The Physiology of Flowering Plants - KHAM PHA MOI

154 TRANSLOCATION OF ORGANIC COMPOUNDSsieve tubes. Different functions for the sieve plates have beensuggestedinconnectionwithalternativeviewsonthemechanismofphloemtranslocation.Alternatives to the mass-flow hypothesisAlternatives to mass flow have from time to time been proposed. Theconcept of spreading along interfaces visualizes the translocated moleculesforming a monomolecular film on surfaces (P-protein fibrils?),like oil films at an air–water interface. This film would then remainintact, with molecules added at the source end and removed at thesink. The hypothesis does not account for the flow of water, whichdoes occur in the phloem. It is also difficult to conceive of surfaces ableto form films with all the variety of compounds that can be carried inthe phloem – sugars, amino acids, mineral ions, ATP, fluorescent dyes,hormones, etc. Some sieve tubes moreover contain very little P-protein.Protoplasmic streaming has been claimed to proceed in the sievetubes, with transcellular strands running from cell to cell through thesieve plate pores, but after the original observations were published inthe 1960s and 1970s, other workers have been unable to confirmthem. The apparent streaming rates of 3.5 cm h –1 are also far tooslow compared with the velocity of translocation which reaches severalm h –1 . Submicroscopic tubules passing through the sieve platepores, and pumping the tubule contents along by contractions (as inperistalsis) have been proposed without any solid evidence; the samegoes for the idea of lashing protein filaments, like miniature cilia,anchored to the outsides of the hypothetical tubules and to sieve tubewalls, wafting along a flow in the bulk of the cell. The theory ofelectroosmosis regards the sieve plates as pumping stations for ions: anelectric potential difference (PD) is built up across the plate, by pumpingof H + ions from companion cells into the sieve tube upstream ofthe plate, and out of the sieve tube into the companion cells downstreamof the plate. This would result in the movement of the positivelycharged K + ions in the phloem sap across the plate, towards thenegatively charged downstream side and the K + ion flow would carrywith it water and other solutes. The attractiveness of the electroosmotichypothesis is that it ascribes both a specific function to the high K +content of the phloem sap, and a physiological function to the sieveplates. But the idea has no experimental data to support it. Theenergy expenditure for the pumping would be high; there wouldhave to be localized differentiation of sieve elements and companioncells with the direction of H + ion movement between thecells reversed over a micrometre or two near sieve plates, and allother reported cases of H + pumping involve pumping of the ions outof cells. Quantitatively, too, with sieve tube cells often over 100 mmlong, it seems hardly credible that so much force could be built upby sieve plates so far apart. In summary, no viable alternative toturgor-driven mass flow in the phloem has been proposed. But it isconceivable that in species with very narrow sieve plate pores thereis facilitation of mass flow through the pores from some mechanism