Effect of host instar on host discrimination of heterospecific‐parasitised hosts by sympatric parasitoids

1. Interspecific competition among hymenopteran parasitoids may shape their behavioural strategies for host resource exploitation. In order to reduce or prevent competition, many parasitoid species have evolved the ability to discriminate between unparasitised hosts and hosts parasitised by another parasitoid species (i.e. heterospecific host discrimination). However, discriminatory ability might be affected by host instar. 2. This study reports the first results on whether host instar can influence the use of heterospecific‐parasitised hosts by sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae). 3. Aphytis melinus and Aphytis chrysomphali discriminated between unparasitised and heterospecific‐parasitised hosts when they found a third‐instar host (high quality), with a tendency to multi‐parasitise. However, this discrimination was not observed in the second instar (lower size). 4. The behavioural strategies adopted towards multi‐parasitise third‐instar hosts varied between both species. Aphytis chrysomphali reduced its clutch size in heterospecific‐parasitised hosts, whereas A. melinus tended to probe them for longer than healthy hosts. 5. Overall, our results highlight the importance of host instar in the study of intrinsic competition between parasitoids.     

The Basis for Different Sensitivities of Photosynthesis to SO2 in Two Cultivars of Pea

Alscher, R., Bower, J. L. and Zipfel, W. 1987. The basis fordifferent sensitivities of photosynthesis to SO₂ in two cultivarsof pea.—J. exp. Bot. 38: 99–108. The response of several physiological parameters to exposureto SO₂ (0?8 ppm and 0?6 ppm) was studied in two cultivars ofPisum sativum in which photosynthesis showed a different sensitivityto SO₂. Leaf conductance was slightly reduced during exposureto SO₂ in the sensitive but not the insensitive cultivar. Moresulphite accumulated in the leaves of the sensitive than inthose of the insensitive cultivar. Total leaf content of reducedglutathione in the insensitive cultivar increased during exposureto SO₂, while in the sensitive cultivar there was no increaseuntil the post-exposun period. The activities of fructose l,6-bisphosphataseand glyceraldehyde-3-phosphate dehydrogenase did not decreasegreatly in either cultivar, although activities of enzymes fromthe sensitive cultivar were more affected by SO₂ than were thoseof the insensitive cultivar. Exposure to SO₂ also had littleeffect on either coupled or uncoupled electron transport ofisolated thylakoids from the leaves of either cultivar. Increasedglutathione in the insensitive cultivar may protect the photosyntheticapparatus against SO₂ Key words: SO₂, photosynthesis, light modulation, glutathione     

Cytotaxonomic observations in tropical Vaccinieae (Ericaceae)

Chromosome numbers are reported for 36 accessions representing 31 species from nine genera of the tribe Vaccinieae, family Ericaceae. The plants are tropical and come from Southeast Asia and Central and South America. The taxonomy of the tribe is outlined in these regions. Genera are often poorly defined and taxa were chosen to reflect the range of variation of the Vaccinieae. Most Southeast Asian Vaccinium species were diploid (2 n = 24) as were those of Agapetes subgenus Agapetes (apart from the Himalayan A. flava ), Agapetes scortechinii and Costera endertii. All other accessions were found to be polyploid. The correlation between polyploidy, geographical distribution and the possession of an 'anatomical complex' of the leaf and stem in Vaccinieae of New Guinea and the neotropics is discussed.     

Resolving the differences in plant burial responses

Burial is one of the major factors influencing plant ecology in deserts and coastal areas. Consequently, many studies have measured the responses of dune plants to sand burial. However, there remains little agreement about the mechanisms and characteristics constituting the burial response of plants. In particular, stimulation of growth has been reported as the most common plant burial response; however, stimulation has not been reported consistently among studies. Here, a literature survey showed that the depth of burial relative to the height of the plant determined whether the growth of a species was stimulated by burial. Growth stimulation was limited to shallow burial depths, while burial depths greater than the height of the plant consistently resulted in reduced growth. As studies used widely differing burial depths or units of growth measurement, the variation in reported stimulation of plant growth can be partly attributed to differences in experimental procedure. The stimulation of growth in many species was accompanied by an increase in photosynthesis over a limited period and by a shift in biomass allocation from root to shoot. Most plants demonstrated stimulated growth (up to 200%) in response to shallow burial indicating that some burial response mechanisms are general to many species. However, a few specialist dune species displayed a much greater ability to respond to burial (up to 700% stimulation of plant mass). Although allocation shifts and increased photosynthesis have been shown to be associated with dune plant burial response, there remains a need for field measurements that focus on the diversity of mechanisms underlying plant response to burial.     

Distribution and feeding of larval Chironomidae in a gravel-pit lake

SUMMARY. Replicated benthic samples were collected from a wet gravel-pit in the English Midlands at fortnightly intervals during 1978. The aggregation patterns of the four most numerous genera, Chironomus, Polypedilum, Procladius and Tanypus , were analysed by fitting the means and variances of replicated sampling units to Taylor's power law and Iwao's plot of mean crowding against mean density. Chironomus larvae were not aggregated; all other genera showed an increased aggregation with density. The Iwao plots indicated that Tanypus larvae tended to be aggregated at low as well as high densities. Gut contents were analysed in larvae of the above genera and also in Cryptochironomus larvae, to investigate the relationship between dispersion pattern and feeding. Chironomus and Polypedilum larvae fed mainly on detritus, Tanypus on algae, Procladius on algae and crustacea and Cryptochironomus on oligochaetes. The extent of overlap in feeding niche was measured by Levins' index and shown to be low in all except the pair feeding upon detritus. Differences in the feeding niche may partly account for the dispersion patterns of the genera investigated.     

On the structure and evolutionary relationships of the Cerro Cuadrado fossil conifer seedlings*

Seedlings from the Cerro Cuadrado (Jurassic) Petrified Forest in Argentina have been shown to be comparable to extant araucarian seedlings from the sections Eutacta, Bunya and Colombea. Anatomically they are compared to the cone axes and embryos in the fossil cones of Araucaria mirabilis and Pararaucaria patagonica. Two seedlings previously described by Wieland are shown on pith structure to belong to P. patagonica. The remaining turbinate to top-shaped structures are compared with first year extant Bunya and Columbia seedlings and the cone A. mirabilis. Corm-like structures may represent an older seedling stage. From earlier work on the embryonic anatomy and germination stages described here, P. patagonica is deduced to have been epigeal and A. mirabilis hypogeal.