Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues4: II. PATHWAYS OF ION TRANSPORT FROM THE XYLEM TO THE PHLOEM

It has been shown that the time lapse of 2–3 hours foundby Peel (1963), between the introduction of an inactive ioninto the xylem of a stem segment and its appearance in honeydewcollected from that segment, is not an artefact caused by detectionmethods. A delay of similar magnitude was found in the presentexperiments when radioactive rubidium, ³²P-phosphate, or ³⁵S-sulphatewas used. Pretreatment of stem segments with the respectiveinactive ion caused a very considerable increase (20 hours)in the time lapse when the radioactive ions were subsequentlysupplied to the segment. Experiments with bark strips demonstratedthat the time lapse was due to the presence of a barrier tothe radial movement of ions from the xylem of segments, sinceusing bark strips the time lapse was reduced to less than onehour. This barrier could either be the xylem itself, the cambium,or both tissues. Pretreatment of bark stnps with inactive ionsincreased the time lapse in the case of ³²P-phosphate and ³⁵Ssulphate, but had no detectable effect with radioactive rubidium.It is considered that these results indicate that rubidium maybe able to pass directly into the sieve tube from the bathingsolution in bark strips. Phosphate and sulphate, however, mayhave to move into the sieve tube via the storage cells of thebark. It has been shown that phosphorus-32 activity in the sievetube is not confined to inorganic phosphate, but is certainlypresent in other compounds of which three sugar phosphates havebeen identified.     

A comparison of spike and spikelet survival in mainstem and tillers of barley

Initiation and abortion of spikelet primordia on primary tillers and mainstem were compared for plants of spring barley, Maris Mink, sown on two sowing dates during 1983, in plots at the Scottish Crop Research Institute (SCRI) and at Long Ashton Research Station (LARS). Sowing date had no effect on the number of spikelets initiated on either mainstem or surviving tillers and many more spikelets were initiated on equivalent stems at LARS than at SCRI but more spikelets aborted at the former site. The final number of spikelets produced was eventually similar. Spikelet abortion was complete by anthesis and coincided with the period of rapid stem and rachis elongation and with the time over which tiller death occurred. Despite the fact that plants at SCRI produced fewer fertile tillers they outyielded those at LARS because much heavier grains were produced on the mainstem and tiller 1 of plants at the Scottish site. An interval of 2 wk between sowing had only small effects on the components of yield.     

Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues3: I. INTERFERENCE BETWEEN SOLUTES AND RATE OF TRANSLOCATION MEASUREMENTS

Certain aspects of the secretion of solutes into, and removalfrom, the sieve tubes of isolated stem segments and rooted cuttingsof Salix viminalis have been studied. Sieve-tube sap was obtainedeither as honeydew from whole individuals or via the severedstylets of the aphid Tuberolachnus salignus (Gmelin). It was shown that interference occurred between the chemicallyunrelated solutes, sucrose and the cations potassium and rubidium.On raising the potassium concentration in the sieve-tube sapby passing a solution of this ion through the xylem, the sucroseconcentration declined. When the sucrose concentration fellover a period of days due to respiratory loss of carbohydratesfrom an isolated stem segment, a concomitant rise in eitherthe potassium or rubidium level in the sap occurred. When a solution of sodium was passed through the xylem, theconcentration of this ion in the sieve-tube sap rose, whilstthat of potassium fell at first, but later rose higher thanits initial value, indicating that both antagonism and synergycan occur between these ions. On introducing both these cationsinto the xylem simultaneously, more sodium than potassium wastaken up by the segment, though the increase in the sodium concentrationin the sieve-tube sap was less than that of the potassium. Perfusingthe xylem with a calcium solution had no effect upon the concentrationof potassium in the sieve tube. It has been shown that the rate of translocation of a solutealong the sieve tube, as measured by the two colony technique,depends upon the rate of removal of this solute from the sievetube. The amount of such lateral loss from the sieve tube isrelated to the potential gradient for a solute between the sievetube and surrounding cells.