Interspecific interference between Apoanagyrus lopezi and A. diversicornis, parasitoids of the cassava mealybug Phenacoccus manihoti

The parasitoids Apoanagyrus lopezi De Santis and A. diversicornis (Howard) (Hymenoptera: Encyrtidae) have been introduced into Africa for the biological control of the cassava mealybug Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae). We have studied competition between these species to investigate if they can coexist. Here we report on the influence of the simultaneous presence of non-conspecific adult females on searching efficiency on patches. Wasps of either species foraged on discs of cassava leaf with mealybugs, while at the same time different numbers of non-conspecifics were also depleting the patch. Patch area per parasitoid and number of hosts available to each parasitoid were equal in all treatments.In both species, the presence of other foragers clearly affected several aspects of the parasitoids' behaviour. Patch residence time increased with the number of non-conspecifics in A. diversicornis. In both parasitoid species, the proportion of hosts left unparasitized after the patch visit decreased with increasing numbers of females on the patch. The proportions of super- and multiparasitism did not change with the number of females. Both species produced more offspring during a patch visit in the presence of more non-conspecifics. These behavioural changes did not, however, lead to a change in the offspring production rate on patches. A. diversicornis produced offspring at a rate three times that of A. lopezi when one A. lopezi and one A. diversicornis foraged simultaneously. This is the first report of an aspect of interspecific competition where A. diversicornis has an advantage over A. lopezi. Interference between adult females thus promotes coexistence of the two species on P. manihoti.     

LIFE-HISTORY ADAPTATION AND REPRODUCTIVE ISOLATION IN A GRASSHOPPER HYBRID ZONE

Patterns of life-history adaptation and reproductive isolation were investigated in the acridid grasshoppers Melanoplus sanguinipes and M. devastator, which hybridize along an altitudinal gradient in the Sierra Nevada of California. Melanoplus sanguinipes females crossed with M. devastator males produced eggs that were approximately half as viable as eggs from other crosses. Diminished viability was not attributable either to infection by Wolbachia pipientis or to failure of sperm transfer. When offered an opportunity to choose a mate, females from all populations discriminated against males of the other species, whereas in no-choice tests measuring copulation duration only females from the tails of the clines showed preferences. Melanoplus sanguinipes, found at high elevations where the growing season is short, exhibited faster egg hatch, faster larval development, smaller adult body sizes, and smaller clutch sizes than M. devastator. Melanoplus devastator, from California's Central Valley, endured a hot and dry summer in a reproductive diapause that was absent in M. sanguinipes. Clines in reproductive diapause and clutch size coincided with the region of reproductive incompatibility. Development time, body size, and hatch time also changed across the hybrid zone, but the regions of largest transitions in these traits were either difficult to locate using the limited populations studied here or were not coincident with the zone's center. A method is described for combining ecological and phylogenetic analyses to address the unknown issue of whether life-history divergence has conributed to reproductive isolation in this system.     

Stable carbon isotopes as indicators of increased water use efficiency and productivity in white spruce (Picea glauca (Moench) Voss) seedlings

The relationship among water use efficiency (WUE), productivity and carbon isotopic composition (δ¹³C) in white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings representing 10 controlled crosses were planted in the field in individual buried sand-filled cylinders. The soil water content in the cylinders was measured using time domain reflectometry over two growing seasons and seedling water use determined by water balance. Two watering treatments were imposed: irrigation and dry land. There was significant (1.6–2.0%c) genetic variation in needle δ¹³C. Ranking of crosses in terms of δ¹³C was generally maintained over watering treatments and there was not a significant genetic versus environmental interaction. There was a positive correlation between δ¹³C and both intrinsic and long-term WUE (more positive δ¹³C with increased WUE) and between δ¹³C and productivity, suggesting a correlation due to variation in photosynthetic capacity. Root to shoot ratios did not increase in water-stressed plants, indicating that responses to drought were primarily at the level of gas exchange, rather than through morphological changes. Our results indicate that it should be possible to use δ¹³C as a surrogate for WUE and to select white spruce genotypes for high WUE without compromising yield.     

Genotypic variation in zinc efficiency and resistance to crown rot disease (Fusarium graminearum Schw. Group 1) in wheat

A crown rot disease in wheat caused by the fungusFusarium graminearum Schw. Group 1 is a widespread problem in chronically Zn-deficient Australian soils. A link between crown rot and Zn deficiency was established by Sparrow and Graham (1988). This paper reports a test of a further hypothesis, that wheat genotypes more efficient at extracting zinc from low-zinc soils are more resistant to infection by this pathogen. Three wheat cultivars (Excalibur, Songlen and Durati) of differential Zn efficiency were tested at three zinc levels (0.05, 0.5 and 2.0 mg Zn kg⁻¹ of soil) and three levels ofF. graminearum S. Group 1 inoculum (0.1 g and 0.3 g kg⁻¹ live chaff-inoculum and control having 0.1 g kg⁻¹ dead chaff inoculum). Six weeks after sowing dry matter production of shoots and roots was decreased byFusarium inoculation at 0.05 mg and 0.5 mg kg⁻¹ applied Zn.Fusarium inoculum at 0.1 g was as effective as 0.3 g kg⁻¹ for infection and decreasing dry matter. The infection at the basal part of culm decreased significantly by increasing the rate of Zn application. Excalibur, a Zn-efficient cultivar (tolerant to Zn deficiency) produced significantly more shoot and root dry matter, and showed less disease infection compared with Zn-inefficient cultivars (Durati and Songlen) at low (0.05 mg Zn kg⁻¹ soil) and medium (0.5 mg Zn kg⁻¹ soil) Zn fertilization rates. Higher rate of Zn fertilization (2.0 mg Zn kg⁻¹ soil) reduced the disease level in Durati to the level of Excalibur but the disease level of Songlen was still high, indicating its high Zn requirement and or sensitivity to crown rot. The data on Zn uptake show that Excalibur, being Zn-efficient, was able to scavenge enough Zn from Zn-deficient soil, we suggest that besides sustaining growth Excalibur was able to build and maintain resistance to the pathogen; inefficient cultivars needed extra Zn fertilization to achieve performance comparable to that of Excalibur. The present study indicates that growing Zn-efficient cultivars of wheat along with judicious use of Zn fertilizer in Zn-deficient areas where crown rot is a problem may sustain wheat production by reducing the severity of the disease as well as by increasing the plant vigour through improved Zn nutrition. ei]Section editor: R Rodriques-Kalana     

Relative growth rate, biomass allocation pattern and water use efficiency of three wheat cultivars during early ontogeny as dependent on water availability

We have investigated the water use efficiency of whole plants and selected leaves and allocation patterns of three wheat cultivars (Mexipak, Nesser and Katya) to explore how variation in these traits can contribute to the ability to grow in dry environments. The cultivars exhibited considerable differences in biomass allocation and water use efficiency. Cultivars with higher growth rates of roots and higher proportions of biomass in roots (Nesser and Katya) also had higher leaf growth rates, higher proportions of their biomass as leaves and higher leaf area ratios. These same cultivars had lower rates of transpiration per unit leaf area or unit root weight and higher biomass production per unit water use. They also had higher ratios of photosynthesis to transpiration, and lower ratios of intercellular to external CO₂ partial pressure. The latter resulted from large differences in stomatal conductance associated with relatively small differences in rates of photosynthesis. There was little variation between cultivars in response to drought, and differences in allocation pattern and plant water use efficiency between cultivars as found under well-watered conditions persisted under dry conditions. At the end of the non-watered treatment, relative growth rates and transpiration rates decreased to similar values for all cultivars. High ratios of photosynthesis to transpiration, and accordingly high biomass production per unit of transpiration, is regarded as a favourable trait for dry environments, since more efficient use of water postpones the decrease in plant water status.     

The development of phloem anastomoses between vascular bundles and their role in xylem regeneration after wounding in Cucurbita and Dahlia

The differentiation of phloem anastomoses linking the longitudinal vascular bundles has been studied in stem internodes of Cucurbita maxima Duchesne, C. pepo L. and Dahlia pinnata Cav. These anastomoses comprise naturally occurring regenerative sieve tubes which redifferentiate from interfascicular parenchyma cells in the young internodes. In all three species, severing a vascular bundle in a young internode resulted in regeneration of xylem to form a curved by-pass immediately around the wound. The numerous phloem anastomoses in these young internodes were not involved in this process, the regenerated vessels originating from interfascicular parenchyma alone. Conversely, in mature internodes of Dahlia, the regenerated vessels originated from initials of the interfascicular cambia, and their phloem anastomoses did not influence the pattern of xylogenesis. On the other hand, in old internodes of Cucurbita, in which an interfascicular cambium was not yet developed, the parenchyma cells between the bundles had lost the ability to redifferentiate into vessel elements, and instead, regenerated vessels were produced in the phloem anastomoses. Thus, the wounded region of the vascular bundle was not bypassed via the shortest, curved pathway, but by more circuitous routes further away from the wound. Some of the regenerated vessels produced in the phloem anastomoses were extremely wide, and presumably efficient conductors of water. It is proposed that the dense network of phloem anastomoses developed during evolution as a mechanism of adaptation to possible damage in mature internodes by providing flexible alternative pathways for efficient xylem regeneration in plants with limited or no interfascicular cambium.This paper is dedicated to the memory of the late Isaac Blachmann (deceased 19 November 1995), father-in-law of the senior author, for encouragement and advice throughout the yearsThis research was supported by an International Scientific Exchange Award to R.A. from the Israel Academy of Sciences and The Royal Society.     

胶孢镰刀菌产生串珠镰刀菌素的不稳定性

从陕北克山病病区分离到的两株串珠镰刀菌素产生菌株——胶孢镰刀菌(Fusarium subglutinans Wollenw.et Reinking)陕-6号和2-17号进行单孢分离,分别得到23和19个单孢分离株。这些单孢菌株可分为两种培养型:一种形态上与原始菌株相似,产生串珠镰刀菌素,产色素,具有大、小分生孢子,转管八次产毒量有下降;另一种则不产串珠镰刀菌素和孢子,无色素,后者在二株菌的单孢分离菌中所占比例分别为60.9％和15.8％。由此可见,胶孢镰刀菌产毒稳定性受异核体和该菌单核变异性共同影响。     

生长光强对亚热带自然林两种木本植物稳定碳同位素比、细胞间CO2浓度和水分利用效率的影响

生长于１００％、４０％和１６％自然光下的荷木和黧蒴幼苗叶片稳定碳同位素比（δ１３Ｃ,－‰）,细胞间ＣＯ２浓度（Ｃｉ）和水分利用效率（ＷＵＥ）有一定的差别。１６％和４０％弱光下,δ１３Ｃ的负值增加－０．５４Ｊ‰到－０．８９‰。Ｃｉ增大８．１－１３．２μｉＬ－１,ＷＵＥ则下降６－２４％。结果表明,叶片的水分和气体交换特性受生长光强的调节,叶片的δ１３Ｃ值可反映其生长过程中受光的强弱状况。     

Energetic constraints and foraging efficiency

Previous research considers foraging options that differ interms of their gross rate of gain b and rate of energy expenditurec. This research argues that maximizing efficiency b/c willmaximize net energetic gain when there is an upper limit onthe amount of energy that can be assimilated. This analysisdoes not include the expenditure during the time for which theanimal is unable to forage because of this constraint. Whenthis expenditure is included, maximizing efficiency is no longeroptimal. Instead the best feeding option is the one with thehighest value of b/(c – c₁), where c, is the metabolicrate when the animal is not foraging.