Potassium cycling in Helianthus : ions of the xylem sap and secondary vessel formation

Abstract

Recent work has shown that dyes travelling in the transpiration stream in dicotyledon leaves become concentrated in the vessels of the finest veins where water enters the symplast. Such concentrations are called sumps. Using X-ray microanalysis of frozen-planed Helianthus leaf tissues, this paper investigates whether natural ions in the transpiration stream behave similarly and become concentrated in the fine veins. The most abundant ion in the xylem sap was potassium (K): concentrations of up to ~ 200 mM were found in some vessels of some leaf veins. Occurrence of such high concentrations was irregular and unrelated to vein order, leaf age, time of day or transpiration conditions. High K concentrations were not especially characteristic of the fine veins, and it appears that sumps are not formed (as with the dyes) by selective uptake of water to the symplast: absence of K sumps is probably the result of uptake of K by the cells surrounding the vessels. The origin of K was sought in the stem, where evidence was found that differentiating secondary vessel elements accumulate very high concentrations (~ 500 mM) of K, releasing it into mature open vessels when they mature themselves. I propose the hypothesis that the K in the leaf vessels is derived from the K of the maturing secondary vessel elements of the stem. It arrives irregularly because the vessel maturation is spasmodic and the destiny of the released K depends upon the particular downstream connections of the new vessel to leaf traces. I further propose that the K in the leaf veins is taken up by bundle sheath cells and phloem parenchyma cells, and part of it is returned via the phloem to the cambium of the stem where it may be used again to provide osmoticum in the expansion of newly differentiating secondary vessel elements. When high concentrations of ions are present in vessels that are embedded in tissues whose cell walls have non-zero reflection coefficients (low diffusivities), osmotic pressures would develop in them. Such pressures may counteract tensions in the xylem sap generated by transpiration, and help to account for the small values of these tensions measured recently with the xylem-pressure probe.