Preference–performance relationship in the gall midge Rabdophaga rosaria: insights from a common‐garden experiment with nine willow clones

1. Oviposition preferences of herbivorous insects are predicted to match offspring performance on different host taxa or on conspecific host genotypes. In gall‐inducing insects, host‐plant properties such as growth rate and gall size, which are determined by plant genotype and growing conditions, may have a significant impact on offspring performance and, hence, should influence oviposition site selection. 2. The present study investigated host preference of the European rosette willow gall midge Rabdophaga rosaria (Loew) in relation to offspring success on seven clones of Salix myrsinifolia Salisb. and two naturally hybridised S. myrsinifolia × phylicifolia L. clones growing in a replicated design in an experimental field under two fertilisation regimes. For each clone, the average growth rate, number of shoot tips, and leaf and gall size were determined, and their effects on midge preference and larval survival were examined. 3. Main shoot height, number of shoot tips, and gall size were significantly affected by clone. The midges clearly preferred certain clones over the others, but preferences were not related to willow growth traits or to gall size. Survival probability was higher in large than in small galls, but females did not prefer large‐leaved clones that produced the biggest rosette galls. Midge oviposition was also uncorrelated with prior rates of leaf‐rust infection and with feeding preferences of voles and folivorous insects. 4. The weak preference–performance relationship of R. rosaria within S. myrsinifolia is probably explained by evolutionary constraints that prevent generalist insects from achieving an ability to discriminate among conspecific hosts of variable quality.     

Effects of 2,3,5-Triiodobenzoic Acid on Plantlet Formation from Cultured Tissues of Taro, Colocasia esculenta (L) Schott (Araceae)

Cultures of a Californian cultivar of Colocasia esculenta varantiquorum, UCI Runner, produced abnormal structures in additionto plantlets on Linsmaier-Skoog (LS) medium supplemented with1 0 mg 1^–1 adenine-N-benzyl-tetrahydro-2H-pyran-2-yl or6-dimethylaminopurine and 0 1 mg 1^–1 napthaleneaceticacid after at least 32 weeks of culture A number of substitutionsand combinations of growth regulators were tested in an attemptto stimulate normal plantlet development These included trialswith 2,4,5-trichlorophenoxyacetic acid, aminocyclopropane-1-carboxylicacid, and 2,3,5-triodobenzoic acid (TIBA) When tissues werecultured on LS medium without hormones, and supplemented with1 mg 1^–1 TIBA, plantlet production occurred in 2 to 4weeks and the number of abnormal structures was reduced Auxin, calloid, callus culture, cytokinins, micropropagation, development     

Growth Regulators in Populus tremula

Shoots grown out from aspen root sections in light or in darkness were extracted and the content of growth-regulating substances in various fractions was determined with the Avena coleoptile straight-growth assay. The most obvious difference was obtained for the inhibitor β fraction. This was about ten times more active in the light-grown than in the dark-grown shoots. Some indications of more growth inhibitory material in ether-insoluble fractions of light-grown plants were also obtained. The stimulation obtained at the Rf of indol-3-yl-acetic acid was somewhat lower in the etiolated shoots than in the light-grown ones.     

Dispersal and connectivity in metapopulations

This paper reviews characteristics of dispersal that influence metapopulation functioning, such as releasing factors, density dependence, timing and types and health of dispersers. Economic thresholds, intraspecific conflicts and avoidance of inbreeding arc often regarded as the key ultimate or proximate (or both) causes of dispersal, but there is no consensus about the most important mechanisms. Dispersing individuals arc often considered to differ genetically from the residents but good supporting evidence has only been presented for some insect species. Sex and age differences in dispersal rates are most common in polygamous species and in long-lived species with many litters per female. A bimodal distribution of dispersal distances, earlier thought to be a common pattern, is probably an artifact, caused by habitat heterogeneity and varying survival of settled individuals. Dispersal distances are longer in poor environments. Habitat specialists are more affected by boundaries during dispersal than generalists. Dispersal just before or during the early reproductive season is common in certain species occupying early successional habitats. Dispersal increased both population and metapopulation size and persistence in plants, insects and small mammals.     

Effect of Indoleacetic Acid on the Abscisic Acid Level in Stem Tissue

Excised stem sections from growing plants of Populus tremula L. and Pisum sativum L. including lateral buds were treated with indole-3-acetic acid in a phosphate buffer solution. In control sections the level of the abscisic acid-like inhibitor decreased strongly during 24 h as did the level of the endogenous auxin. Exogenous indoleacetic acid counteracted the decrease in the inhibitor level to a considerable extent. Implications of this auxin effect in relation to apical dominance are discussed.     

Growth Regulators in Populus tremula IV. Apical Dominance and Suckering in Young Plants

Experiments with small plants of Populus tremula L. growing in solution culture indicate that polarly transported auxin is an important factor in the control of axillary bud growth. If the auxin supply from the growing apex is eliminated, the number of buds released is influenced by factors translocated in the transpiration stream from the roots. Suckers may be induced to develop from aspen roots, the age of which is six weeks or more. Removal of the growing apex and the axillary buds or stoppage of shoot growth by short day treatment were effective in inducing abundant suckering in small aspen plants. Some mature leaves had to be maintained, indicating the dependence of sucker formation on carbohydrate supply. These treatments are known to decrease auxin production in the shoots. Extraction and biological assay showed a decrease in the content of auxin in the roots as a consequence of removal of growing shoot parts. The results indicate that suckering in roots of intact aspen plants is prevented by auxin transported into the roots from growing shoot parts.     

Growth Regulators in Populus tremula

The seasonal variation in capacity to form suckers and in auxin level in bark and wood was determined in root segments of aspen (Populus tremula L.). Auxin occurred in the roots from May to October but not in November. The highest auxin level was found during the period of shoot growth. During this period the capacity of root segments to form suckers was low. Auxin level decreased in isolated root segments during the first 24 hours after excision and was low during the period of sucker induction. The relation of endogenous auxin level to control of sucker formation is discussed. The experiments do not exclude the possibility that the auxin effect is mediated through inhibitors.     

Recognition of a probable secundo–primo event in the Early Silurian

The early Telychian graptolite extinctions discussed by Loydell in the Stimulograptus utilis Sub-zone agree completely with the predicted consequences of a secundo–primo event, the first to be identified. The event would have included brief inhibitions in vertical circulation, causing a drop in primary planktic productivity from an already low level and causing extinctions; less well-oxygenated deep waters would enhance preservation of organic-walled fossils. The succeeding primo episode would instigate more vigorous vertical circulation with increased primary planktic productivity and higher downward transport of oxygen.