Sodium fluxes in sweet pepper exposed to varying sodium concentrations

The sodium transport and distribution of sweet pepper (Capsicumannuum L.) under saline conditions were studied aftertransferring the plants to a sodium-free nutrient solution. Sodium stressup to 60 mM did not affect the growth of sweet pepper, as it appears ableto counteract the unfavourable physiological effects of sodium efficiently.Sodium was particularly accumulated in the basal pith cells of the stem andin the root cells, while almost no sodium was directed to the leaves or thefruits. The sodium concentration in the pith cells and xylem sap graduallydecreased towards the shoot tip. Removal of sodium from the medium resultedin a 50% release of sodium from the plant after 1 week without affectingthe gradient in the pith cells. In contrast, the concentration profile inthe xylem sap was completely changed: the sodium concentration in the xylemsap at the stem base was similar to that at the top.Phloem transport wasstudied in a split root experiment, in which both portions of the rootswere exposed to 15 mM NaCl and one part was fed with additional²²NaCl. During continuous exposure to 15 mM NaCl nolabel was detected in unlabelled root parts. However, after transferringthe plants to a sodium-free solution, ²²Na wasrapidly released from the unlabelled roots, indicating a downward phloemtransport.It was concluded that pith cells, the intermediates between thexylem and phloem, play a decisive role in the recirculation of sodiumthroughout the plant. Release of sodium from the plants following transferto a sodium-free solution may be explained by changes in the diffusionresistance for passive sodium efflux from the cells.Keywords: Xylem, phloem, sodium, fluxes, sweet pepper