NEW APPROACHES TO STUDYING SIEVE TUBE PHYSIOLOGY USING THE APHID STYLET TECHNIQUE

New techniques were developed to measure turgor pressure and membrane potentials continuously and directly in functioning sieve tubes. The techniques were then used to make preliminary investigations into the role of turgor regulation in sieve tubes.;In one set of experiments, using intact plants, exudate osmotic potential was estimated from its refractive index and sucrose content while phloem water potential was estimated from leaf water potential. In these experiments the measured turgors were almost always greater than the calculated values. The discrepancy between the two was most likely due to the presence of appreciable amounts of potassium and amino acids in the exudate.;In a second set of experiments, using bark strips, the exudate osmotic potential was calculated directly from its freezing point depression while phloem water potential was taken as the potential of the solution irrigating the cambial surface. In these experiments the calculated and measured turgor agreed within experimental error.;Sieve tube membrane potentials were measured in the phloem of willow bark strips by inserting a measuring microelectrode into sap exuding from severed aphid stylets. Data taken from 20 bark strips gave an average potential of -155 (+OR-) 9 mV. Evidence is presented for an electrogenic component of the sieve tube membrane potential. The occurrence of a saturable sucrose-induced membrane depolarization and the inhibition of this depolarization by p-chloromecuribenzene sulfonic acid is consistent with the concept of sugar accumulation by a sucrose/H('+) cotransport mechanism.;Turgor pressures were measured directly by gluing capillary micromanometers over exuding aphid stylets with cyanoacrylate adhesive. Pressures of up to 12.5 bars were measured in willow sieve tubes with an estimated experimental error of (+OR-) 0.3 bars or better.;To investigate the possibility of osmo- or turgor-regulation in willow sieve tubes, the response of the turgor pressure to changes in external water potential was measured. Because a relatively large flow of exudate through a stylet is required to attain pressure equilibrium in a micromanometer, pressure changes were measured directly by using a pressure transducer sealed to an exuding stylet. Rapid changes external water potential resulted in prompt changes in turgor pressure. Successive decreases in potential gave successive decreases in turgor but the change in pressure was usually only 60-80% of the change in the external water potential.;When a solution of low water potential was applied for several hours to the cambial surface of bark strips, the turgor was reduced and remained constant after its initial decrease. No increase in turgor was observed even after 10.5 hours, indicating that no accumulation of solutes had occurred.;The lack of a 1:1 relationship between the change in water potential and the change in turgor pressure is tentatively attributed to the change in the sieve tube volume. Thus the loss of turgor was partially compensated by the wall elastic modulus and a resultant slight increase in the concentration of the sieve tube contents. No evidence was found for active osmoregulation in the sieve tubes of willow bark strips.