Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration: II. EFFICIENCY OF NITROGENASE FUNCTIONING

Changes in nitrogenase activity (C₂H₂ reduction and H₂ production),nodulated root respiration and the efficiency of nitrogenasefunctioning were measured in response to progressive dehydrationof nodules on intact well-watered plants of subterranean clover(Trifolium subterraneum L.) cv. Seaton Park. The nodulated rootsof vegetative plants grown to the 14-leaf stage were incubatedin a gas exchange system through which a continuous dry airstreamwas passed over an 8 d period. The root tips were immersed inan N-free nutrient solution during this time so that water andion uptake was unimpeded. The decline in nodulated root respirationresulting from nodule drying was associated with a continualreduction in respiration coupled to nitrogenase activity. Asnodule water potential (_nod) decreased, the proportion of totalnodulated root respiration which was nitrogenase-linked declinedfrom 50% (day 1) to 33% (day 8). This was accompanied by a 79%reduction in specific nitrogenase activity (from 3.79 to 0.81umol C₂H₄ g^–1 nodule dry weight min^–1). Nodule dehydrationalso induced a decline in hydrogen (H₂) production in air. Therelative decline in hydrogen production exceeded that of acetylenereduction activity and this resulted in an increase in the relativeefficiency of nitrogenase functioning. However, the carbon costof nitrogenase activity progressively increased above 2.0 molCO₂ respired per mol C₂H₄ reduced as rood decreased below –0.4to –0.5 MPa. Consecutive measurements of the rates ofhydrogen evolution, ¹⁵N₂ fixation and acetylene reduction activityon intact unstressed plants resulted in a C₂H₄/N₂ conversionfactor of 4.08 and an electron balance of 1.08. These resultsindicated that the pre-decline rate of acetylene reduction activitymeasured in a flow-through system provided a valid measure ofthe total electron flux through nitrogenase. Key words: Subterranean clover, dehydration, efficiency, nitrogenase activity     

Effects of metabolic inhibitors and drugs on ion transport and oxygen consumption in isolated frog skin

Active ion (NaCl) transport across isolated frog skin is discussed in relation to sodium and potassium composition and to O(2) consumption of skin. A distinction is made between processes in skin related to "unidirectional active ion transport" and processes related to "maintenance electrolyte equilibrium;" i.e., ionic composition of skin. Several metabolic inhibitors were found that could be used in separating maintenance electrolyte equilibrium from unidirectional active ion transport. Fluoroacetate (up to 1 x 10(-2)M/liter) did not affect maintenance electrolyte equilibrium, but severely diminished the rate of active ion transport. This could also be accomplished with azide and diethyl malonate when 1 x 10(-3) molar concentrations were used. When applied in higher concentrations, these two inhibitors, and several others, diminished active ion transport, but this was associated with changes in maintenance electrolyte equilibrium (gain of Na(+) by and loss of K(+) from skin). Similar observations were made when skins were subjected to K(+)-deficient media. Mersalyl and theophylline, in low concentrations, stimulated active ion transport without leading to changes in maintenance electrolyte equilibrium. Inhibition of active ion transport was found accompanied by decrease, increase, and unaltered over-all O(2) consumption, depending on the kind of chemical agent used. A provisional scheme of the mechanism of unidirectional active ion transport is proposed. It is conceived as a process of metabolically supported ion exchange adsorption, involving a carrier, forming complexes with K(+) and Na(+), a trigger, K(+) ions, and two spatially separated metabolic pathways.     

Foraging strategies in orb-spinning spiders: Ambient light and silk decorations in Argiope aetherea Walckenaer (Araneae: Araneoidea)

Abstract Many species of orb-spinning spiders construct silk decorations within the structure of the orb-web. The evolutionary significance of these decorations is poorly understood, but the silk decorations of many species reflect UV light, suggesting that they may function to attract insects. In these species, relatively more silk decoration may be required under dimmer light conditions in order to maintain a constant UV-reflecting signal, and hence level of insect attraction. We investigated experimentally whether the orb-spinning spider Argiope aetherea adjusts the amount of silk decoration added to the web according to light conditions. Consistent with the prey-attracting function, we found that spiders adjusted the quantity of silk decoration to their webs, adding more silk decoration when the web was located in dim light rather than bright light.     

Gynandromorphs of the pycnogonid Anoplodactylus portus

A large sample of the pycriogonid Anoplodactylus portus , collected from the Pacific side of the Panama Canal Zone, contains a high percentage of gynandromorphs. The literature on aquatic arthropod gynandromorphs is outlined. The nature and variations of the sexual mosaic population are described and discussed. The several morphological differences between normal and mosaic members of the population are illustrated, and the possible origin of these gynandromorphs is discussed.     

Seed dispersal in a cycad,Macrozamia riedlei

The influence of animals on patterns of seed dispersal has not been well explored, despite the importance of seed dispersal in plant population biology. Furthermore, the exploitation of animals as dispersal agents has been considered one reason for the replacement of gymnosperms by angiosperms in the Cretaceous. We studied seed dispersal in a cycad, Macrozamia riedlei, and showed that in the absence of animals, ripe seeds drop directly beneath the parent not more than 40 cm from the base. Several animals eat the fleshy outer integument of the seeds, and possums (Trichosurus vulpecula), transport seeds up to 24 m from the parent. The postdispersal positioning of seeds gave a log density-log distance relationship of y = 7.20 – 2.85x (r²= 0.96, N = 17). Effective local dispersal may have contributed to the persistence of cycads in the world flora for the last 200 million years.     

Flowering phenology,seed set and bird pollination of five Western Australian Banksia species

Flowering phenology and seed set characteristics of five species of Banksia were studied in relation to the nectarivorous birds which feed at their inflorescences. Within the Banksia woodland at the study site near Perth, the flowering seasons of the Banksia species were sequential and only slightly overlapping, providing a year-round nectar source. Although honeyeaters visited alt five species, seed set was very low in each case. Caging experiments indicated that, in B. attenuata at least, alternative pollinators may play a more important role in pollination than do nectar-feeding birds. It is suggested that non-avian pollinators, predatory insects, and characteristics of the breeding system may also have been important in the evolution of the observed flowering phenology and patterns of seed set.     

Linking intronic polymorphism on the CHD1‐Z gene with fitness correlates in Black‐tailed Godwits Limosa l. limosa

We show that variation in an intronic length polymorphism in the CHD1‐Z gene in Black‐tailed Godwits Limosa l. limosa is associated with fitness correlates. This is the second example of the CHDZ‐1 gene being correlated with fitness, a previous study having established that Moorhens Gallinula chloropus carrying the rare Z* allele have reduced survival. In Godwits, however, carriers of the Z* allele (374 bp) fared better than those with the more frequent Z allele (378 bp) with respect to body mass, plumage ornamentation, reproductive parameters and habitat quality. The Z* allele was found in 14% of 251 adult birds from nature reserves, but was absent from 33 birds breeding in intensively managed agricultural lands. Males and females with the Z* allele had less extensive breeding plumage, and females had a higher body mass, bred earlier and had larger eggs. There were no significant differences in annual survival between birds with and without the Z* allele. DNA isolated from museum skins demonstrated that this polymorphism was present at low frequency in 1929. We speculate that strong asymmetrical overdominance may explain the low frequency of the Z* allele and that genetic linkage to causal genes might be an explanation for the phenotypic correlations. Our findings suggest a degree of cryptic genetic population structuring in the Dutch Godwit population.