Sodium fluxes in sweet pepper exposed to varying sodium concentrations

The sodium transport and distribution of sweet pepper (Capsicum annuum L.) under saline conditions were studied after transferring the plants to a sodium-free nutrient solution. Sodium stress up to 60 mM did not affect the growth of sweet pepper, as it appears able to counteract the unfavourable physiological effects of sodium efficiently. Sodium was particularly accumulated in the basal pith cells of the stem and in the root cells, while almost no sodium was directed to the leaves or the fruits. The sodium concentration in the pith cells and xylem sap gradually decreased towards the shoot tip. Removal of sodium from the medium resulted in a 50% release of sodium from the plant after 1 week without affecting the gradient in the pith cells. In contrast, the concentration profile in the xylem sap was completely changed: the sodium concentration in the xylem sap at the stem base was similar to that at the top. Phloem transport was studied in a split root experiment, in which both portions of the roots were exposed to 15 mM NaCl and one part was fed with additional 22NaCl. During continuous exposure to 15 mM NaCl no label was detected in unlabelled root parts. However, after transferring the plants to a sodium-free solution, 22Na was rapidly released from the unlabelled roots, indicating a downward phloem transport. It was concluded that pith cells, the intermediates between the xylem and phloem, play a decisive role in the recirculation of sodium throughout the plant. Release of sodium from the plants following transfer to a sodium-free solution may be explained by changes in the diffusion resistance for passive sodium efflux from the cells.