Regulation of Rubidium Uptake in Sunflower Roots

Uptake of Rb⁺ was investigated in 12-day-old intact plants of sunflower (Helianthus annum L. var. californicus) which had been cultivated or pretreated in nutrient solutions with various K⁺ concentrations. The relationship between Rb⁺ influx and K⁺ concentration of the roots indicated regulation of Rb⁺ uptake by allosteric inhibition of the uptake mechanism. A constant passive influx occurred contemporaneously with the active uptake as shown by experiments at 0°C or with 2,4-dinitrophenol. The allosteric regulation of ion carrier activity occurred after a time lag of up to 1 h after the change of external solution. In experiments involving Rb⁺ treatments of K⁺-deficient plants, the synthesis of carriers for transport of Rb⁺ could be demonstrated. A model including allosteric regulation of Rb⁺ uptake in roots is discussed.     

Can acaricide‐impregnated leg bands fitted to female red grouse reduce sheep tick parasitization of chicks and increase chick survival?

In parts of northern England, North Wales and the Scottish Highlands, increasing numbers of sheep ticks Ixodes ricinus (Ixodida: Ixodidae), and the louping ill virus they can carry, are considered to be important factors that reduce red grouse Lagopus lagopus scotica productivity. The present study tested this hypothesis by fitting adult female grouse with leg bands impregnated with the acaricide cypermethrin to experimentally control ticks on their chicks on two managed grouse moors in northeast Scotland. The chicks of females fitted with acaricide leg bands showed reduced tick infestations and improved survival in one of the two study years, relative to chicks of control females. Acaricide leg bands constitute a potential management technique that may be adopted by grouse moor managers in circumstances of high tick infestations on grouse chicks.     

Diurnal Cycling in Root Resistance to Water Movement

The occurrence of diurnal changes in root resistance of cotton was studied by measuring the flow of water through 35-to70-day-old root systems under a pressure of 3.10 bars or a vacuum of 0.88 bar. The volume of exudate obtained under constant pressure or constant vacuum was 2 to 3 times greater near midday than near midnight indicating that the root resistance apparently was 2 to 3 times greater at night than during the day. The salt concentration of the exudate also cycled; the concentration was lowest at midday and highest at night, hence there was little diurnal variation in the total amount of salt moved per hour. The cycle for volume of exduate, salt concentration, and apparent root resistance had a period of 22 to 26 hours at 24°C. The cycle gradually died away 2 to 3 days after removal of the shoots. The diurnal variations appeared to be controlled by signals from the shoots because the phase of the cycles could be reset by changing the light-dark cycle under which the plants were grown. Cycling was eliminated by exposure to 8 or more days of continuous light before removing the shoots, and cycling could not be entrained by a 6 hour light-6hour dark cycle. Bubbling nitrogen gas through the nutrient medium stopped cycling. A possible role of ion or growth regulator action is discussed.     

Use of Bullfrog Tadpoles (Rana catesbeiana) to Examine the Mechanisms of Lead Neurotoxicity

Sublethal exposure to lead elicits changes in behavior, particularlylearning. Previously, we had shown that bullfrog tadpoles exposedto 1 mg Pb/liter for 7 days exhibit learning deficits in a discriminateavoidance learning assay. The precise mechanisms involved inthese lead-induced learning deficits are not understood, butCNS monoamine neurotransmitter systems have been implicatedin the learning process. In the present study, we exposed bullfrogtadpoles to 1.7± 0.2 mg Pb/liter for 7 days and thencompared concentrations of neurotransmitters from whole brainsamples with those of controls. Serotonin or 5-hydroxytryptamine(5-HT) was significantly decreased in the lead exposed groupwhile the serotonin metabolite, 5-hydroxyindoleacetic acid,(5-HIAA) was similar to controls. No changes were observed inthe catecholamines epinephrine and norepinephijine followinglead exposure. The ratio of 5-HIAA/5-HT was significantly increasedin lead exposed animals as a result of the decrease in 5-HT,suggesting a decrease in 5-HT biosynthesis rather than an increasein 5-HT metabolism. These findings are the first to suggestthat lead exposure in bullfrog tadpoles affects the monoamineneurotransmitters that are implicated in the learning process.The results of the present study, in conjunction with previousevidence of learning deficits following lead exposure, offerthe possibility of correlating lead exposure with learning deficitsand alterations in CNS neurotransmitters in bullfrog tadpoles.The use of this tadpole model shows promise as a means to examineand understand the mechanisms involved in lead neurotoxicity.     

Landscape structure influences tree density patterns in fragmented woodlands in semi‐arid eastern Australia

Landscape and local‐scale influences are important drivers of plant community structure. However, their relative contribution and the degree to which they interact remain unclear. We quantified the extent to which landscape structure, within‐patch habitat and their confounding effects determine post‐clearing tree densities and composition in agricultural landscapes in eastern subtropical Australia. Landscape structure (incorporating habitat fragmentation and loss) and within‐patch (site) features were quantified for 60 remnant patches of Eucalyptus populnea (Myrtaceae) woodland. Tree density and species for three ecological maturity classes (regeneration, early maturity, late maturity) and local site features were assessed in one 100 × 10 m plot per patch. All but one landscape characteristic was determined within a 1.3‐km radius of plots; Euclidean nearest neighbour distance was measured inside a 5‐km radius. Variation in tree density and composition for each maturity class was partitioned into independent landscape, independent site and joint effects of landscape and site features using redundancy analysis. Independent site effects explained more variation in regeneration density and composition than pure landscape effects; significant predictors were the proportion of early and late maturity trees at a site, rainfall and the associated interaction. Conversely, landscape structure explained greater variation in early and late maturity tree density and composition than site predictors. Area of remnant native vegetation within a landscape and patch characteristics (area, shape, edge contrast) were significant predictors of early maturity tree density. However, 31% of the explained variation in early mature tree differences represented confounding influences of landscape and local variables. We suggest that within‐patch characteristics are important in influencing semi‐arid woodland tree regeneration. However, independent and confounding effects of landscape structure resulting from previous vegetation clearing may have exerted a greater historical influence on older cohorts and should be accounted for when examining woodland dynamics across a broader range of environments.     

Field resistance in cowpea, Vigna unguiculata to the cowpea moth, Cydia ptychora

Field evaluation of 26 selected cowpea cultivars showed that several of them had good levels of resistance to Cydia ptychora; TVu 946, Adzuki, ‘Vita 5’ and Igbirra were the most resistant. Percentage seed damaged by the larvae was positively correlated with days to 50% flowering (r= 0.39*), days to 50% pod ripening (r= 0.48*) and weight of 100 seeds (r= 0.56**). All four cultivars were early maturing, had semi-determinate or determinate plant types and carried their pods above the leaf canopy. No single morphological character could be related to the degree of seed damage but some association with phenological characteristics was indicated. Earliness, determinacy and uniform pod maturity were important plant characters associated with the expression of field resistance. Even if these characters are not causative, they provide a rational basis for screening of a large germplasm to identify or synthesise materials with greater resistance to this moth.