Sap ascent in vascular plants: Challengers to the cohesion theory ignore the significance of immature xylem and the recycling of Munch water

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. 0.7 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. 2.5 MPa. Also an independent method based on mercury penetrometry provides evidence that sap tensions of at least 2.0 MPa can be demonstrated directly implying, that serious limitations arise from the pressure probe method itself. Without tensions exceeding 2.0 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 ('Munch 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. (C) 1996 Annals of Botany Company