Summary The Cohesion Theory considers plant xylem as a ‘vulnerable pipeline’ isolated from the osmotically connected tissue cells, phloem and mycorrhizas living in symbiosis with plant roots. It is believed that water is pulled exclusively by transpiration‐induced negative pressure gradients of several megapascals through continuous water columns from the roots to the foliage. Water under such negative pressures is extremely unstable, particularly given the hydrophobicity of the inner xylem walls and sap composition (lipids, proteins, mucopolysaccharides, etc.) that prevents the development of stable negative pressures larger than about −1 MPa. However, many plant physiologists still view the Cohesion Theory as the absolute and universal truth because clever wording from the proponents of this theory has concealed the recent breakdown of the Scholander pressure bomb (and other indirect methods) as qualified tools for measuring negative pressures in transpiring plants. Here we show that the arguments of the proponents of the Cohesion Theory are completely misleading. We further present an enormous bulk of evidence supporting the view that – depending on the species and ecophysiological context – many other forces, additional to low tensions, can be involved in water ascent and that water can be lifted by a series of watergates (like ships in staircase locks).

The concept of encapsulated-cell therapy is very appealing, but in practice a great deal of technology and know-how is needed for the production of long-term functional transplants. Alginate is one of the most promising biomaterials for immunoisolation of allogeneic and xenogeneic cells and tissues (such as Langerhans islets). Although great advances in alginate-based cell encapsulation have been reported, several improvements need to be made before routine clinical applications can be considered. Among these is the production of purified alginates with consistently high transplantation-grade quality. This depends to a great extent on the purity of the input algal source as well as on the development of alginate extraction and purification processes that can be validated. A key engineering challenge in designing immunoisolating alginate-based microcapsules is that of maintaining unimpeded exchange of nutrients, oxygen and therapeutic factors (released by the encapsulated cells), while simultaneously avoiding swelling and subsequent rupture of the microcapsules. This requires the development of efficient, validated and well-documented technology for cross-linking alginates with divalent cations. Clinical applications also require validated technology for long-term cryopreservation of encapsulated cells to maintaining a product inventory in order to meet end-user demands. As shown here these demands could be met by the development of novel, validated technologies for production of transplantation-grade alginate and microcapsule engineering and storage. The advances in alginate-based therapy are demonstrated by transplantation of encapsulated rat and human islet grafts that functioned properly for about 1 year in diabetic mice.

SUMMARYControl of ion loading into the xylem has been repeatedly named as a crucial factor determining plant salt tolerance. In this study we further investigate this issue by applying a range of biophysical [the microelectrode ion flux measurement (MIFE) technique for non-invasive ion flux measurements, the patch clamp technique, membrane potential measurements] and physiological (xylem sap and tissue nutrient analysis, photosynthetic characteristics, stomatal conductance) techniques to barley varieties contrasting in their salt tolerance. We report that restricting Na + loading into the xylem is not essential for conferring salinity tolerance in barley, with tolerant varieties showing xylem Na + concentrations at least as high as those of sensitive ones. At the same time, tolerant genotypes are capable of maintaining higher xylem K + /Na + ratios and efficiently sequester the accumulated Na + in leaves. The former is achieved by more efficient loading of K + into the xylem. We argue that the observed increases in xylem K + and Na + concentrations in tolerant genotypes are required for efficient osmotic adjustment, needed to support leaf expansion growth. We also provide evidence that K + -permeable voltage-sensitive channels are involved in xylem loading and operate in a feedback manner to maintain a constant K + /Na + ratio in the xylem sap.

Xylem parenchyma cells (XPCs) control the composition of the transpiration stream in plants and are thought to play a role i n long-distance signaling as well. We addressed the regulation, selectivity, and dependence on the apoplastic ion concentrations of two types of outward rectifiers i n the plasma membrane of XPCs, to asses the physiological role of these conductances. In whole-cell recordings, the membrane conductance at depolarization was under the control of cytosolic Ca2+: at physiological Ca2+ levels (1 50 nM) the K+ outward-rectifying conductance (KORC) predominated, whereas at elevated Caz+ levels (5 p~) , only the nonselective outward-rectifying conductance (NORC) was active. No such regulatory effect of CaZ+ was observed i n inside-out experiments. l h e voltage dependence of whole-cell KORC currents strongly depended on apoplastic K+ concentration: an increase in apoplastic K+ resulted in a positive shift of the current-voltage curve, roughly following the shift in Nernst potential of K+. KORC is impermeable to Na+, but does translocate Ca2+ in addition to K+. In contrast to KORC, NORC selected poorly among monovalent cations and anions, the relative permeability P,+/P,-being about 1.9. Cating of NORC was largely unaffected by the leve1 of K+ in the bath. Under ali ionic conditions tested, NORC tail currents or single-channel currents reversed close to O mV. Using an in vivo xylem-perfusion technique, tetraethylammonium (an inhibitor of KORC) was shown to block K+ transport to the shoot. These data support the hypothesis that release of K+ to the xylem sap is mediated by KORC. The molecular properties of these two conductances are discussed in the light of the distinct physiological role of XPCs.XPCs, the cells surrounding the xylem vessels, are thought to play a key role in salt transport, long-distance signaling, and the ascent of the transpiration stream. In the root, in particular, these cells release mineral nutrients to the xylem that conducts the transpiration stream, forming the main pathway for long-distance transport of salts from the root to the shoot (Clarkson, 1993). Only recently, interest has focused on two other aspects of xylem transport. Based on direct measurements of the pressure relations, the cohesion theory of water movement in the xylem has been This work was supported by the Studienstiftung des Deutschen Volkes (L.H.W.).

To identify mechanisms for the simultaneous release of anions and cations into the xylem sap in roots, we investigated voltagedependent ion conductances in the plasmalemma of xylem parenchyma cells. We applied the patch-clamp technique to protoplasts isolated from the xylem parenchyma by differential enzymic digestion of steles of barley roots (Hordeum vulgare 1. cv Apex). In the whole-cell configuration, three types of cation-selective rectifiers could be identified: (a) one activated at membrane potentials above about -50 mV; (b) a second type of outward current appeared at membrane potentials above +20 to +40 mV; (c) below a membrane potential of approximately -1 10 mV, an inward rectifier could be distinguished. In addition, an anion-specific conductance manifested itself in single-channel activity in a voltage range extending from about -100 to +30 mV, with remarkably slow gating. In excised patches, K+ channels activated at hyperpolarization as well as at depolarization. We suggest that salt is released from the xylem parenchyma into the xylem apoplast by simultaneous flow of cations and anions through channels, following electrochemical gradients set up by the ion uptake processes in the cortex and, possibly, the release and reabsorption of ions on their way to the xylem.