Photosynthetic utilization of HCO, in leaves of Pot(tmogeton and Elodea occurs at the lower leaf side, with subsequent OH~ release at the upper side. It is accompanied by transport of cations, in the present experiment K^, across the leaf. The resulting pH and K"^ concentration changes near the leaf surface were recorded with miniature electrodes. From the pH and K^ concentration the concentrations of the different inorganic carbon species were calculated and compared with photosynthetic O2 production, HCO3' utilization is accompanied by a drastic increase in the free CO2 concentration near the lower epidermis. Experiments with CO2-and HCO3"-free solutions showed an oscillating acidification near the lower epidermis and alkalinization near the upper epidermis. It is concluded that the acidification results from the activity of light-dependent H"^ pumps. The finding that an increase in pH at the upper side always coincided with a decrease at the lower in these experiments shows that the H ^ pumps and the OH " extruding mechanism are coupled although occurring in different cell layers. Previously we have suggested that the first step in the process of photosynthetic HCO.^ utilization is external conversion of HCO3" by acidification caused by light-dependent H^ pumps. The present results strongly support this hypothesis. Two possible pathways for the accompanying Kt ransport are discussed. The model presented here explains the known inhibiting effects of buffers and high pH on photosynthetic HCO^ utilization.

The utiiLzation of HC03-as carbon source for photosynthesis by aquatic angiosperms results in the production of 1 mole OH-for each mole CO2 assimilated. The OH-ions are subsequently released to the medium. In several Potamogeton and Elodea species, the site of the HC03-influx and OH-efflux are spatially separated. Described here are light-and darkinduced pH changes at the lower and upper epidermis of the leaves of Potamogeton lucens, Elodea densa, and Elodea canadensis.In the light, two phases could be discerned. During the first phase, the pH increased at both sides of the leaves. geton and Elodea are accompanied by cation transport from the lower to the upper side of the leaf and by the formation of an electrical PD across the leaf, making the upper side negative with respect to the lower side (5, 6, 7). Comparable PD were also observed in Chara corallina cells. In these latter cells, electric currents are generated between the alkaline and acid regions and may result in electrical PD of 7 mv between the centers of the bands (4, 18, 28). The highest value we observed so far with a Potamogeton lucens leaf was 40 mv, upper side negative.We analyzed the time course of the light induced pH changes at both sides and the trans-leaf PD of leaves of Potamogeton and Elodea with miniature pH and reference electrodes positioned against the leaf surface.For comparison, we studied the pH changes at the leaf surface of Vallisneria spiralis, a HCO3 -fixing aquatic species with no separation of the HCO3 and OH-transport sites, and of Ludwigia natans, a waterplant that used CO2 only as a C source for photosynthesis. MATERIALS AND METHODSP. lucens and Elodea canadensis were grown in concrete tanks outside the building, whereas Elodea densa, V. spiralis and L. natans were grown inside under artificial light as described earlier (21). All plants were cultivated in H20 at pH 7.8 to 9.5.The pH near the surface on both sides of the leaves was measured simultaneously with two miniature pH electrodes (Microelectrodes Ml 440) with the sensing bulb touching gently on the leaf surface. Two reference electrodes (Microelectrodes Ml 401) filled with 3 M KCI were also positioned with their tips very near the leaf surface on both sides. These reference electrodes were also used to measure the electrical PD across the leaf (Fig. 1). The pH-sensitive part of the glass electrode is a hemisphere of radius 0.75 mm and the wall is not of uniform thickness. The end which touches the leaf surface is much thinner and, thus, is the most sensitive part. To enlarge the contact between the sensing bulb and the leaf surface, we folded the leaf slightly around the bulbs (Fig. 1B)

The effect of oxygen on the trans-root potential (TRP) of excised roots in Plantago media L. and P. maritima L. was investigated. Two distinct reactions were found. In some experiments (type A roots) the reaction of TRP to anoxia was bi-phasic, and this reaction fits well into a model, assuming the presence of two spatially separated proton pump sites in the roots: one at the plasmalemma of epidermal and cortical cells and the other at the symplast/xylem interface. The two pumps work in opposite directions. In other experiments (type B roots) no hyperpolarization as a response to anoxia at the inner symplast membrane was observed. There is evidence that the inner pump is also present in these roots, but only in an inactive or electroneutral state. It is concluded that O2-deficiency prevails more often in the central part of the root than in epidermal and cortical cells, when roots are brought gradually under anoxia. This causes the pump located at the symplast/xylem interface to be inhibited more quickly than the other at decreasing O2-concentrations in the bathing solution.

A common language, or terminology, for representing what clinicians have said and done is an important requirement for individual clinical systems, and it is a pre-requisite for integrating disparate applications in a distributed telematic healthcare environment. Formal representations based on description logics or closely related formalisms are increasingly used for representing medical terminologies. GALEN's experience in using one such formalism raises two major issues, as follows: how to make ontologies based on description logics easy to use and understand for both clinicians and applications developers; what features are required of the ontology and description logic if they are to achieve their aims. Based on our experience we put forward four contentions: two relating to each of these two issues, as follows: that natural language generation is essential to make a description logic based ontology accessible to users; that the description logic based ontology should be treated as an "assembly language" and accessed via "intermediate representations" oriented to users and "perspectives" adapting it to specific applications; that independence and reuse are best supported by partitioning the subsumption hierarchy of elementary concepts into orthogonal taxonomies, each of which forms a pure tree in which the branches at each level are disjoint but nonexhaustive subconcepts of the parent concept; that the expressivity of the description logic must include support for transitive relations despite the computational cost, and that this computational cost is acceptable in practice. The authors argue that these features will be necessary, though by no means sufficient, for the development of any large reusable ontology for medicine.

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