Localisation of adenosine triphosphatase activity in mature sieve elements of Tetragonia.

In the mature phloem of Tetragonia expansa strong ATPase activity is present on the plasmalemma of the companion cells and sieve tubes, and probably on the ER of the latter. The sieve tube wall sometimes shows a lead deposit. When the tissues is pre-treated with DNP there is often a diffuse deposit also on the P-protein and on the fibrillar protein from ruptured plastids. This is believed to be a diffusion artefact, and not to represent genuine P-protein activity.

The influence of dinitrophenol on phloem transport along the stolon of Saxifraga sarmentosa.

Dinitrophenol in concentrations of 5×10(-3) M applied to the centre 30 cm of 60-70 cm stolons of Saxifraga produces a strong and reversible inhibition of the phloem transport of (137)Cs or (14)C-assimilates. There is every reason to believe that this effect is localised in the sieve tubes; callose formation does not occur. This evidence is very difficult to reconcile with the Münch hypothesis; it seems on the contrary to demand a theory of active pumping.

The simultaneous movement of two ions in the phloem of the Saxifraga stolon.

Paris of tracers were applied simultaneously to the long thin stolons of Saxifraga sarmentosa. After several hours of translocation the very precise pattern of exponential fall-off was examined and interpreted in the light of a model of mass flow with leakage. (42)K appears to leak faster than (22)Na; (86)Rb is very close to (42)K. The anion (82)Br shows a lower fall-off than (137)Cs; this is tentatively regarded as due to a much-reduced leakage, though it might imply a higher velocity. The implications of these findings for sieve-tube mechanism are uncertain.

The effect of nitrogen on the movement of tracers down the stolon of Saxifraga sarmentosa, with some observations on the influence of light.

The movement of applied (137)Cs and of naturally-assimilated (14)C down the long stolon of Saxifraga is strongly inhibited by confining a length of 10 to 30 cm of the stolon in an atmosphere of nitrogen. The inhibition is reversible, normal transport being restored after less then 4 h when the stolon is returned to air from 5 h in nitrogen. Callose formation does not seem to be involved. There is evidence that local darkness has a similar adverse effect on phloem transport.These findings are considered antagonistic to the pressure-flow hypothesis, but favourable to the active mass-flow theories.

Movement of [(14)C]sucrose along the stolon of Saxifraga sarmentosa.

The characteristics of (137)Cs transport along the stolon of Saxifraga previously reported have been confirmed for applied sucrose and natural assimilate. Long-distance transport is strictly unidirectional, with a symmetrical short-distance spread from the point of application. Only the latter takes place in a long piece of excised stolon. Transport is readily reversed when the parent plant is darkened and the daugther, plantlet allowed to photosynthesise. These findings strongly support a mass-flow mechanism for the stolon. They also confirm the value of (137)Cs as a tracer for assimilate movement, though in contrast to assimilate it suffers appreciable lateral leakage. Pulse labelling of the subtending leaf failed to produce a sharp peak of activity in the stolon. A flattening with time of the (14)C profile is considered to be due to differing linear velocities in parallel sieve tubes.

The effect of protease digestion (in situ) on the slime substance of mature sieve tubes.

The sieve tube slime of Tetragonia and Lycopersicum petiolar phloem was subjected to proteolytic enzymes both before and after embedding and sectioning. Subsequent electron microscopical examination confirmed that the fibrillar material is protein in nature, and provided evidence that the protein is acidic rather than basic.

Unidirectional movement of tracers along the stolon of Saxifraga sarmentosa.

When radioactive tracer is applied locally to the stolon of Saxifraga its long-distance movement after 18 hours is found to be strongly polarised; there is in addition a short-distance movement which is unpolarised. With caesium, the long-distance movement is predominantly in the phloem; with strontium in the xylem. These interpretations, a priori probable, were confirmed by artifically reversing, separately, the xylem and the phloem currents. With long pieces of excised stolon only the unpolarised short-distance movement is observed. These results constitute evidence against simultaneous bidirectional translocation in the same sieve tube, and are consistent with either the Münch or the electro-osmotic theory.

The state of the pores in functioning sieve plates.

Electron microscopy of Helianthus stem and Saxifraga sarmentosa stolon after first plunging the material into boiling water indicates that the natural condition of the sieve plate pores is to be plugged fairly compactly with slime substance. Electron microscopy of pre-wilted plants demonstrates the same thing. These and other arguments indicate that the slime-filled condition of the pores is not an artifact induced by enzyme action or turgor release, nor is it a consequence of rapid translocation of slime; it is the natural state. This conclusion, very damaging to the pressure-flow hypothesis, is favourable to the electroosmotic theory.

The sieve tube wall and its relation to translocation.

The sieve tube wall possesses a broad inner layer often with pronounced radial striations. The plasmalemma of the sieve tube appears to penetrate this wall in the form of a "brush border" of irregular microvilli, greatly increasing its surface area. It is suggested that this is the site of active transport of potassium, which circulates electroosmotically through the sieve plate pores and back through the thick wall. The function of the companion cells is the "care and maintenance" of the active "brush border" sites; in conjunction with their activity in supplying high-energy intermediates movement in the column acts regeneratively and fully polarises the plates. Many of the lamellar stacks and curvilinear membrane aggregates hitherto regarded as endoplasmic reticulum are, it is suggested, plasmalemma displaced from the wall. These findings have important consequences for the electroosmotic theory.

The fine structure of the sieve tubes of Salix caprea (L.) and its relation to the electroosmotic theory.

The sieve plate pores of Salix caprea in preparations fixed in glutaraldehyde are normally found to be occupied by slime fibrils showing periodic banding such as occur in a number of other species. Arguments are put forward to suggest that the occurence of fibrils in this position is natural and not an artefact of preparation. The sieve tubes further possess prominent and persistent nucleoli showing a radiating structure of tubules. The endoplasmic reticulum often occurs in parietal stacks reminiscent of other species.This evidence is discussed in relation to the electroosmotic theory of translocation.

Electro-osmotic and biopotential measurements on phloem strands of Nymphoides.

Electroosmotic measurements on the excised vascular strand from the petiole of the water plant Nymphoides peltatum have been carried out, and the Onsager coefficients obtained. About 120 water moleculer per ion are carried electroosmotically when the strand is in 10(-4) M KCl, about 30 in 10(-1) M KCl. Potential measurements made on an intact and functioning petiole are discussed in the light of the pressure-flow and electroosmotic theories of translocation.