Effect of Reduction of Sink Strength in Developing Seeds on Vein Loading Patterns of Photoassimilate in Source Leaves of Pisum sativum L

The effect of sink strength reduction in developing seeds onvein loading of photoassimilate has been studied in Pisum sativumL. The sink strength was manipulated by means of the ‘openseed coat technique’. Sink strength of the operated ovuleswas controlled by the osmolality of a substitute medium replacingthe embryo. A high osmolality of the medium (400 mM mannitol)or a low osmolality of the medium (without mannitol) was usedto maintain a high or low sink strength, respectively. To studythe effect of sink strength reduction on vein loading, macro-autoradiographywas used. After applying ¹⁴CO₂ to the source leaf for 10 to20min autoradiographs of plants with different sink strengthshowed differences in distribution of ¹⁴C-photoassimilate overthe mesophyll and the veins. Under low sink strength conditionsvein loading of ¹⁴C-photoassimilate was reduced. After longertime spans (40 to 60 min) no clear differences in vein loadingwere visible in the autoradiographs. However, measurement of¹⁴C in plant parts along the path of transport from source tosink showed a decrease of the rate of export of ¹⁴C from thesource leaf under low sink strength conditions. Apparently,accumulation of ¹⁴C-photoassimilate into the phloem and exporttowards sink regions can be reduced by lowering the sink strength.A signal must have been transferred from sink all the way tothe source regions. The mechanism of such a signal is discussed. Key words: Pisum sativum, phloem loading, photoassimilate transport, seed development, sink-source interactions     

The restricted distribution of potato leafroll luteovirus antigen in potato plants with transgenic resistance resembles that in clones with one type of host genemediated resistance

The distribution of virus-infected cells was examined, by fluorescence microscopy, within plants of a range of potato clones infected with potato leafroll luteovirus (PLRV). This range included nine PLRV-resistant clones, of which four were transgenic lines carrying the PLRV coat protein gene and five were conventionally bred. Plants of these clones were resistant to PLRV multiplication and accumulated less virus antigen in leaf tissue than did susceptible clones. Indirect fluorescent antibody staining of thin sections from carbodiimide-fixed petiole tissue revealed that in plants of PLRV-susceptible clones, virus-infected cells were abundant within both external (abaxial) and internal (adaxial) phloem bundles. In plants of the PLRV-resistant conventionally bred clones and in resistant transgenic lines of cv. Pentland Squire, virus-infected cells were much fewer in number and largely restricted to internal phloem bundles. In resistant transgenic lines of cv. Désirée, this restricted distribution of PLRV antigen was only detected in petioles of young leaves. The results suggest that the transgenic and a host-mediated type of resistance that restricts virtis multiplication have underlying similarities.     

Spatial and temporal response of stream bacteria to sources of dissolved organic carbon in a blackwater stream system

1. We hypothesized that changes in bacterial colony growth would be correlated to shifts in riparian vegetation (via leachate quality) along a river continuum of a south-eastern, blackwater stream (U.S.A.). Spatially, we expected bacterial assemblages from downstream reaches to utilize more sources of leachate and at higher concentrations than bacteria collected from headwater reaches. Temporally, we predicted higher colony growth on leachate from autumn-shed (senescent) leaves compared with leachate from fresh, green leaves.
2. We examined spatial differences in assemblage growth by culturing bacteria sampled along the stream continuum on gradient plates using leachates from four common riparian species ( Taxodium distichum , Carya spp., Acer rubrum and Decumaria barbara ). Bacteria from the lowest site were able to use all sources provided and at all concentrations, whereas bacteria from upper reaches could not. Colony density was correlated to relative leachate concentration at all sites along the continuum.
3. Leachates from fresh and senescent A. rubrum leaves were used to determine temporal differences. Winter assemblages of bacteria could not grow on fresh leaf leachate at any concentration but grew well on autumn leaf leachate at higher concentrations. Differential response of bacterial assemblages indicated local adaptation to potential sources of dissolved organic matter.
4. Growth response of stream bacterial colonies appeared to be dependent on the timing and source of leachate as well as on sources of dissolved organic carbon from further upstream. Growth of bacterial assemblages exhibited 'generalist' characteristics in headwater reaches and 'specialist' characteristics at the mouth of our study stream drainage. Thus, our findings lend support to the argument that variable resource habitats favour a small, generalist assemblage, while environments with stable resource supplies allow for highly diverse assemblages dominated by specialists.     

Influence of ultraviolet-B radiation on the drift response of stream invertebrates

1. Recent studies have shown that ultraviolet (UV) radiation (280–400 nm) has increased by ≈ 8% in temperate regions over the past decade, but little effort has been devoted to understanding the ecological effects on temperate ecosystems. This research examined the effects of artificial ultraviolet-B (UVB; 280–320 nm) radiation on the drift response of immature stream insects in laboratory microcosms.
2. Two experiments involved natural populations of stream invertebrates, collected from the Cache la Poudre River (September 1994) and the Arkansas River (October 1995) in Colorado. UVB lamps were turned on from 10.00 to 14.00 h each day, and drifting animals were collected on days 1, 3, 5 and 7 during the exposure period. Levels of artificial UVB used in these experiments were similar to levels that stream organisms experience during clear, mid-day conditions at Fort Collins, Colorado (longitude 105°30'; latitude 40°35').
3. Drift was significantly higher in microcosms exposed to UVB than in controls and was dominated by Baetis sp. (Ephemeroptera: Baetidae), Trichoptera (caddisflies) and Simulium sp. (Diptera: Simuliidae). The increased drift of some stream invertebrates in UVB-exposed streams may be a behavioural response and/or a result of injury.
4. Stream organisms may be particularly sensitive to predicted increases in UV radiation, because streams are generally shallow with clear water. As a result of this potential sensitivity, we recommend that research be directed to understanding the ecological effects of UV radiation on these habitats.