Summary. Uptake of water and magnesium chloride solution was investigated through the outer surface of twigs of Picea abies (L.) Karst. Water uptake was determined by using pressure/volume (P/V) curves of the twigs as a basis for calculation to avoid problems of superficial extraneous water. When water was sprayed on bark and needles of 3-to 7-year-old twigs at a xylem water potential of -1.00 MPa, they absorbed as much as 80 mm 3 water in 200 min/g twig dry weight as the twig water potential recovered to -0.15 MPa. With fluorescent dyes, pathways for absorption of water and solutes through the twig bark were found, particularly through the radially orientated ray tissue. In addition to uptake by mass flow, magnesium could also diffuse along a concentration gradient from the twig surface into the xylem. In the field, the magnitude of these uptake processes would depend on the concentration of elements deposited by atmospheric precipitation, the concentration gradient between the plant surface and the xylem sap, the xylem water potential and the intensity and duration of each precipitation event.

In recent years the cohesion theory has been attacked on the grounds that direct measurements made with the pressure probe indicate that sap tensions are much less (maximum tension approx. 0n7 MPa) than indicated by parallel measurements made with the more conventional methods : osmotic methods, pressure bomb, or psychrometer. It has also been claimed that other direct methods do not support the cohesion theory. Thus a re-examination using the Renner technique indicated sap tensions of approx. 2n5 MPa. Also an independent method based on mercury penetrometry provides evidence that sap tensions of at least 2n0 MPa can be demonstrated directly implying, that serious limitations arise from the pressure probe method itself. Without tensions exceeding 2n0 MPa mangroves would be unable to extract fresh water for transpiration from seawater. It is suggested that the pressure probe is susceptible to bias because it investigates the least mature xylem conduits while they are still under varying degrees of turgor pressure and only partially interconnected with the main xylem system. This supposition is supported by claims that the xylem sap sampled by the probe contains significant concentrations of solutes. Additionally water, supplied by reverse osmosis from the sieve tubes (' Mu$ nch water '), is continually being liberated in the vicinity of the outermost xylem vessels hydrating them to an atypical degree which can explain several of the discrepancies claimed. These results, which are supported by the work of others, demonstrate that the challenges to the cohesion theory for the ascent of sap are ill-founded. The release of water from the phloem can explain not only some discrepancies claimed by the cohesion challengers, but also explain the refilling of cavitated xylem conduits : a hitherto unsuspected role for the phloem transport system.

Most myeloproliferative neoplasm (MPN) patients lacking JAK2 mutations harbour somatic CALR mutations that are thought to activate cytokine signalling although the mechanism is unclear. To identify kinases important for survival of CALR-mutant cells, we developed a novel strategy (KISMET) that utilizes the full range of kinase selectivity data available from each inhibitor and thus takes advantage of off-target noise that limits conventional small-interfering RNA or inhibitor screens. KISMET successfully identified known essential kinases in haematopoietic and non-haematopoietic cell lines and identified the mitogen activated protein kinase (MAPK) pathway as required for growth of the CALR-mutated MARIMO cells. Expression of mutant CALR in murine or human haematopoietic cell lines was accompanied by myeloproliferative leukemia protein (MPL)-dependent activation of MAPK signalling, and MPN patients with CALR mutations showed increased MAPK activity in CD34 cells, platelets and megakaryocytes. Although CALR mutations resulted in protein instability and proteosomal degradation, mutant CALR was able to enhance megakaryopoiesis and pro-platelet production from human CD34+ progenitors. These data link aberrant MAPK activation to the MPN phenotype and identify it as a potential therapeutic target in CALR-mutant positive MPNs.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K(+):Na(+) ratio and low levels of Ca(2+) and free H(+). Sucrose and K(+) (together with its associated anions) accounted for 75% of the phloem-sap solute potential (Ψs). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K(+) levels. Measurements of phloem-sap Ψs and xylem water potential (Ψ) indicated that this resulted in a partial maintenance of phloem turgor pressure Ψp. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K(+) levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H(+) co-transport mechanism of phloem loading.