Electrical Polarization of Tobacco Cells by Ca2+ Ion Channels

It has been suggested that neighbouring cells in higher plantsco-ordinate their direction of growth by sensing the electricalpolarities of their neighbours. The present work sets out toexamine the role played by calcium in the response of theirindividual cells to externally-applied fields. The transcellular currents of cultured tobacco cells were investigatedwith a vibrating probe before and after the application of anartificial electric current with a density of 250 µA cm^–2,giving a potential difference of approximately 3 mV across thecell. When calcium was omitted from the experimental medium,the externally-applied current had little effect on either thedirection or magnitude of the cells' own transcellular currents.When 01 mM calcium was present, the external current repolarizedthe cells so that their own currents tended to flow in the samedirection as the current applied. This was due to a large localizedincrease in inward current in the region nearest the positiveelectrode, with the outward current being more evenly spread. Adding cobalt ions (a Ca²⁺ -channel blocker) in the presenceof external calcium had little immediate effect on the transcellularcurrents themselves, but they lost their ability to change inresponse to the artificially applied current. This suggeststhat the cells may have detected the applied current by enhancedcalcium ingress through calcium channels in the most hyperpolarizedregion of the membrane. An hypothesis is presented which proposesthat asymmetric calcium entry results in the electrical polarizationof cells by a mechanism involving both the opening of calcium-gatedion channels and the lateral movement of electrophoreticallymobile channels. Key words: Cell-culture, cobalt, plant electrophysiology, polarity, vibrating-probe     

The Electrical Control of Growth in Plant Tissue 7

Goldsworthy, A. and Rathore, K. S. 1985. The electrical controlof growth in plant tissue cultures: The polar transport of auxin.—J.exp. Bot. 36: 1134–1141. The reasons for a many-fold stimulation of shoot formation anda 60–70% stimulation of growth in tobacco callus causedby passing a very weak electric current (1 or 2 µA) betweenthe callus and the culture medium have been investigated. Thestimulation of callus growth occurred only when the callus wasmade negative to the medium and then only when IAA was added.It was abolished, even in the presence of IAA, by the additionof TIBA which is an inhibitor of polar auxin transport, andalso when the IAA was substituted by either IAN or the syntheticauxin 2,4-D, neither of which show significant polar transport.This suggests that the electrical treatment may have alignedthe physiological polarities of the callus cells so as to promotethe polar transport of IAA into the tissue when the callus wasnegative to the medium. If so, the enhanced shoot formationmay have been due to the parallel orientation of the growthaxes of individual cells so as to make the production of organformingmeristems more likely. The mechanism of the effect and its relationshipto the natural forces controlling differentiation is discussed. Key words: —Auxin, electrophysiology, polarity, tissue-culture, tobacco, transport     

Exploitation of distant Antarctic waters and close neritic waters by short‐tailed shearwaters breeding in South Australia

Identifying the primary foraging grounds of abundant top predators is of importance in marine management to identify areas of high biological significance, and to assess the extent of competition with fisheries. We studied the search effort and habitat selection of the highly abundant short‐tailed shearwater Puffinus tenuirostris to assess the search strategies employed by this wide‐ranging seabird. During the chick‐rearing period 52 individuals were tracked performing 39 short foraging trips (1–2 days), and 13 long trips (11–32 days). First‐passage time analysis revealed that 46% of birds performing short trips employed area‐restricted searches, concentrating search effort at an average scale of 14 ± 5 km. Foraging searches were more continuous for the other 54%, who travelled faster to cover greater distances, with little evidence of area‐restricted searches. The prey returned indicated that continuous searchers consumed similar prey mass, but greater prey diversity than area‐restricted search birds. On long trips 23% of birds travelled 500–1000 km to neritic (continental shelf) habitats, showing weak evidence of preference for areas of higher chlorophyll a concentration, and foraged at a similar spatial scale to short trips. The other 76% performed rapid outbound flights of 1000–3600 km across oceanic habitats commuting to regions with higher chlorophyll a. The spatial scale of search effort in oceanic habitat varied widely with some performing broad‐scale searches (260–560 km) followed by finer‐scale nested searches (16–170 km). This study demonstrates that a range of search strategies are employed when exploiting prey across ocean basins. The trade‐offs between different search strategies are discussed to identify the value of these contrasting behaviours to wide‐ranging seabirds.     

The Occurrence of High CO2-Compensation Points in Amaranthus Species

When the CO₂-compensation points of detached leaves of Amaranthusedulis plants grown undera range of controlled environmentswere determined, they were all found to be near zero. However,similar plants grown in a greenhouse occasionally yielded high-compensation-pointleaves. The occurrence of these could not be correlated withleaf age, moisture stress, or the prevailing environmental conditions.These high-compensation-point leaves still showed a typicalKranz anatomy. Starch was found external to the bundle sheathin both high- and low-compensation-point leaves, but the distributionof the starch appeared to vary with light intensity.     

Tocopherol Levels and Ageing in Wheat Grains

The levels of tocopherol and other anti-oxidants were measuredin healthy and aged wheat grams No major reduction in anti-oxidantlevels was observed after either natural or artificial ageing,even when carried to the total loss of germination It is concludedthat there is little to be gained with respect to seed longevityin wheat by breeding for high tocopherol levels Triticum aestivum L., wheat -tocopherol, vitamin E, anti-oxidant, ageing     

Lipoprotein transformations during adipokinetic hormone action in Locusta migratoria

ABSTRACT. Detailed studies of lipoprotein A⁺ formation during AKH action have been made in Locusta migratoria L. using gel filtration combined with the use of radiolabeled haemolymph protein probes. In addition, we have assessed the quantitative contribution of the C_L-proteins to A⁺ formation by direct measurement of the changes in concentration of free C_L-proteins and report some properties of the C-I and C-II proteins: they appear to be glycoproteins of 20,000 and 16,000 MW respectively, but do not bind to concanavalin A. We have confirmed earlier observations (using different techniques) which showed that liproprotein A_yellow is not involved per se in A⁺ formation during the first 15 min of AKH action. In contrast, the (two) C_L-proteins take part in A⁺ formation without any apparent delay after hormone injection. Our observations show that A⁺ formation is essentially complete within 30 min of AKH injection, although further C_L-protein binding and lipid-loading do occur subsequently. After 30 min there is no further decrease in the A_yellow titre. It is argued that much, if not all, C_L-protein is located at the surface of the A⁺ particle. From the changes in titres which occur in A_yellow and C_L-proteins during AKH action we estimate that A⁺ is formed from 1 mole of A_yellow and approximately 28 moles of C_L-proteins. Using these figures we calculate an apparent molecular weight for A⁺ within the range of 1.65–2.12×10⁶, which is in reasonable agreement with estimates derived from gel exclusion chromatography data. These studies emphasize the dynamic and fully reversible nature of lipoprotein A⁺ formation and highlight the complex nature of the lipoprotein transformations occurring during hormone-stimulated lipid transport in locusts.     

An Instrument for Measuring Crop Density by Light Absorbance

A portable instrument is described which measures simultaneouslythe light intensity above and below a leaf canopy, calculatesthe absorbance (extinction) and displays it continuously ona meter. It measures the capacity of the crop to absorb lightand so provides an estimate of its density and, by inference,the approximate photosynthetic potential. The measurements,which are simple and non destructive, have been shown to correlatewith seed-rate, sowing date, seed-vigour and final yield inwheat. Although originally designed for cereals, the instrumentmay also find application with other crops. crop density measurement, creals, light abrbanace