Effects of synthetic plant growth retardants and abscisic acid on root functions of Brassica rapa plants exposed to low root-zone temperature

Possible interactions of two synthetic plant-growth retardants during the short-term response of Brassica rapa L. ssp. oleifera (DC.) Metzger plants to low root-zone temperature were investigated by pretreating with mefluidide or paclobutrazol. Water and solute transfers were studied by measuring xylem sap volume flow (under root pressure exudation) and ion flow from the roots. Relations with nitrate uptake rate were also considered. Root pretreatment with paclobutrazol strongly restricted the cold-inducible processes which normally restore water and solute flow from the root xylem. Paclobutrazol decreased the rates of nitrate uptake and exudation flow from the root xylem (principally by reducing root hydraulic conductivity) with dramatic consequences for ion flow, especially that of nitrate.
The effects of root ABA pretreatment on plant response to root cooling were then studied separately or in association with a pretreatment with paclobutrazol. Despite a slight decrease in nitrate uptake rate, ABA pretreatment of the roots enabled the plant to develop rapid mechanisms for adaptation to cold constraint at the root level. Moreover, this action of exogenous ABA greatly reduced the effect of a simultaneous paclobutrazol pretreatment and partly restored water and solute flows.
Thus, the improvement of plant resistance to cold conditions brought about by treatments with mefluidide and paclobutrazol (previously shown in long-term experiments) cannot simply be explained by their short-term effects.     

Taxonomic revision of the Mediterranean Plakina Schulze (Porifera, Demospongiae, Homoscleromorpha)

Only three species of the sponge genus Plakina Schulze have been described from the Mediterranean since 1880, in spite of a large amount of allegedly intraspecific variability in morphological characters. However, recent genetic studies based on electrophoretic techniques have revealed extensive cryptic speciation in north-western Mediterranean Plakina , demonstrating that most of this variation was interspecific rather than intraspecific. We describe in detail the morphology and anatomy of four new species of Plakina from the Mediterranean– P. crypta, P. weinbergi, P. endoumensis and P. jani –of which the latter two were discovered through allozyme electrophoresis. Plakina monolopha Schulze and P. trilopha Schulze are redescribed, and their morphological and geographical limits are discussed along with those of P. dilopha Schulze. Accurate analysis of the internal anatomy and of the shape and ramification pattern of lophose spicules in scanning electron microscopy provides new, powerful morphological criteria for species discrimination in Plakina. More widespread use of such new taxonomic characters should provide evidence against the alleged cosmopolitanism of some Plakina species, thus generating an increase in estimates of the biodiversity of plakinids.     

Heterologous Antiserum to Thymus-derived Cells in the Guinea-pig

USEFUL antisera specific for thymocytes and peripheral T lymphocytes have only been widely available in the form of antisera to thymic isoantigens of the mouse¹. We describe here the preparation and properties of a heterologous antiserum to guinea-pig thymocytes which is rendered specific for T lymphocytes after absorption with a pure population of B lymphocytes. We have already described^2,3 the properties of the transplantable acute lymphatic leukaemia L₂C of inbred strain two guinea-pigs. The L₂C leukaemia cell is characterized as a B cell by the presence of surface immunoglobulin of the λ₂ class, the secretion of a small amount of λ₂ immunoglobulin and the presence of a receptor for antigen-antibody-complement (C3) complexes characteristic of the B cell population⁴. Because, as will be shown, the antiserum is specific both for thymocytes and thymus derived lymphocytes, it will be referred to as anti-thymus derived cell (anti-TDC) serum. The availability of such an antiserum for a species in which the in vivo and in vitro manifestations of delayed hyper-sensitivity are so easily demonstrated may prove to be highly advantageous. illustration

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E-type Prostaglandins and Gastric Acid Secretion in the Rat

IT is known that prostaglandins of the ? series (PGEs) inhibit gastric acid secretion^1–4, but the relative potencies of prostaglandin E₁ and prostaglandin E₂ have not been evaluated. We report observations which indicate that orally administered PGE₂ has a considerably longer duration of action than an equipotent oral dose of PGE₁ in inhibiting pentagastrin-induced gastric acid secretion in the rat and that this inhibitory action appears to be due to a local action on the gastro-intestinal wall rather than to absorption of prostaglandins into the systemic circulation.     

Patterns of macroinvertebrate colonization on fresh and senescent alder leaves in two Michigan streams

SUMMARY.

• 1 Communities of invertebrates colonizing senescent autumn and fresh summer alder leaves (Alnus rugosa) were compared. Leaf packs for each treatment were placed in two hardwater streams in the Upper Peninsula of Michigan in late summer and early autumn. One stream has a cobble-bottom and the other a sand-bottom and both receive fresh leaf inputs by beaver fellings.
• 2 Fresh leaf packs remained intact after 26 days immersion, but thereafter were processed faster than were the autumn leaf packs in both streams.
• 3 In the cobble-bottom stream taxon richness (S), numbers of individuals and biomass were higher on fresh than on autumn leaves.
• 4 Fresh leaves in the sand-bottom stream supported a more diverse (H'), richer (S) and more equitably distributed (J') insect fauna than did the autumn leaves.
• 5 We discuss the simultaneous lack of fresh leaf loss and the presence of more complex insect communities on those leaves during the first 26 days of the study. Invertebrates in both mid-latitude heterotrophic streams and in tropical lowland wet forest streams may rely on fresh leaf inputs, which have received little attention.

     

Cytokinins and Morphological Aspects of French-bean Roots in the Presence of Rhizobium

Typical abnormalities were observed on French-bean roots when the plants were cultivated in the presence of Rhizobium phaseoli, under hydroponic conditions. Substantial quantities of cytokinins were detected in the plant culture medium and their concentration remained high until the 9th day after inoculation, when the first nodules appeared. The presence of both partners, plant and bacteria, was required for this synthesis. Exogenous cytokinin production was responsible for morphological alterations of the roots observed before nodulation.     

Role of Glycerol Permeation in the Bloodstream Form of Trypanosoma brucei*

SYNOPSIS. Under aerobic conditions, we have determined glycerol uptake in the long slender (LS) bloodstream form of Trypanosoma (Trypanozoon) brucei brucei by studying glycerophosphate accumulation in the parasites. The coupled enzyme theory applies to the permeation-phosphorylation sequence. Glycerol passage through the plasma membrane is asymmetric, the efflux process being favored over the influx process. No free diffusion of glycerol can be detected even under conditions under which free glycerol accumulates within the cells; most probably, glycerol permeation is mediated by a specific transport system. In the absence of respiratory activities, glycerol is known to be an end-product of T. brucei glycolysis; its production from glycerophosphate should allow ATP synthesis. The observed efflux of free glycerol following intracellular accumulation of glycerophosphate confirms the hypothesis that glycerol production occurs through reversal of glycerol kinase activity. We conclude that in vivo the role of the carrier-mediated asymmetric permeation process is to prevent inhibition of the reversal of the glycerol kinase-mediated reaction by removing free glycerol.     

Changes in Na, K, Ca, Mg and Al content of submersed leaf litter, related to ingestion by the amphipod Gammarus pulex (L.)

SUMMARY. 1. Changes in the amounts of Na, K, Ca, Mg and Al in fallen leaves of oak, alder and mat grass were measured during 13 weeks of submersion in seven streams differing in pH and ionic content.
2. In the first 2 weeks, Na, K and Mg leached from the leaves, but in the following weeks there was a net increase of Al, K and Mg and, in some instances, Na and Ca. Increases of K and Mg were relatively larger in circumneutral streams whereas the greatest increases of Al occurred in acid streams. Net gains are attributed to absorption of cations from water by saprophytic micro-organisms degrading the leaves, direct adsorption of Al on to leaf tissues - especially in acid streams - and capture of small particles of magnesium and aluminium silicates (clays).
3. Daily ingestion of elements was calculated as a percentage of whole body contents in Gammarus pulex (L.) and compared with daily loss rates. Leaf litter may be an important source of some major elements for ionic balance in juvenile G. pulex and a major source for uptake of potentially toxic aluminium in acidic, soft waters.