The Physiology of Flowering Plants - KHAM PHA MOI

THE MECHANISM OF PHLOEM TRANSLOCATION 153has resulted in formation of P-protein strands and depositionof P-protein material on sieve plates rather than loss of discretestructures. This was observed by confocal microscopy when sievetubes were deliberately wounded (Knoblauch & van Bel 1998). Theblocking of pores on wounding confines leakage of sap to thedamaged cell.Metabolic activity in the phloemAccording to the original mass-flow hypothesis, the sieve tubesact as passive conducting channels, with no necessity for theexpenditure of metabolic energy on the way, for the energyinput would take place at the source during loading and possiblyalso at the sink during unloading. Normally the transport phloemhas a high rate of respiration; sieve tube sap contains ATP at anaverage concentration of 0.4 mmol L –1 and the turnover rate ofATP is high, suggesting an expenditure of ATP all along thepathway. This has been quoted as an argument against themass-flow hypothesis. It has become clear, however, that allalong the transport route there is leakage of translocate, andwhilst some of this goes to nourish the axial tissues, a majorproportion is retrieved by energy-requiring membrane pumps. Ahigh rate of turnover is claimed for phloem proteins and proteinsynthesis requires an energy supply. Perhaps the unusual state ofthe sieve tube cells, lacking nuclei, and traversed by a rapid flowof sap, requires a higher than usual rate of maintenance respiration.The high respiratory rate in the phloem therefore cannot betaken as a contraindication to mass flow driven by an osmoticgradient built up locally in the loading region.The function of the sieve platesFor mass flow, sieve plates appear to be obstructions and theirpresence has been interpreted as being against the mass-flowhypothesis. They do nevertheless perform an important functionin preventing sap loss from damaged phloem: on injury, thedeposition of callose narrows the pore diameters, P-proteinsurges towards the cut because of release of turgor and thenpiles up against the sieve plates forming slime plugs over thealready narrowed pores, making them impassable. Plastids burst,releasing starch grains which help to plug the pores, and possiblythey also release chemicals which promote P-protein coagulation.As noted (Section 5.2.3), from most species there is little exudationfrom cut phloem in spite of the fact that the sieve tubecontents are under considerable hydrostatic pressure. The systemcan be desensitized by repeated rubbing or even beating in thecase of robust woody specimens so that cutting no longer inducesblockage, and this effect is utilized when sugar palms are tapped.The sieve plates may also give some mechanical support to the