during off year. Auxin to cytokinin ratio was 6.6 and 6.1 in on year and 6.3 and 5.7 in off year at flowering and 3 months after flowering respectively, suggesting that this ratio itself may not play a major role in flowering during on and off years, though the ratio was slightly more during on year. Results of the study indicate that efficient utilization of metabolites and nutrients in on year may render vine weak in the subsequent year coupled with poor remobilization into developing berries, which could make it an off year.

Expression of symptoms in black pepper plants (Piper nigrum) infected with Piper yellow mottle virus (PYMoV) vary depending on the season, being high during summer months. Here, we explored the influence of temperature on symptom expression in PYMoV infected P. nigrum. Our controlled environment study revealed increase in virus titer, total proteins, IAA and reducing sugars when exposed to temperature stress. There was change in the 2-D separated protein before and after exposure. The 2-D proteomics LC-MS identified host and viral proteins suggesting virus-host interaction during symptom expression. The analysis as well as detection of host biochemical compounds may help in understanding the detailed mechanisms underlying the viral replication and damage to the crop, and thereby plan management strategies.

Black pepper is mainly grown as a rainfed crop. Total rainfall and its distribution influence pepper productivity. Heavy rains during flowering reduce the rate of pollination and continuous heavy rainfall promotes vegetative development and limits flowering. On the other hand, break in the rainfall experienced at different stages during critical period following flower initiation leads to severe reduction in yield. Black pepper needs to be irrigated in summer to harvest good crop. Low spike intensity in rainfed situation is due to staggered and delayed spiking, lower bisexual flowers, anthracnose incidence, and spike shedding. Both plant height and leaf area are affected by water stress. Tolerant genotypes maintain higher root growth, higher relative water content, and lower cell membrane leakage under stress condition. In general, tolerant genotypes accumulated all the amino acids in higher quantities compared to susceptible ones during water stress. A, gs, and E decreased drastically, while Tleaf increased after 6 days of stress induction. Water stress reduced the activity of catalase and acid phosphatase and increased the activities of peroxidase, glutathione reductase, and polyphenol oxidase enzymes. Lipid peroxidation was high in drought-susceptible cultivars compared to tolerant cultivars. There was no difference in the expression profile of isoforms of catalase and peroxidase enzymes between control and stress, while super oxide dismutase showed an additional isoform in some genotypes under stress condition. Studies on 2D SDS gel electrophoresis indicated that the proteins relevant to photosynthesis were downregulated during drought suggesting reduced photo oxidative stress and protection of photosynthetic machinery during stress. Studies on exogenous application of ABA indicated a role for ABA in stress tolerance.

We report on the behavior of cylindrical walls formed in a substrate-free nematic film of PCH5 under the action of an in-plane ac field. In the film, with vertical molecular alignment at all the limiting surfaces, annular Brochard-Leger walls are induced well above the bend-Freedericksz threshold. They exhibit, at high field strengths, a new type of instability not encountered in sandwich, or any other, cell configuration. It manifests as a shearing of the loop-wall between the opposite free-surfaces. The shear strain is measured as a function of time, field strength, frequency, and temperature. Significantly, the strain is linear in field strength. The origin of shear and its dependence on field variables are explained through an adaptation of the Carr-Helfrich mechanism of charge separation. The sheared wall is stable against pincement up to several times the threshold field, and divides itself into two fragments under a large enough strain. With the shear distortion, linear defects appear in the opposite splay-bend regions, just as Neel lines in Bloch walls of magnetic systems. At very low frequencies, flexoelectric influence on distortion is revealed.

We characterize the structural transitions in an initially homeotropic bent-rod nematic liquid crystal excited by ac fields of frequency/well above the dielectric inversion point fi. From the measured principal dielectric constants and electrical conductivities of the compound, the Can-Helfrich conduction regime is anticipated to extend into the sub-megahertz region. Periodic patterned states occur through secondary bifurcations from the Freedericksz distorted state. An anchoring transition between the bend Freedericksz (BF) and degenerate planar (DP) states is detected. The BF state is metastable well above the Freedericksz threshold and gives way to the DP state, which persists in the field-off condition for several hours. Numerous +1 and -1 umbilics form at the onset of BF distortion, the former being largely of the chiral type. They survive in the DP configuration as linear defects, nonsingular in the core. In the BF regime, not far from fi, periodic Williams-like domains form around the umbilics; they drift along the director easy axis right from their onset. With increasing fi, the wave vector of the periodic domains switches from parallel to normal disposition with respect to the c vector. Well above fi, a broadband instability is found.

We report a novel dc field-driven propagative instability associated with the thermally induced layer undulations of the smectic C phase in a phenyl benzoate. While the undulations are two-dimensional, the drift is observed only along the wave vector q parallel to the c director; undulations with orthogonal q and c remain stable. The drift, which is nonhysteretic, takes place in a hopping way between equilibrium positions; it has a well-defined threshold in a given region, but the threshold varies rather widely for different regions. The average propagation velocity increases linearly from zero with the control parameter epsilon until epsilon approximately 2 but tends to saturate thereafter. Significantly, the drift direction reverses on switching the field polarity. The mechanism responsible for the drift appears to involve a coupling between the transverse field gradients due to the conductivity anisotropy and the transverse component of the flexoelectric polarization.

We report on the inverse flexoelectric effects observed in a nematic liquid crystal with a small positive dielectric anisotropy subject to static and very low frequency (

This paper concerns the instability behavior of a nematic liquid crystal in the region of dielectric inversion frequency for different waveforms of the exciting electric field. The critical frequency separating the regimes of dielectric and electroconvective primary bifurcation states shows a notable dependence on the waveform. In particular, it is found to undergo a large downshift for square-wave and sawtooth-wave fields as compared to sine-wave and triangle-wave fields. This seems to underscore the significance of harmonics in nonsinusoidal fields for the evolution of patterned electroconvective states. The study also deals with the flow pattern associated with the periodic state and the sequence of secondary instabilities occurring at higher fields to emphasize the role of the Carr-Helfrich mechanism for the instabilities in this region. The relevance of dielectric heating to the formation of transient structures is also pointed out.

The electric Freedericksz transition is a second-order quadratic effect, which, in a planarly aligned nematic liquid crystal layer, manifests above a threshold field as a homogeneous symmetric distortion with maximum director-tilt in the midplane. We find that, upon excitation by a low frequency (

Electric field experiments have been carried out on +1 and −1 defects formed in alignment inversion walls, in a planarly aligned nematic phenyl benzoate. The results show that the defects are non‐singular in the core and exhibit a flexoelectric response to an applied d.c. or low frequency a.c. field. When the c‐director flux lines are circular, as in a +1 defect in a wall parallel to the easy axis, flexoelectro‐optic switching characterized by an azimuthal angle variation is observed. When the c‐director flux is radial, the response is seemingly through polar angle variations involving no rotation of the extinction brushes due to crossed polarizers. This conclusion follows from the field‐induced structural distortions observed at a −1 defect having a combination of radial and tangential c‐director fields.

We present the results of optical studies on the instabilities in a substrate-free nematic 8CB film, subject to an in-plane electric field. The initial director field involves a −1/2 strength disclination loop, separating the central pseudoisotropic zone from the splay-bend (SB) birefringent boundary. Three regimes of sample thickness are distinguishable on the basis of field-induced instabilities. Thick (∼75 μm) films display growth of SB zones (independently of disclination movement), wall-formation, and reversible transition between walls and disclinations. Moderately thick films, a few μm in the central part, exhibit distinctive undulations at the border of advancing SB layers. Submicron thin films, with smectic-like homeotropic central plateau, show spectacular isotropic vortex-pairs at either end of this plateau. Further, the end regions of the birefringent zone exhibit both electro-convective flows and reorientational effects. The latter are associated with the formation of open and closed walls, and loop-wall emission. The final high field instability involves jet-like flows at the two ends of the film.

Studies of electric field-induced orientational effects are carried out on substrate-free nematic 8CB films, held between parallel electrodes and containing a peripheral-1/2 loop. The loop separates the birefringent boundary from the central homeotropic zone. Static and dynamic characteristics of the splay-bend layers contiguous to the electrodes are investigated by polarizing microscopy. A model based on director field symmetry is proposed to explain (a) the growth of the transition layers independently of the associated singularities and (b) the formation or otherwise of static solitons (Brochard-Leger walls). Linear dependence of the growth rate of transition layers on field strength is compared with a similar behaviour for line defects. Bifurcation of a soliton at elevated voltages into a pair of - 1/2 lines, through a non-pincement process, is described.

We report experimental results on the formation, dynamics, and annihilation of edge dislocations of opposite topological charge in the electroconvective inplane vortex state of a bent core nematic liquid crystal. The approach of paired, oppositely charged defects toward each other is a two-step process. Near constant velocity at large separation and accelerated motion close to annihilation are found, as in the case of nematic rolls belonging to standard electroconvection. Periodic arrays of dislocations of alternating polarity form upon a sudden, strong elevation of the control parameter. Chevron structures that appear between undulatory defect chains are metastable, and their decay with time is accompanied by an exponential reduction in the dislocation density. The initial periodicity of defect chains also drops exponentially with increasing field.

The Bobylev-Pikin striped-pattern state induced by a homogeneous electric field is a volume flexoelectric instability, originating in the midregion of a planarly aligned nematic liquid crystal layer. We find that the instability acquires a spatiotemporal character upon excitation by a low frequency (0.5 Hz) square wave field. This is demonstrated using a bent-core liquid crystal, initially in the 90\ifmmode^\circ\else\textdegree\fi{}-twisted planar configuration. The flexoelectric modulation appears close to the cathode at each polarity reversal and, at low voltage amplitudes, decays completely as the field becomes steady. Correspondingly, at successive polarity changes, the stripe direction switches between the alignment directions at the two substrates. For large voltages, the stripes formed nearly along the alignment direction at the cathode gradually reorient toward the midplane director. These observations are generally attributed to inhomogeneous and time-dependent field conditions that come to exist after each polarity reversal. Polarity dependence of the instability is attributed to the formation of intrinsic double layers that bring about an asymmetry in surface fields. Momentary field elevation near the cathode following a voltage sign reversal and concomitant gradient flexoelectric polarization are considered the key factors in accounting for the surfacelike modulation observed at low voltages.

Nematic 8CB and MBBA slabs, with a homeotropic zone between two parallel -1/2 disclinations (and associated splay bend zones), are subject to an electric or magnetic field acting transversely to the disclinations. Optical microscopic study of the static and dynamic characteristics of the boundary zones is carried out for the first time. The growth rate of the zones is linear in field-strength. As the field is progressively elevated, the zones widen nonlinearly, and merge to form a splay-bend wall in the mid-region. At a high enough field, the homeotropic region is squeezed out of the wall and a pair of wedge disclinations forms in its place. One of these combines with a -1/2 line originally present in the field off state, forming a loop that progressively collapses.

Optical, DSC and X-ray studies show that Cn H2n+1, O Cm H2m OH (n=16; m=4; n=18 and 20, m= 2 and 3; n = 22, m=3)/aqueous H3 PO4 systems exhibit mesophases in the order middle - viscous isotropic - ...

The observation of two polarity-sensitive electrical responses found in the low-frequency (1 Hz) regime of a square wave field is reported for an achiral rodlike smectic-$C$ liquid crystal with negative dielectric and conductivity anisotropies and in the 90\ifmmode^\circ\else\textdegree\fi{}-twisted configuration. The first involves a transient director modulation appearing at each polarity reversal and vanishing under steady field conditions. The instability is polarity sensitive, with the maximum distortion localized near the negative electrode instead of the sample midplane. This is inferred from the wave-vector orientation alternating in the two halves of the driving cycle between the alignment directions at the two substrates. Various electro-optic characteristics of this temporal phenomenon are also described. Following a similar observation in nematic liquid crystals, we associate the transient periodic order with the Carr-Helfrich mechanism assisted by quadrupolar flexoelectric polarization obtaining under electric field gradients. The second polarity-sensitive effect manifests in the relative shift of the periodic Fr\'eedericksz pattern upon field reversal. The shift, which is linear in field for low fields, tends to saturate for large fields. It is interpreted as due to flexoelectric polarization associated primarily with the c director twist about the layer normal. A model involving a periodic wedgelike band, which has the twist localized within it and is flanked by two uniformly and transversely aligned regions, accounts for the flexoelectric shift of the optical pattern.

We report, for a rodlike nematic liquid crystal with small positive dielectric and conductivity anisotropies, and in the 90°-twisted configuration, low frequency (

Differential development of seeds and their abortion in multi-ovulated fruits is explained by a process of self-organized movement of resource units into the developing ovules. This process involves movement of resource units would be autocatalytically favored and hence dominate over others at a rate defined by the sink-drawing ability of the ovules of a species. Such a process leads to varying levels of seed abortion that are independent of resource or pollen limitation and also predicts a positive relationship between the sink-drawing ability of a species and the extent of seed abortion in its fruits. We tested this prediction by (1) manipulating the sink-drawing ability of ovules in phaseolus vulgaris Linn., Vigna unguiculata Walp., Crotolaria medicaginea Lam., and Solanum seaforthianum Andr.; (2) analyzing the relationship between the in vivo sink-drawing ability and seed abortion in 15 species; and (3) using data from the literature. Our results seem to indicate that, as sink-drawing ability of ovule...

We have studied the electroconvective behaviour of MBBA, with initial homeotropic boundary conditions, employing a.c. fields acting transversely to the line of sight. Two major types of convective structure occur. They may be described as the distorted-homeotropic (V D ) and distorted-planar (H D ) states. The first bifurcation is to the V D state and takes place at a threshold voltage which varies parabolically with the electrode-separation. The second bifurcation corresponds to V D -H D transformation occurring close to turbulence. The H D structure, once nucleated, propagates in a highly organized and coherently winding manner so that an array of + 1 linear defects form at the axes of circulation. The growth speed of H D front is strongly voltage dependent. Field induced undulatory structures develop along the V-H interface below the threshold of electroconvection.

A new type of highly organized, large scale propagation of vortical flow has been observed in MBBA subject to a transverse electric field. This is a transient phenomenon associated with the transformation, occurring at elevated voltages, between electrically distorted homeotropic and planar states. The convective flows generate an array of linear defects which persist even after removal of the field.

Bent-core nematic electroconvection is a relatively less explored area, particularly in the low frequency regime. We focus here mainly on the instabilities occurring below 100 Hz in an initially planar monodomain of a bent-core nematic liquid crystal, which is negative in both conductivity and dielectric anisotropies. An unprecedented observation is the occurrence of three distinct bifurcation modes in a narrow region (10–17 Hz) that manifest, in the order of increasing threshold, as longitudinal, oblique and normal rolls. Whereas the second of these is the flexoelectrically enabled Carr–Helfrich mode, the other two are nonstandard electroconvection modes. Significantly, the first two instabilities remain unquenched even after bifurcation into the normal roll state below their respective codimension-2 points. The hybrid roll states display complex flows and morphologies. The study includes measurement of electrical parameters relevant to the discussion of results.

We report on the anisotropic electrohydrodynamic states formed over a wide temperature range (∼45 °C) in a planarly aligned bent-core nematic liquid crystal driven by fields of frequency in the range 0.1 Hz-1 MHz. Three different primary bifurcation scenarios are generated in the voltage-frequency (V-f) plane, depending on the temperature T. These, under increasing T, are characterized by the pattern sequences (i) in-plane longitudinal rolls (ILR)→in-plane normal rolls 1 (INR1), (ii) Williams rolls (WR)→ILR→INR1, and (iii) WR→INR2→INR1. Temperature-induced ILR→INR2 transition, the first example of its kind, points to elastic anisotropy as possibly the determining factor in wave vector selection. In the ILR and INR states, at threshold, the director modulations are predominantly azimuthal, and the streamlines, mainly normal to the wave vector, lie in the sample plane. Well above threshold, growing director deviations lead to narrow disclination loops that evolve in regular arrays, with their area density being exponential in voltage. The defects drift in a coordinated manner along the flow lines with a speed that scales nonlinearly with voltage; they mediate in the eventual occurrence of turbulence. The current theories of anisotropic convection based on static electrical parameters fail to account for the observed high-frequency instabilities. The study includes (i) a quantitative characterization of the critical parameter functions V(c)(f), V(c)(T), q(c)(f), and q(c)(T), with q(c) denoting the critical pattern wave number, and (ii) measurement of electrical and elastic parameters of relevance to electroconvection; the latter show anomalous features supporting the cluster hypothesis.

We report on measurements of dielectric permittivity epsilon, electrical conductivity sigma, elastic moduli k(ii), and rotational viscosity gamma for a bent-core nematic liquid crystal. The static permittivity anisotropy epsilon(a) = epsilon(parallel)-epsilon(perpendicular) is negative; at a given temperature in the interval 107-123 degrees C, epsilon(parallel) shows two relaxations falling in the frequency bands 20-200 kHz and 0.9-2 MHz; epsilon(perpendicular) also shows a relaxation between 0.9 and 5 MHz. The conductivity anisotropy sigma(a) = sigma(parallel)-sigma(perpendicular) is negative at low frequencies; it changes sign twice at frequencies f(1) and f(2) that increase with temperature, in the ranges 6.5-10 and 95-600 kHz, respectively. Surprisingly, the splay modulus k(11) is considerably greater than the bend modulus k(33) in the entire nematic range. Viscous relaxation is more complex than in calamitic systems involving at least a two-step process. The gamma values are an order of magnitude greater compared to calamitics.