ALBERT

Notes: The distribution of NO3− reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO3−, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 mM NH4+ and 0.5, 1.5 or 10 mM NO3−. Xylem transport of NO3− was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO3− reduction at all 3 levels of NO3− nutrition. However, the contribution of the shoots to the whole plant NO3− reduction increased with increasing external NO3− availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO3−, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO3− translocation from roots to shoots. It is proposed that the lack of NO3− export to the shoots at low NO3− uptake rate resulted from a competition between NO3− reduction in the root epidermis/cortex and NO3− diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO3− nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO3− assimilation, principally occurring in the roots, was mainly incorporated in the shoots.

Notes: The intracellular distribution of K+ and Na+ ions has been determined by compartmental analysis of isotope exchange. The simultaneous measurement of electrical potentials allowed us to show that the distribution of K+ was close to thermodynamic equilibrium while the internal concentration of Na+ was well below the value predicted for the equilibrium. The efflux of Na+ was more sensitive to temperature than its influx. Both ouabain and variations in the external levels of KCl produced weak and inconsistent effects, observations which would emphasize the difference between the Na+ extrusion mechanism of plants and animals. The Na+ extrusion system of Acer cells ceased to be functional in Na+-depleted cells but recovered its function if the cells were placed in 10 mM NaCl, which suggests that the extrusion system was induced by the level of internal Na+ ions.

Notes: Summary The purified (H+ATPase from corn roots plasma membrane inserted spontaneously into preformed bilayer from soybean lipids. The yield of the protein insertion, as measured from its H+-pumping activity, increased as a function of lipids and protein concentrations. In optimum conditions, all the (H+)ATPase molecules were closely associated with liposomes, exhibiting a high H+-pumping activity (150,000% quenching· min−1·mg−1 protein of the probe 9-amino-6-chloro-2-methoxyacridine). The insertion was achieved within a few seconds. No latency of the (H+)ATPase hydrolytic activity was revealed when lysophosphatidylcholine was added to permeabilize the vesicles. This indicated that the (H+)ATPase molecules inserted unidirectionally, the catalytic sites being exposed outside the vesicles (“inside-out” orientation), and thus freely accessible to Mg-ATP. The nondelipidated (H+)ATPase could also functionally insert into bilayer from PC∶PE∶PG or PC∶PE∶PI, due to the presence of both hydrophobic defects promoted by PE, and negative phospholipids specifically required by the (H+)ATPase from corn roots. The detergent octylglucoside facilitated the delipidated (H+)ATPase reinsertion probably by promoting both a proper protein conformation and hydrophobic defects in the bilayer. Lysophosphatidylcholine facilitated the delipidated protein insertion only when hydrophobic defects were already present, and thus seemed only capable to ensure a proper protein conformation

Notes: Abstract The effect of HCO 3 - on ion absorption by young corn roots was studied in conditions allowing the independent control of both the pH of uptake solution and the CO2 partial pressure in air bubbled through the solution. The surface pH shift in the vicinity of the outer surface of the plasmalemma induced by active H+ excretion was estimated using the initial uptake rate of acetic acid as a pH probe (Sentenac and Grignon (1987) Plant Physiol. 84, 1367). Acetic acid and orthophosphate uptake rates and NO 3 - accumulation were slowed down, while 86Rb+ uptake and K+ accumulation rates were increased by HCO 3 - . These effects were similar to those induced by 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid/2-amino-2-(hydroxymethyl)-1,3-propanediol (Hepes-Tris). They were more pronounced when the H+ excretion was strong, were rapidly reversible and were not additive to those of Hepes-Tris. The hypothesis is advanced that the buffering system CO2/H2CO3/HCO 3 - accelerated the diffusion of equivalent H+ inside the cell wall towards the medium. This attenuated the surface pH shift in the vicinity the plasma membrane and affected the coupling between the proton pump and cotransport systems.

Notes: Abstract The characteristics of sulphate uptake into right-side-out plasma-membrane vesicles isolated from roots of Brassica napus L., Metzger, cv. Drakkar, and purified by aqueous polymer two-phase partitioning, were investigated. Sulphate uptake into the vesicles was driven by an artificially imposed pH gradient (acid outside), and could be observed for 5–10 min before a plateau was reached and no further net uptake occurred. The uptake was partially inhibited in the presence of depolarizing agents and little uptake was observed in the absence of an imposed pH gradient. Uptake was strongly pH-dependent, being greatest at more acidic pH. After imposition of a pH gradient, the capacity for uptake decreased slowly (t1/2〉10 min). The uptake had a high-affinity component which was strongly dependent on the external proton concentration (K m=10μM at pH 5.0, 64 μM at pH 6.5). The K m for protons varied from 0.4–1.9 μM as the sulphate concentration was reduced from 33 to 1 μM. A low-affinity component was observed which could be resolved at low temperatures (0 °C). Microsomal membranes that partitioned into the lower phase of the two-phase system gave no indication of high-affinity sulphate transport. Sulphate uptake into plasma-membrane vesicles isolated from sulphur-starved plant material was approximately twofold greater than that observed in those isolated from sulphate-fed plant material. Isolated vesicles therefore mirror the well-known in-vivo response of roots, indicating an increase in the number of transporters to be, at least in part, the underlying cause of derepression.

Notes: Abstract The uptake of sulphate into roots of barley seedlings is highly sensitive to phenylglyoxal (PhG), an arginine-binding reagent. Uptake was inhibited by 〉80% by a 1-h pre-treatment of roots with 0.45 mol · m−3 PhG. Inhibition was maximal in pre-treatment solutions buffered between pH 4.5 and 6.5. Phosphate uptake, measured simultaneously by double-labelling uptake solutions with 32P and 35S, was less susceptible to inhibition by PhG, particularly at pH 〈6.5, and was completely insensitive to the less permeant reagent p-hydroxyphenylglyoxal (OH-PhG) administered at 1 mol · m−3 at pH at 5.0 or 8.2; sulphate uptake was inhibited in -S plants by 90% by OH-PhG-treatment. Root respiration in young root segments was unaffected by OH-PhG pre-treatment for 1 h and inhibited by only 17% after 90 min pre-treatment. The uptake of both ions was inhibited by the dithiol-specific reagent, phenylarsine oxide even after short exposures (0.5–5.0 min). Sulphate uptake was more severely inhibited than that of phosphate, but in both cases inhibition could be substantially reversed by 5 min washing of treated roots by 5 mol · m−3 dithioerythritol. After longer pre-treatment (50 min) with phenylarsine oxide, inhibition of the ion fluxes was not relieved by washing with dithioerythritol. Inhibition of sulphate influx by PhG was completely reversed by washing the roots for 24 h with culture solution lacking the inhibitor. The reversal was dependent on protein synthesis; less than 20% recovery was seen in the presence of 50 mmol · m−3 cycloheximide. Sulphate uptake declined rapidly when -S roots were treated with cycloheximide. In the same roots the phosphate influx was little affected, small significant inhibitions being seen only after 4 h of treatment. Respiration was depressed by only 20% in apical and by 31% in basal root segments by cycloheximide pre-treatment for 2 h. Similar rates of collapse of the sulphate uptake and insensitivity of phosphate uptake were seen when protein synthesis was inhibited by azetidine carboxylic acid, p-fluorophenylalanine and puromycin. Considering the effects of all of the protein-synthesis inhibitors together leads to the conclusion that the sulphate transporter itself, or some essential sub-component of the uptake system, turns over rapidly with a half-time of about 2.5 h. The turnover of the phosphate transporter is evidently much slower. The results are discussed in relation to strategies for identifying the transport proteins and to the regulation of transporter activity during nutrient stress.

Notes: Abstract We have studied the mechanism of the response to iron deficiency in rape (Brassica napus L.), taking into account our previous results: net H+ extrusion maintains a pH shift between the root apoplast and the solution, and the magnitude of the pH shift decreases as the buffering power in the solution increases. The ferric stress increased the ability of roots to reduce Fe[III]EDTA. Buffering the bulk solution (without change in pH) inhibited Fe[III]EDTA reduction. At constant bulk pH, the inhibition (ratio of the Fe[III]EDTA-reduction rates measured in the presence and in the absence of buffer) increased with the rate of H+ extrusion (modulated by the length of a pretreatment in 0.2 mM CaSO4). These results support the hypothesis that the apoplastic pH shift caused by H+ excretion stimulated Fe[III] reduction. The shape of the curves describing the pH-dependency of Fe[III]EDTA reduction in the presence and in the absence of a buffer fitted this hypothesis. When compared to the titration curves of Fe[III]citrate and of Fe[III]EDTA, the curves describing the dependency of the reduction rate of these chelates on pH indicated that the stimulation of Fe[III] reduction by the apoplastic pH shift due to H+ excretion could result from changes in electrostatic interactions between the chelates and the fixed chargers of the cell wall and-or plasmalemma. Blocking H+ excretion by vanadate resulted in complete inhibiton of Fe[III] reduction, even in an acidic medium in which there was neither a pH shift nor an inhibitory effect of a buffer. This indicates that the apoplastic pH shift resulting from H+ pumping is not the only mechanism which is involved in the coupling of Fe[III] reduction to H+ transport. Our results shed light on the way by which the strong buffering effect of HCO 3 - in some soils may be involved in iron deficiency encountered by some of the plants which grow in them.

Notes: Abstract The article presents the final conclusions of a series of studies (secondary analyses of statistical data and investigations) carried out by the Centre de Sociologie Européenne between 1964 and 1967 and re-evaluated in the light of the new trends which emerged after 1968 (more open system of education offset by diversification of schemes of study with unequal returns in terms of career openings or prestige). In order to present the problem of the democratisation of higher education by reference to unambiguous data, the authors propose a simple principle for calculating the opportunities for entry to a given level of instruction, or into a given field, as a function of social origin and sex. This systematic method of measuring inequality of academic opportunity, linked to sociological variables, has (a) technical (b) methodological and (c) theoretical advantages. o li](a) From the technical point of view, this calculation which relates the number of students (of a given level) possessing certain social characteristics with the cohort of young people of the same age possessing the same social characteristics, makes it possible to eliminate the ambiguities and statistical fallacies of most analyses of the democratisation of education, which are confined to a comparison of the percentages (or the change over time of the percentages) of different social categories represented within the student population, without taking account of the numerical representation (and the change over time of the numerical representation) of these same categories in the active population. li](b) From the methodological point of view, one has thus at one's disposal an indisputable method of presenting unambiguous conclusions with regard to the chronological development of academic inequalities for a given country, or with regard to international comparisons. li](c) From the theoretical viewpoint, this method translates into statistical terms a principle of sociological theory of much more general application and one which is exemplified in the article; this principle, which might be termed structuralist, states that no value (quantitative or qualitative) should be appraised in itself and for itself, rather that it must always be evaluated in its operational or transformational context by taking into account its positional value in the whole system of relations which link it to other values. By applying this principle, it becomes clear that the sciences are not, in France, more democratic than other fields of study, as is asserted by some authors who are content to compare the percentage of working-class students in the science faculties with the percentage achieved in the other faculties, without relating this rate to the extremely low one for working-class students in the Grandes Écoles scientifiques (the École Polytechnique, etc.): in fact, making such a comparison would highlight the weak positional value of scientific subjects in universities in the total system of scientific institutions of higher education and it would reveal the function of relegation (or reduced acceptance) of children from lower-class families assumed by the science faculties in a system dominated by the Grandes Écoles. On a more general level, this principle of relational intelligibility guides the authors' interpretation of opportunity for entry to higher education in France from 1962 to 1966. During this period, the structure of academic inequalities which in 1962 ranged from one chance in 100 (for the least favoured category) to 50 chances in 100 (for the most highly favoured category) has moved upwards, since, in 1966, the range of opportunities extends roughly from 2% to more than 70%. In other words, the total increase in the rate of enrolment in higher education of 18 to 21 year olds has been distributed, grosso modo, between the different social categories in proportion to existing inequalities. Despite a slight statistical narrowing of this range, the authors' psychosociological analysis attempts to show that the general system of expectations and attitudes of different social classes vis-à-vis the University cannot have been modified by a change which amounts to an upward translation of the structure of inequalities. It is therefore concluded that the policy of increasing enrolment during this period had an almost zero effect (sociologically speaking) on the phenomenon of the democratisation of the university, defined as a process equalising the academic opportunities of different social categories. Again it is the same principle, applied to the analysis of the relative scarcity of academic qualifications, which makes it possible to demonstrate that the expansion of higher education devalues degrees (both in the job market and in the symbolic prestige market) as it “democratises” them, thus reconstructing again at a higher level, or in fields of study yielding high returns, the academic privileges of the favoured classes.