This study dealt with the visualization of the sieve element (SE) cytoskeleton and its involvement in electrical responses to local cold shocks, exemplifying the role of the cytoskeleton in Ca 2+ -triggered signal cascades in SEs. High-affinity fluorescent phalloidin as well as immunocytochemistry using anti-actin antibodies demonstrated a fully developed parietal actin meshwork in SEs. The involvement of the cytoskeleton in electrical responses and forisome conformation changes as indicators of Ca 2+ influx was investigated by the application of cold shocks in the presence of diverse actin disruptors (latrunculin A and cytochalasin D). Under control conditions, cold shocks elicited a graded initial voltage transient, DV 1 , reduced by external La 3+ in keeping with the involvement of Ca 2+ channels, and a second voltage transient, DV 2 . Cytochalasin D had no effect on DV 1 , while DV 1 was significantly reduced with 500 nM latrunculin A. Forisome dispersion was triggered by cold shocks of 4°C or greater, which was indicative of an all-or-none behavior. Forisome dispersion was suppressed by incubation with latrunculin A. In conclusion, the cytoskeleton controls cold shock-induced Ca 2+ influx into SEs, leading to forisome dispersion and sieve plate occlusion in fava bean (Vicia faba).

Phloem transport stops transiently within dicot stems that are cooled rapidly, but the cause remains unknown. Now it is known that (1) rapid cooling depolarizes cell membranes giving a transient increase in cytoplasmic Ca 2+, and (2) a rise of free calcium triggers dispersion of forisomes, which then occlude sieve elements (SEs) of fabacean plants. Therefore, we compared the effects of rapid chilling on SE electrophysiology, phloem transport and forisomes in Vicia faba. Forisomes dispersed after rapid cooling with a delay that was longer for slower cooling rates. Phloem transport stopped about 20 s after forisome dispersion, and then transport resumed and forisomes re-condensed within similar time frames. Transport interruption and forisome dispersion showed parallel behaviour -a cooling ratedependent response, transience and desensitization. Chilling induced both a fast and a slow depolarization of SE membranes, the electrical signature suggesting strongly that the cause of forisome dispersion was the transient promotion of SE free calcium. This apparent block of SEs by dispersed forisomes may be assisted by other Ca 2+ -dependent sealing proteins that are present in all dicots.

Summary1. This account presents information on all aspects of the biology of Rosa spinosissima L. (R. pimpinellifolia L.) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation. 2. Rosa spinosissima is a small, deciduous shrub forming clonal patches from root suckers. In Britain it is common on stabilized coastal dunes, more or less base-rich heaths, and on open, dry habitats on chalk or limestone inland. It is naturally distributed across temperate Europe and western and central Asia. 3. The small leaves and leaflets, in combination with many straight or slightly curved bristles and prickles, and the black hips, make R. spinosissima easily recognizable in Britain. It normally does not exceed 10-40 cm but cultivars often grow taller. 4. The seeds are dispersed by birds and other animals, particularly as the hips are a favoured diet of birds, because of the high vitamin and anthocyanin content. Vegetative spread is frequent through the root suckers. 5. Cultivars are commonly planted for amenity and in gardens. The origin of the cultivars is not known, but genetic exchange of these with natural British populations may lead to introgression with genotypes which are not locally adapted. The invasive Rosa rugosa may be a threat in some coastal habitats as this species is displacing R. spinosissima on the German North Sea coast.

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