Toward More Productive,Efficient, and Competitive Nitrogen-Fixing Symbiotic Bacteria

The prospects of developing strains of legume nodule bacteria that provide higher productivity of leguminous plants are described. The generic, biochemical, physiological, regulatory, and economic constraints that govern the ability of private and public efforts to construct better inoculants for legume nodulation are discussed. Success in constructing better inoculants requires a two-pronged approach. First, strains need to be improved in order to compete successfully with indigenous strains for root nodulation of legumes. Several loci have been identified to date that affect competitiveness for strain nodule occupancy. Usually mutations in these loci affect the ability of a strain to form nodules rapidly and efficiently. Other loci, such as those that confer antibiotic production, can be added to strains to enhance nodulation competitiveness when co-inoculated with antibiotic-sensitive strains. Second, the inoculum strains must be improved with respect to symbiotic nitrogen fixation. Efforts to enhance the symbiotic productivity of legume nodule bacteria either by mutation or genetic engineering are also described. The best characterized example of these is the hydrogenase system. Due to nitrogenase-dependent catalysis of proton reduction, diazotrophs evolve large amounts of H₂. An approach to maximize the efficiency of symbiotic N₂ fixation, and therefore of legume productivity, is to construct strains of Rhizobium with the ability to oxidize this otherwise wasted H₂. The electrons produced by H₂ oxidation are funneled through energy-conserving electron transport chains. Our knowledge of the genetics and biochemistry of H₂ oxidation in Bradyrhizobium japonicum and Rhizobium leguminosarum has developed rapidly in recent years. At least 20 genes are needed for these bacteria to manufacture and efficiently express a nickel-containing H₂-uptake hydrogenase. These genes include those encoding regulatory elements, posttranslational processing enzymes, nickel-sensing and nickel-metabolism proteins, and electron transport components for integrating the electrons from H₂ oxidation into the respiratory chain. Some of the components for oxidizing H₂ in the symbiotic N₂ fixing bacteria are distinct from the analogous components in (nonsymbiotic) H₂ oxidizing bacteria.     

Physical and chemical plant characters inhibiting the searching behaviour of shape Trichogramma chilonis

Several plant characters are known to affect the searching behaviour and parasitization efficiency of Trichogramma spp. (Hymenoptera: Trichogrammatidae). In this study, plant characters contributing to the low Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) egg parasitism levels on pigeonpea (Cajanus cajan (L.) Millspaugh) were investigated. The efficiency of T. chilonis on pigeonpea was dependent on the plant structure on which the host eggs were found. In a cage experiment, more than 55% of eggs placed on leaves were parasitized, while 1% of eggs on calyxes and no eggs on pods were parasitized. In a filter paper bioassay, parasitoids were deterred by acetone and hexane surface extracts from pigeonpea pods but showed no response to water extract. The searching behaviour of the parasitoids was not affected by different solvent extracts from the surface of pigeonpea leaves. In a four-armed airflow olfactometer, T. chilonis was repelled by volatiles from pigeonpea pods but showed no response to volatiles derived from hexane extract of pod surfaces. Volatile infochemicals and hexane surface extracts from pods of two wild Cajanus species, C. scarabaeoides (L.) Thours and C. platycarpus (Bentham) van der Maesen, were similarly deterrent to T. chilonis. The movement of the parasitoids on pigeonpea pods and calyxes was inhibited by long trichomes and wasps were trapped by sticky trichome exudates. Parasitoids walked significantly faster on leaves than on pods. The walking speed on both pods and leaves increased significantly after washing with hexane. The results presented in this paper show that the plant growth stage and the plant structures preferred by H. armigera for oviposition are the least suitable for T. chilonis, contributing to the low parasitoid efficiency on pigeonpea.     

Speed of invasion in lattice population models: pair-edge approximation

 We propose a simple approach to approximating the speed of invasion in lattice population models. Approximate critical parameter values for successful invasion are then found by solving for zero wave speed. The approximation is based on describing the occupied region by the ordinary pair approximation, and using quasi-steady-state pair approximations to describe the leading edge of the wave front. We illustrate this idea using the basic contact process on the 1 and 2 dimensional lattice (with and without nearest-neighbor migration), finding very good agreement between the approximation and simulation results. The approximate critical values obtained by our approximation are significantly more accurate than those obtained by the ordinary pair approximation. Received 4 September 1996     

Rapid temperature compensation of volitional swimming speeds and lethal temperatures in tropical tunas (Scombridae)

Synopsis Observations on continuously swimming tunas were used to determine effects of temperature upon volitional locomotory activity and to determine upper and lower lethal temperatures. Experimental subjects were 10 skipjack tuna, Katsuwonus pelamis, 9 kawakawa, Euthynnus affinis, and 3 yellowfin tuna, Thunnus albacares.Our results: lower and upper lethal temperatures for the euthynnids (K. pelamis and E. affinis) were 15° and 33° C, respectively. Swimming speed for the euthynnids did not decrease with temperature within most of the zone of thermal tolerance; we observed either temperature independence or increases in speed as the temperature decreased. Yellowfin tuna swam slower as the water temperature decreased, but swimming speed changes lagged behind the water temperature changes. This effect was most certainly due to the large thermal inertia that is a property of tunas. The lag between swim speed and water temperature was eliminated by utilizing an estimate of red muscle temperature, rather than water temperature, as a covariate. Yellowfin tuna swim speed was best correlated with red muscle temperature rather than ambient water or brain temperatures.     

Effect of light colours and weather conditions on captures of Sogatella furcifera (Horváth) and Nilaparvata lugens (Stål)

To study the phototactic responses of white‐backed planthopper, Sogatella furcifera (Horváth) and brown planthopper, Nilaparvata lugens (Stål) to different wavelengths, four colours of light traps (blue, green, yellow and red light‐emitting diodes) were placed in the same rice field along with a traditional black light trap. This study revealed that S. furcifera and N. lugens are more attracted to blue and green lights than that to yellow and red lights. During the 24 nights, compared with the black light trap, the blue LED trap could catch more rice planthoppers at 17 nights. Furthermore, catches of other species (moths and beetles) were substantially reduced in blue LED light traps. Multiple regression models were developed to assess the effect of weather factors on light trap catches of rice planthoppers. Rainfall and mean air temperature at a night mainly affected light trap catches of S. furcifera. Higher rainfall and lower temperature increased light trap catches of S. furcifera. However, wind speed was the main factor affecting the catches of N. lugens, and the lower incidence of catches was found in the night when wind speed exceeded 3.08 m/s. S. furcifera may be flying against wind at light wind nights by 0.3–1.5 m/s, whereas N. lugens may be flying down at strong wind nights by 1.5–3.08 m/s. Relative humidity did not significantly influence on trap catches. Consequently, light wavelengths, precipitation, average temperature and wind should be considered when monitoring rice planthoppers by light traps. Therefore, the blue LED light traps are worth using for monitoring planthoppers.     

Flight and echolocation behaviour of three vespertilionid bat species while commuting on flyways

This study compares the flight and echolocation behaviour of three vespertilionid bat species while they commute on flyways. We measured the bats’ spatial position relative to vertical background contours and relative to the ground while recording their echolocation behaviour. In Myotis daubentonii, we found a significant influence of spatial context on the position and dimensions of flyways as well as on echolocation behaviour. In gap situations, flyways tended to be narrower and located closer to background structures, flight speeds were lower and the bandwidth of echolocation signals was larger than in edge situations. Differences in background structure did not affect flight and echolocation behaviour. When commuting in the same gap situation flyway positions and dimensions for M. daubentonii and Myotis brandtii were similar but differed from those of Pipistrellus pipistrellus, which were slightly higher and further out than those used by the Myotis species. In M. brandtii, flyway positions and dimensions remained constant over 3 years. We found species-dependent differences in signal structure, but pulse interval and flight speed were similar across all species. The influence of available space on the position of flyways, on flight speed and on echolocation behaviour is discussed.     

Seedling growth variation in response to sand burial in four <Emphasis Type="Italic">Artemisia</Emphasis> species from different habitats in the semi-arid dune field

We compared seedling growth of four Artemisia species dominated at different habitats to determine whether interspecific seedling growth variation of a same genus in tolerance to burial can be used to explain plant distribution in the sand dune field. Interdune lowland species, Artemisia gmelinii, stabilized dune species, A. frigida, semi-stabilized dune species, A. halodendron, and active dune species, A. wudanica were selected. Seedlings grown for 3 weeks were treated at five burial depths for three burial times in pot experiments. Species from the habitats with little burial had smaller survival rate, dry weight and stem elongation speed than those from the habitats with intensive burial when buried. Furthermore, when buried, the former tended to adjust biomass allocation between shoot and root and produce adventitious buds, while the latter tended to maintain a constant root:shoot ratio and produce adventitious roots. We conclude that (1) seedlings of species with a long evolutionary history of exposure to sand burial (from the active sand dune), show quicker stem growth when buried than do seedlings of species from the habitats with little or no sand burial; (2) seedlings of species which do not change root:shoot ratio might be more tolerant of sand burial than those do; (3) seedlings of species from the habitats with intensive sand burial is prone to produce adventitious roots and seedlings of species from the habitats with little or no sand burial tend to produce adventitious buds when buried.     

Habitat preferences of Ukrainian brook lamprey Eudontomyzon mariae ammocoetes in the lowland rivers of Central Europe

The pattern of microhabitat preferences of Ukrainian brook lamprey Eudontomyzon mariae ammocoetes was examined in two rivers of central Poland: the Pilica River (the Vistula River basin) and the Grabia River (the Odra River basin). A comparison of abiotic factors of the rivers revealed differences in water speed and principal components: PC1 (determining gradient from decreasing medium sand to the increasing share of three fractions of gravel), PC2 (a gradient from the declining share of very coarse and coarse sand fractions to the growing content of fine sand) and PC3 (correlated with an increasing proportion of silt). The sites did not differ significantly in terms of water depth. Relative abundance and frequency of ammocoete occurrence in the Grabia River were higher than in the Pilica River. Only speed, PC1 and PC2 made a significant contribution to the prediction of larval occurrence. Eudontomyzon mariae larvae preferred substrata with a reduced amount of medium sand and increased content of gravel (PC1) as well as with a lower content of coarse sand and higher proportion of fine‐grained sand (PC2). The ammocoetes also preferred areas with a water speed of 0·2 m s^?1 but avoided speeds ≥0·6 m s^?1. The abundance of E. mariae was affected by water speed, as well as by all PCs. The mean ± s.e. optimal current speed was 0·265 ± 0·007 m s^?1, while abundance decreased with increasing amounts of gravel (PC1) and increased with increasing amounts of fine sand and silt in the bottom substratum (PC2 and PC3). Comparison of ammocoete microhabitat use in the Pilica and Grabia Rivers showed the lack of differences in distribution in the preferred values of current speed, PC1 and PC2.     

Insecticide‐mediated behavioral avoidance by bean bug,Riptortus pedestris (Heteroptera: Alydidae)

Bean bug, Riptortus pedestris (Fabricius) (Heteroptera: Alydidae), is an economically important pest of soybean and tree fruit. Behavioral responses of R. pedestris to olfactory cues from insecticide‐treated foliage were assessed in a Y‐tube olfactometer. Ten insecticides were applied to kidney bean plants at the recommended dosage, and then bug avoidance was assessed at 0, 3, 5, 7 and 13 days after treatment (DAT). Each bug was allowed 5 min to choose one of the olfactometer's arms. Even though the degree of avoidance was variable to the insecticides tested, bean bugs significantly avoided the insecticide‐treated leaf. The pyrethroid bifenthrin was the most avoided of the insecticides tested. In all treatments, avoidance decreased as DAT increased. Also, the movement speed was higher to the untreated leaf than to the insecticide‐treated leaf. The degree of avoidance was positively correlated to the vapor pressure of the chemical; this finding implies the interaction of the treated chemicals and plant volatiles may be used as cues for bean bug selection of the host‐plant.