A comparison of attack rates in a native and an introduced population of the parasitoid Cotesia glomerata

The attack rate of a population of the braconid parasitoid Cotesia glomerata, introduced into the USA over 100 years ago as a parasitoid of Pieris rapae, was compared with that of a native British population, which normally attacks P. brassicae, and with that of a P. rapae specialist, Cotesia rubecula. British C. glomerata attacked P. brassicae at a much higher rate than it attacked P. rapae. In comparison with British C. glomerata, C. rubecula showed a higher attack rate with P. rapae. American C. glomerata attacked P. rapae at a slightly higher rate than did British C. glomerata, but not at as high a rate as that achieved by C. rubecula. The differences in each comparison were statistically significant. The possible causes of the differences between British and American C. glomerata attacking P. rapae are discussed. They may be due to genetic or environmental effects. Egg load did not appear to be a factor limiting the number of hosts parasitized under the conditions of the experiments.     

UPTAKE OF INORGANIC CARBON BY CLADOPHORA GLOMERATA (CHLOROPHYTA) FROM THE BALTIC SEA1

Carbon uptake in the green macroalga Cladophora glomerata (L.) Kütz. from the brackish Baltic Sea was studied by recording changes in pH, alkalinity, and inorganic carbon concentration of the seawater medium during photosynthesis. The use of specific inhibitors identified three uptake mechanisms: 1) dehydration of HCO₃ ^? into CO₂ by periplasmic carbonic anhydrase, followed by diffusion of CO₂ into the cell; 2) direct uptake of HCO₃ ^? via a 4,4′‐diisothiocyanato‐stilbene‐2,2′‐disulfonate‐sensitive mechanism; and 3) uptake of inorganic carbon by the involvement of a vanadate‐sensitive P‐type H ⁺ ‐ATPase (proton pump). A decrease in the alkalinity of the seawater medium during carbon uptake, except when treated with vanadate, indicated a net uptake of the ionic species contributing to alkalinity (i.e. HCO₃ ^? , CO₃^{2 ?} , and OH ^? ) from the medium, where OH ^? influx is equivalent to H ⁺ efflux. This would suggest that the proton pump is involved in HCO₃ ^? transport. We also show that the proton pump can be induced by carbon limitation. The inducibility of carbon uptake in C. glomerata may partly explain why this species is so successful in the upper littoral zone of the Baltic Sea. Usually, carbon limitation is not a problem in the upper littoral of the sea. However, it may occur frequently within dense Cladophora belts with high photosynthetic rates that create high pH and low carbon concentrations in the alga's microenvironment.     

Testing the efficiency of three 15N-labeled nitrogen compounds for indirect labeling of grasshoppers via plants in the field

In field studies of plant–insect herbivore interactions it is often difficult to establish which herbivore has fed on a particular plant. We investigated the suitability of three different ¹⁵N‐labeled nitrogen compounds (ammonium, nitrate, and glycine) for indirect marking of three grasshopper species [Omocestus viridulus (L.), Chorthippus parallelus (Zett.), and Chorthippus biguttulus (L.) (Orthoptera: Acrididae)] through labeling their food plants in the field. In two short‐term experiments grassland plots of 1 m² were separately labeled with either one of the different nitrogen compounds. Grasshoppers were caged on three food‐plant species [Dactylis glomerata L., Holcus lanatus L. (Poaceae), and Trifolium repens L. (Fabaceae)] present in these plots for 72 h. Significantly enriched δ¹⁵N values in grasshoppers were found in all plant/grasshopper combinations. Enrichment in grasshoppers was positively correlated with the enrichment of plants and labeling with nitrate resulted in highest ¹⁵N enrichment. In a long‐term experiment, individuals of C. biguttulus were placed in a cage covering an area of 1 m² for 37 days, with sampling of grasshoppers at regular intervals. δ¹⁵N values of the grasshopper and a common food plant, D. glomerata, increased steadily over time, up to 40‐fold by the end of the experiment. Our results demonstrate that ¹⁵N‐labeling of plants is an appropriate tool for the investigation of insect–plant interactions under natural conditions.     

INFLUENCE OF VARIOUS EXPOSURE PERIODS ON THE BIOMASS AND CHLOROPHYLL A OF CLADOPHORA GLOMERATA (CHLOROPHYTA)1

The biomass of Cladophora glomerata (L.) Kütz. was estimated at selected sites in the Colorado River between Glen Canyon Dam, Arizona, and River Kilometer 354. C. glomerata biomass was significantly higher at sites above Lees Ferry (25 km downstream from the Dam) than sites below the Ferry. Biomass and chlorophyll a were significantly reduced when C. glomerata was subjected to one-time exposures to the atmosphere for 12 daylight h in more. Repeated 12/12 h and 24/24 h (exposure/submergence) cycles over a two-week period also showed a significant reduction in biomass. The adaptations of C. glomerata to “stranding” during regulated flows are discussed.     

Feulgen Densitometry: Importance of a Stringent Hydrolysis Regime

Abstract: Feulgen densitometry is still a widely used method for DNA content measurements, but experimental procedures and results are often controversial. The present note is concerned with a recent report in the literature that optimum Feulgen staining required a remarkably longer hydrolysis time with 5 M HCI in Dactylis glomerata L. than in Hordeum vulgare L. (i.e., 62 min versus 20 min at 25 C). As this result is prone to question the usual practice of maintaining unified hydrolysis times for test material and internal standard, we established hydrolysis curves for D. glomerata, H. vulgare, Pisum sativum L. and Allium cepa L. at 20 C and 25C for 0 to 100 min. No striking differences between the species and, in particular, between Doctylis and Hordeum were found. Optimum staining occurred after 60 min with hydrolysis at 20 C and after 25 min at 25 C. It is strongly recommended to conduct the quantitative Feulgen reaction at a precisely controlled temperature instead of an inexact room temperature. The broader plateau of optimum staining at 20 C makes this regime preferable.     

Increasing growth temperature reduces the stimulatory effect of elevated CO2 on photosynthesis or biomass in two perennial species

We examined how anticipated changes in CO₂ concentration and temperature interacted to alter plant growth, harvest characteristics and photosynthesis in two cold-adapted herbaceous perennials, alfalfa ( Medicago sativa L. cv. Arc) and orchard grass ( Dactylis glomerata L. cv. Potomac). Plants were grown at two CO₂ concentrations (362 [ambient] and 717 [elevated] μmol mol⁻¹ CO₂) and four constant day/night temperatures of 15, 20, 25 and 30°C in controlled environmental chambers. Elevated CO₂ significantly increased total plant biomass and protein over a wide range of temperatures in both species. Stimulation of photosynthetic rate, however, was eliminated at the highest growth temperature in M. sativa and relative stimulation of plant biomass and protein at high CO₂ declined as temperature increased in both species. Lack of a synergistic effect between temperature and CO₂ was unexpected since elevated CO₂ reduces the amount of carbon lost via photorespiration and photorespiration increases with temperature. Differences between anticipated stimulatory effects of CO₂ and temperature and whole plant single and leaf measurements are discussed. Data from this study suggest that stimulatory effects of atmospheric CO₂ on growth and photosynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.     

Direct and indirect inhibition of single leaf respiration by elevated CO2 concentrations: Interaction with temperature

Two herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and orchard grass (Dactylus glomerata L. cv. Potomac), were grown at ambient (367 μmol mol⁻¹) and elevated (729 μmol mol⁻¹) CO₂ concentrations at constant temperatures of 15, 20, 25 and 30°C in order to examine direct and indirect changes in nighttime CO₂ efflux rate (respiration) of single leaves. Direct (biochemical) effects of CO₂ on nighttime respiration were determined for each growth condition by brief (<30 min) exposure to each CO₂ concentration. If no direct inhibition of respiration was observed, then long-term reductions in CO₂ efflux between CO₂ treatments were presumed to be due to indirect inhibition, probably related to long-term changes in leaf composition. By this criterion, indirect effects of CO₂ on leaf respiration were observed at 15 and 20°C for M. sativa on a weight basis, but not on a leaf area or protein basis. Direct effects however, were observed at 15, 20 and 25°C in D. glomerata; therefore the observed reductions in respiration for leaves grown and measured at elevated relative to ambient CO₂ concentrations could not be distinguished as indirect inhibition. No inhibition of respiration at elevated CO₂ was observed at the highest growth temperature (30°C) in either species. CO₂ efflux increased with measurement and growth temperature for M. sativa at both CO₂ concentrations; however, CO₂ efflux in D. glomerata showed complete acclimation to growth temperature. Stimulation of leaf area and weight by elevated CO₂ levels declined with growth temperature in both species. Data from the present study suggest that both direct and indirect inhibition of respiration are possible with future increases in atmospheric CO₂, and that the degree of each type of respiratory inhibition is a function of growth temperature.     

Chronic exposure to increasing background ozone impairs stomatal functioning in grassland species

Two species found in temperate calcareous and mesotrophic grasslands (Dactylis glomerata and Leontodon hispidus) were exposed to eight ozone treatments spanning preindustrial to post‐2100 regimes, and late‐season effects on stomatal functioning were investigated. The plants were grown as a mixed community in 14 L containers and were exposed to ozone in ventilated solardomes (dome‐shaped greenhouses) for 20 weeks from early May to late September 2007. Ozone exposures were based on O₃ concentrations from a nearby upland area, and provided the following seasonal 24 h means: 21.4, 39.9 (simulated ambient), 50.2, 59.4, 74.9, 83.3, 101.3 and 102.5 ppb. In both species, stomatal conductance of undamaged inner canopy leaves developing since a midseason cutback increased linearly with increasing background ozone concentration. Imposition of severe water stress by leaf excision indicated that increasing background ozone concentration decreased the ability of leaves to limit water loss, implying impaired stomatal control. The threshold ozone concentrations for these effects were 15–40 ppb above current ambient in upland UK, and were within the range of ozone concentrations anticipated for much of Europe by the latter part of this century. The potential mechanism behind the impaired stomatal functioning was investigated using a transpiration assay. Unlike for lower ozone treatments, apparently healthy green leaves of L. hispidus that had developed in the 101.3 ppb treatment did not close their stomata in response to 1.5 μm abscisic acid (ABA); indeed stomatal opening initially occurred in this treatment. Thus, ozone appears to be disrupting the ABA‐induced signal transduction pathway for stomatal control thereby reducing the ability of plants to respond to drought. These results have potentially wide‐reaching implications for the functioning of communities under global warming where periods of soil drying and episodes of high vapour pressure deficit are likely to be more severe.