Phloem transport in the stolon of Saxifraga sarmentosa

Abstract

A study was made of assimilates and ionic tracer movementalong the stolon of Saxifraga sarmentosa. When ionic tracers or 14 C-sucrose are applied to the transloadting stolon they undergo a longdistance transport which is strictly polarised, with a short-distance movement which is symmetrical with respect to the point of application. The distribution along the axis after a suitable length of time shows an accurate exponential fall-off pattern, except with naturally assimilated or applied 14C-sucrose where it is fairly level. In this case the linear velocity of movement was estimated as about 20 cm h-1.

Transport of materials is readily reversible by interchanging the roles of parent and daughter plants as source and sink. The transpiration stream can be similarly reversed. By these means it was confirmed that 14 C-assimilates and 137Cs move in the phloem, whereas 89Sr moves in the xylem. The sieve tubes are thus not inherently polarised in this organ.

Inhibitors such as nitrogen, cyanide and DNP applied overlengths of 20--30 cm exert a more or less complete and reversible inhibition of the movement of 14C-assimilates and 137Cs. Cyanide gas is considerably more effective than solution. A variety of approaches, including the use of 14C-cyanide, confirmed that the inhibition was effective in the sieve tubes themselves, and not merely at the terminal sites. Electron microscopic examination showed that callosing in the sieve plates was not involved; however, in all cases, including the normal one, the sieve plate pores appeared blocked with P-protein.Tests with Valinonycin which possesses a particular affinity for potassium, were ineffective. This may have been due to its insolubility in water and to the large size of the molecule.

Comparative studies were carried out by applying two ionic tracers together. These gave consistent and precise results in termsof the slopes of the plots log activity against distance Of the tracers used (22Na, 42K, 86Rb, 137Cs and 82Br) the comparison of 137Cs and 82Br was perhaps the most interesting; the anion is transported more or less equally with the cation, but its lateral leakage appears to be less.

The results of this study provide a well-established case for the following conclusions relevant to the controversy over mechanism: (1) Phloem transport in the stolon is strictly unidirectional. (2) Nitrogen, Cyanide and DNP exert a strong effect localised in the sieve tubes themselves (as well as at the terminals); it is reversible. (3) The inhibition of transport is not due to callose blockage. (4.) Anion transport follows a similar pattern to cation.

These conclusions are very adverse to the Munch hypothesis, and to any diffusion-analogue theory; they favour a theory of activemass flow. The electro-osmotic theory faces the difficulty of anion transport.