The Physiology of Flowering Plants - KHAM PHA MOI

150 TRANSLOCATION OF ORGANIC COMPOUNDSthe xylem, or the water might be utilized at the sink end, as notedearlier.Evidence in favour of the mass-flow hypothesisMany observations can be quoted in favour of a mass flow in thephloem. When one single sieve tube unit is pierced by an aphidstylet, the volume of sap exuded in an hour is around 1 mL, equivalentto the volume of some 2500 individual sieve tube cells; the flow cancontinue for days at this rate. On a macroscopic scale, the phloem sapof sugar palms and agaves is tapped for sugar production or fermentationinto an alcoholic drink. One tree can exude for several monthsand yield several thousand litres of sap during this period. This ismanifestly a mass flow of liquid! Moreover, the sugar concentrationremains steady during the period of exudation in both the aphidstylet exudate and in the palm sap, so that the flow of liquid cannotbe attributed to a leakage of water into punctured sieve tube cells fromthe nearby xylem. If mass flow is rejected, all the above-mentionedexudations of sap must be ascribed to an injury reaction unrelated tonormal translocation in the phloem. NMR techniques (Section 5.3.1)have demonstrated water flow in the phloem in intact plants.The Münch hypothesis requires that sieve tube contents should beunder a positive turgor pressure, and that there should be a gradientof turgor pressure (and of osmotically active solutes), decreasing fromsource to sink. The sieve tubes are certainly under positive turgorpressure; this is demonstrated even by the simple fact that sap canexude from cuts. Aphid stylets contain only a very narrow channel,offering a considerable resistance to the flow of the viscous sap; apressure of 1–3 MPa is required to force the sap through the stylets.Direct measurements of the turgor of sieve tubes have been achieved,either by insertion of sensitive pressure probes, or by the externalapplication of pressure cuffs (analogous to the apparatus used formeasuring blood pressure). Such measurements are in practicefraught with great difficulty; nevertheless, not only have positivepressures been recorded, but some workers have succeeded indemonstrating gradients of decreasing turgor passing down trees,away from source leaves. Gradients of sugar concentration have beenobserved. In soybean (Glycine max(soja)) stalks, for example, the sugarconcentration in the sieve tubes of the leaflet stalk has been found tobe 10.5–12.5%, when the sieve tubes of the root contained only4.4–6.3%.All this is good evidence for the Münch mass-flow hypothesis. Butthe hypothesis cannot be accepted until it has been shown to befeasible quantitatively as well as qualitatively: until it is shown thatmass flow at the experimentally measured velocities can be driven by theactually existing pressure gradients, through channels of the dimensionspresent in the sieve tubes. It is difficult to prove this with completecertainty, the main problem being the interpretation of the finestructure of the sieve plate area.