Inorganic ions modulate the path of phloem loading of sucrose in Ricinus communis L seedlings

The impact of inorganic ions on sucrose fluxes in the cotyledons and on the pathway of phloem loading was studied in Ricinus communis L. seedlings. Th e cotyledons were incubated in defined solutions which contained either pot assium, sodium, magnesium or calcium as chloride salts, or the sodium salts of sulphate or phosphate. Sucrose uptake from the medium into the cotyledo ns was only slightly affected by the salts. Sucrose efflux to the medium wa s increased by phosphate and sulphate and to a lesser extent by sodium and potassium. Phloem loading from the apoplasm and the symplasm was analysed b y addition of labelled sucrose to the medium, determination of the specific radioactivity of sucrose in sieve-tube exudate and quantification of expor t into the seedling axis. Potassium and sodium stimulated the apoplasmic ro ute of phloem loading of sucrose, mostly at the expense of loading from cot yledon sucrose pools. In contrast, sulphate and phosphate strongly inhibite d the apoplasmic route whereas the (small) symplasmic flux from the cotyled on sucrose pools was less affected. Magnesium ions inhibited phloem loading by both pathways. The potential of ions in modulating the pathways of sucr ose export in day to day operation of plants is discussed.