The effect of changes of environment on the electrical and ionic pattern of muscle

The resting and action potentials of sartorius muscles of the toad, Bufo marinus, have been measured under varying conditions of external environment. At the same time, analyses for Na(+) and K(+) content were carried out. There was a slight elevation of 2 mv. when the measurements were made in phosphate-Ringer instead of in bicarbonate-Ringer. The R.P. was independent of the hydrogen ion concentration between pH 6.5 and 8.5, although at these pH's there was marked alteration in the level of Na(+) and K(+) in the muscle. Alteration of the external K(+) level between 0 and 50 m.eq./liter has little influence on the internal K(+) concentration. When the log of the external K(+) concentration is plotted against the R.P. there is not a linear relationship until the external K(+) is raised above 12 m.eq./liter, at which point the cell is unexcitable. Above this value a straight line with a slope of 58 mv. per ten-fold change in concentration is obtained, but the absolute values at any point are about 35 per cent higher than those which would be given by the Nernst equation. Alteration of the external Na(+) level within a range of 45 to 650 m.eq./liter resulted in marked changes in the internal Na(+) content, without, however, having any effect on the ratio Na(+) (out)/Na(+) (in). This ratio has remained at about 3 in spite of marked fluctuations in the absolute value of the internal and external Na(+) levels. When the Na(+) level is lowered there is a decrease in the height of the action potential although there is no alteration in the ratio Na(+) (out)/Na(+) (in). As the Na(+) level is raised the height of the action potential is not affected even in the presence of a fivefold increase in Na(+) in the Ringer. The results do not support the conclusion that the bioelectric potentials can be calculated from the ionic ratios by means of simple physical chemical hypotheses such as the Nernst or Goldman equations. The maintenance of the normal K(+) content of the cell cannot be accounted for by a Donnan mechanism. No definite evidence has been produced to explain the mechanism of a Na(+) "pump." In other words, the concept of a Na(+) pump requires that there shall be a physico- or organochemical mechanism which will distinguish between Na(+) and K(+) (or other) ions. There is evidence that Na(+) can be extruded against a concentration gradient. On the other hand the cell is able to maintain a constant ratio of external to internal Na(+) even when the cell has been severely damaged by very high external Na(+) levels.     

Evaluation of the risk of overwintering Helicoverpa spp. pupae under irrigated summer crops in south-eastern Australia and the potential for area-wide management

Surveys were undertaken to determine the distribution of overwintering pupae of two species of Helicoverpa in south‐eastern Australia. The results indicate that significant populations of H. armigera have the potential to overwinter as pupae in the region under residues of their summer crop hosts. H. punctigera, conversely, was found not to overwinter in the region to any significant degree. The results also suggest that a high proportion of the overwintering H. armigera population are located in relatively few high risk fields. The overwintering population represents an ideal opportunity for control on an area‐wide basis using post harvest cultivation or “pupae busting”. The risk of overwintering H. armigera pupae occurring was largely associated with crop type, the mechanism being related to the date that the crop flowers, the level of pupal parasitism and the use of larval control measures. The results are discussed in terms of recommendations for farmers and it is suggested that a concerted effort to cultivate high risk fields has the potential to significantly reduce the population on an area‐wide basis.     

The hatching stimulus for eggs of Aedes punctor (Diptera: Culicidae)

ABSTRACT.

• 1 Eggs of the British woodland mosquito, Aedes punctor (Kirby), were subjected to a variety of media in an attempt to determine the hatching stimulus.
• 2 It was found that continuous immersion in distilled water evoked almost no hatching (0.2%); intermittent removal from the water gave low hatching (8.9%).
• 3 Slow deoxygenation of the medium by either bacterial growth or the introduction of nitrogen produced the highest level of hatching (26.0–90.0%). Eggs of A.punctor were shown to hatch within a 4 h period of the oxygen concentration in the medium reaching zero.

     

Sink-Regulation of Photosynthesis in Relation to Temperature in Sunflower and Rape

Stimulation of the rate of photosynthesis at 2·0 kPaO₂ in comparison with 21 kPa O₂ and carbohydrate accumulationover 4h were measured during exposure of sunflower (Helianthusannuus L.) and rape (Brassica napus L.), grown at 30 °Cand 13 °C, to temperatures between 7 °C and 35 °C.The effect of reducing source: sink ratio by shading on theresponse of photosynthetic rate to temperature was also determined.Stimulation of photosynthesis by 2·0 kPa O₂ in comparisonwith 21 kPa O₂ decreased over 4 h at cool temperatures in sunflowerplants grown at 30 °C but not in rape grown at 30 °C.Stimulation did not decrease over 4 h in plants grown at 13CC. Sucrose was the main carbohydrate accumulated over 4 h;its accumulation increased with decreasing temperature. Starchaccumulation either decreased or remained the same with decreasingtemperature. In plants grown at 30 °C more carbohydrateaccumulated between 8 °C and 21 °C in sunflower thanin rape, but more carbohydrate accumulated at 30 °C in rapethan in sunflower. In plants grown at 13 °C much less carbohydrateaccumulated between 13 °C and 23 °C than in plants grownat 30 °C. Photosynthetic rate in plants grown at 30 °Cexposed to between 20 °C and 35 °C over 32 h (14 h light-10h dark-8 h light), declined over 32 h at 20 °C and 25 °Cin sunflower and at 20 °C in rape. This fall over 32 h,especially at 20 °C in sunflower, was significantly reducedby shading the rest of the plant. Shading had little effecton photosynthetic rate above 25 °C. The work confirms thatlow temperature imposes a sink-limitation on photosynthesiswhich occurs at higher temperatures in sunflower than in rape.This limitation may be relieved by decreasing the source:sinkratio. Key words: Sunflower, rape, photosynthesis, carbohydrates, sink demand, temperature     

Provisioning habitat with custom‐designed nest‐boxes increases reproductive success in an endangered finch

Manipulating resource availability to assess the strength and effects of resource limitation on an animal population is relatively straightforward, and thus, common in the scientific literature. Resource quality, however, is rarely manipulated, as this requires a priori knowledge of a relationship between some measurable variation in the resource and individual preference and/or fitness. Recent research on nest‐site selection was used to inform the design of custom‐built nest‐boxes for an endangered hollow‐nester, the Gouldian finch (Erythrura gouldiae). By provisioning breeding populations with these ‘high quality’ artificial nest sites over 3 years, we experimentally investigated the strength and effects of nest‐site limitation in this species. Breeding pairs using nest‐boxes initiated nesting earlier, produced larger clutches and fledged more offspring per season than those in natural hollows. Total and mean reproductive output also increased in nest‐box provisioned sites over the duration of the study. All of these effects were predominantly driven by pairs initiating clutches earlier, potentially because of the reduced intra‐ and interspecific competition for nest sites at optimal breeding times. Our findings suggest that reproduction in wild Gouldian finch populations may be limited by the availability of high quality nest sites in the landscape, and that nest‐boxes could be used as a tool for enhancing reproduction in recovering populations. Furthermore, we conclude that resource quality is equally, if not more important than quantity when supplementing resource‐limited habitats.