The effects of external NaCl on thylakoid stacking in lettuce plants

The average degree of thylakoid stacking was determined for loose-leaf lettuce plants which were grown in complete nutrient solutions containing either 10 or 100mol m^?3 NaCl. Digitonin fractionation and differential centrifugation were used to assay the level of thylakoid stacking. Based on a comparison between 10mol m^?3 NaCl-grown and 100mol m^?3 NaCl-grown lettuce plants of equal ages, digitonin assays indicated that significantly less stacking occurred in 100mol m^?3 NaCl-grown plants. Isolated thylakoid membranes from 100mol m^?3 NaCl-grown plants were also characterized by a greater capacity to absorb divalent cations and by a higher chlorophyll a/b ratio. Since plants from both growth salinities were capable of a marked increase in thylakoid stacking upon a transition from high to low irradiance, the observed differences in thylakoid stacking were not attributed to a salinity-related impairment of stacking mechanisms. Instead, the salinity-induced differences in thylakoid stacking appear to represent a process of controlled adjustment.     

Riparian vegetation diversity along regulated rivers: contribution of novel and relict habitats

1. The creation and maintenance of spatial and temporal heterogeneity by rivers flowing through floodplain landscapes has been disrupted worldwide by dams and water diversions. Large reservoirs ( novel ecosystems ) now separate and isolate remnant floodplains ( relict ecosystems ). From above, these appear as a string of beads, with beads of different sizes and string connections of varying lengths.
2. Numerous studies have documented or forecast sharp declines in riparian biodiversity in relict ecosystems downstream from dams. Concurrently, novel ecosystems containing species and communities of the former predam ecosystems have arisen along all regulated rivers. These result from the creation of new environments caused by upper reservoir sedimentation, tributary sedimentation and the formation of reservoir shorelines.
3. The contribution of novel habitats to the overall biodiversity of regulated rivers has been poorly studied. Novel ecosystems may become relatively more important in supporting riverine biodiversity if relict ecosystems are not restored to predam levels. The Missouri River of the north-central U.S.A. is used to illustrate existing conditions on a large, regulated river system with a mixture of relict and novel ecosystems.     

Stabilizing microtubules with taxol increases microfilament stability during freezing of rye root tips

We have used double fluorescence labelling to investigate the effect of freezing on microtubules and microfilaments in root-tip cells of rye (Secale cereale L. cv Rymin). Freezing to -5°C (which does not kill these cells) caused partial depolymerization of both, but microfilaments were more resistant than microtubules. When microtubules were stabilized against freeze-induced depolymerization by pre-treating seedlings with taxol, microfilaments exhibited enhanced stability as well. Almost all the frozen cells containing taxol-stabilized microtubules also contained microfilaments. When seedlings were treated with the microtubule-destabilizing drug APM prior to freezing, microfilaments became more susceptible to freeze-induced depolymerization than in controls. These data suggest a physical interaction between microtubules and microfilaments in these cells.     

The relationships among three habitat scales and stream benthic invertebrate community structure

1. The relationships between three habitat scales and lotic invertebrate species composition were investigated for the 15 540 km² Yakima River basin in south-central Washington, U.S.A.
2. The three spatial scales were sample (the sampled riffle), reach (a length of ten–twenty stream widths) and segment (a length of stream of nearly uniform slope and valley form having no change in stream order).
3. Physical variables were highly correlated between scales and expressed a relationship between altitude, basin form and small-scale physical structure.
4. Multiple discriminant function analyses indicated that segment- and reach-scale variables discriminated among species-defined groups better than sample-scale variables.
5. Species composition varied along a complex altitudinal gradient of changing basin form and resultant land use.
6. There was no clear relationship between species richness and altitude on a site basis. However, when viewed at the basin scale, maximum richness was observed at the transition between montane and valley sites.     

Propagation of lettuce (Lactuca sativa) breeding material by tissue culture

A tissue culture method using Murashige and Skoog's (MS) medium was devised to propagate healthy plants from field grown lettuce plants selected for seed production. Explants (2–3 mm long) from axillary buds were successfully grown on MS + 1.0 or 2.0 mg litre^-1 kinetin and 6.4 mg litre^-1 IAA to promote shoot growth. Concentrations of 0.5 and 4.0 mg litre^-1 kinetin gave poor shoot growth. The cultures were successfully rooted after 3–4 wk on MS + 6.4 mg litres^-1 IAA after transfer from MS + 1.0 mg litre^-1 kinetin and on MS + 4.8 mg litre^-1 IAA after transfer from MS + 2.0 mg litre^-1 kinetin. Concentrations of 3.2 and 8.0 mg litre^-1 IAA gave poor root initiation. Root initiation was more successful when cultures were grown at 40 Wm^-2 than in cultures grown at 5 Wm^-2. Rooted cultures were established in compost with a 90–95% success rate and the regenerated plants flowered c. 18 wk after the cultures were initiated.     

Studies on the seed transmission of cucumber mosaic virus in chickweed (Stellaria media) in relation to the ecology of the virus

Cucumber mosaic virus (CMV) was transmitted in the seed of infected Stellaria media plants. The rate of seed transmission varied both in manually infected plants (3–21%) and in plants grown from infected seed (21–40%). In naturally infected plants the rates of transmission found were 4–29%. Seeds recovered from field soil carried 4–5% infection and in infected seed placed in the soil the virus persisted for at least 5 months. Seed transmission of CMV also occurred in infected Lamium purpureum (4%), Cerastium holosteoides (2%) and Spergula arvensis (2%) but it could not be demonstrated in six other more common weed species in five botanical families. Seed transmission in Stellaria media occurred with a British (W) and an American (Y) strain of CMV. The virus was shown to occur in S. media pollen. The importance of CMV-infected S. media seed in the soil in relation to the epidemiology of the virus is discussed.