ALBERT

Notes: The photoluminescence (PL) properties of the II3V2 compound semiconductor Zn3As2, grown by metalorganic vapor phase epitaxy, is investigated in the temperature range of 4.2–350 K. Several lines are reported, believed to be due to acceptors and donor-acceptor complexes in the epitaxial material. From variable temperature PL, the band gap is deduced to be indirect in nature (in contrast to most experimental findings to date which have indicated the band gap to be direct in nature) and ∼25–30 meV larger than the indirect band gap. The direct band gap energy has been extracted from the PL spectra in the range of 110–350 K. A linear relationship is observed in this temperature range with slope dEg/dT=−4.00×10−4 eV/K. PL yielded a value of 1.005 eV for the direct gap at room temperature in good agreement with values between 0.989 and 0.999 eV obtained from transmission and reflection measurements. © 1999 American Institute of Physics.

Notes: The generation of electrostatic drift wave turbulence is modeled by the Charney–Hasegawa–Mima equation. The equilibrium density gradient n0=n0(x) is chosen so that dn0/dx is nonzero and spatially variable (i.e., v*e is sheared). It is shown that this sheared diamagnetic flow leads to localized turbulence which is concentrated at max(∇n0), with a large dv*e/dx inhibiting the spread of the turbulence in the x direction. Coherent structures form which propagate with the local v*e in the y direction. Movement in the x direction is accompanied by a change in their amplitudes. When the numerical code is initialized with a single wave, the plasma behavior is dominated by the initial mode and its harmonics. © 1999 American Institute of Physics.

Notes: We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin-walled sieve tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin- and thick-walled sieve tubes within this C3 species. Lack of connections between vascular parenchyma and sieve tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore-plasmodesmatal connections between companion cells and sieve tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell-sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles.The degree of cellular connectivity and the potential isolation of the companion cell-sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell-thin-walled sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s-1) longitudinal and radial transport in the bundle sheath-vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the sieve tubes to the vascular parenchyma. Injection of the companion cell-sieve tube complex, did not demonstrate backward connectivity to the bundle sheath.We conclude that the low plasmodesmatal frequencies, coupled with a two-domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.

Notes: Pyrophosphate: d-fructose-6-phosphate 1-phosphotransferase (PFP; EC 2.7.1.90) and ATP: d-fructose-6-phosphate 1-phosphotransferase (PFK; EC 2.7.1.11) activities were determined in sugarcane varieties differing in sucrose content. For this purpose, activities were measured in those internodes where the maximum rate of sucrose accumulation occurs. The specific activity of internodal PFP varied significantly between the sugarcane varieties and was inversely correlated with the sucrose content. There was also a highly significant inverse correlation between PFP and sucrose content in a segregating F1 population. PFK activity was comparable to, or lower than, PFP activity and no relationship was evident between PFK activity and sucrose content. In all tissues investigated, the fructose 2,6-bisphosphate levels were probably sufficient to ensure full activation of PFP. The levels of PFP activity appear to be controlled by the expression of the β-subunit of the protein. The molecular mass of the PFP subunit polypeptide(s) was approximately 63 kDa. There was an inverse correlation between sucrose content and the partitioning of radiolabel into respiration in internodal tissue slices labelled with [U-14C]glucose across 3 sugarcane lines. The estimated flux of carbon into respiration correlated directly with PFP activity.

Notes: Uptake of sugar by Phaseolus vulgaris cell suspension cultures from a sucrose supplemented medium is predominantly in the hexose form. This is due to a rapid cleavage of the sucrose by an apoplastic acid invertase activity and an apparent very low demand for and uptake of carbon from the medium prior to induction of cell growth and division. Glucose is preferentially taken up, leading to an accumulation of fructose in the medium. However, when the glucose is depleted the cells do take up the fructose at a rate similar to that of glucose. When glucose or fructose is supplied individually to cell cultures, both are utilised very efficiently with growth slightly better on the fructose medium. Hexose uptake is largely an active process with diffusion uptake even at the highest concentrations (〉 50 mM) contributing less than 30%. The hexose uptake system of the cells has a greater affinity for glucose (Km= 240 µM) than for fructose (Km= 960 µM) but the maximum uptake (Vmax) is similar. The major difference in the kinetic properties of hexose uptake is that glucose is a strong inhibitor of fructose uptake, while fructose has little effect on glucose uptake. The differences in the kinetic properties of the uptake system for the two hexoses can largely explain the observed pattern of hexose utilisation when both glucose and fructose are present in the medium.

Notes: The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) is one of the most destructive insect pests of cereals world-wide. Although resistant cultivars have been bred, the biochemical mechanism of resistance is unknown. The aim of this work was to gain information on the mechanism of resistance which could contribute to more directed breeding of resistant cultivars in the future. The effect of RWA infestation on the inter- and intracellular β-1,3-glucanase activities was studied in different resistant wheat (Triticum aestivum L.) cultivars containing the Dn-1 gene for RWA resistance and corresponding near-isogenic susceptible cultivars. The activity was determined spectrophotometrically by measuring the release of glucose from laminarin. Infestation differentially induced the intra- and intercellular activities to much higher levels in resistant than susceptible cultivars within 48 h. According to immunological studies induced enzyme activities were due to increased protein levels. The intracellular β-1,3-glucanase contained about 8% exo-activity. The exo-activity made an insignificant contribution to the intercellular activity. The genetic background into which the resistance gene was bred did affect the level of activity that corresponded to the resistance performance. Seven apoplastic isoforms of β-1,3-glucanase, varying from acidic to basic, were resolved by isoelectric focusing. All isoenzymes were equally induced and no specific one could be linked to resistance or susceptibility. The RWA induced β-1,3-glucanase activity in resistant cultivars closely resembles defence responses during pathogenesis and seems to be part of a general defence response like the hypersensitive reaction (HR), which confers resistance to the RWA. This knowledge might be helpful in future to identify genes for RWA resistance. The increased β-1,3-glucanase activity after RWA infestation might serve as an additional measure to biochemically trace resistance in crosses during breeding.

Notes: The response of Phaseolus vulgaris L. cv. Contender grown under controlled environment at either ambient or elevated (360 and 700 μmol mol-1, respectively) CO2 concentrations ([CO2]), was monitored from 10 days after germination (DAG) until the onset of senescence. Elevated CO2 had a pronounced effect on total plant height (TPH), leaf area (LA), leaf dry weight (LD), total plant biomass (TB) accumulation and specific leaf area (SLA). All of these were significantly increased under elevated carbon dioxide with the exception of SLA which was significantly reduced. Other than high initial growth rates in CO2-enriched plants, relative growth rates remained relatively unchanged throughout the growth period. While the trends in growth parameters were clearly different between [CO2], some physiological processes were largely transient, in particular, net assimilation rate (NAR) and foliar nutrient concentrations of N, Mg and Cu. CO2 enrichment significantly increased NAR, but from 20 DAG, a steady decline to almost similar levels to those measured in plants grown under ambient CO2 occurred. A similar trend was observed for leaf N content where the loss of leaf nitrogen in CO2-enriched plants after 20 DAG, was significantly greater than that observed for ambient-CO2 plants. Under enhanced CO2, the foliar concentrations of K and Mn were increased significantly whilst P, Ca, Fe and Zn were reduced significantly. Changes in Mg and Cu concentrations were insignificant. In addition. high CO2 grown plants exhibited a pronounced leaf discoloration or chlorosis, coupled with a significant reduction in leaf longevity.

Notes: Pollination greatly accelerates petal senescence. The first observed event after pollination is an increase in the flower's sensitivity to ethylene, followed by an increase in ethylene biosynthesis. Our objectives were to study the mode of action of the increase in ethylene sensitivity and the possible involvement of short-chain saturated fatty acids (SCSFAs) in this process. Application of SCSFAs, ranging in chain length from 7 to 10 carbons onto stigmas of Phalaenopsis (Phalaenopsis hybrid, cv. Herbert Hager) flowers increased their sensitivity to ethylene in the same way as pollination. Following pollination, there was a significant increase in the endogenous content of these fatty acids in the flower's column and perianth, with octanoic acid (C8) being the main SCSFA observed. The increase in SCSFA content was observed as early as 6 h after pollination and began to decline 6 h later. Incorporation of octanoic acid into liposomes or microsomal membranes isolated from Phalaenopsis petals resulted in a decrease in lipid order that was detected by fluorescence polarization of dansyl pyrrolidine (DNSP) but not of 1,6-diphenyl-1,3,5-hexatriene (DPH). At peak ethylene sensitivity, 10 h after pollination, there was a significant decrease in the lipid order of microsomal membranes isolated from Phalaenopsis columns and perianths, again as detected by DNSP but not by DPH. Stigmatic application of octanoic acid mimicked the effect of pollination on membrane lipid order. We suggest that SCSFAs may be the ethylene ‘sensitivity factors’ produced following pollination, and that their mode of action involves a decrease in the order of specific regions in the membrane lipid bilayer, consequently altering ethylene action.

Notes: The pattern and sequence of cellular degradation during the course of leaf senescence remains obscure and the nature of the trigger that induces cell senescence is unknown. In order to probe the pre-mortem phase of senescence temporal changes in cell ultrastucture were studied in aging leaves of light-grown achlorophyllous Hordeum vulgare L. cv. Dyan mutant seedlings. Electron microscope examination of the ultrastructure of mesophyll cell plastids revealed the absence of ribosomes and a highly disorganized prolamellar body. Both the number and size of plastoglobuli increased with aging and this change coincided with depletion of starch grains and dilation of lamellar membranes. Aging of mesophyll cells occurred coincident with a decline in ribosome content of the cytoplasm and loss of matrix granularity. Loss of ribosomes associated with the outer nuclear envelope membrane and a reduction in chromatin were also apparent. Only after 10 days was there evidence of loss of internal membrane integrity and swelling of mitochondrial cristae. Compartmentation was thus maintained during the aging process with membrane dissolution occurring late in senescence. These results suggest that an inability to produce chlorophyll and carotenoids and form thylakoid stacks due to the absence of plastid ribosomes, contributes to the rapid onset of senescence in light-grown achlorophyllous seedlings. Furthermore, disruption of chloroplast ribosome synthesis/assembly may constitute part of the plastid signal involved in triggering cell senescence.

Notes: South Africa's entry into the 1990s has been dramatic. Changes are taking place on political, social and economic fronts at an increasingly rapid pace and past practices are being openly challenged. Changes invoke concerns, fears and hopes. Rather than resisting the reform process, one must concentrate on managing it. The changes taking place in South Africa at present have influenced all people in some or other way. The nature and consequences of this process of changes are, as yet, not fully understood.