The Physiology of Flowering Plants - KHAM PHA MOI

82 WATER RELATIONSthat xylem sap is quite rich in solutes, contrary to the generallyaccepted view that it is extremely dilute (except in early spring,before transpiration commences). Another idea (Canny 1995) isthat, whilst transpiration provides the driving force, there is nobuilding up of large tensions because the living turgid cells of thexylem exert a positive pressure on the vessels and tracheids, as doesthe phloem, where cells are under a high positive pressure.In defence of the cohesion–tension theoryThe majority of plant physiologists nevertheless support the cohesion–tensiontheory. The new postulates of Zimmermann and Cannyrely on tension measurements from one single type of instrument,the pressure probe, whereas evidence for strong tensions has beenobtained with several methods, with consistent results. The discrepancymay be due to problems with handling the pressure probe(Milburn 1996). Its insertion could cause cracks in the xylem cellwalls, letting air in and causing cavitation. The probe diameter of10 mm is not negligible compared with the diameters of the xylemvessels probed, 50–90mm; the insertion of the probe could do appreciabledamage. With living cells, the plastic cell contents pressingagainst a pierced wall could seal up cracks, but in the xylem conductingcells there is nothing to protect against air entry. Claims for ahigh solute content could derive from the probe sampling preferentiallythe young xylem (which is outermost), where immature cellsmight still contain solutes derived from the breakdown of cell contents(Milburn 1996); such cells might also be under more positivepressures. Later experiments by other workers have succeeded inmeasuring stronger tensions in the xylem with the probe, down to–1 MPa, and have shown agreement between pressure-bomb andpressure-probe data. The osmotic theory implies a high energyexpenditure by the living xylem parenchyma cells for pumpingsolutes into the xylem, but the low O 2 level in the xylem of woodyaxes precludes high aerobic respiration rates and the limited volumeof living cells would moreover have to control a large volume of deadconducting cells (Richter 1997). There would also have to be amechanism for recycling the solutes. Mercury is pulled up by aleafy, transpiring twig inserted into the top of a water-filled glasstubing, which has its bottom end in a reservoir of mercury. The twigsucks the mercury into the xylem and right into very narrow taperingcell tips, which would require a tension of –2 MPa, and there is ofcourse no question of osmotic forces acting on the mercury. Strongpositive pressures, postulated to be exerted by living xylem parenchymaand by the living phloem, would probably have little effect onthe conducting cells with their rigid walls.At present, the bulk of the evidence seems to be in favour of thecohesion–tension theory; but then one has to account for the factthat airlocks can be introduced into the xylem in nature by cavitationowing to water stress, freezing, mechanical damage – or experimentally,without permanently or even temporarily stopping the overall